CHAPTER II. 60

Works of construction: Earthworks, Culverts, Bridges, Foundations,

Screw piles, Cylinders, Caissons, Retaining walls, and Tunnels.

Earthworks.--Under this heading may be classified cuttings and

embankments of earth, clay, gravel, and rock.

When setting out a line and adjusting the gradients, an endeavour is

usually made to so balance the earthworks that the amount obtained

from the cuttings may be sufficient to form the embankments. With

care, this may be effected to a considerable extent; but there will be

places where the material from cutting is unavoidably in excess, and

others where the cuttings are too small, or contain good rock, or

gravel, which can be more advantageously used for building and

ballasting purposes than for ordinary embankment filling. Or there may

be a large cutting which will provide enough material to form three or

four of the adjoining embankments; but the distance, or _lead_, as it

is termed, to the far embankment may be so long, and, perhaps, on a

rising gradient, that it would be cheaper to run the surplus cutting

to _spoil_, and _borrow_ other material for the far embankment from

side cutting or elsewhere. A long lead adds materially to the cost and

time of forming an embankment, as it not only necessitates a

considerable length of _service_, or temporary permanent way, but also

occupies much time in the haulage of the earth waggons. For distances

of half a mile and upwards, a small locomotive is more suitable than

horses for conveying the waggons.

To run to _spoil_ is the term applied to such of the material from a

cutting which, not being required or utilized in the formation of the

line embankments, is removed and tipped into mounds, or

_spoil-banks_, in some one or more convenient sites near the mouth of

the cutting. Sometimes the surplus material is disposed of by

increasing the width of the embankments. Material excavated in a 61

tunnel, and hoisted through the shafts to the upper surface, has to be

deposited in spoil-banks along the centre line of the tunnel.

To _borrow_ material to form an embankment is the term used when the

earthwork filling is not obtained from the cuttings

on the line. This borrowing is generally done by excavating

a trench on each side of the line, of such width and depth as

will supply sufficient material to form the embankment. Fig.

47 gives an example of an embankment thus made from side

cutting. In some cases a piece of high ground adjacent to the

embankment can be utilized for obtaining a portion, or even

the whole of the filling.

Increased material is sometimes obtained by widening the

cutting, or flattening the slopes, or both.

The degree of slope of a railway cutting must be regulated by the

nature of the material excavated. A slope of 1½ to 1, which gives for

every foot of vertical height a width of one foot 6 inches of

horizontal base, as in Fig. 48, is usually adopted for cuttings in

ordinary earth, good clay, sand, or gravel. There are some

descriptions of strong clay and marl which will stand at a steeper

slope, even at 1 to 1; but, on the other hand, there are some kinds of

clay which must ultimately be taken out to 2 to 1, and even 3 to 1.

It frequently occurs that the slopes of a clay cutting, taken out to

1½ to 1, appear to stand well for a time, but after exposure to the

frost and rain of one or two seasons, the material becomes loosened,

and forms into slipping masses, which slide down on to the line,

stopping all traffic, and have to be cleared away before train

operations can be resumed.

Cuttings through solid rock may be taken out to a slope of ¼ to 1, as

shown in Fig. 49, provided the material is compact, and there is not

too great a dip in the strata or rock-beds. Where the rock-beds lie at

a considerable angle, the slope on the high side will have to be made

flatter than the slope required on the low side, as shown in Fig. 50,

and great care must be taken to remove from the high side all loose or

disconnected pieces of rock which might come away and slide down on to

the line.

[Illustration: Fig. 47, 48, 49, 50, 51]

Strong dry chalk will generally stand at a slope of ⅓, or ½ to 1, but

when wet and mixed with flints it will be necessary to increase the

slope to not less than ¾ to 1. Where the rock is loose and

disintegrated, a slope of not less than ½ or ¾ to 1 will be required, 63

and at many points there will be detached threatening masses of rotten

rock which must be cleared away to a much flatter slope for safety. In

cuttings of this description it is frequently found necessary to clear

out a portion of the loose pieces of the lower cavities and build in

their place a facework of masonry to support the superincumbent rock.

Springs of water rising in the rock, or running over any part of the

rock slopes, must be properly provided for, and conducted to the

nearest channel. They should be carefully watched during the winter

season, when the frost, acting on the water penetrating the crevices,

splits and separates large pieces which were previously firm and

secure.

Instances will occur where a cutting has to be made through a thick

bed of rock and several feet of soft loose strata underneath. The

effect of forming a cutting through the soft strata is to induce the

heavy bed of rock above to squeeze or force out the softer material

below, and unless proper means were taken to avert such a disturbance,

the entire cutting would have to be excavated to a very flat slope.

The method adopted in such a case is to build strong face-walls of

masonry, brickwork, or concrete, underneath the rock, as shown in Fig.

51, with strong inverts placed at short distances. Suitable

arrangements must be made to take away the drainage water which will

collect at the back of the walls, and weeping-holes or outlets must be

left in the lower part of the walls to convey the water into the

water-tables on the line.

Where there is a depth of earth cutting on the top of the rock, the

earth should be cut away so as to leave a bench or space of 3 or 4

feet between the edge of the rock cutting and the foot of the earth

slopes, as shown on Fig. 52.

In cases of shelving rock, with earth or clay on the top, as shown in

Fig. 53, it is frequently found necessary to remove the whole of the

clay on the high side to prevent the possibility of its sliding off

the rock on to the line below.

