With reference to fissures in a slope which tend to produce slips as

they allow water to trickle down them, which must either be absorbed by

the earth or find an outlet; a slippery surface is thus created and the

tenacity and continuity of the soil impaired or destroyed; separation

also takes place in non-homogeneous earths such as boulder clay and in

embankments formed of rock and earth tipped together. It is practically

impossible to fill or pun every fissure that may appear in a cutting or

an embankment, but as there are generally places where slips are more

probable than others, it may be advantageous in treacherous soils to

adopt a regular system of filling the fissures, especially before the

wet season commences.

Coverings can also be made of a thin coating of burnt ballast, hard

chalk, or gravel, which will reduce the number of cracks or crevices, or

a mattress of fascine work can be used at the toe of the slope in

submerged work, but care should be taken that the mattresses overlap and

that there are no open places between them, or, instead of being a

protection they will then be a source of danger by conducting water

between the joints; for this reason, as with any other material having

loose openings, it is advisable to use them over a continuous surface so

that percolation may be uniform and not simply for weak places or for

the purpose of repairing a slip in an earth embankment. The chief aim in

fascine work is to thoroughly bind the work together so that it is of

equal strength in all directions, and a little time after construction

should be allowed before deposition in order that the material may

settle, as an even surface is important. The most durable material

should be used in making a fascine covering, willow being the best, or

it may require constant renewing. Alder, aspen, and the best available

brushwood are also employed, and straw and ordinary matting for shallow

embankments, say up to 8 feet in height, which lasts only six to twelve

months. The best time for cutting should be locally ascertained, and

when they commence to deteriorate after being hewn, generally from three

months to a year, depending upon the season in which they are cut, &c.,

&c. In England, thorn switches have been used in lengths of 5 to 6 feet,

tied up with tarred rope in bundles having a diameter of about 1 foot,

every endeavour being made to bind and interweave them. Care must be

taken that the mattresses are well loaded or they will float; the

loading should commence at the centre, and be equally continued in all

directions, as that has been found to be the best method; they should be

made to sink evenly if they have to be lowered through water, and they

must be prevented from curling up at the edges. The props which fix them

must not be too close to the border, or near the centre, as then the

extremities will be torn away or bent upwards. The system of fascine

work may be very useful for protecting sandy and soft foundations in

such situations, to prevent slips in the slopes of an embankment and in

providing a firm bed, and also for making training banks, groynes, and

spurs, for correcting and directing a current in a desired channel, and

to secure freedom from slips in a river-bank. Depending upon the degree

of looseness of the sand or sandy bed, loaded fascine mattresses will

assume a slope of from 1 to 1 TO 5 to 1 if allowed to sink in the bed,

and after they have settled and reached the angle of repose which the

action of water will effect; the large number of structures standing

upon them for many years in considerable depths of water proves, when

they are properly made, that they are to be trusted in comparatively

unexposed situations.

A covering of stone pitching may be necessary, but in unsubmerged work

it has some disadvantages for, having joints, it allows unequal

penetration of water through them and impedes the equal discharge of

water from the earth, although its weight is a recommendation as it

opposes and may balance a pressure of water behind the slope. However,

unless it is bedded upon a layer of soil of equal permeability, such as

gravel laid upon a nearly impermeable bed of clay, which should always

be mixed with sand to prevent it fissuring and bursting by heat or

water, so as to convey the water that has percolated and also that which

exudes through the slope; it may cause a localization of the flow, and,

except in peculiar cases and provided the slopes are covered, there is

no occasion for pitching if merely used to prevent slips and not

erosion, as obviously weight and a secure protection may be obtained by

other means and at less expense. For the protection of the slopes of

rivers or canals or submerged work the case is different, as then

pitching prevents erosion and may be the only secure preservative in

exposed situations, although generally the most expensive. The pitching

should rest upon a bed of permeable material, and this layer should have

a power of suction and distribution more than equal to the quantity of

water that may penetrate the joints of the pitching; there should be a

bed of impermeable material next to the soil, and in exceptional cases

even two to carry off any water that has percolated, so as to obviate

any lodgment of water, due provision being made for the land drainage

discharge.

Sir James Brunlees, Past-President Inst. C.E., found by experiment that

at a slope of 2 to 1 pitching has the greatest resistance to extraction,

_i.e._, it requires a greater effort to extract a brick at that slope.

Taking the slope of 2 to 1 as unity, the relative resistances were found

to be as follows:—

1 to 1 slope 0·71

2 to 1 slope 1·00

3 to 1 slope 0·97

4 to 1 slope 0·66

The resistances at 2 to 1 and 3 to 1 are practically the same.

Should it be determined to pitch the lower part only of a slope, in

order to prevent slips, the pitching must continue to such a height that

a flow of water down the slope is impossible, or it may become detached.

If the pitching is not rough squared on the joints, but of various

forms, it is preferable that the face having the largest area be laid

downwards, smaller stones being carefully wedged in between the

interstices. Any defective execution is usually followed by a falling of

the stones, and should this happen the surface will be broken and

erosion and slips will ensue. In laying the stones they should be so

placed that if a few become removed those above or upon the sides will

not be disturbed, and in loose soil no pitching should be laid until an

embankment has had time to settle and consolidate, which the necessities

of rapid execution may prevent, for however even the pitching may be

when first laid, it will settle and become more or less uneven in such

soil as sand or estuary slake, and every effort should be made to leave

no hole or exposed surface, but to cause a continuous close covering.

Fascine mattresses may have to be used in such a situation, as they will

follow the contour of the slope. Should the slope be subject to

considerable wave action, smooth surfaces offering the least frictional

resistance, obviously aid the travel of a wave, which is undesirable,

and provided the embankment is sufficiently strong a rougher surface is

to be desired. Projecting stakes tend to subdivide a wave.

Should pitching be required to be placed partly upon the face of a

cutting and partly upon the slope of an embankment, as in the case of a

canal upon sidelong ground, care must be taken that it does not settle

unequally, and provision should be made by setting it upon a pervious

layer, so that any damming back of drainage waters may be prevented. The

weight of pitching upon a slope affords a counter-pressure to that of

any water in the slopes which may be trying to emerge. Chalk rubble,

also hard chalk, or gravel has been used in lieu of stone pitching for

protecting slopes against erosion.

With the exception of retaining walls, the preceding may be considered

as the principal means, used separately or in reasonable combination,

for covering the surface of a slope in a cutting or an embankment.

A frequent cause of instability of the slopes in countries having severe

winters is the melting of unequal masses of snow, the result of drifts,

and also from the thawing of a considerable snowfall. The chief

preventive measures against snowdrifts and consequent protection to the

slopes, although not a covering, may be stated to be as follows:—

1. Locate the line in a naturally sheltered position, which it is most

improbable can be done throughout its length.

2. Adopt tunnels, covered or sheltered galleries where the district is

subject to avalanches or snow-slips.

3. Prevent drifts by permanent or portable screens such as earth mounds,

trees, hedges, fences, &c.

4. Obstruct drifts by having deep trenches some distance from the top of

the slopes.

5. Avoid cuttings as much as possible, and make the line upon an

embankment of such a height as to be above the depth of the drift snow

in similar situations.

6. Construct the embankments from side cutting, instead of cutting, so

that the trenches may catch the snow before it reaches the railway.

7. Make the slope much flatter than its angle of repose.

8. Increase the width of the formation so as to lessen the depth of a

drift, and to enable a snow-plough to more readily discharge its

excavation.

With regard to the preventive measures enumerated.

No. 1.—As a rule this can only be practised to a limited extent

consequent upon traffic requirements and the configuration of the

country.

No. 2.—Shelter-galleries are found to be generally effectual, but

obviously are expensive; they may, however, be the only means to adopt,

as any obstruction to a snow-slip is avoided. The slope of the top of a

covered or shelter-gallery should not be steeper than that of the hill,

so as to offer no impediment to the free passage of the snow.

No. 3.—Experience has proved that permanent screens are to be preferred

to portable shields, and that ultimately they are the more economical.

The excavated material, either from a cutting, drains or fence-ditches,

is generally used for the earth mounds so as to save expense; their

height, which is often from 7 to 10 feet, of course is governed by the

greatest depth of the snowfall, degree of exposure and usual direction

of storms, wind, and drift. Should these vary much, in an open air line

it may be impracticable to entirely shut out or prevent drifts, but the

protective works will lessen drifting and may reduce it to manageable

proportions.

A simple earth dam is seldom sufficient, owing to the impelling power of

the wind upon the snow-flakes and the considerable height at which it

acts; therefore, fencing has been fixed in the top of the mound, or

trees and shrubs planted, also old sleepers have been successful when

simply closely driven into the surface of the original ground. A screen,

consisting of one or more rows of trees or shrubs placed at a little

distance from the top of the slope of a cutting, is that most generally

preferred. It is made either double or single, as may be thought

necessary. In Germany, hedges of fir trees similar to Christmas trees

have been found to afford excellent protection. In France, pine trees

planted upon the cess have effectually prevented serious snowdrifts.

In sloping ground, mounds or dams, screens, plantations of trees and

hedges in very exposed places have been insufficient to prevent some

drifting and accumulation of snow in a cutting, and a mound or fence has

consequently been erected upon the plateau at the top of the hill a

sufficiently safe distance from its face that snow-slips, which might be

increased into avalanches, were prevented from reaching the slopes or

formation. A drain should be cut upon the higher side of such a hill-dam

or fence so as to carry away the snow-water upon a thaw, and prevent the

earth becoming in an unstable condition.

No. 4.—This system is generally used when an earth mound is also adopted

and it has to be deposited from side cutting. The trench, of course,

should be on the side of the mound farthest from the main cutting. As a

snow-catcher it is successful, especially in shallow embankments, for it

not only retains snow that would otherwise heap, but drains an

embankment.

No. 5.—This depends upon the configuration of the country, but as

cuttings act as traps to catch snow, if possible they should be avoided;

and particularly when of little depth, as it is found they quickly

become choked, and want as much protection as deeper excavations, and

from being more exposed to the cold air, the snow in them becomes caked

and frozen, and requires breaking up before it can be removed.

Embankments of a height a little above the ground, and the greatest

depth of the uniform snowfall of the country, are to be preferred.

No. 6.—Generally approved, as not only affording an advanced

catch-trench, but because it acts as a permanent drain.

No. 7.—This is a controverted system. Some engineers object to it as

facilitating the deposition of snowdrifts; others approve, upon the

ground that by flattening the slopes the snow can drift freely and will

fall equally upon the formation. The balance of experienced opinion

seems to be rather against the adoption of flattened slopes, unless they

are made very flat, such as from 4 to 1 TO 10 to 1, at which latter

slope snow it is found does not usually accumulate but passes on

depositing only its general depth: and additional means of protection

are afforded, and appears to indicate that the unaided system is only

well adapted for countries in which winds of great force are generated,

such as the “blizzards” of North America.

No. 8.—It is found that increased width of the formation of a cutting is

an advantage, as it retards choking, facilitates clearing operations,

and provides room for shovelling snow from the permanent way, whether

effected by a snow-plough or by manual labour.

With regard to the formation width, _i.e._, the width of the bottom of a

cutting or the top of an embankment, and the prevention of slips in

earthwork, ample breadth is necessary in cuttings for the purposes of

drainage, although less in a rock cutting than in that of ordinary soil,

and the extent of the top of an embankment has some influence in the

promotion of stability. The required width of the formation must be

principally regulated by the character of the earth, the amount and

suddenness of the rainfall, the height of an embankment or depth of a

cutting, the degree of exposure, the exigencies of the traffic, and the

required drainage. In wet cuttings the formation should be wider than in

dry earth, and the side ditches should be made larger, especially when

there is a steep gradient in a long cutting, in order to keep the

formation and the permanent way in as dry a state as possible and aid

traction, for the coefficient of friction of the wheels of a locomotive

will then be greater than when the rails are damp and greasy. In cold

climates the width of the formation is often increased so as to lessen

the depth of snowdrifts, and to enable a snow-plough or men to more

readily deposit the excavated snow and clear the track, as has been

referred to in the immediately preceding pages; and in severe climates

in wet places ample width in cuttings is found necessary, as drains

frequently have to be cut in them as deep as 4 to 5 feet to afford a

free flow for and to prevent an accumulation of water. In temperate

climates the width can be much reduced. When the formation is narrow the

simple percolation of water through the slopes of an embankment, unaided

by any aqueous action caused by fissures, may gradually saturate the

mass, and as the wider the formation the larger the cross-sectional area

of an embankment, it follows that increased resisting power to

deterioration is obtained by widening the formation, as there is more

earth to absorb the water.

A train upon the permanent way will make a force act downwards until it

meets with sufficient resistance to cause reaction. The direction of the

resolution of these forces is towards a slope, therefore, the farther

the surface of a slope is from the line of action and reaction, the

greater is its distance from the disturbing force and the lateral

resistance it receives from the mass. In the case of a solid rock

foundation and a homogeneous embankment in the same state of

consolidation throughout its mass, the direction of the forces might be

accurately delineated, but as such a uniformly homogeneous and equable

condition seldom exists in railway embankments, it cannot be absolutely

said that the forces act throughout upon certain lines; however, it is

advisable to ascertain the probable direction of the forces.

The allowance for lateral settlement should be liberal and be regulated

by the nature of the earth, the height of an embankment, and other local

conditions of situation and rainfall that affect earth, many of which

are named in other chapters. It will vary greatly, and may be anything,

from 5 per cent. to 100 per cent. additional width of formation. An

addition of from 5 to 10 per cent. of the height of an embankment is

usually sufficient, but in clay sand and such soils an embankment may be

slowly washed away by rain until it has shrunk to half its required

width. As an embankment settles or weathers the width of the formation

should be maintained without having to steepen the slope, widen the top,

or erect a wall upon the formation in order to hold the added earth. The

additional formation width of embankments is also a provision against

the effects which the “lurching” of an engine may cause by its weight

temporarily acting upon one rail, and the pressure to be in the 4 feet

8½ inches gauge at a distance of about 2 feet 6 inches from the centre

of the permanent way. For some distance upon each side of the point of

junction of two high tip heads the top width should be increased, _vide_

Chap. IX., and in sidelong ground it is advisable to widen an embankment

more on the upper side than the lower, as slips seldom occur upon the

higher side; and in the case of railways, should the lower slope move,

the rails can be placed towards the hill, and perhaps away from the

slipped portion of the embankment.

With respect to the deteriorating influence of vibration as regards the

slope of a cutting or embankment, it is well established that vibration

will cause movement in a retaining wall which would otherwise be stable,

and that soils possessing considerable powers of cohesion are not so

easily affected by vibration as those of a looser character consisting

of particles having more or less rounded surfaces; but the action of

water may produce seams in such earths as clay, and create a smooth

greasy sliding surface upon which any reposing mass may but require the

least disturbing force, on the principle that the least impact is

sufficient to impart motion to the largest body; such as a man walking

upon it or the tread of an animal, to unbalance the delicate state of

equilibrium. This action may be gradual, continuous, and increasing, as

the earth will always be subject to changes of weather. On the contrary,

vibration in soils having particles insoluble in water, provided water

does not dissolve any cementing material between them, may cause them to

equally settle and become firmer by being pressed together than if they

were not subject to such operation, and should the particles wedge by

shaking and the slopes be sufficiently flat, vibratory motion may, under

these circumstances, tend to consolidate certain earths in an

embankment; but not so in the slopes of a cutting which vibration must

disturb by reason of it agitating and loosening the surface and making

it less dense. For instance, in sandy soils the surface friction on a

cylinder, when sinking operations are not being prosecuted and when the

material is being raised from the interior, is different; the latter

resistance being from 20 to 25 per cent. less than the former,

consequent upon the disturbance, and although fine, soft drift sand

usually presents greater frictional resistance than firm sand, it

obviously cannot be taken as equal to that of firm sand, as it is

quickly dissipated.

The conditions of earth being so very diverse no rules can be deduced,

for even the effects of such stupendous force as that of earthquake

vibrations vary according to the nature of the ground, however, the

weakening effects of vibration are undoubtedly very considerable. It is

known that the lateral thrust of earth is thereby much augmented, and,

therefore, that the strain upon the frictional resistance and the

cohesion of the earth is increased; and experiments have shown that when

a wall is nearly strained to the point of overturning, slight vibration

will quickly destroy the equilibrium, thus demonstrating that it adds to

the lateral pressure. An analysis of some reliable experiments proves

this increase to usually range from 10 to 60 per cent., but it is

evident that the practical effect of any increase may be very much

greater than a mere computation of the increment, for it may supply the

additional strain, however small, necessary to initiate a movement,

hence the danger.

As collateral testimony to the important effects of vibration may be

mentioned:—

A comparison of the coefficients of friction during motion and those at

the commencement of movement or of repose.

The existence of a nearly vertical face generally assumed, when unshaken

by artificial means, in the bared top earth in a quarry, shallow well,

gravel or sand pit, pond, and even a river-bank or a cliff, which

experience has proved could not be maintained when subject to vibration.

The fact that timber piles, which are chiefly supported by the

frictional resistance of their surfaces, will not sustain a rolling,

_i.e._, a vibratory, load equal to that of a fixed load, and also that

the method of driving in soils easily disturbed, such as sand, is also

considered as reducing the safe load according to the percussive action

and frequent vibration caused by a pile-driver, whether worked by steam,

hand, or by means of an explosive substance. As further proof may be

named that in pile-driving, especially in open soil, piles continually

driven penetrate the earth considerably quicker and easier than if

driven at intervals, as the latter system allows the soil to settle

round them and the loosening and friction destroying effects of

vibration are lessened.

The experiments made by Mr. J. A. Longridge, M. Inst. C.E., for Mr. G.

R. Stephenson, Past-President Inst. C.E., in Morecambe Bay, England,

showed that by vibration the bearing power of driven timber piles was

reduced to one-fourth or one-fifth of that when subject to a steady

non-vibratory load.

Its deleterious effect on the structure of such a solid substance as

iron, &c., particularly when it is loaded beyond its elastic limit.

It is generally agreed that a substance is broken sooner when a load is

intermittingly imposed than when the same load is permanently placed

upon a structure.

The experiments of Professor Stokes, 1849; M. Phillips, 1855; M.

Renaudot, 1861; M. Bresse, 1866; and recently of Dr. Winkler and others,

show that the increase of the intensity of strain consequent upon the

dynamic effect of a suddenly-applied moving load may be as much as 33

per cent. more than that of the computed statical pressure.

The fact that in masonry piers of considerable height and small

dimensions, as in piers of viaducts, the vibration caused by trains

loosens the brickwork or masonry and necessitates frequent repairs.

The theory that the particles of all solid bodies may be in a state of

continuous vibration and motion, though there may be no means of

rendering their motion visible, has not been refuted by deductive

reasoning; but, on the other hand, it is in accord with the theory that

“motion communicates itself among material bodies, and is never lost;

when it appears to be so, it in fact only passes from the moving body

into other bodies which are at rest, or are endued with a less velocity,

and at length it becomes insensible in consequence of its enormous

diffusion. In fact, motion can only be destroyed by motion; resistances

and friction disperse it, but do not destroy it.”

The laws of statics and dynamics are well established, and are fully

described in many admirable works upon mechanical philosophy; so far as

the subject of this book is concerned that which is required to be

answered is the question. Have the deleterious effects of vibration upon

earthwork in various conditions been determined so as to be of practical

value?

Not by experiment upon a large scale, nor is it probable that they will

be; but they have been deduced from experience, reasonable inference,

and experiments on a small scale, as has been before mentioned.

The effect of vibration is usually more marked in cuttings than in

embankments, although it may nearly approach when they are of little

depth or height: because a train in a cutting is contained within the

area excavated, whereas in an embankment it is without the area

deposited. In a cutting vibration commences upon the formation level and

the toe of the slopes, the latter the most vulnerable parts and those

most strained. In an embankment it proceeds through the formation to the

base and the toes of the slopes which are necessarily the most distant.

On the other hand, the material in an embankment is generally in a

looser and lighter condition, and therefore more inclined to move and to

suffer from vibration. It may also be greater upon one side consequent

upon the “lurching” of the engine and carriages.

Obviously, vibration is increased with the speed and weight of a train;

probably a short, heavy train travelling at high speed causes a more

deleterious effect than a long heavy train travelling at a slow speed:

also the higher an embankment or the deeper a cutting the greater the

area of the cross section. Assume a train weighs 100 tons, and the

weight of the earth is 112 lbs a cubic foot, or 0·05 of a ton; it might

be considered that the effects of vibration would be less as the areas

increased. Consider the formation to be 18 feet in width and the slopes

1½ to 1, the respective areas of cross section would be as follows:—

───────────┬───────────────────────┬───────────┬───────────┬───────────

Height. │ Area. │ Weight of │ Weight of │ Ratio of

Feet. │ │ the │the Train. │the Weight

│ │Embankment.│ Tons. │ of 1 Foot

│ │ Tons per │ │ Lineal of

│ │ Lineal │ │ an

│ │ Foot. │ │Embankment

│ │ │ │ to the

│ │ │ │ Weight of

│ │ │ │the Train.

───────────┼───────────────────────┼───────────┼───────────┼───────────

│Square Lineal Ton. │ │ │

│ Feet. Foot. │ │ │

10 │ 330 × 1 × 0·05 = │ 16·5│ 100 │0·165 to 1

20 │ 960 × 1 × 0·05 = │ 48·0│ 100 │0·480 to 1

30 │1,890 × 1 × 0·05 = │ 94·5│ 100 │0·945 to 1

40 │3,120 × 1 × 0·05 = │ 156·0│ 100 │1·560 to 1

50 │4,650 × 1 × 0·05 = │ 232·5│ 100 │2·325 to 1

60 │6,480 × 1 × 0·05 = │ 324·0│ 100 │3·240 to 1

───────────┴───────────────────────┴───────────┴───────────┴───────────

NOTE.—For the purpose of a comparison of ratios it is not necessary

to consider the length of the train.

A simple inspection of the above ratios would lead to a supposition that

the effects of vibration would be no less than 3·210/0·165 = 19·6 times

greater in a 10 feet than in a 60 feet embankment. Merely comparing the

weight of a train with that of an embankment, and assuming that the

results of vibration at the same rate of speed are so governed is

incorrect, for the effect of vibration at the formation level is not

regulated by the height of an embankment or the depth of a cutting. The

weight of a train may bear a very small relation to that of the quantity

of earth slipped, yet the soil may have gradually become in such a state

of delicate equilibrium that at last the least vibration will destroy

it, even a little of the top soil falling upon the slope; and it

frequently occurs that a slip commences by the detachment of a few small

lumps and increases until it becomes of serious dimensions; therefore,

the area of a cutting or embankment cannot necessarily be considered as

reducing vibration although the source of disturbance may be more

distant; but, of course, the heavier the mass the greater the weight and

speed required to cause the whole to vibrate.

Lighthouses and such exposed works being constantly subject to

vibration, the experience gained through their behaviour may be

considered as indicating the direction in which the stability of

structures in analogous situations has to be sought. It is generally

agreed that it favours weight and bulk, as they are unchangeable, and

shows that although the form, execution, and the material may be

perfect, a light fabric will gradually be deteriorated by constant

tremor and vibratory motion, at length culminating in the loosening and

separation of the parts.

Except from actual experiment in each case, it is impossible to

determine the greatest weight of and the speed at which a train should

be allowed to travel so as to prevent any deleterious effect from

vibration, and the circumstances are so various that a practical rule

cannot be deduced, except by assuming conditions from experience alone,

which would so modify a formula as to make it show any desired result,

and cause it to be regarded as too complaisant to be trusted. Perhaps

the best test of the effects of vibration that any earthworks can

receive is when a temporary railway is laid upon the formation or cess

for the carriage of materials, and in a lesser degree a steam excavator,

as the weight and vibratory action may show the weak places in a

cutting, or so shake portions of an embankment that should the earth be

unstable a slip will soon occur.

CHAPTER VIII.

EARTHWORKS IN OR UPON SIDELONG GROUND.—SOME INSECURE

CONDITIONS.—PRECAUTIONARY MEASURES.—EMBANKMENTS UPON SOFT

GROUND.—EMBANKMENTS COMPOSED OF SOFT EARTH.—THE PROMOTION OF

STABILITY AND CONSOLIDATION.

With regard to earthworks in or upon sidelong ground, the configuration

of the surface conduces to a movement of the hill-slope of a cutting,

and the centre of gravity of an embankment is not in the middle, and in

a narrow embankment may be outside the central portion, the tendency,

therefore, is for the slope of an embankment on the upper or hill-side

to assume a steeper inclination than that on the lower, as the earth is

tipped against the inclined ground until the point is reached where a

perpendicular erected upon the surface of the ground cuts the top of the

inside slope. This slope, if protected from weathering, might be allowed

to stand at the inclination at which it remains when tipped, and be

simply trimmed and covered; but the toe of the slope on the lower or

valley side is especially likely to slip, and therefore, every

precaution should be taken to prevent movement. A retaining wall is a

certain protection, benching the ground, weighting the foot of the

slope, dividing it into benches about 10 to 15 feet apart, varying in

width from 5 feet and increasing according to the depth; the benches

having sufficient inclination to prevent an accumulation of water;

counterforting the slopes in dry and mild weather with simple earth

counterforts 5 to 12 feet in width at intervals, and in length about

twice their height with a reasonable slope, the height being about half

that of the embankment; a dry retaining wall may also be erected in

temperate climates and in a comparatively unexposed situation: are all

precautionary measures that can be adopted. But in the case of a cutting

in drift or permeable soil upon rock or an impermeable surface liable to

become wet, which not infrequently occurs, the conditions of equilibrium

may easily be destroyed, and the slope upon the higher side may not

stand at a less inclination than that of the hill. Obviously this slope

cannot be obtained either in cuttings or embankments, therefore,

provided the drift is of considerable depth and treacherous, means must

be taken to prevent movement, and drains should be inserted so as to

make the earth capable of standing at a steeper inclination than it

naturally assumes; and in order to drain the ground at the seat of an

embankment, a trench on the valley side filled with stones or supported

by other means acting as a wall and drain can be made. Every precaution

should be taken to prevent any accumulation of water in earthworks in

sidelong ground, and to gently control them, and to promote this object

it is well that lateral outlets be opposite to each other so as to

afford through drainage and an unobstructed flow.

As drift soil upon the inclined surface of a rock may be held by

friction, the least change may impair it sufficiently to cause movement,

therefore it is advisable to have a narrow gullet or heading excavated

in short lengths, followed _pari passu_ with protective works, whether a

retaining wall or not, so that the earth is weakened as little as

possible by exposure and deprivation of its usual support; and there are

many places in which, unless the system of excavating in short lengths

be adopted, failure and slips must result.

In sidelong ground the formation is frequently partly in cutting and

partly in embankment, if so, and the drain is impermeable and can be

thoroughly relied upon, it may be advisable to make the formation

incline towards the hill, so as to drain it, lessen, and perhaps prevent

percolation into the semi-embankment or under its seat, for the soil

being loosened by the process of excavation and deposition is in the

mass more porous than in a cutting.

The chief aim of all drainage operations for the preservation of

earthworks in sidelong ground is to prevent the hill waters guidelessly

flowing into, upon, or under them, their accumulation, or any

obstruction to their easy discharge; and to attain this object it is

necessary to know the location of the sources of the supply, the

quantity, and other particulars, _vide_ Chap. IV. A proper system of

drains must therefore be made upon the hill-side to gather and gently

guide the waters, to lead them by channels across the formation and

discharge them outside it, care being taken to protect and line the

surface of the drains so that no leakage occurs or any erosion of the

face. In countries where the rainfall is excessive or sudden at certain

places or entirely, it may be sufficient to protect the slopes so as to

allow of a flow over them, and particularly upon the valley slope of an

embankment, and in a lesser degree upon the hill-slope of a cutting.