[Illustration: Fig. 52, 53, 54, 55, 56]

In large cuttings it is usual to push forward a gullet of sufficient

width for one or two lines of waggons, as shown in Fig. 54. When this

has advanced some distance, strong planks or half balks of timber are

placed across the gullet, and the sides or wings of the cutting can be

excavated, the material wheeled to the gullet, and tipped from the

barrows into the waggons beneath. By this arrangement the work can be

carried on very expeditiously, as one set of men can be engaged 65

advancing the gullet and laying the track, while others are following

up and taking down the sides. A large number of waggons can thus be

filled in a day, and a small locomotive kept fully employed.

Occasions will arise where the material from a large cutting, situate

on a continuous gradient, as in Fig. 55, has to be carried in both

directions to embankment.

In wet weather, or if the cutting is at all wet, it would be almost,

if not quite, impossible to carry on the excavation at the upper end

to the proper formation level. The water would collect at the lower

level, and not having any means of escape, except by pumping, would

stop the work. In such a case the best way is to take out the cutting

at the upper end to a slight rising gradient, as shown in the sketch,

sufficient to carry away all water, and afterwards take out the lower

portion in the working from the other end of the cutting.

Cases will arise where it will be necessary to make a shallow cutting

through boggy peaty ground. If the boggy material be very soft, and

its thickness from the formation level to the solid ground below be

not great, it may be advisable to remove this extra thickness down to

the hard lower bed, and fill in up to formation level with strong

material. If, however, the bog or peat be too thick to justify its

entire removal, it should be excavated say down to two feet below

formation level, and a thick layer of branches of trees and strong

brushwood closely laid and packed the full width of the road-bed. On

this preparatory foundation must be placed good clean ballast to carry

the permanent way. Two or three extra sleepers should be allowed to

the rail length, and in some instances it will be necessary to

introduce two, or even four, rows of strong longitudinal timbers--half

balks--under the transverse sleepers. The object of all this extra

timber is to obtain a large increase of bearing area on the soft

yielding surface of the boggy material. Notwithstanding these special

precautions, the trackway will sink down a little during the passage

of an engine or train, but will generally return to its former level.

Good side drains or water-tables should be formed at each side of the

cutting to take away all rain and surface water.

In all cuttings it is desirable to have the line of formation on a

slight gradient, sufficient to carry away all rain water or spring

water which may be collected in the water-tables; but more 66

particularly so is this necessary in a rock cutting, where the

material, being non-absorbent as compared with earth or gravel,

requires that all drainage must be carried away to the mouth of the

cutting.

In carrying out railway embankments and road approaches, it is usual

to form the sides to a slope of 1½ to 1, as shown on Fig. 56.

Occasionally the cuttings produce material which might stand at a

rather steeper slope, but considering the effects which might

afterwards be produced by heavy rains falling on the sides, it is more

prudent to adopt the flatter slope of 1½ to 1. Some descriptions of

clay will not stand at the above slope, but require a slope of 2 to 1,

or even 3 to 1.

When proceeding with the earthworks, it is customary to first remove

and lay aside a layer, say 9 inches in depth, of soil and earth from

the seat of the embankments and top widths of the cuttings, to be used

afterwards in soiling the trimmed and finished slopes of the cuttings

and embankments. This soil being removed, the actual work of the

excavation can be commenced. The working longitudinal section will

give all the necessary particulars as to position of the mouths of the

cuttings and the depths at the various chain-pegs, and the top widths

of the cuttings can be ascertained by calculation, if on even ground,

or from the cross-sections if on side-lying ground, according as the

material may be earth, clay, or rock.

For facility of carrying on the works, reliable bench marks, or

reduced level stations, must be established at convenient distances

along the route of the line, and from these and the fixed chain-pegs

the correct line of formation level can be checked from time to time

as the work proceeds.

For ordinary earth or clay cuttings, the usual tools are picks and

iron crow-bars for loosening, or _getting_ the material, and

shovels for filling into barrows, carts, or waggons. For heavy

earthworks, steam excavators are now largely employed. Great

improvements have been made in this class of machinery, in the way of

perfecting the method of excavating lifting, and filling the material

into the earth-waggons.

In nearly all rock cuttings the greater portion of the material has to

be taken out, or loosened, by blasting with gunpowder, dynamite, or

other explosive. The number and extent of the charges will depend upon

the nature of the rock and its stratification, and also on its position 67

as regards proximity to buildings or residential property.

Where the rock is loose, or disintegrated, the pieces can generally be

readily separated by picks and bars without having to resort to any

great extent of blasting.

[Illustration: Fig. 57, 58, 59, 60, 61, 62, 63, 64, 65, 66]

The first of the material excavated in the cuttings is generally

conveyed in wheelbarrows to form the commencement of the adjoining

embankments. When the wheeling distance becomes too far for economical

barrow work, ordinary carts or three-wheeled carts, sometimes termed

_dobbin carts_, are brought into operation where the cuttings and

embankments are light; but where the earthwork is heavy, both in

excavation and filling, a service or temporary road of light rails and

sleepers is usually laid down to carry strong _tip_ earth-waggons. For

moderate distances these waggons are hauled by horses, but for

distances over three-eighths of a mile a small locomotive is more

speedy and economical. Fig. 57 shows one form of dobbin cart; the

wheels are made with good broad tyres, so as not to sink too deep into

the soft ground, and the body being attached to the framework by a

pivot or trunnion on each side, can be readily tilted over, and the

earth tipped out, by releasing the holding-down catch. Where the

ground is soft and wet, or of a very loose sandy nature, the work of

hauling these dobbin carts is very heavy on the horses, and in such

cases it soon becomes an advantage to lay down a service road of rails

and sleepers. This service road is formed of light rails manufactured

for the purpose, or old, worn rails no longer fit for main-line work,

spiked down on to rough transverse wooden sleepers. The end of the

embankment in course of formation, and where the earth is being

tipped, is termed the _tip head_. Two or more roads are required at

the tip head to form the embankment to its full width. Fig. 58 gives a

sketch plan of a service road near the tip head. The width is shown as

for a double line. The earth-waggons are hauled along the line from

the excavation, and brought to a stand at the point A. If a

locomotive has drawn the waggons, it is then detached, moved forward,

and shunted back into the siding BC. A horse accustomed to tipping

then takes one full waggon at a time over one or other of the two

turn-outs, DEF or DGH, to the tip head, sufficient impetus being

given to the waggon to run the front wheels off the ends of rails on

to cross-sleepers laid close, with a steep rise, and backed up with 69

earth. This suddenly checks the frame of the waggon, and the body

containing the excavated material revolves on its trunnion, tilts up,

and shoots out the material well forward, so that the man in charge of

the tip head, who also knocks up the “tail-board catch,” is able to

level off the filling without assistance. The empty waggon is then

hauled back, and turned into the siding BC, and another full waggon

taken forward and tipped, until all the waggons of the rake are

emptied. Ten waggons generally form a rake when the work is pushed

forward vigorously, each waggon holding about three tons. The tip head

horse pulls the waggon by a trace-chain having a spring catch at the

end, by which the driver releases the horse at the right moment. It is

very important that this spring catch should be kept in good order,

because occasionally too much impetus is given to a waggon, which,

running over the tip head down the slope, would drag the horse with it

if the spring catch did not act properly. Good firm foothold must be

provided for the tipping horse.

The tip head should never be carried across culverts or bridges until

they have been well backed up, and protected by a thick covering of

earth or clay, wheeled in with barrows to an equal height on each side

of the masonry, so as to prevent undue side pressure.

Fig. 59 gives a sketch of one form of end-tipping waggon. In some

cases the wheels are made of cast-iron, but as these are readily

broken during the rough handling to which earth waggons are exposed,

it is questionable whether the light wrought-iron wheels, with light

steel tyres, used on some works, are not more economical in the long

run. The framework and body are made of strong undressed timber, well

bound and bolted together. The tail-board catch keeps the body of the

waggon in its proper horizontal position while loading or running, but

when released leaves the body free to tilt up, and to revolve on the

front trunnion by means of the circular clip A. The same principle

is also applied to side-tipping waggons which are used for the

widening of embankments, or formation of platforms and loading-banks.

The permanent way of these service roads is generally made as simple

as possible. A pair of movable rails are used instead of switches, as

shown in Fig. 60. These rails are linked together by iron tie-rods,

and pulled or pushed over into position for one or other of the roads 70

by means of the handle at A. A stout iron pin, or iron

clamping-plate, serves to retain the rails in position during the

passing of the waggons. In a similar manner, a short rail working on a

pin, or pivot, is made to answer the purpose of an ordinary crossing.

The rails are laid complete and continuous for the one road, and for

the second road the outer rail is laid sufficiently high to cross over

the rail of the first road. A piece of rail is then secured by a

centre pin, or pivot, to the cross-sleeper, as shown on Fig. 61. This

pivoted rail is pulled over into the position shown by the dotted

lines, to allow the passage of waggons on the one road, or pulled

across to the end of rail at B, for waggons to pass on or off the

other road. In the latter case an iron pin or clamp serves to keep the

pivoted rail in position. As these service roads are merely laid down

on the soft loose material brought forward for filling, they require

constant packing and lifting to prevent them working into depressions,

which might cause the waggons to leave the rails.

To indicate the height of the embankment filling, strong stakes or

poles must be firmly set in the ground at each chain-peg. On each of

these poles two cross-bars must be fixed, the lower one placed to the

correct height of the embankment, and the upper one to show the amount

allowed for subsidence. The excavated material, as brought from the

cuttings, is in a soft, loose condition, and an allowance must be made

for its settlement, or subsidence, as the embankment becomes

consolidated. This allowance will, of course, depend on the height of

the embankment and the quality of the material, but for ordinary earth

and clay it is customary to allow about one inch to the foot of

height, which is equal to about 8 per cent.

When forming embankments over very side-lying ground, it is necessary

to cut steps in the sloping surface on which the filling material has

to be placed, as shown in Fig. 62. These steps give a hold to the new

earthwork, and check the tendency to slide down the hillside.

Embankments have frequently to be carried over ground which is low,

soft, and wet, but not boggy. If the culverts and drains are

sufficiently large, and properly arranged, these places are not likely

to cause much future trouble.

For a thoroughly soft deep bog, however, it is most difficult to make

any accurate calculation as to the amount of embankment filling which 71

will be necessary to form a permanent foundation for the line; and the

construction of a high heavy embankment across such a place is one of

those undertakings which every engineer is most anxious to avoid. A

large quantity of material may be tipped into the bog, and seem to

stand fairly well for a time, and then suddenly disappear altogether.