When the configuration of the country allows, it is advisable to avoid

cuttings, and particularly embankments, in or upon drift soil lying upon

an inclined surface of rock or impermeable soil, because it can so

easily be disturbed, become unstable, and of the difficulty of restoring

it to a state of equilibrium; in fact, this may be impossible, as

vibration may cause it to move. In such a case the only means of

preventing a slip is by erecting a wall at the toe of the slope, and as

it is necessary that the foundations be in the rock beneath the surface

earth, the simple operation of excavating may impel the drift down the

hill. Also should an embankment be required upon drift soil its weight

may be sufficient to cause the mass to slide and a diversion of a

railway may be imperative, for the cost of a retaining wall, because of

its height and the great pressure it would have to sustain, or be built

to sustain, would prohibit the adoption of such a remedy. Therefore, an

embankment is more to be feared than a cutting in such earth. Making

benchings in the rock below the drift to resist movement of an

embankment will most probably be impracticable, and the soil may be in

such a delicate condition that a shock or very little additional weight

may make it slip, and when once it has moved, its stability may not be

permanently restored. Such a case may be considered as one of the worst

that can be encountered, and certainly the simple removal of detached

masses of rock that may slide upon the formation is easier to effect

than to treat drift soil upon an inclined rock bed.

It is always well to remember that in sidelong ground a hill may be

reposing at the steepest slope of stability, and when the earth is rock

it may dip in the direction of a cutting or be imposed upon a perishable

stratum, such as porous or inferior shale. Support upon the valley side

is then absolutely necessary to restore its normal equilibrium, and

every care should be taken to prevent water reaching the embankment or

trickling under its base, and to control all water that may permeate

through fissures in the rock face, especially should it be loosely

bedded. Earthworks in such positions require to be regularly watched.

In forming an embankment upon sidelong ground every means should be

adopted to increase the friction between the tipped material and the

soil; consequently all turf or herbage should be removed in order that

the embankment may be upon the bare earth, care being taken that no

loose top mould or turf is deposited, but only the firm soil. It may be

advisable to only tip earth excavated from the solid hill-side, and not

top soil such as that obtained from side cutting, and unless in rock or

firm soil to avoid partly side cutting and embankment.

Should an embankment have to be tipped upon rock, care must be taken to

prevent the surface water flowing under its base, and to counteract

sliding movement the ground should be benched and a drain made on the

higher side; also in a deep cutting the slope or face should be stepped,

or have a cess at about mid-height, or where desirable, to lessen the

effect of the trickling of the surface water and to prevent a flow in a

direct line, which might make the velocity of any leakage waters

dangerous, especially after a rapid thaw. On a mid-cess a catchwater

drain should be cut to intercept the surface water, which may not

percolate but flow upon the slope, and, unless diverted and conducted to

an outlet, may saturate or erode the bottom.

Before a cutting in drift soil upon the side of a hill can be drained,

it may be necessary to sink a shaft upon the higher side some distance

from the slope and construct a complete series of drains under the

formation. Also should an embankment in sidelong ground be near to a

river, the exposed river-bank face must be protected, and in any case,

when a pier of a bridge has to be placed close to the edge of a rock,

the face of the earth should be preserved by a substantial Portland

cement concrete covering in order to prevent disintegration culminating

in a slip or subsidence.

In setting out a line of railway or a canal care should be taken to

avoid, if practicable, either cuttings or embankments in soil resting

upon inclined rock, as slips are almost sure to occur. However, should

it be unavoidable, their depth and height should be reduced to a minimum

and, if possible, the formation be raised until the bare rock crops out,

and this can frequently be effected within the limits of deviation. Also

deep cuttings in clay or earth, having particles soon affected by

weather, should be avoided, but they may be necessary; and should any

have to be located upon a sharp curve, it is advisable to excavate the

projecting spur on the convex side of the curve more than that required

to obtain the formation width, as it is peculiarly exposed, and

therefore more likely to slip. When cuttings are upon the escarpment of

a clay hill the slope upon the valley side need not be so flat as that

against the hill. An inclination of 3 or 4 to 1 may be necessary upon

the hill-side, whereas a 1½ to 1 slope may be sufficient upon the other.

When a slip may involve the destruction or injury of any adjacent

buildings, the best plan is to erect a retaining wall at the foot of a

slope, not less than about one-fourth of the height of an embankment or

the depth of a cutting, ample provision being made for drainage and the

expansion of the earth. It is a sure protection and support, and the

consequences of a slip may be most serious in such a position. The

protection of the slope and a breast wall may, however, be alone

necessary upon the valley-side.

The construction of embankments upon soft soil is here referred to so

far as regards lateral movement, or where vertical settlement causes

such action by drawing asunder. Whenever the surface of the ground

bulges or upheaves upon earth being tipped upon it or pressure applied,

it shows that the disturbing harbingers of instability are in action and

that its sustaining power has been reached, its surface ruptured, and

any more soil placed thereon will only sink and displace the earth until

it meets the solid ground beneath, or the material becomes sufficiently

compressed and dense for it to sustain the load; such a condition of

unrest generally proceeds from over-pressure and will loosen the soil,

increase percolation, and aid disintegration. When it is known that

soft, peaty, muddy, or silty earth overlies level solid ground, and it

is certain it cannot escape, and can only be condensed by weighting

until it will permanently bear the strain required, it can be built

upon; although should there be more pressure at one place than another,

or the character of the soil vary, in certain places be swampy and in

others dry, the surrounding ground will probably rise where the weight

is the least; therefore, increased bearing area of an embankment maybe

necessary until the sustaining power of the earth is not exceeded, or

slips and subsidences will occur, for the weight of an embankment in

such a situation cannot be diminished by retaining walls, as stable

foundations are not to be obtained. It is obvious the width of the

formation cannot be reduced and therefore the only way of lessening the

load upon the base per square foot, assuming the embankment to act as a

mass, and of distributing its weight, is by flattening the slope and

making a platform under the seat of an embankment. All earths, however,

that are not hard rock subside upon being weighted, and in the case of

tenacious clay soils, upon a considerable load being applied over a

small area, a trifling contiguous uplifting of the ground will generally

be noticed. If such rise of the soil does not exceed about one-sixth of

the settlement of the clay under the load it may be disregarded as not

likely to cause a slip or serious lateral movement.

When the foundation consists of a firm stratum of gravel, sand, or solid

earth overlying soft soil such as quicksand or silt, _i.e._, mud and

sand; or alluvial deposit, care must be taken that the latter is not

forced up at a weak place by the additional weight of an embankment, and

the firm stratum undermined, as then it will sink, fissure, and be

unsafe. Any disturbance of the bed or water of an estuary or lake other

than by the tides, currents, or wind is an indication that the earth

beneath the firm soil is overloaded, and that subsidences and slips will

ensue.

Before determining upon the necessary precautions that must be taken,

the deposits should be thoroughly examined and tests made of the amount

of soluble and insoluble material in a certain mass, as although the

earth may have the appearance of being mud, it may contain a

preponderating proportion of insoluble particles of sand, and its

character may hardly be that of mud or silt. When a marsh rests upon

clay it usually requires only reasonable care, but if upon mud or loose

sand it may always be in a more or less unstable condition.

The chief means of consolidation in the construction of embankments of

soft soil upon soft ground may be stated to be as follows:—

1. Draining, which must be deep, and should be effected before the earth

embankment is deposited.

2. Weighting the ground outside the slopes with firm material so as to

consolidate the soil by intermixture with it.

3. The formation of terraces of earth, and by allowing the ground to

subside until movement is arrested, but there may be a porous seam of

harder soil, such as gravel, under the soft ground, then care must be

taken not to dam back the flow of the underground waters by the

subsidence of the added earth or the whole embankment may slide, become

softened and finally be swept away; therefore, through drainage must be

maintained under an embankment.

4. Making the load as light as possible consistent with due

consolidation.

5. Constructing an embankment in equal and regular layers of ashes or

other dry, light, stable soil, and by drying the earth to be deposited

by separation, wind, sun, or by burning.

To prevent slips in such situations, draining, although causing

subsidence, should be executed as long as possible before the embankment

is tipped, is more effectual and certain in its action than any system

of lessening the load or of counterweighting the soil, which may thereby

temporarily be brought to rest but afterwards give way; and any damming

back of water may cause additional sinking, and change the condition of

the earth from a state of dampness to that of saturation. It may be easy

to calculate the weight per square foot that will have to be sustained

by the ground from an embankment and the heaviest train, but to make the

necessary allowance for the effects of vibration is not so readily

computed. The ground can be weighted to ascertain the limit of its

sustaining power. The test should extend for a period of several weeks

to be thoroughly reliable, which time can seldom be afforded upon works;

therefore a considerable excess of sustaining power over the load should

be allowed.

In draining the site of an embankment in peat soil, it is advisable

first to ascertain the depth to which trenches can be cut without the

sides falling in. It generally varies from about 1 foot 6 inches to 3

feet. As the ground becomes firmer the drains can be deepened if

required. The width should not be less than 2 feet 6 inches to 3 feet.

Cross drains can be cut at every 20 to 40 feet according to the state of

the soil. Peat usually drains freely at the surface when its level is

not at or below the water-bearing line of the country. A successful

method of thoroughly draining the surface of peat moorland is to first

make a drain about 1 foot 6 inches in width and the depth of an ordinary

spade, and leave it for a time until the top soil to that depth becomes

firmer, then repeat the operation until a depth of 3 feet is obtained,

the sides being excavated to a perpendicular face, the top turf being

carefully cut and stacked upon the surface to dry. Along the centre of

the bottom of the trench a small cut is made about 1 foot in depth and 6

inches in width, leaving a 6-inch ledge upon each side, then the dry

stacked turf 1 foot 6 inches in width previously excavated, is placed

upon the ledge, the grass downwards and the layers reversed in rotation

to the order in which they were excavated. This system in moss peat land

is cheaper and better than tile or rough stone drains, as they often

settle unequally. The underneath drain will not close up provided the

excavated turf is carefully deposited, and water will not disintegrate

the peat. These drains should be cut about 20 feet apart.

A more extensive system may be necessary, and a comparatively large area

may be required to be drained in order to obtain a firm foundation and

to prevent breaking away of the ground. In such event the first drain on

each side parallel to an embankment should be at a distance from the

centre line of the formation at least equal to the bottom width of an

embankment, and as deep as the soil will allow, so as to drain the land

within the fences. A cut should also be made outside the fencing at a

distance of not less than 10 feet from the inside drain, to catch the

surface waters and prevent their flowing upon the enclosed land. Cross

drains may be required at intervals of from 15 to 50 feet according to

the character and depth of the bog. The sides of the open drains may

require to be supported. Branches of trees and rough fascine work laid

in them have been used to an extent so as not to interfere with the

flow; and when a spoil or fence bank has to be deposited it should be

made a reasonable distance from the drains or they may become closed.

Every effort should be directed to make the drainage of the same degree

throughout the area; it is therefore necessary to know whether the water

can be discharged into a channel or adjacent river, or if the bottom of

the drains will be above the general water-discharge level of the

district so as to prevent the ground being in the condition of a sponge

always full of water and becoming a floating mass. When the peat soil is

of little depth and rests upon sand or clay, drainage can be effected by

side drains reaching to the underlying stratum, and the drains can be

open and be filled with gravel to prevent the sides falling or the

bottom rising. Few serious slips will take place in bog-land when the

precautionary works named in this chapter are more or less executed;

subsidence is then the chief difficulty. It cannot be entirely

prevented, but it may be lessened. It is frequently very considerable,

depending principally upon the depth, situation, and character of the

deposit and the proportion of water in it. Deep bog-lands upon being

drained have subsided as much as 4 to 5 feet the first year, and 10 to

12 feet the second, and in the worst situations they will continue to

subside until the earth is sufficiently compact to support the insistent

weight, and they have sunk as much as 30 feet before the ground became

consolidated. Burnt ballast, fine gravel, or sandy gravel, is to be

preferred as ballast upon peat embankments, as affording an even coating

and tending to consolidate the earth, whereas broken rock ballast breaks

up the surface, as the pieces are not uniform in size and are

fragmentary. The cost of maintenance on bog or marsh-land is sure to be

heavy for two or three years, but then the road will generally have

become firm. The chief points in maintenance are to cause equal loading

and to promote thorough drainage. Baulks laid under the sleepers, or the

longitudinal system of permanent way, is not advantageous in soft soil,

and even should a longitudinal bearer be placed upon the cross sleepers

the road is difficult to lift or pack, and less timber will be required

by a reduction of the distance between the cross sleepers. The rail

joints should also be stiffened so as to make the depth of the

undulations consequent upon the passage of a train as regular as

possible, and the number of the sleepers can be increased with this

object.

The sustaining power of peat moss, and peat which is sometimes found

under a bed of gravel and upon a substratum of clay or marl; and

bog-land varies greatly. As a rule high bog-land will bear the greater

weight. The only reliable method of ascertaining its sustaining

capabilities is by an actual test, and such experiments are the more

requisite in unreliable soils such as mud and slake, alluvial deposits,

peat and bog-land. It is known that some mossy peat and bog-land will

not even resist the weight of a stone of ordinary size, but will upheave

and shake upon very little pressure being applied. By draining and other

consolidating operations such land may be rendered capable of sustaining

a load if the weight be spread over a considerable area upon a platform

of poles, timber, or fascine work, as it then as it were, floats upon

the surface; but piling is not successful, as the piles disunite the

particles, destroy the continuity of the layers and make a passage for

water. The elasticity of bog-land is manifest even when a road-bed has

become sufficiently firm and even-bearing for traffic, by the agitation

of the water in any adjoining drains.

In low bog-land, peat-moss, or peat-land, it is important to ascertain

whether the deposit extends to a considerable depth, as in Holland it

has been found that some marshes simply rest upon a bed of water, being

nothing but floating peat moss, although having as great a thickness as

20 feet. It has also been noticed that marsh-lands often rest upon peat

mosses with a small layer or film of loam upon them, and that they are

floating masses, although the water upon which they float may not be

deep; its depth may be known by the insertion of a bar or boring-tool,

for when it sinks suddenly it has probably reached the water, and until

its downward movement stops it shows that water exists.