More material has to be brought forward, and will most likely

disappear in a similar manner. The filling material being heavier than

the bog on to which it is thrown, falls through, and displacing the

soft semi-liquid matter, continues to sink down lower and lower until

it is stopped by a harder stratum underneath. In a measure the

operation somewhat resembles the tipping of earth into a lake; the

material will go down until it meets with a solid bottom, and in going

down it assumes its own natural slope, and forms for itself a width of

base corresponding to its height. It will be readily understood what

an enormous amount of filling material will be swallowed up in

following out such a process. On a very soft bog, say 20 feet in

depth, over which an embankment 20 feet high has to be formed, the

extent of the actual earthwork filling will very probably closely

approach the outline shown in Fig. 63. The upper portion, ABCD,

representing the embankment proper, will contain about 133 cube yards

to the yard forward, whereas the lower portion, CDEF, which has

displaced the soft boggy matter, will contain about 266 cube yards to

the yard forward, or, in other words, the filling which is out of

sight will be double the filling which is in view above the section

ground line.

Apart from the large amount of filling consumed in forming this

semi-artificial island, the progress of the work itself is very

perplexing. A long length of the bank may have been raised again, once

or twice, to the proper height, and may have carried rails and

earth-waggons for some weeks, and then sink all at once several feet.

The sinking, too, may not be uniform, but may produce fissures,

depressions, and separation of the earthwork which will necessitate

much care when bringing forward fresh filling material. The bog may

not be of the same consistency throughout, there may be some layers of

harder material, such as imbedded trunks of trees, and these may

sustain the filling for a time, and then yield under the increasing

weight of the superincumbent mass. Even when the embankment is

finished throughout, and shows no sign of sinking, it should be very

carefully watched for a long time for any indication of further 72

movement.

When the bulk of the material has been taken out of an earth or clay

cutting, the work of trimming the slopes should be put in hand, so

that any surplus left on the wings, or sides, may be removed, and

carried away before stopping the earth-waggons. The angle of slope

having been decided, a battering rule of light wooden boards is made

to correspond to the slope, and in form similar to that shown in Fig.

64. A plumb-bob is suspended from a fixed point, A; the lower end,

B, is then held against a peg or mark which indicates the correct

level and width of the cutting at the place, and the upper end, C,

is raised or lowered until the plumb-bob string coincides with the

vertical line marked on the rule from A to D, and the plumb-bob

rests steadily in the space cut for it at D. With this battering

rule a length of seven or eight feet, according to the size of the

rule, is first trimmed to the correct slope, and by continuing the

application of the rule up the side, a correct slope line is obtained

from bottom to top of slope at that place. By repeating the process at

convenient distances along the cutting, a series of correct slope

lines are obtained, and the intermediate space can readily be trimmed

to correspond.

The same form of battering rule and method of working is applicable

for trimming the slopes of the embankments.

When the slopes of the cuttings and embankments have been trimmed,

vegetable soil, which has been laid aside, or reserved as previously

described, should then be spread evenly over the slopes to the uniform

thickness of not less than four inches, and the whole sown with good

grass seeds to form a strong sward.

The trimming, soiling, and sowing of the slopes not only gives a more

finished appearance to the earthworks, but the strong grass, when once

well grown, binds the surface together, and helps to resist the

injurious effects of heavy rains and melting snow.

There are many places abroad where a neat finish to the earthworks is

considered quite a secondary matter, or where it would be difficult to

obtain suitable soil to spread on the slopes. The earthworks are

hurried forward to allow the iron highway to be laid down as quickly

as possible, the slopes of the cuttings and embankments are only

roughly trimmed, and nature is left to supply such grass or vegetation 73

as may spring up, or be self-sown.

The fencing in of a line of railway serves the double purpose of

defining the boundary of the company’s property, and of forming a

barrier for the prevention of trespass of persons and animals on to

the line. For our home lines, fencing is compulsory, and the same

obligation exists on many foreign railways. In our colonies, and out

in the far West of the United States, and in newly opened out

countries, fencing, except near towns and villages, is rather the

exception than the rule; people and animals roam at will from one side

of the railway to the other wherever they find a convenient crossing

place, and the cowcatcher of the engine has to be depended upon for

throwing aside any animal which may be standing, or resting, on the

line of rails at the passing of a train.

The description of fence will be influenced by the locality, and the

materials conveniently obtainable. Where stone is plentiful, perhaps

brought forward out of the cuttings, and labour cheap, a masonry wall

will be found a most suitable permanent fence. Any fence to be of

service should not be less than four feet high. A wooden post and rail

fence is much in favour in some districts, the posts being firmly set

or driven into the ground, and four or five stout bars nailed on to,

or set into, the upright posts. This fencing does not last very long,

the pieces are small in size, and soon fail from decay. Quick or

hawthorn hedges, when fully grown, make a good fence, but require

careful attention to prevent gaps being made by roving cattle. They

also require constant trimming and cutting. The quicks are generally

planted in a mound formed by cutting a continuous ditch, or gripe, as

shown in Fig. 65. The ditch serves as a drain to take away water

running down the slopes of the embankments, small openings in the

mounds, or drain pipes through them, forming leaders to conduct the

water to the ditch or gripe. The outer edge of the ditch represents

the boundary of the railway property, unless specially arranged

otherwise.

Galvanized iron-steel wire fencing, if not made too light, is strong

and durable, and very easily kept in order.