Peat bogs consist of decomposed mosses, grasses, aquatic plants, and

mud. In mountainous districts when they are superimposed upon hard or

non-weathering rock, such as quartz, they have little thickness; if upon

clay rock, or any that decomposes under weather influences, they are

usually of considerable depth. As a rule, the greater the specific

gravity of peat, the firmer it is and also the darker. The top layers

are generally the most fibrous and the driest, although spongy and

containing vegetable matter, and are of a light brown colour, and of

less specific gravity than the next deposit, which is of a darker brown

tint, and is denser and more decomposed; the peat then becomes brown

black or black in hue and approaches a coaly condition. Compact turf

usually contains little water, but should it be of a mossy nature it is

generally saturated. Wet bog, peat, or moorland may contain as much as

80 per cent. of water, and any soil having so large a proportion must

necessarily subside and change upon being thoroughly and regularly

drained, and have its bearing power increased in a ratio approximate to

the percentage of water in it before draining and that after such

operation has been effected, other conditions being similar. The depth

and character of the top layers will indicate the best method of

procedure, and whether it is advisable to excavate them and deposit an

embankment upon the lower and firmer bed.

In Holland, where the peat is superimposed upon sandy soil, in order to

prevent slips in embankments it is found advisable to excavate a trench

about 15 to 20 feet in width and as wide as the formation width of a

railway or road, until it reaches the subsoil, and to fill it with sand;

and when an embankment has to be formed of very porous earth, to make a

trench at the toe of each slope, and to fill it with sand so as to act

as a counterfort. Peat and sand have been found to laterally spread the

most, clay and sand less, and therefore the latter soil stands at a

steeper slope.

On the South Austrian system of railways marsh-land has been made firm,

and malaria-fever almost annihilated by depositing ashes over the earth.

When they are mixed with the bog or swamp deposit, it is found that they

cause it to become fertile.

Peaty soils and peat bogs, which latter it should always be remembered

may be almost floating upon water and have a surface layer of vegetation

and moss, must be differently treated to other soft soils, and thorough

drainage may be economically impossible in deep deposits. In such a

situation, as the peat is alone capable of bearing the pressure, it

should not be disturbed or the particles disunited, for the continuity

of the layers will then be destroyed, and probably cause an upward flow

or other passage of water upon pressure being applied. This state is

frequently met with where inland waters have existed, as in that case

the peat moss covering has sunk, the depression depending upon the depth

of the peat underneath it and the distance at which the firm ground is

reached. It is, therefore, advisable to ascertain this, as then some

idea of the probable subsidence may be known.

The system of a platform of poles, hurdles, or fascine-work is well

known, and has been proved to be effectual in preventing any dangerous

lateral or vertical movement. Sand, fine gravel, ashes, or other binding

material, spread over the surface of peat consolidates it and assists in

preventing movement, and it is well if such a covering extends for some

distance in front and at the sides of an embankment to keep the ground

from rising. Upon soft soil, a thick layer of clean sand, well wetted

and consolidated, 5 feet or more in depth, has been used in lieu of

piles for forming a bed upon which to deposit a Portland cement concrete

foundation to receive heavy masonry piers of a bridge, and at much less

expense and with greater expedition, and no slip or subsidence of the

ground has occurred.

It is obvious the earth should be deposited in even layers, similarly to

the general method of closing reclamation embankments from the bottom

upwards; staging is, therefore, necessary for the waggons to run upon

and discharge their contents, but piles should not be driven, as they

disturb and upheave the earth and do not consolidate any soil. A

successful system of depositing embankments of bog or peat earth is to

thoroughly cut up, pun, and tread down the soil in order that it may not

shrink in cakes. If an embankment subsides and must be raised, only

light earth or material should be used. “Forming” or embankments of

little height are usually made from the drain excavation, and when more

earth is required the outer ditches are widened and deepened so that the

fenced-in land is made as firm as possible upon which a railway or a

road has to be maintained.

Fascine mattresses, to prevent slips and subsidence in an embankment in

very soft ground, such as loose sand, are to be recommended, and

experienced engineers have expressed their belief that they are the best

means of procuring a firm foundation and securing it from scour in loose

sandy and soft deposits; and their employment may become necessary when

the earth is very wet, and the contour of the district renders effective

drainage or consolidation of the soil difficult or impracticable. Should

the ground after drainage be only damp, brushwood, or heather, so laid

as to offer the firmest and most even bed may be sufficient; and if sand

can be used with it, it is very advantageous; but the fascine or a more

solid method of distributing weight may have to be adopted. Fascines

should be laid over the whole base of an embankment and extend some

little distance upon each side, beyond the toe of the slope, and be made

stronger according to the load they have to support, which, of course,

increases towards the centre of an embankment, where it is greatest, in

order to prevent their breaking apart and becoming detached.

In peaty soils an embankment should be formed of light dry materials, be

spread out, have equal bearing, and be of as little height as possible,

and all hollows and depressions should be carefully filled. The worst

state of peaty or bog soils upon which an embankment has to be tipped is

when they are no better than a floating mass.

Where a stratum of sound firm soil, such as gravel, lies upon a soft bed

of great depth, by increasing the bearing area upon the firm earth and

not interfering with it, a stable foundation for an embankment may be

obtained. In such a case a cutting should be avoided, for if care be

taken not to impair or injure the firm top layer it may stand without

serious subsidence, but under other conditions it would be unstable, and

when a soft stratum is affected and its natural condition altered, it

may be very difficult to restore it to a state of permanent equilibrium

in consequence of it being in almost a constant state of mutation.

CHAPTER IX.

THE DEPOSITION OF AN EMBANKMENT.—PREPARATION OF THE GROUND UPON

WHICH AN EMBANKMENT HAS TO BE DEPOSITED.—METHODS OF

PROCEDURE.—CONSIDERATION OF SOME OF THE DIFFERENT SYSTEMS.—THE

EFFECT OF THE HEIGHT OF A TIP AND THE LENGTH OF A LEAD.—THE STEAM

NAVVY AND EMBANKMENTS.

With regard to the deposition of an embankment, one of the chief objects

to be accomplished is to make it homogeneous and prevent the formation

of solid layers, which may become detached along the line of

stratification from less compact strata. Careless and intermittent

tipping conducted in dry and wet weather and with different kinds of

earth, probably in various conditions, will cause slips and subsidences;

as also any local disturbance of the soil, which is always more porous

and absorbent of water than when in its natural unexcavated state,

although it may be free from water-pressure which may exist in a

cutting: and particularly so when fresh-tipped, as then the pressure and

strain upon it is at the maximum, its tendency to unstableness gradually

decreasing as it becomes consolidated. In hard granular soils this is

almost certainly the case, but consequent upon percolation of water into

a mass deposited in a dry state that expands; it may not be so in

aluminous or other earth having particles affected by moisture, for an

additional or an unequal load after a settlement may cause further

movement and irregular density. In countries where the ground becomes

caked or parched during the dry season, the change to a wet or saturated

condition affects them much more than in variable climates, as they are

then generally in a moist state.

It is easy to decree that no embankment shall be made of any earth of a

treacherous character, if the circumstances are known to conduce to

instability, and to rigidly specify what shall be done and what shall

not be done, but the exigencies of an undertaking may cause such

stipulations to be impracticable. For instance, it may be found that no

other earth may be available except that which is proscribed. No one

with a knowledge of earthwork would desire to erect an embankment of

considerable height of yellow or sandy clay, or excavate a deep cutting

in such soil unless a considerable allowance be made to provide against

contingencies; nevertheless, it has to be done, and will have to be

done. The precisianism and delicate refinement of a specification

replete with good intentions has, therefore, very frequently to be toned

down in order to allow of the execution of work.

It is obvious the care that is bestowed upon a reservoir embankment,

such as the damping, punning, rolling, mixing of the material, and

raising it in layers, is not necessary in a railway or ordinary

embankment; however, certain precautions should be observed and may be

effected at a small cost, and it is well to remember that errors of

construction will cause undue strain upon particular parts. A fruitful

cause of a slip or subsidence in an embankment is the variation of the

character and condition of the tipped earth. It seldom happens that the

soil of a cutting of considerable extent is the same throughout in

character and condition; and the earth in an embankment tipped at one

end may be different to that deposited at the other, and therefore the

point of contact of the two earths will probably be troublesome. When a

seam of unstable soil occurs in a clay cutting, it should not be tipped

but be run to spoil. Such layers are usually of small extent, yet in an

embankment they may cause portions to run in rainy or frosty weather.

Side cuttings are a protection against slips in embankments when they

are sufficiently far from the toe of the slope, as they form drainage

channels, but the excavation being top soil is loose and porous,

therefore, when the lower portion of an embankment is made from side

cutting and the upper from a cutting, the firm material may be at the

top and the more open earth at the base; consequently, if an embankment

in soil of a doubtful character must be partly constructed from side

cutting and partly from cutting, especial care is required in forming it

so as to prevent slips and subsidence, and also, in excavating any

ground for such purposes, it is advisable not to interfere with any

ditches, or join them, as by concentration a stream may be created. In

countries where land is of little value, contractor’s plant expensive,

not to be obtained in the district, and carriage costly, embankments in

most cases can be more cheaply and quickly deposited from side cutting

than cutting, the excavation from the latter being run to spoil instead

of tipped by waggons into the embankment, therefore, their erection by

such means becomes imperative for reasons of economy.

In depositing embankments it is well to remember that experience has

proved that materials uniform in size and homogeneous in character form

the most compact and impenetrable masses. The great stability of

breakwaters formed of materials of uniform size and the firmness of

macadamized roads are proofs of this. The same rule applies to soils. It

is the separation of the larger bodies from the smaller that causes a

want of cohesiveness and weight-sustaining power.

To prevent embankments of little height as in “forming,” spreading,

slipping or weathering, as they are usually constructed from side

cutting and loose top soil, the sods over the site of the side cutting

should be removed and a turf wall be made of them on each side of the

formation, the excavation being deposited within them, when it will

settle equally and become consolidated, the wall preventing it spreading

and also saving expense in maintenance.

In an embankment of soft earth that weathers quickly, the tipped

material should be allowed to take its natural slope, and then every

effort should be made to prevent its equilibrium being destroyed.

Where heavy and sudden rainfall occurs the edges of the formation have

been purposely tipped and maintained from 6 to 12 inches above the

height of the centre, in order to prevent during construction gutters or

eroded channels being formed upon the slopes, the central portion being

drained and the water led away. However, aqueous action may not

deleteriously affect earth when tipped into an embankment

notwithstanding that the soil cannot be made in the same state as before

excavation, and that deposited material is more open and subject to

percolation than the solid unexcavated earth; on the other hand,

although the percolation and degree of exposure to the atmosphere are

greater, in some soils this may tend to drain and render them harder and

more stable. In fact, special circumstances may alter the general

behaviour of any earth, but the material forming an embankment should be

regarded as in the same condition as if it had been exposed to

meteorological influences, and it should be remembered that in railway,

dock, or canal works the deposited earth is seldom uniform throughout,

it being either soft at one place and hard at another, or intermixed in

a manner unknown in nature, and that exceptionally difficult cases may

occur which can hardly be treated by any particular method of procedure

or even in the way which experience has proved to be effectual in

several instances in similar soils under apparently like conditions.

The system of tipping has some influence upon the stability of an

embankment and the prevention of slips and subsidences.

The first operation is the preparation of the ground upon which an

embankment will be deposited. Solid hillocks or firm mounds that in any

way tend to arrest motion should not be removed, but vegetable or bush

growth should be destroyed, and where slips and subsidences are likely

to occur, turf and all soft soapy matter should be stripped so that the

deposited earth rests upon a sound stratum. In the dyke countries of

North West Europe, before an embankment is deposited, care is taken to

remove all trees and roots so as to effect a thorough connection between

the ground and the earth forming the embankment.

It is well if the ground be ploughed, raked, or harrowed, so as to offer

a rough and similar surface to the tipped material, and also to obviate

any arrest of the percolation of water, which upon reaching a turf or

smooth or less permeable surface, may form a water seam at the foot of

an embankment. It may not be necessary to strip the turf for the full

width of the base of an embankment, but only for some 20 feet from the

toe of the slope on each side. As the earth is bared it should be

covered in order not to expose it to the weather, for should the surface

be coated with mud, water will accumulate, as it cannot drain away or

evaporate as quickly as when the ground at the seat is laid bare; and

also if the top soil be mere dust moisture will convert it into mud.

When the surface of the ground is inclined it can be benched or bared,

and it is open to question whether baring and harrowing the surface is

not a better practice than benching, as it gives uniform support and

increases friction and prevents a sliding surface, and is generally a

quicker and cheaper method to adopt as the soil need not be so prepared

for a greater depth than 6 to 9 inches. Benching may be the better

system to adopt, in loose and non-cohesive earths which require support

to prevent rolling, but it must be carefully made with the necessary

slopes and inclination of cess to prevent a localization of water, or

the ledges may slip and carry away the material deposited upon them. The

ground at the seat of an embankment can also be prepared by ploughing

over it and removing the disturbed earth with a scraper.

The drainage of the ground is most important in order that water cannot

reach the seat of an embankment. Many systems are referred to in Chapter

IV., and others are herein named. A complete system of drains may be

necessary, but much depends upon the character and sound condition of

the earth and whether the slopes and formation are protected. Simply

covering the whole area of the seat, or that upon which the base of the

slopes rests, with broken stone obtained from the cuttings may suffice,

the stone being so deposited that water flows from the centre to the

sides, with a rough drain here and there, or central, diagonal, and

lateral drains may be required; but the pressure of the deposited earth

frequently causes land springs to issue in the seat of an embankment,

the existence of which was not conjectured, hence the effect of tipping

should be watched.