The wires may be secured to strong wooden posts, which should be

creosoted, and not placed too far apart, or to iron posts or standards

of angle iron or tee-iron section. The straining-posts, whether of iron 74

or timber, must be stronger than the intermediate posts, firmly fixed

into the ground, and well stayed, to withstand the pulling and

tightening of the wires. There are many places where a quick fence

would not grow, and where the ground is too soft to carry a wall. In

such cases a good galvanized-wire fencing will fulfil all

requirements. The strand wire is better than the plain wire, as its

method of manufacture necessitates the use of a superior material, and

it is easier to straighten and keep in good order. An extra strong

fence is often made of six, eight, or more rows of round rod-iron

secured to wrought-iron uprights of bar-iron or tee-iron.

In hot countries abroad an excellent fence is obtained by planting a

species of cactus or aloe in a similar manner to the quick fences at

home, and as shown in Fig. 66. These cactus plants are readily

obtained, are very hardy and quick in growth, and with their large

spike-shaped leaves form such an almost impenetrable barrier that few

animals will attempt to pass.

Road approaches to bridges over or under the line, or to public road

level crossings, may be fenced in the same manner as the line proper.

If quicks are adopted, it will be necessary to put up a light wooden

fence also to protect the young plants until they are well grown. Near

towns and villages it is frequently found advisable to adopt a

specially strong wooden fence, or close-boarded fence, where the

approach is an embankment, and too newly made to carry a wall.

Gates for farm or occupation level crossings may be made of wood or

iron. As a rule, iron gates are preferred, as they can be supplied at

the same cost as wood, and are very much more durable. Gates for

public road level crossings have to be so placed that they will either

close across the railway or across the road; their length will

therefore depend upon the width and angle of the road crossing. It is

better to make these gates of wood, so that, in the event of a train

running through them, there may be less risk of injury to life and

rolling-stock than if they were made of iron. For footpath crossings,

small gates, wickets, or stiles may be adopted of such form as may be

found most suitable for the requirements.

Culverts and Drains.--Before proceeding with the formation of the

embankments, it is necessary to construct the culverts and drains

which will be covered over by the earthworks. Any existing drains

which may be of too light a description must be reconstructed in a 75

more substantial manner. It is a simple and comparatively inexpensive

matter to rebuild a drain before the earth filling is brought forward,

but it is a costly work to open out an embankment, and rebuild a

culvert afterwards. Unless the seat of an embankment is well drained

and kept free from the accumulation of running water, the earthwork

will be exposed to washing away of the lower layers, and consequent

subsidence. Each watercourse or open drain must be provided for either

by a separate culvert of suitable size or, as may be done in some

cases, by leading two or more watercourses into one, and thus passing

all through one culvert of ample capacity. When fixing the sizes of

the culverts they must not be limited to the normal flow of water, but

a large margin must be allowed sufficient to meet extraordinary

floods. The depth of the bed or invert of a culvert is a very

important point. If laid too high, and the stream above should at any

time deepen, the high invert would check the flow of the water, and

would also incur the risk of being undermined and gradually carried

away. If, on the other hand, the invert be laid too low, it will

gradually silt up to the level of the stream-bed alongside, and there

will be so much of the culvert space lost for all practical purposes.

In cases when the invert of a culvert has to be laid at a special low

depth to allow for future improvements in drainage, it is advisable to

give extra height from the invert to the crown, or top, so as to

provide ample waterway in the event of any silting up in the mean

time. Particular care should be taken when building the foundation of

a culvert. It has to be laid on the site of the watercourse, or on a

new channel which will ultimately form the watercourse, and it should

be built sufficiently deep into the ground to avert as far as possible

the chance of water finding a course through below the foundation.

The invert may be of stone pitching or brick if the current is not

rapid, or liable to bring down stone boulders from its gravelly bed.

With a stream-course having considerable fall, and which carries with

it large stones, roots of trees, and other _débris_, the invert should

consist of strong pitching, composed of large-sized, rough-dressed

stones of hard, durable quality, capable of withstanding the pounding

of the boulders brought down during floods. A soft description of

stone would be quite unsuitable for the invert of such a stream; the

pitching would wear away quickly, break, and become detached, leaving 76

the foundation and side walls exposed to the cutting inroads of the

water.

Where large flat bedded stones or flags of tough quality can be

obtained, they form good covers, or tops, for culverts up to two feet

in width. They should have not less than nine inches bearing on the

side walls, and their contact edges should be fairly dressed, so as to

fit sufficiently close to prevent the embankment filling from falling

through.

Where the stream, or run of water, is very small, strong earthenware

pipes, 9 inches or 12 inches in diameter, well bedded, may be

sufficient to carry away all the water likely to arise. For small

springs in low swampy ground, dry stone drains may in many cases be

used with advantage. These are made by cutting a trench, say two feet

deep by twelve or eighteen inches wide, in the seat of the embankment

from side to side, and filling it up with dry rubble stones, not

boulders, hand-laid, the upper layer placed on the flat to keep the

earthwork as much as possible from filling in between the stones.

In soft boggy ground, where the depth to a hard bottom is very

considerable, wooden culverts are frequently adopted. Although these

cannot be classed as permanent structures, still, when they are made

of sound well-creosoted timber, and substantially put together, they

last for a number of years. Sometimes they are made cylindrical in

section--a species of elongated cask with strong iron hoops every few

feet. Others are rectangular in section, made with two strongly

trussed side frames connected and covered with cross-planking and

longitudinal tie-planking on the top and bottom.

Wooden culverts are seldom made of very large size, rarely exceeding

an opening of 3 feet, and it is considered preferable to use two of

these culverts of moderate dimensions than one of large size. Figs. 67

and 68 give sketches of wooden culverts of cylindrical and rectangular

section, and Fig. 69 of flag top culverts of 12-inch, 18-inch, and

2-foot openings. In masonry culverts the side walls are shown to be of

rubble stonework, but brickwork can be used instead, provided the

bricks are well burnt, hard, and capable of withstanding the action of

the water.

[Illustration: Fig. 67, 68, 69, 70, 71]

In Figs. 70 and 71 are shown types of arch-top culverts of 4 feet and

6 feet span respectively. The arch portion is shown to be of brick,

which, as a rule, is cheaper than stone rings, which must be cut and 78

dressed to suit the small radius of the arch. The side walls may be of

brick of good quality. Occasionally they are built of concrete. The

wing walls may either be carried out in the direction of the stream,

as in the sketch of the 6-foot culvert, or they may be built

transverse, as shown on the 4-foot culvert, whichever arrangement is

found to work in the best for the case in question.

For arch culverts on very steep side-lying ground it is better to

build the arch-top in steps, as shown in Fig. 72, instead of forming

it parallel to the invert, or slope, of the stream-course. The level

portions of the arching give a better hold for the embankment than

could be obtained on a long inclined surface of brickwork or masonry.

The writer has built a large number of culverts of this type for

mountain streams on steep hillsides, and has found them to prove

satisfactory in every way.

In embankments alongside tidal rivers, or across the corners of

estuaries of the sea, culverts have frequently to be so constructed

that they will permit the passage of the drainage water from the land,

or high side, without admitting the tidal water. This can be arranged

by placing at the lower end of the culvert close-fitting hinged-flap

valves opening outwards. When the tide has gone down the weight of the

fresh, or land, water swings the flap-valve sufficiently open to allow

of a free passage; and, on the other hand, when the tide rises, the

pressure of the water against the face of the flap-valve keeps it

tightly closed, and prevents ingress of the salt water.

Culverts are sometimes fitted with lifting-valves or doors, which can

be raised or lowered to serve irrigation purposes. The door, which

works in guides, is made sufficiently heavy to fall with its own

weight, and the raising is effected by means of a screwed

suspension-rod working in a well-secured fixed nut.

In cases of soft or treacherous ground, timber-piling or wide

bed-courses of cement concrete are necessary to form firm foundations

for culverts. Drains and streams which are intersected by a railway

cutting have to be dealt with according to their size and their height

above the finished rail level. The water from a small drain or field

spring may be conducted in pipes down the slope of the cutting into

the water-table, or side drain, at formation level, and will be thus

carried away to the lower level at the entrance of the cutting. In

many cases streams can be diverted, and the water led away to some 79

lower point without the necessity of actually crossing the railway.

With a large stream, where it is essential that the water should be

conveyed across the line and continue on its ordinary course, it may

be carried over in iron pipes or iron trough if there is ample

headway, or in iron syphon pipes where the height is not sufficient.

The iron pipes or trough can be supported on masonry or brick piers,

or cast-iron columns, the height from the rails to the underside of

the conduit being not less than that adopted for the over-line

bridges.

Occasionally the pipes can be carried across on an over-line bridge,

either by placing them under the roadway or on small brackets outside

the parapet.

With the syphon arrangement the iron pipes must be laid down the

slopes of the cutting and under the road-bed of the permanent way. The

pipes must be continuous, strong, and firmly connected at the joints

to prevent leakage. The inlet and outlet ends of the pipes should be

securely built into receiving-tanks of masonry, brickwork, or

concrete, to ensure an uninterrupted flow of the stream, and also to

prevent any of the water from percolating through under the pipes and

on to the railway. As a precautionary measure, it is well to place

iron gratings some little distance in advance of the syphon pipes to

intercept and collect any brushwood, straw, or other things which

might be brought down with the stream.

Fig. 73 gives an example of the syphon arrangement as constructed with

two cast-iron pipes placed side by side.

Railway works carried out in cities and large towns, whether they take

the form of cuttings, embankments, arching, or tunnels, are certain to

cause a very considerable disturbance of existing drains, corporation

sewers, gas-pipes, water-mains and underground telegraph wires. Some

of these underground works may be so peculiar and complicated as to

necessitate a slight deviation from the course originally intended for

the line. Suitable provision will have to be made for each of the

items interfered with by the railway, and the substituted work must be

carried out to the satisfaction of the constituted authorities within

the municipal boundaries.

[Illustration: Fig. 72, 73]

Bridges.--Amongst the many bridges and viaducts which have to be

built during the making of a railway those constructed over rivers and

waterways are generally the most important The bridging across any 81

navigable river or tidal water can only be effected in compliance with

conditions imposed by the authorities controlling the navigation

rights. These conditions will place restrictions as to the number and

distance apart of the piers, as well as the height from high water

level to the under side of the arches or girders. For rivers having a

constant traffic of sea-going vessels of large tonnage and lofty masts

the authorities will demand great height or headway as well as large

spans; and if to this be added a deep water-way and bad foundations,

the work to be constructed becomes one of considerable magnitude. The

banks of the river must be carefully studied to find the most

favourable point for crossing, and in some cases it may be prudent to

make a detour of two or three miles. The crossing at a great height

involves the construction of the approach lines at a great height

also. If the river is in a deep valley with high sloping sides the

natural contour of the ground facilitates the formation of the

approach lines; but with a river on a low, wide, open plain, inclined

approach lines add enormously to the cost of construction, as well as

to the cost of permanent working.

If the number of sailing craft passing up and down the river be

moderate, and, perhaps, only passing at high water, the authorities

may permit a low-level viaduct with an opening bridge.