Slips and subsidences are induced in embankments by the material being

deposited when the soil is dry, and in every state of humidity from

dampness to saturation; consequently the earth settles unequally, is

denser in parts, and a stratification is caused as if it were composed

of dissimilar earths. Should any soil be deposited in the same condition

throughout, and be punned or rammed in layers, it becomes as near its

original texture as is possible to quickly attain; but in depositing

railway embankments, which cannot be punned or rammed, the perfect

incorporation of any soil is not effected by the simple operation of

tipping from a spurn head, and stratification of earth is

disadvantageous, inasmuch as the layers may not be homogeneous, and in

railway embankments are, with a few exceptions, in a different state

owing to changes in the character of the soil, the effects of weather,

mode of excavation, and in a lesser degree the height and length of the

lead or distance from the cutting to the tip; the stratification,

therefore, becomes varied and irregular, and depressions are formed

which may hold water, and none should be permitted to accumulate in or

upon a freshly tipped embankment, for then it is in its most permeable

condition, and in retentive soil especially the existence of a wet place

may cause it to become so deteriorated that it cannot be made stable

without being drained, and when the earth is liable to become

alternately very wet and dry, every means must be taken to lessen the

deleterious effects.

Provided an embankment is tipped of one kind of earth, although it will

be in a more open state than the solid unexcavated ground, the slopes

given to it are flatter than the angle of repose, the weight brought

upon it does not exceed the safe load, that it is deposited upon level

ground in regular and equal particles and the soil of ordinary

character, there is no reason that it should slip or subside unequally

if it be not undermined by aqueous action. Under these conditions

settlement would be equal, or nearly so, and as all deep embankments

must necessarily subside from about 10 to 40 per centum, and in extreme

cases one-half, the object to be gained is to cause the settlement to be

even and regular towards the centre. Benching the ground so as to

prevent a movement of the toe, does not aid equal settlement nor keep

the portion of an embankment above the level of the ground from

spreading, except from the resistance offered by the cohesion and

friction of the soil; whereas, if the ground slopes inwards towards the

centre the earth has a tendency to rest and come together, and therefore

not to slip upon the slopes. This system is subsequently referred to.

All saturated or wet earth should, if practicable, be run to spoil, but

it is not easily effected, as it cannot well be filled into one waggon

of a set, for it may be present throughout the whole surface of a

cutting, nor in variable weather can operations be conveniently

suspended until the exposed faces of a cutting are dry, but means are

generally available by which any serious deterioration may be lessened.

No turf, mould, mud, peat-moss, soft pasty earth, frozen soil, or snow,

should be deposited in an embankment, but only firm earth; and no solid

lumps of large size intermixed with shovelled material unless they are

broken up and trimmed when tipped, as the absorption of water will be

greater in the earth having the smaller particles; and should it happen

that several waggon-loads of such material are tipped and are succeeded

by others containing large lumps, an embankment cannot be a homogeneous

mass, but will consist of more or less consolidated portions in a

comparatively dry state, and others which are less impervious and

therefore more subject to the effects of water and settlement. All snow

or frozen soil or muddy earth should be cleared away from the spurn

head, and if it can be avoided no material should be tipped in bad

weather.

With respect to the loosening of the soil by the process of tipping,

taking into consideration that railway embankments are almost invariably

formed in masses of 2, 3, or 4 cubic yards, according to the capacity of

a waggon, it is obvious almost the whole bulk has been disturbed, and

that the condition of the solid earth in a cutting is not maintained,

for the soil in addition to being unbound by the process of excavation,

casting into waggons, and shaking during transition, is ejected with

force down the slope of the tip, the impetus having to be dispersed. It

is certain the size of earth waggons cannot conveniently be much

increased, and that little would be gained if a cubic yard or two were

added to their contents; and that the higher an embankment the greater

the velocity of the soil down the slope of the tip and the loosening

action which causes the earth to be lighter and to take a flatter slope

than when deposited in layers from a moderate height, also the greater

the length of the lead, the greater the vibration and agitation. In the

case of certain soils the particles of which become soft or dissolved

when in a wet state, such as sandy clay, loamy soil, and some varieties

of clay, _vide_ Chapter II., the effect of a long lead or even a short

one will be that the more solid portion of the contents of a waggon will

settle, leaving the loose or mud at the top, and when a waggon is tipped

the loose top “slurry” will roll down the tip almost as a fluid and

proceed beyond the spurn head, thus making a wet sliding surface, the

bottom toil frequently remaining in the waggons and requiring to be

excavated from it. Nothing can be done in such a case but wait until the

earth has had time to dry, or preferably the wet mud should be run to

spoil. In fact a long lead, especially when aided by deposition from a

considerable height, will cause friable soil possessing particles

readily impaired or dissolved in water to become dust when in a dry

state, or mud if saturated.

The gradients and leads chiefly determine the manner in which earthwork

can be economically executed, and are, in great measure, governed by the

configuration and roads of the country; therefore, it will usually

happen that the length of the lead cannot be reduced, and the only

resource is to lessen the height of the tip, and, consequently, the

momentum of the earth down the slope, and to reduce the inclination of

the temporary road at the tip head to that sufficient to cause the

contents of a waggon to be freely ejected and no more. This cannot be

effected at the same cost as depositing an embankment to the full

height, although in the case of an uphill lead a down gradient upon

which the waggons will run freely by force of gravity can be made from

the commencement of a cutting to the required width, the embankment

being raised to its desired height without extra expense but assuming

the case of an embankment 60 feet in height, and that in order to make

it as dense and firm as possible it has to be tipped in three heights

averaging 20 feet, it involves the laying, maintaining, and removal of

three temporary roads instead of one, and cannot be erected without

extra expenditure. The large majority of railway embankments have been

deposited to the full height and width, and they stand; although the

effect of tipping loose soil upon loose soil, which has not had time to

consolidate, is at each addition to cause a movement of the surface. It

is obvious the higher the tip the greater the disturbance, but it may be

said in firm and hard granular earth, if always dry, this unbinding,

although temporarily a disturbing element, may really tend to produce

ultimate homogeneity and stability; on the other hand, in tipping dry

clay from a considerable height it often separates and becomes loose and

mere dust.

In an embankment of moderate height the day’s excavation from a cutting

will increase its length several yards, and the material from each set

of waggons will only be exposed to the weather for a short time. On the

contrary, in a high embankment, the effect of the deposition of the

contents of a set of waggons upon the tip head is hardly perceptible,

and is, until the toe of the slope is approached, the superimposition of

a thin layer of earth which a shower of rain can convert into mud or

cause to be in a soft or disunited condition, especially in the case of

soils having easily soluble particles. The deteriorating influence of

high deposition may be judged from the closing of an embankment of

considerable height, as it is often a tedious and somewhat anxious

undertaking, as might be expected from excavated, shaken, and loosened

earth being deposited upon soil in a similar condition, instead of upon

the solid ground like the other portions of an embankment. Also water

frequently percolates and trickles down the approaching slopes, and

penetrates them and loosens the soil, whereas if one tip is proceeding,

any surface waters may flow away upon the slope and the solid ground,

and the earth has time to become in a similar state throughout. To

lessen these disintegrating effects, if it can be done without

interfering with the due progress of the works, it is well to allow one

tip end to consolidate for some time, and to complete the closure of the

embankment solely from the other, suitable means being adopted to effect

a firm junction. Should closing from a single tip head be impracticable,

the earth from the meeting point of the toe of the slopes of the tip

heads should be similar in character and be deposited in the same

condition; in any case additional care should be taken to adopt every

reasonable precaution to secure equal consolidation. As evidence of the

dissipation of earth at and near to the junction of two high tip heads

may be stated that they require more material to close them than would

appear to be necessary from a computation based upon measurements taken

from cross sections. In non-granular earths few cases will occur in

which high tipping is an advantage; one of the few is when loose rock

and firm earth are deposited, then the rock having larger particles, and

consequently being heavier, will roll to the foot of the tip before the

smaller material, and so form a broken stone seat for the embankment. In

granular earths, such as gravel or sand, the height of a tip head is not

a matter requiring much careful attention, as the particles are not

deleteriously affected by water. When they are of nearly equal size, an

embankment will settle equally, but in gravelly sand the stone will

separate to some extent during transit and the process of deposition

will be at the base; a varying slope may then be assumed, and it may

here be named that it is known when sand is differently deposited,

although its appearance is unaltered, it will exert dissimilar thrusts.

An embankment of little height may be deposited in bad weather without

slipping or subsidence, but when a high embankment is similarly tipped

the surface of the earth is in an unfit state, and operations should be

suspended for a few days.

With regard to the systems of tipping and the prevention of slips, as a

general rule an embankment should be deposited to the full width, for if

it be not erected at one operation the earth may be of a different

character in diverse conditions, and will be more exposed to the

vicissitudes of weather; and in wide embankments, should the system be

adopted of tipping two outer roads and one central road, the three tips

should equally proceed in order that the earth may be in a similar state

and be subject to equal exposure; or weathered surfaces will be created

down which water will more easily percolate than through the solid mass.

In the three tip system the two sides have a tendency to lean towards

each other, and cause the greatest pressure to be upon the inner

material during construction, and therefore the embankment is supposed

to be more consolidated and less likely to slip; but, unless the

conditions are exactly similar, the advantage of this arrangement is

more fanciful than real, for should the inside slopes of the outer roads

meet first, the earth deposited from the centre tip has not an equal

distance to descend and, therefore, the mass has not been tipped from

the same height, the looser material, temporarily or permanently, being

upon the outer tips and the denser in the centre; whereas the slopes,

being the most exposed, should be the more compact, and when they settle

towards the centre without an outward movement at the foot, so much the

better for the stability of the embankment. The pressure upon the seat

is more regular when it is deposited to the full width, as weight is

then not irregularly added, probably after some settlement. Instead of

dividing the tips into two outer roads and one central road, and in that

way endeavouring to obtain permanent stability, it is better to diminish

the height the earth has to be cast, and to deposit it as far as

practicable in lifts, and to the full width. In treacherous soils such

as some of the clays, embankments that would not stand at a depth of 30

feet have been permanently stable when the seat was drained, and they

were tipped in lifts of 15 to 20 feet in soft clay, and 20 to 25 feet in

firmer clay, at double the height at which they previously slipped.

This method involves the expense of moving the temporary roads, but

allows more time for subsidence, and the mass of an embankment is not so

exposed to the weather; however, in a dry season, an embankment can

often be tipped to the full height, whereas in wet or changeable weather

it would not stand if so deposited, and two lifts may be required. Care

should be taken that the width is always sufficient to receive the top

lift. It is more in the direction of equally reducing the height the

material has to be tipped, especially in treacherous soils, that

solidity is to be attained, than in an attempt to consolidate earth by

endeavouring to cause it to fall together by deposition from two or more

separate parallel roads at the same level.

Many slips and failures of embankments have been caused by the central

portion being deposited of one material, such as clay, and the slopes

afterwards made of a different earth, as rock. The latter will then slip

upon the greasy surface of the clay, the porous nature of the broken

rock readily admitting water and air. In fact, a more flagrant example

of effecting that which on no account should be done is not easy to

imagine. In treacherous soil the system of tipping an embankment wider

than that required from considerations of lateral and vertical

settlement, and allowing it to stand at a steeper slope than its

permanent angle of repose, and paring down the top unnecessary width is

not to be commended, as the earth is then strained, and the lower

portion is in the looser condition and the toe the weakest part:

however, in the construction of a single line of railway with a narrow

formation width, it is very convenient as affording room for roads for

loaded and empty waggons, and in any soils other than aluminous and

calcareous earths it may be done with impunity, provided all slimy and

slippery surfaces are removed. No side tipping should be allowed in

depositing a new embankment, and any addition of soil to a slope should

be avoided as much as practicable, although in trimming some filling may

be necessary, but the slopes should be rough trimmed by the bank-head

men as the earth is deposited so as to prevent hollows and depressions,

form a comparatively even and regular surface, cause the mass to be

equally exposed to the weather, and prevent any lodgment of water. The

form a tip head naturally assumes will afford some indication of the

stability, for should it be regular and approach a semicircular shape it

shows the earth is of a comparatively uniform character and is subsiding

equally. When it is uneven and jagged, with streaks down the tip, it

indicates unevenness of soil and condition.

Circumstances arise which necessitate a departure from any generally

approved method, for frequently an embankment must be widened to provide

for increased traffic, and tipping upon a consolidated surface must be

effected. Chapter VIII. and this refer to such a condition of work. It

sometimes occurs when a slip has taken place that only a narrow tip can

be adopted, as the soil may not temporarily be able to bear the weight

of a larger mass unless equally diffused; then there is no other

reasonable course to pursue than to complete an embankment to the

required width and slopes by dry side-filling, such as ashes, broken

bricks, or other absorbent firm material regularly and carefully

deposited. In order to prevent slips and subsidence and increase

friction, when an embankment must be widened by tipping upon its side,

the existing slope should be stripped of all turf or covering, but only

so as to leave no uncovered surface exposed to the weather; and the

bared earth should be made to present an even surface. When the slope is

benched, care should be taken that the benchings do not localise water,

and that it cannot percolate down the face of the old bank at its point

of junction with the new; also in tipping an embankment upon sidelong

ground or upon the slope of an old bank, the seat should be bared,

particularly on the lower side, as the earth meets with no resistance

from the ground except from friction; but upon the upper side, the

material being tipped partly against the hill has a less distance to

travel, and, therefore, the upper or lesser slope is not so loose as the

lower or longer slope, which latter is the more likely to admit water,

and requires a flatter inclination than the denser portion of the

embankment. In the absence of other protective works, it is advisable to

form on the lower side an earth wall or counterfort covered with turf at

the toe of the slope in advance of the tip, and to make it of the

hardest and largest material deposited and with a foundation upon bared

soil, as it is the part most liable to injury. This is a simple

precaution against slips, and will save its cost in lessening the

expense of repairing and trimming the slopes.

As the steam navvy or excavator has now become necessary plant upon most

large public works, and greatly accelerates the speed at which cuttings

can be executed, always provided the excavation is not of a treacherous

nature, and is in such quantity that if not so used it would have to be

run to spoil, many narrow valleys in the near future will probably be

wholly or nearly closed with an earthen embankment, thereby effecting an

important saving of time and expense; or the number of spans of a

viaduct will be lessened, and only made sufficient to cross a road or

allow the required waterway for a river or the discharge of any drainage

or surface waters. The tendency, therefore, will be to increase the

height of embankments of earth; and in countries where the first cost of

an iron, masonry, or brickwork viaduct is too great, and the use of

timber rendered necessary, earth embankments, except in treacherous soil

or upon soft ground, are to be preferred to wooden pile and trestle

bridges or culverts, whose average life in America, which probably now

possesses as many as all other countries, is from eight to ten years,

and that of timber truss bridges nine to eleven years.

In addition to other recommendations, the steam navvy usually excavates

the earth in small pieces of nearly uniform size, and as it will perform

the work of many men it may be most useful in treacherous soil, as it

may enable a cutting to be excavated in dry weather and be protected

before the commencement of the wet season. The slopes should be

excavated in such earth as soon as possible after the gullet has been

removed so as to prevent a slip, and it may be necessary to

close-sleeper or consolidate the ground in order that the weight of the

machine can be supported upon soft ground.

CHAPTER X.

NOTES UPON THE LOCATION, PRESERVATION, AND PROTECTION OF SEA, ESTUARY,

RECLAMATION, CANAL, AND RESERVOIR EMBANKMENTS OF EARTH CONSTRUCTED

TO CONTAIN OR EXPEL WATER.

In the first place, care should be taken in determining the site of an

embankment across an estuary that there shall be no concentration or

alteration of the general direction of the currents, or scouring action

will be created; for the erosive and other deleterious effects of wave

action upon a shore are influenced by the angle at which they are

impelled against it. A prudent course to adopt is to carefully preserve

the usual channels by means of bridges, particularly in ground of a

loose character, such as is usually found in partly landlocked waters;

for if the velocity is increased, the earth which has been deposited by

the original current being reduced or impeded resulting in the suspended

matter in the water falling to the bottom, will be subject to a force

that will again disturb and cause it to return to its previous

suspensory condition; and any disturbance of the normal currents may

destroy the equilibrium of stability and alter the flow, and when they

are affected it may be most difficult to restore them to their original

state, for water will always endeavour to obtain the easiest channel.

It is essential to know the heaviest flood discharge of any river that

may flow into an estuary, the greatest depth and velocity of the river,

the normal and flood channels, their sectional area and direction, and

the extent and shape of the catchment area, so as to establish the

required opening to give the natural waterway; for it is important not

to interfere with the tidal capacity of an estuary or the volume or flow

of any upland waters into the sea, as a navigable channel may become

filled with silt, especially when the shore is flat or sandbanks exist;

and upon a sandy coast an embankment across an estuary with openings for

a navigable channel will probably cause it to become difficult to

navigate and, perhaps, impossible, without constant dredging and other

works of maintenance, as an embankment may obstruct and deflect the

currents and prevent them carrying away the suspended matter. An open

viaduct instead of an embankment is almost always to be preferred, and

may be necessary; for the power of a current to scour or move particles

is greatly augmented by a small increase of the velocity, and the earth

may be in such a delicately balanced condition that any increase of

scouring action may destroy the seat of an embankment.

To prevent leakage and scour of the base of an embankment near a river

resting upon loose soil, curtain walls are sometimes inserted upon both

sides extending to a considerable depth, thereby affording security

against an embankment merely resting upon a mound which may gradually

erode, with the result that it must finally slip and be destroyed; and

it may happen, unless the foundations are carried down into impervious

soil, that water may escape underneath and undermine it; such action is

obstructed by carrying the slopes a few feet below the ground so as to

prevent through surface percolation.

As a rule, an embankment across an estuary with one or two openings in

it is to be avoided, and it should be most carefully considered whether

it will not be better to expend a larger sum and erect a pile viaduct

which will not interfere with the currents or channels and only require

ordinary precautions to be taken against erosion, instead of depositing

an embankment with openings at the channels and the necessary protective

works which may consist of pitching the slopes, covering them with

fascines, mattress work, or sods, erecting short or long, low or high,

groynes, as the former may be ineffectual in causing a deposit or a

shoal in front of the toe of the slope and in preventing scour, for in

loose silty and sandy soil of considerable depth and not sufficiently

firm to resist erosion, they will probably fail by reason of the space

between them being washed away; and a complete covering of the foreshore

may be requisite, or a protecting apron of homogeneous impermeable soil,

and continuous training walls to prevent the creation of shoals: all of

which protective works will constantly need to be repaired, and the

failure of any one may cause not only an embankment to give way but the

remaining preservative works. Undoubtedly there are many estuaries upon

which if an embankment even with many openings for channels had been

deposited, it would have failed, the soil being in such a tender state

that the least additional weight upon the surface would destroy the

equilibrium. Also interference with the littoral currents is a risky

operation in any but a hard rock bed as regards the foundations, quite

apart from injury to navigable channels. When there are numerous

ditches, creeks, or channels crossing the line of a proposed estuary

embankment of variable stability, especially should hills be near and

the earth be so porous as to be saturated every tide, without doubt the

safer plan is to erect a low viaduct, as the old waterways will be an

endless source of trouble should the ground be anything but firm clay,

and interference with the currents will not be preventable, as either

their velocity will be increased or decreased, the result being scour or

fresh deposits which will affect the channels; and such alteration may

change the direction of the motion of the waters, which must be

prevented in front of an estuary embankment; or waves will be created by

the water travelling and rushing over shoals, for the even configuration

of the bottom is a wave lessener, and should there be a deep channel

near an estuary embankment severe wave action may be created. The piers

or piles of such a viaduct should be cylindrical, thus offering no flat

surface for the waves to break against, and yet temporarily to divide

them without serious shock to a structure. When of very considerable

length, a projecting embankment deposited upon a flat shore of a tidal

estuary has caused a heaping up of the water on one side at low tide,

and therefore it is unequally strained. This was found to be the case

with an estuary embankment on the Scheldt, 2½ miles in length.

The form of the slope has also to be considered, but all the principles

that determine the best profile in each case of a pier or breakwater do

not necessarily apply to an embankment in an estuary. The recoil of the

waves washing away the ground in front and at the toe is particularly to

be guarded against, and the action of spray or a broken mass of water

falling upon the formation, as also the direct action of the waves. A

concave form should not be adopted throughout, as waves roll up until

they approach the top portion, when they turn over and fall upon the

flatter portion and often breach it, but when the face is straight,

excepting for a few feet at the toe, the waves are diffused in

travelling up, although they proceed to a higher point upon a flat slope

than a steep one, and the recoil is greatly diminished, but with the

view of avoiding direct wave action, when the ground is inclined in

front of a sea or estuary embankment, it is well to curve the lower part

of the slope and to make it cycloidal to the surface of the ground and

the slope for a little distance, so as to reduce obstruction to a

minimum. The method of making a level terrace or stepping the slope

instead of a curved face has the advantage of checking the rising of the

sea up the face, altering its direction and acting as a wave-breaker,

and also combines these effects with giving as large an area of the base

as can be obtained by a curved face, and in bringing the centre of

gravity of the cross section of the embankment nearer to the seat, but

the face must be securely protected. The width of the cess should

increase according to the degree of exposure. If an embankment consists

of earth the slope of the cess should be from 6 to 10 to 1, or the

system is better avoided and a continuous face adopted.

Short groynes will often protect the toe of an embankment and prevent

any longitudinal current undermining it. As a rule, in a tidal estuary

the cost of protecting the slopes is considerable, and more above

low-water level than below it. In adopting groynes formed of single or

double rows of piles, in order to prevent erosion of the toe and a slip

and subsidence in an estuary or sea embankment, the littoral currents

must be considered before determining their direction and position, the

object of their erection being to prevent the waves, and especially the

prevailing waves, from scouring the shore, and also to cause a general

deposition of shingle, and, therefore, they are usually placed at an

angle to the set of the waves, so as to cause the latter to be diffused.

The angle will vary; the best guide is to examine the effects of any

that may be erected in a similar position to that to be built. An angle

of 50° to 70° with the foreshore, in a leeward direction is frequently

adopted, and with respect to the distance apart, this depends

principally upon the direction of the current and prevailing wind,

contour of the shore, degree of exposure, and the length of the groynes.

When the whole of an open coast has to be protected, no natural defence

existing, the line of the prevailing set of the current and wind on a

straight shore can be set off at the end of a groyne, and before the

point at which it meets the foreshore another can be erected. They are

generally successful when properly placed, and are most frequently

straight; if not, they have a concave face to the direction of maximum

eroding force; a convex must be avoided, as it will not permanently

retain the deposits; their practical effect being that the breach is

heaped up or retained upon the prevailing wind, set of current, or

wind-wave side to a height of some feet above the leeward side,

therefore, a groyne should be so constructed that planks can be added to

it as required. They have been proved to prevent the formation of bars

when judiciously located, but their success greatly depends upon their

proper position and direction, or in loose soils such as sand they may

cause a deposit upon the windward and prevailing current side, but an

erosion and falling away upon the other; and when placed in front of a

reclamation embankment upon shifting sand until the ground at the back

of the embankment becomes dry by the tidal waters being excluded, or the

surface is impermeably coated, they may be of comparatively little use

to prevent a slip or movement of the shore; for as the tide recedes

below the level of the toe of the slope, a seaward flow of the tidal and

land waters will be created in very porous soil under the seat of an

embankment, and may continue until the tide returns and rises to the

level of the ground, thus causing the shore to be constantly changing

place and permanently established accumulation impossible. Short

transverse spurs have been adopted to lessen this action but their

effect is hardly noticeable. As failure of a groyne will probably cause

erosion of the foot of an embankment and a slip, a cheap and effective

means, attested by the experience of nearly half a century, of

preserving the timber from the attacks of most marine worms may here be

named. It consists in scorching the piles, thus preventing fermentation

of the sap, and immediately tarring them: also in placing the wood in

the opposite direction to that in which it grew; the latter operation

has been found to increase the durability 50 per cent., the reason, it

is believed, being that the capillary tubes in the trees are so adjusted

as to oppose the rising of moisture when the wood is inverted.

In excavating for an enclosure embankment, the earth should not be

disturbed nearer than is economically necessary, and a cess should be

left of about 40 to 50 feet in loose permeable soil, the width being

governed by the character of the earth, the depth of the excavation, and

the height of the embankment. Should dredging have to be executed near

an embankment, a considerable distance should be left between the toe of

the slope and the line of operations. Before commencing an estuary or

enclosure embankment, it is advisable to notice whether a deposit is

left upon the shore by the incoming tide, and to ascertain whether it

forms in some degree a protective covering, for if this should be the

case, any increase of the velocity of the flowing water which might be

caused by dredging or a concentration of the littoral currents should be

avoided, or the tidal matter in a state of suspension will not gradually

sink to the ground. The tidal deposit, although the earth forming the

embankment should not be tipped upon it, may also be of importance as

tending to prevent or lessen any percolation of water through the

foreshore and under the seat of an embankment; for instance, on the

Nile, the deposited slime is found to make a practically watertight

covering on the loose sand. Mr. Thomas Stevenson has also stated that,

according to the depth below the surface of low water that mud reposes,

may be approximately judged the force of wave disturbance and degree of

exposure; the less the depth, the less the power possessed by the waves.

In the case of treacherous soil which circumstances compelled to be

partly used in a reclamation embankment of moderate height, but which it

was found would gradually become firm by compression, rough sheet piles

with a plank at top have been inserted, giving the outline of the

finished slope of the material to be afterwards tipped upon it, the

piles and planking not being removed, and, therefore, affording the

required temporary support before the earth became consolidated and

stable by compression and time.

In Chapter IX. the deposition of embankments is referred to as it

affects slips and subsidences in earthwork, but in an enclosure

embankment an additional precaution is particularly necessary, namely,

that immediately tipping from the ends increases the velocity of the

flow of the outgoing or incoming tidal water through the opening, and

consequently augments its scouring action; its deposition from a spurn

head should be abandoned, and the embankment be uniformly raised from

the base. The employment of cofferdams, piling and planking for

effecting a closure is now generally discarded in favour of the

horizontal system of equally raising the height of an embankment from

its base; and is even to be preferred to fascines, unless the latter are

merely used to distribute the weight over the base, to protect the

surface of a slope, or form a shield against scour either temporarily or

permanently.

In previous chapters the protection of a slope is examined; here some

reference is made to the particular preservation of the slopes of an

estuary or reclamation embankment.

When the protection afforded is not uniform care must be taken that

although it makes one part secure it does not weaken another. In the

case of river-banks in a soil that is in a delicate state of

equilibrium, it may occur that soon after one portion has been

protected, another is being scoured, whereas previously it was stable;

therefore, to prevent localization of the erosive action, whether on the

foreshore of an estuary, reclamation, or a river-bank, and consequent

slips and subsidences, the covering should extend over a considerable

length. In a sheltered position simply sodding the slopes may be

effectual. Some other means of protection are a hard chalk or gravel

counterfort founded a few feet below the ground at the toe of an

embankment, and a covering of similar chalk or gravel upon the slope,

should the soil be favourable. When an embankment of earth in an estuary

or river is of firm soil and only requires to be made proof against wave

action, stones may be simply deposited evenly upon a slope and so that

they will not be washed out, and pitching be not required, as the rough

face will tend to break up the waves; but where a simple covering is

adopted, whether close or comparatively loose, the slope should be

straight, as a concave form causing a recoil of the waves will in time

damage or separate the face shield. A coating of clay about 2 feet in

thickness, upon a slope with stakes driven into it, and large bushy

boughs of trees fixed thereon with the tops downwards, is frequently

used in India as a protective cover to a crumbling bank of a river, and

to training spurs erected to prevent erosion and slips. Mattresses,

fascine or wattled work, besides being expensive, will slide down a

slope unless well secured to it, and therefore a constant strain is

produced as in all stake-held coverings; it has also been observed by

the experienced that although so largely and successfully used in

Holland and on its coast, there is very little ground swell on the Dutch

shores, and that in a very exposed situation, or where heavy ground

swells exist, they may not answer, and may become disintegrated by the

much greater weight and force of the sea; and this, notwithstanding the

surface breakers of thin water and little mass broken up by the wind may

produce more visible agitation.