There are thus the two systems: the high-level viaduct, which allows

trains to pass over and vessels to pass under at any and all times,

and the low-level viaduct with opening bridge, which, if open for

vessels, is closed for trains, or _vice versâ_.

Every crossing of a navigable river will have to be considered and

dealt with according to its own individual requirements. An

arrangement suitable for the one may not be admissible or prudent for

the other. A frequent and important train service might be much

interfered with by an opening bridge, and, in a similar manner, an

opening bridge might cause much interruption and detention to the

navigation of the vessels on the river.

[Illustration: Fig. 74, 75, 76, 77]

Where a low-level viaduct with opening bridge can be adopted, there

will be a very great saving of expenditure; and there are numbers of

such viaducts in existence, accommodating a large railway and river

traffic without inconvenience. Even with a low-level viaduct the

height from water-level to the under side of the girders of the various 83

fixed-spans will generally be sufficient for the passage of barges and

small craft, leaving the opening portion to be used by the larger

vessels.

The principal openings for these large river viaducts are generally

constructed for girders, partly on account of the greater facility of

girder work for large spans, and also for the advantage of having one

uniform height, or headway, from pier to pier.

For a high-level viaduct across a deep-water river, the cost of the

lofty piers forms a very important part of the undertaking. Each pier

will require its own cofferdam, caisson, or other appliance for

obtaining a suitable foundation. The deeper the water, the more costly

the arrangement for foundation; and the higher the pier to rail-level,

the greater the amount of material in the construction of the pier.

The consideration of these two points will at once show that it is

very desirable not to have more of these costly piers than is actually

necessary, and in studying out the design it will be a question for

calculation how far the spans may be increased so as to dispense with

one or more piers.

In every work of this description there is a relative proportion

between span and height, which will give the most economical result

from a cost point of view; the proportion varying according to the

depth of the water and description of ground for foundations. An

increase in the span will naturally necessitate an increase in the

thickness of the pier; but where a cofferdam, or arrangement for

putting in the foundations, must in any case be made, a small addition

to its width may not necessarily form a large increase to its cost.

Figs. 74, 75, 76, and 77 are sketches of high-level railway viaducts

which have been constructed with great height, or headway, to allow

large vessels to pass under at all times without interruption. This

description of work is very costly, not only in the deep-water

foundations, but also in the heavy scaffolding and appliances

requisite for building piers and girders at such an elevation above

the ground-level. The hoisting of the material alone forms an

important item where such vast number of pieces have to be lifted to a

height of 80, 90, or 100 feet.

[Illustration: Fig. 78, 79]

Figs. 78 and 79 are sketches of low-level viaducts constructed with

one large opening span, or swing-bridge, for the passage of vessels.

The girders and roadway of such opening span are usually constructed 85

as a compact framework, which revolves on a centre placed in the

middle of a circular roller path or species of turn-table. The

portions of the rotating opening bridge, although not always the same

length on each side of the centre-pin, are generally very carefully

balanced, to preserve the equilibrium of the entire mass when swinging

round for the passage of vessels. To ensure stability in working, and

steadiness during heavy gales, a liberal diameter should be given to

the roller path of all swing-bridges having large span and great

weight.

Lattice, or truss, girders are preferable to plate girders for

swing-bridges of considerable opening, as they present less surface

area to the action of the wind.

The opening and closing of these bridges is effected by wheel-gearing

actuated by hydraulic, manual, or other motive-power. The revolving

machinery should be set solid and true, well protected from the

weather, and, at the same time, readily accessible for constant

inspection, lubrication, or repair.

Figs. 80 to 85 are sketches of various types of railway bridges

constructed for smaller openings across narrower rivers, water-ways,

or canals. Fig. 80 is an example of what is known as a _bascule_

bridge. This particular bridge is made in two halves, meeting in the

centre of the span, the tail end of each half being provided with

heavy counterweights to assist in opening or tilting up the bridge for

the passage of vessels, or lowering it down for railway traffic. Each

half of the bridge swings on horizontal axles, and the raising or

lowering is effected by means of hand winches or other motive-power,

actuating wheel-gearing working into toothed vertical segments

attached to the tail end of each half. The same principle has also

been applied to bridges having only one leaf to tilt up to clear the

passage way.

Railway bridges of this pattern are now very rarely adopted. They have

the great drawback that when raised to the vertical position, a very

large area is presented to the action of the wind, and this defect

might lead to very serious consequences in the case of a bridge

situated in an exposed locality. An open-work floor diminishes the

wind area, but a very large surface must necessarily remain.

[Illustration: Fig. 80, 81, 82, 83, 84, 85]

Fig. 81 illustrates what is known as a _traversing bridge_. In this

case the width of the opening passage-way and the adjoining span are 87

made the same, and the girders for the two spans are constructed in

one continuous length. By means of gearing attached to the fixed

portion of the work, the continuous length of girder, with its

roadway, is first slightly raised or lowered, and then drawn back on

rollers sufficiently far to leave the opening span quite clear for the

passage of vessels. A reverse movement of the gearing causes the

movable girders and roadway to travel back and return to their

original position ready for the train traffic.

Opening bridges are sometimes constructed on this system in cases

where the level of the rails is only a few feet above the level of the

water, and where there is only one water opening, and that not more

than 20 to 30 feet wide. In such bridges the movable portion is rolled

back along iron rails, or plates secured to masonry walls, or strong

pile-work. This class of bridge is cumbersome, slow to move, and is

now but very rarely adopted.