To prevent a river-bank slipping, and also to maintain a channel in a

muddy river, half-tide longitudinal training walls made of wattled work

or fascines have been used, so as to cause the deposition of the

suspended matter in the tidal water and to gradually restore the

impaired slope and secure it from crumbling into the river. The stones

brought down by heavy floods have also been used to maintain a river

channel and protect its banks from slipping, the interstices gradually

becoming filled with mud deposited by the water when the floods subside.

When sudden and unexpected scour of the slope or bed near an estuary

embankment upon soft soil has to be immediately arrested to prevent a

slip, gunny bags filled with sand afford a ready means of repairing any

cavities, the interstices between the bags usually being filled rapidly.

Material should be added as required and any concentration of the

erosive currents should be avoided.

The required height has to be determined of an embankment in an estuary

or the sea to prevent any flow over it or waves falling upon the inner

slope; 4 to 5 feet above the highest water mark appears to be adopted in

the lower reaches of the Thames and unexposed estuaries in England. In

Holland 10 to 15 feet, depending upon the degree of exposure. It is of

paramount importance to prevent any waves washing over the top, as

damage and, perhaps, a breach may be caused thereby. The height of the

highest known wave must therefore be ascertained.

Should the shore be sandy and loose, a characteristic of estuarine

accumulations, although its usual bed may be preserved in any storm,

when an embankment is erected the rapid and ceaseless process of wasting

of the sand and loose soil by the recoil of the waves from the face may

in time lay bare the toe of a steep slope and undermine it; for it has

been found where the foundation was sand and a rubble mound, which

should be so constructed that its interstices become filled in order to

make it more solid and stable, was placed upon it, and the

superstructure upon the mound, that the sea being resisted by a vertical

wall recoiled and made the soil a quicksand, although the sand would be

stable at its natural slope in still water. Obviously the less the

action of the waves is impeded, the less the looseness of the sand.

Experiments have shown that a slope of 1 to 1 will reflect waves, on a

flatter slope they are broken. In such situations light structures,

offering little resistance to the action of the waves and not causing an

impediment to the current, should be adopted in preference to a massive

or solid erection; but when an embankment is necessary, it should have a

long sloping mound or foreshore upon which the waves will gradually

become lessened and dispersed, the desired object being to prevent deep

water close to the work. When a railway or road follows the shore,

instead of erecting a retaining wall to protect an embankment, a

preferable plan to adopt may be to have open trestle-work offering the

least possible obstruction, and when the formation is at the base of a

cliff of variable and doubtful soil it is the best construction, as the

cliff is not touched, and slips and subsidences are avoided. Should the

deposition of an artificial beach be considered necessary for the

preservation of the foot of a cliff in addition to the trestle road,

experience seems to indicate that the contour affording the most

protection is one in which the slope has a flat terrace or cess at not

above three-fourths of the vertical height, another short slope, and a

nearly horizontal space some distance from the foot of the cliff; but a

storm will straighten the face, and it may be impossible to economically

maintain it; however, should such a slope be assumed it should not be

disturbed. _Vide_ Chapter VI. for information respecting slopes. When a

sea or an estuary retaining wall is necessary in order to prevent the

slipping of an embankment consisting of loose soil, an inner dwarf wall

at the edge of the formation upon the land side should be erected so as

to hold the embankment in a box, and not allow any spray or water

passing over the retaining wall to erode the inner portion.

Deep water is generally required close to the work in railway piers or

jetties to enable vessels to get alongside; a heavy and monolithic wall

must consequently be erected; however, in the case of loose soil, the

vertical system simply should not be used unless the foundation is

thoroughly protected and below the reach or effect of wave action, and

no re-entering or right angles should exist, as they increase the action

of the waves. When a rubble mound is cast in and a vertical structure

placed thereon, great care must be taken that there are no holes, except

the natural interstices between the stones, and that they have a firm

foundation and sink equally, or the random mound may give way and the

superstructure will then necessarily follow.

[Illustration: DATA.—Sand embankment: Width of formation, 30 ft.;

inclination of the slopes, 3 to 1; height of embankment from the surface

of the ground, 30 ft.; depth of water, 20 ft.; height of wave, 5 ft.]

As an illustration of the deleterious effects of the recoil of waves may

be mentioned that a high vertical wall with a parapet has been found to

endanger the toe, but when the parapet has been removed in order to

allow the head of the waves to leap over the top of the work, the

structure remained stable. To prevent the recoil of the sea and the

scooping away of the base and the foreshore in much exposed situations,

and consequent slips and subsidences, breakwaters which simply act as

wave screens, and not as wind screens, are sometimes kept a little below

high-water mark so that the heads of the waves may have free action,

although their onward motion is prevented. On the contrary, light open

work, such as a lattice screen, although it may somewhat lessen wave

action, does not prevent it passing through.

The principal causes of the failure of vertical walls when placed upon

an easily eroded foundation are by the scouring action of the recoiling

waves, therefore their magnitude should be reduced to the lowest limit:

by the impounded air driving out particles of the structure: by waves

travelling upon the top, therefore their forward motion should be

deflected and rendered vertical: by the hammering action of the mass

caused by its being alternately quickly submerged and unsubmerged; the

practical effect being that the foundation is intermittingly released of

a portion of the load and then fully strained, therefore the height of

the waves should be reduced as much as possible. Respecting the action

of falling water, experiments were recently made in India, which proved

that “the greatest intensity of pressure does not exceed that due to a

column of water of a height equal to the fall;” the greatest intensity

of pressure being always fractionally under the hydrostatic head.

As an illustration of the constant change of the load upon the

foundations of an embankment in tidal waters, the following calculations

have been made.

The weight of a cubic foot of sea water is taken as 0·028 ton.

The weight of a cubic foot of sand is here taken as 0·056 ton.

A. The weight of a lineal foot of the embankment when unsubmerged equals

201·60 tons, computed as follows:—

Cubic

ft.

The central portion 30 ft. × 30 ft. × 1 ft. = 900

The two inclined 90 ft. × 30 ft. × 1 ft. = 2,700

portions

————— Ton. Tons.

Cubic contents 3,600 × 0·056 = 201·60.

B. At high water the weight of the embankment is reduced by the weight

of the water displaced, which equals 84 tons, calculated as under.

The submerged contents of the embankment are—

Cubic

ft.

The central portion 30 ft. × 20 ft. × 1 ft. = 600

The two inclined portions 2/30 ft. × 20 ft. × 1 ft. = 1,200

60 ft. × 20 ft. × 1 ft. = 1,200

————— Ton. Tons.

Cubic contents 3,000 × 0·028 = 84.

From this must be deducted the weight of the water resting upon the two

slopes, which equals 33·60 tons—

Cubic ft. Ton. Tons.

60 ft. x 20 ft. X 1 ft. = 1,200 X 0·028 = 33·60.

C. Thus the insistent load at high water upon the whole area of the

foundation is reduced by

Tons. Tons.

84 – 33·60 50·40

—————————— = —————— = 0·25 = ¼.

201·60 201·60

D. At high water a vertical pressure is imposed upon the ground beyond

the toe of the slope due to the 20 feet head of water—

20 ft. × 1 ft. × 1 ft. × 0·028 = 0·56 ton per square foot.

This latter weight and element of stability tends to prevent movement of

the ground, and also the toe of the slope, but is entirely removed at

low water when the insistent pressure at the foot of the embankment is

the greatest.

For the purposes of illustrating the varying load upon the surface of

the ground caused by a rise and fall of a tide, it will be sufficient to

take one slope of the embankment.

E. The weight of a lineal foot of one slope, if unsubmerged =

Cubic ft. Ton. Tons.

90 ft. × 15 ft. × 1 ft. = 1,350 x 0·056 = 75·60.

F. The weight of water resting upon the slope per lineal foot at high

tide =

Cubic ft. Ton. Tons.

60 ft. × 10 ft. × 1 ft. = 600 × 0·028 = 16·80.

G. The weight of the water displaced by a lineal foot of the submerged

portion of one slope of the embankment at high tide =

Unsubmerged portion of one slope Cubic ft. Ton. Tons.

1,350 – (30 ft. × 5 ft. × 1 ft.) = 1,200 × 0·028 = 33·60.

H. The area of the base of the slope per lineal foot =

90 ft. × 1 ft. = 90 square ft.

I. Therefore the insistent pressure upon the surface of the ground at

the base of one slope at low water, when the bed is presumed to be dry,

is,

E

75·60

————— = 0·84 of a ton per square foot.

90

H

J. At high water it is

E F G

(75·60 + 16·80) – 33·60 58·80

——————————————————————— = ————— = 0·65

90 90

H

of a ton per square foot, or 22·60 per cent. less.

It has been shown that the vertical pressure of the water upon the

ground beyond the slope is 0·56 ton per square foot at high tide, _vide_

D., therefore the weight upon the seat of the slope is only in excess of

the normal weight of the water upon the ground beyond the slope,

J D

0·65 – 0·56 = 0·09 of a ton.

An inspection of the diagram shows that the flotation power of the whole

of the shaded portion of the slope is balanced by the weight of water

resting upon the whole of it, the areas of the triangles being similar;

and that the portion W. of the slope is that which loses weight by

immersion.

Calculating the pressures at low and high water upon the base of the

portion W., the relative vertical pressures would be—

AT LOW WATER.

Cubic ft.

30 ft. × 5 ft. × 1 ft. = 150

30 ft. × 20 ft. × 1 ft. = 600

———

750 × 0·056 = 42 tons.

K. The area of the base = 30 ft. × 1 ft. = 30 square ft., consequently

the pressure upon it =

42/30 = 1·40 ton per square foot.

AND AT HIGH WATER.

L. The unsubmerged portion =

Cubic ft. Ton. Tons.

30 ft. × 5 ft. × 1 ft. = 150 × 0·056 = 8·40

The submerged portion =

30 ft. × 20 ft. × 1 ft. = 600 × (0·056 – 0·028) = 16·80/25·20

25·20

Consequently the pressure = ————— = 0·84 ton per square foot,

30

K

a difference of 0·56 ton per square foot, or 40 per cent. less load.

M. And upon the base of the central portion—

AT LOW WATER.

Cubic ft. Ton. Tons.

30 ft. × 30 ft. × 1 ft. = 900 × 0·056 = 50·40.

50·40

The pressure is, therefore, equal to ————— = 1·68 ton per square foot.

30

K

AT HIGH WATER.

N. The unsubmerged portion =

Cubic ft. Ton. Tons.

30 ft. × 10 ft. × 1 ft. = 300 × 0·056 = 16·80

The submerged portion =

30 ft. × 20 ft. × 1 ft. = 600 × 0·028 = 16·80/33·60

33·60

Consequently the pressure = ————— = 1·12 ton per square foot;

30

K

also a difference of 0·56 ton per square foot, or 33 per cent. less

load.

The difference in weight, assuming a wave of 5 feet in height, measured

downwards from high water level, to simultaneously roll against the

embankment upon both slopes and completely recoil, would be equivalent

to the displacement of 5 feet depth of water for a strip =

(30 ft. + 30 ft. + 30 ft.) + (45 ft. + 30 ft. + 45 ft.)

——————————————————————————————————————————————————————— = 105 ft. in

width.

2

The cubic contents per lineal foot are—

105 ft. × 1 ft. × 5 feet = 525 cubic ft.

O. The flotation power =

525 cubic ft. × 0·028 ton = 14·70 tons.

For the purposes of this calculation, this weight is taken as if it were

spread over the whole area of the seat of the embankment at a depth of

15 feet from the top =

45 ft. + 30 ft. + 45 ft. = 120 ft. × 1 ft. = 120 square ft.

The vertical pressure per square foot therefore =

14·70/120 = 0·123 ton = 275 lbs.,

which is equivalent to a hammering action upon the foundations of

275/144 = say, 2 lbs. per square inch occurring each time the 5 feet

waves recoil.

The weight, 1·05 ton upon each slope, of the water upon that portion of

the slope which is alternately submerged and unsubmerged is not

considered.

This wave action may, and generally will, happen upon one side only of

an embankment owing to the direction of the wind, the current, and the

“fetch” of the water. In that event the lateral pressure upon the

embankment will also constantly change, and there will be a varying

horizontal force from the 5 feet in height wave and its percussive

action upon the slope tending to produce unequal strain and movement.

The object of the preceding calculations is to show _the variation of

pressures_ an embankment in an estuary or a tidal river has to sustain

in addition to those of an ordinary embankment upon dry land, and its

especial liability to slip and subside; and also to demonstrate that the

vertical and necessarily the horizontal pressures may be in a perpetual

state of mutation and vary considerably, and that the vertical pressure

of the water outside an enclosure embankment may reach a point when the

water may be forced upward upon the land side. Usually, subsidence is

greatest in the wet seasons and at the time of the lowest tides.

In choosing between two materials for submerged work practically equal

in other respects, the heavier should be preferred, as by reason of its

own weight it has greater power to resist the action of the waves and

scour, and the decrease of its specific gravity by the weight of the

bulk of water displaced is relatively not so large.

With respect to the earthwork of canals and embankments constructed to

hold water, each chapter of this book contains information relating to

the promotion of the stability of the soil, and it is not here intended

to refer to the most approved methods of construction, but only to name

some points requiring attention.

A barge or ordinary ship canal is usually placed at a shallow depth in

the ground and meanders through a district, avoiding deep cuttings and

heavy embankments, such an undertaking as the Suez, or the Panama, ship

canals being altogether exceptional undertakings; and also the

Manchester Ship Canal. Some of the most treacherous soils are named in

Chapter II., but probably the worst earth in which a canal can be made

is peat-bog land; the method of procedure is then different to that

required in making a railway or a road, and in such soil the

construction of a canal should be avoided, as it necessitates most

experienced and skilful treatment and extensive drainage, which causes

subsidence; difficult maintenance to preserve the channel and retain the

water of navigation, control the drainage waters, and keep unimpaired

the towing-path, which must be firmly covered throughout. Fine sand is

also an unfavourable soil as it so readily becomes a quicksand, but

almost all other earths usually met with can be so protected that

failure should be an improbable contingency, except when the cuttings

are of extraordinary depth, as on the Panama Canal; the chief danger

being when there is considerable diversity, irregularity, fissuring,

looseness, and upheaval of the soil, for varieties of almost every

surface earth may be encountered, each with its own characteristics and

behaviour when dry or water-charged, and requiring great attention,

especially at the joints, and not a few separate treatment, not only of

the soil but of the same earth in a cutting and in an embankment.

It sometimes happens that there is not enough clayey earth to form a

canal bank, but if the whole of the excavation, except the mere surface

earth, is used there is sufficient, and that it must be employed for

reasons of economical construction. In such a case the clayey earth

should be deposited upon the water side, and the gravelly or sandy soil

on the land side and as a towing-path covering, great care being

exercised that no stratification of the earth takes place, or it may

separate, and water seams be created. The bottom and slopes of a canal

embankment must be covered with an impervious layer, puddle or concrete

being most frequently used for the bottom, and puddle or a coating of

well rammed vegetable soil sown closely with grass seeds for the slopes,

or stone pitching in a wide canal liable to wash: also in loose or

doubtful soil. In canal cuttings when the water is drawn off the

counter-thrust against the slopes is removed, and unless this is

maintained the earth, if in a delicate or loosened condition, may slip,

as a flow may be caused of previously dammed up waters. Prior to any

works being commenced upon a canal embankment it is advisable to

strengthen it for a distance of not less than about 60 feet in length on

each side of a proposed railway bridge or other structure, particularly

when it will be subject to vibration.

Water will soon find a weak place in any earthwork and, certainly, when

an engineer can maintain heavy earthworks in treacherous soils in canal

construction, where part of one slope is submerged in an embankment and

the other dry, and in a cutting part wet and part dry, he should be able

to do so in any analogous situation, making due provision for vibration,

the chief disturbing agency canals are not subject to, but which is so

potent in its effect upon railway earthwork.

Canal and reservoir embankments are so similar as regards the stability

of earthwork that they are here considered under one head. The object of

a canal, reservoir, or river embankment is in a desired position to hold

water without leakage, subsidence, or deterioration of the earth, or

other works connected with the general construction. In dock, canal, or

any earthworks made for the purpose of containing or expelling water, it

is obvious the position is entirely dissimilar to that of railway works:

for whereas a slip or a subsidence in an embankment upon railway work

may be of slight moment and give comparatively little trouble or

anxiety, any movement in earthwork constructed for the purposes named is

of grave importance, as it may mean the destruction of the work, loss of

life and property, not only upon the site but also in the surrounding

district. No leakage or marked weeping of such an embankment should be

disregarded, as unless arrested it will gradually deteriorate the earth

until the equilibrium becomes so affected that the embankment fails. As

a rule, upon railway works, serious slips, except in very unstable soil,

such as drift earth, or those quickly decomposed by atmospheric

influences, seldom occur, _vide_ Chapter II., until a year or two has

elapsed, the process of disintegration from water being necessarily

slower in its action, the material having an opportunity to return to

its normal condition, and the surface being more equally exposed. In

earthwork for hydraulic purposes, although not subject to frequent and

sudden vibration, the slopes being unequally exposed and part generally

either permanently or temporarily submerged or alternately wet and dry,

and the top and land side unsubmerged and fully open to the action of

the weather, percolation cannot be equal and regular upon its surfaces,

and slips and subsidences therefore usually occur within a short time,

varying from a few weeks to a few months; and after about a year or a

cycle of seasons they are not frequent, unless the inherent design and

construction of an embankment is faulty, permitting leakage through the

submerged slope or bottom, pools of water to collect upon the top, and

the bank to become fissured and unequally wet or dry. Especial care

should be exercised to ensure a complete and impermeable connection with

the earth upon which it is placed.

In embankments to contain or expel water time should always be allowed

for an embankment to consolidate before the admission of the water, but

in the case of a cutting the presence of the navigation water as soon

after construction as practicable may be an advantage, depending upon

the nature of the earth in each case; as it may protect the submerged

portions of the slopes from the deteriorating effects of being in a

constantly changing degree of dryness and wetness, and shield them from

the sun and drying winds. Except under peculiar circumstances the

unstable places will be known in a few months, and provided a canal is

properly constructed, it will not cause much trouble after six months or

a year from earthwork movement if the embankments and cuttings receive

ordinary attention, as the navigation water will quickly indicate the

unconsolidated places. In clay, clay marls, clay loams, and where loamy

soil is intermixed with permeable and water-bearing strata, movement of

the earth in canals will speedily occur unless proper precautions have

been taken in the construction, as they are seldom homogeneous.

Earthworks to contain or expel water should be made proof against even

improbable deterioration and accident, and in proportion as the soil is

less solid and firm it should be consolidated by ramming or other means,

or be covered and protected to prevent it cracking, and only firm and

binding material should be used in canal embankments or those holding or

expelling water in order to prevent slips and subsidences. Chapter II.

treats of some conditions under which slips and subsidences may be

expected. No precaution should be omitted that will render the work

solid and uniform, and nothing should be left to chance. The location is

of great importance, for the earth may vary in stability within a very

short distance, not only in character but as regards the superimposition

of the strata. Upheaved and distorted beds should be avoided, and all

loose and fissured earths, especially rock or other soils having more or

less vertical seams: for all embankments constructed upon fissured soils

are sure to cause anxiety and trouble, and will always be liable to

become in a dangerous state from an excessive rainfall. Impermeable

horizontally deposited earth of considerable thickness is that to be

desired, or firm ground that cannot slide upon another stratum or be

affected by any artificially brought addition to the percolating waters

consequent upon the construction of waterworks. Having carefully

selected a site, which for waterworks purposes must almost invariably be

upon high ground and may have to be upon the top cap soil of a hill and

be peculiarly exposed, the next step to guard against a slip or a

subsidence is to prepare the foundations so that no leakage or trickling

of water can undermine or gradually deteriorate the seat of the

embankment; and, therefore, a thorough connection between the deposited

embankment and the ground must be established and the whole be prevented

from movement.

The selection of the best available earth is one requiring careful

consideration, and it may be necessary to experiment to test the

capability of the soil to be made watertight by compression or other

comparatively inexpensive means. In some cases none may be obtainable;

if so, the only course to pursue may be to consolidate the earth as much

as possible, and protect it with an impermeable homogeneous and durable

covering, and one that experience has shown can be trusted to equally

resist percolation. The whole practice of stable earth dam construction

is comprised in the employment of homogeneous, fine, and tenacious earth

uniformly deposited in thin layers, gently watered sufficiently to aid

ramming and consolidation, rolled, pressed, or trodden down by the

passage of carts, men, or animals, and in its due surface protection.

Heavy rolling is to be preferred, for it is more effective than ramming,

as may be judged by the greater compression; the thickness of the layers

and the weight being so regulated that it compresses the earth to its

state of maximum solidity, and does not pulverize it, as the compression

is more uniform, and irregularly compact masses are not created, which

not only destroy uniformity of condition of the mass, but cause seams

and veins and destroy homogeneity. After rolling, the compressed layer

should be gently watered, as the weight of the roller will have made the

top crust drier than the lower portion, unless it has been made into too

moist a state. The thickness of the layers should not exceed, when the

soil is to be heavy steam-rolled, about 6 inches in earth, and 4 inches

in clay soils; they may be compressed to about four-fifths to two-thirds

of their normal thickness, the degree of compression to ensure maximum

solidity indicating the openness of the original earth. In sand, the

layers when simply wetted and rammed with an ordinary rammer, should be

about 4 to 5 inches in thickness, and ordinary earth about 2 inches. The

volume of the sand will be reduced from 10 to 15 per cent. Simple

ramming of sand will only reduce it about 6 to about 9 per cent., and

water about 4 to 6 per cent., making the total compression as before

stated, the quantity of water used being some 20 per cent. of the volume

of the sand. Embankments of moderate height have been made of sand when

no other earth was available, except clay loamy soil in comparatively

small quantities, the surface of the embankment being worked by thin

bars, and the loam being incorporated with the sand and made to fill its

interstices, surface protection being thus afforded, and all the usual

means to promote consolidation being adopted. Wet earth well mixed with

a grout of quicklime has also been used to make a firm embankment of

little height. Clay is not a good material to use unless it is

incorporated with a considerable percentage of sand to prevent fissures

and to lessen expansion. Many prefer the sand to be in the proportion of

about two-thirds of the mass, only one-third being clay; the soil being

then a loam, and cannot be classed as a clay.

There is much diversity of opinion as to the relative value of a central

puddle wall or protection of the slope and toe. Such a wall is placed in

the centre, not only to give support to an earth embankment and prevent

any through leakage, but also to keep the puddle in a uniformly moist

condition, and therefore to prevent it fissuring and cracking from

exposure to the sun or air. In an embankment erected upon an earth

foundation it may be advantageous and warrant the expense, but when the

embankment of earth is placed upon rock it should not be erected, for it

is impossible to make a watertight joint between rock and clay puddle,

and should it be placed in a rock trench leakage will not be prevented

by it, and water will accumulate when it does not at once pass through

the seat of an embankment until the surface of its base becomes

softened, and the whole mass almost worthless either as a means of

support or of prevention of through percolation; and, moreover, there is

the danger that the clay will draw away from the sides of the rock. What

is wanted is, as it were, to insert as far as the top of an embankment

an artificial rock having a thorough connection with the foundation,

consequently a Portland cement concrete wall should be adopted in such a

case and not one consisting of clay puddle. Two great advantages of a

Portland cement concrete central support over that of a clay puddle wall

are that it does not settle and the weight is evenly distributed,

whereas a puddle wall in subsiding may draw away from the earth of the

embankment and cause cavities and cracks and consequent leakage. The

chief consideration is to produce an embankment having impermeability,

solidity, and homogeneity, and a thorough connection at the foundations;

and as the central puddle wall system rarely does so, and usually only

under circumstances which would probably hardly warrant the expense of

its adoption, it is in a state of obsolescence; and should central wall

support be necessary, which may not infrequently be the case, Portland

cement concrete is being adopted in its place, a solid and durable

material being used instead of one varying in shape and size, according

to its state of dryness or wetness, liable to fissure, and incapable of

being permanently and firmly joined to any material. This system may be

described as one of arresting the percolation of water in the interior

of an embankment; but the great aim is to prevent any percolation into

the mass, as when once a passage is made the water will escape along the

line of least resistance, and a fissure become a cavity, a cavity a

breach.

The protection of the slope and toe has for its object the prevention of

any percolation into the mass, and it may effect all that is requisite,

but its efficiency and completeness much depends upon the preparation

and deposition of the layers during the construction of an embankment,

and also the time that can be allowed for consolidation, so as to

prevent any cracking or fissuring of the face covering from settlement

of the embankment. It is obvious in the case of a reservoir embankment

or a slope alternately submerged and unsubmerged, that an exposed clay

puddle covering cannot be used, as heat or the sun’s rays will cause it

to crack, although if constantly wet it would succeed. A puddled clay

covering with a broken stone bed placed upon it to receive dry or

mortar-set stone pitching has been frequently adopted; a simple cement

concrete facing about 6 to 12 inches in thickness, depending upon the

depth of the water and the nature of the soil, or in conjunction with an

asphalt coating of ordinary thickness. In the two latter cases care must

be taken that they do not separate from the embankment either from the

force of pent-up water, frost, or shrinking of the earth embankment.

As any perforation of the surface must be prevented, in certain

districts the covering should be capable of resisting the attacks of

rodents and crustacea, and, therefore, a stone pitched or concrete

covered slope is necessary, or the puddle towards the surface must be

well incorporated with small stones or ashes or other tough material. In

European countries the effects of the burrowing of rats may be

insignificant, but in warmer climates, as, for instance, on the coast of

Coromandel and in some parts of Bengal, where rats may measure as much

as 2 feet in length, their attacks are not to be disregarded with

impunity.

The causes of failure of water-containing embankments afford an

indication of the direction in which especial care should be exercised

in their construction. Assuming a reservoir embankment to be of the

necessary form and bulk, and to be properly constructed, the principal

causes of failure are as follows:—

1. Leakage along the line of a culvert or pipe passing through the lower

portion of an embankment.

2. Leakage under the seat of an embankment.

3. Water overflowing the top and eroding the land slope and so

destroying the equilibrium.

4. Bursting of springs over the site. _Vide_ Chapter XII. on “boils” in

loose soils.

With regard to the first, the most frequent cause of the failure of a

water enclosure embankment; the culvert or outlet passages have been

rendered unnecessary by conducting the waters in a tunnel passage under

the seat of the embankment and without interfering with it; but this

method is expensive. One of the causes of failure is the weight of the

embankment owing to unequal settlement producing a breach in a culvert,

the probable result of insecure foundations or want of a firm concrete

base to the culvert to evenly distribute the weight; or it being placed

upon clay puddle, which should never be done; or the embankment being

constructed without due care. All culverts should be sufficiently large

for a man to easily pass through and should be equally watertight within

and without, for a leakage from the culvert to the bank is equally

dangerous, and means should be provided so that it can be closed in

short lengths. Reports on the temporary failures of reservoir

embankments almost invariably state, failure occurred from water

penetrating between the puddle and the culvert, from water percolating

between the rock foundation and the central puddle wall, or the

embankment gave way as water issued through interstices in it caused by

settlement of the masonry outlet passage.

Careless construction has often been shown by inspection to be the

reason of so many embankments yielding along the line of the culvert;

but when the latter is properly designed and built it seldom causes a

temporary failure of a well-made reservoir embankment. Respecting the

second cause of failure, it generally proceeds from want of care in

thoroughly binding an embankment to the solid ground and protecting the

toe; by “boils” in the foundations; or by a porous earth seam, such as

fine sand, existing under the stratum upon which the embankment is

erected becoming a quicksand on flowing water reaching it.