Fig. 82 shows a type of simple _lift_ bridge, of which there are but

few examples remaining. In this particular bridge the girders and

roadway form a solid framework, which rests on the abutments during

the passage of the trains. Strong chains, secured to the corners of

the framework, pass over large sheaves on the top of the iron

standards, and then round drums placed below the level of the rails,

and terminate by attachment to heavy counter-weights suspended in iron

cylinders. The counter-weights are adjusted to approximately balance

the bridge, so that a moderate power applied to the wheel-gearing on

the drums is sufficient to raise the roadway to the required height.

This class of opening bridge is only suitable for the passage of

barges and small craft without masts; and it requires the

re-adjustment of the counter-weights when the roadway varies in

weight, in consequence of rain or repairs.

Figs. 83, 84, and 85 are sketches of small _swing_-bridges constructed

for narrow waterways. Although differing in appearance, they are all

practically on the same principle, with centre pin and roller path,

and are similar in general arrangement to the large-size-opening

swing-bridges shown in Figs. 78 and 79.

[Illustration: Fig. 86, 87, 88]

The _swing_-bridge arrangement is so simple in construction,

convenient for inspection, and easy to maintain, that where possible

it is now generally adopted in preference to any other system. The 89

weights on centre pin and roller path may be distributed as considered

most expedient, and by means of suitable appliances the weight may be

altogether taken off the centre and rollers when the bridge is closed

for the passage of trains.

There are many wide rivers which, although not navigable in the

ordinary acceptance of the term, nevertheless require bridges of large

spans to provide free waterway for the floating down of rafts of

timber. Away in the high ground, in the timber-growing districts,

trees are felled, sawn or cut into long poles, logs, or scantlings,

and hauled to the banks of the river. The timbers are then formed into

large rafts of the most convenient form for floating down to the place

of distribution or port for shipment. Even with old experienced

floaters, using their long sweeps in the most skilful manner, it is

difficult to take anything but a very irregular course down the

stream. Under the most favourable circumstances one of these large

rafts is an unwieldy, awkward craft to manage; but in a river full of

twists and turns, with reaches varying from comparative smooth water

to miniature rapids, the current carries the huge mass surging along,

and only a clear, unobstructed channel will enable its navigation to

be carried out with safety. The presence of a pier in the main

waterway might cause destruction to the rafts and loss of life to the

men. The vested interests in floating rights are tenaciously guarded,

and no new bridge would be sanctioned which would in any way interfere

with the waterway or endanger the passage of rafts down the river.

Bridges of this description are much less costly than those over deep

water--navigable rivers. Excepting the large spans, the rest of the

work is comparatively simple. The water is generally shallow, and much

reduced in quantity during the summer months. Good foundations can

generally be obtained without going to any great depth. The headway

may be kept low, or of such height as may best suit the purposes of

the railway, and be sufficiently well up out of the way of the floods

which may take place from time to time on the river.

[Illustration: Fig. 89, 90]

Fig. 86 is a sketch of a bridge constructed over a river much used for

rafting purposes. The large span is over the main channel, and the

small spans are over a wide gravelly foreshore, which is only covered

with water during exceptionally high floods in the autumn or winter. 91

No rafting can be carried on when the river is in flood; the current

would be too strong to permit of the raft being kept under control.

Fig. 87 is a sketch of a similar bridge where the river is confined to

a regular channel between two sloping banks of strong clay.

Fig. 88 shows a bridge erected over a narrow rocky pass in the river.

The channel is hemmed in by the almost perpendicular sides of mountain

granite, there are no banks to overflow, the flood waters cannot

spread laterally, however much they may increase in depth, and with

building-stone at hand in abundance, and foundations formed in the

solid rock, the situation is one of the most favourable for a strong

permanent bridge. The cast-iron arch of 150-feet span has a graceful

appearance, and harmonizes well with the surrounding scenery. A small

masonry arch at each end of the bridge provides for communication

along the banks of the river.

With rivers which are neither under the control of navigation

authorities nor used for rafts of timber, there is much greater

freedom for the designing and carrying out of bridges or viaducts

suitable for the actual physical conditions of the locality. The

headway will be guided only by the height of the railway to be carried

across, and by any flood-water levels which may affect the work. The

size of the spans will be regulated by the width of the river, the

depth of the water, and the nature of the ground into which the piers

have to be built. For broad, shallow rivers with good firm river-beds,

piers may be built at moderate cost, and comparatively small spans

adopted; on the other hand, with a broad deep river it will be better,

as previously explained, to reduce the number of piers and increase

the span. In the one case, for example, a river 150 feet wide may be

crossed with three spans and two piers in the shallow water, as in

Fig. 89; in the other it may be more prudent and economical to cross

in one span, without any intermediate pier, as shown in Fig. 90.

[Illustration: Fig. 91]

Next in importance to the large bridges and viaducts over rivers are

the viaducts which have to be constructed for the crossing of deep

inland valleys. The occurrence of one of these deep valleys between

long lengths of average table-land renders necessary either a series

of cuttings and falling gradients to get down to a low level, or the

erection of high-level works to continue onward the rail-level at the 93

height already attained. A decision to adopt the latter course brings

forward the consideration as to the method of carrying out the work.

To form a high embankment across such a valley would entail an

enormous expenditure for earthwork, and several openings, or bridges,

would have to be made in the embankment for streams, rivers, and

roadways. Instead, therefore, of making this part of the line entirely

of embankment, it is usual to carry the earthwork forward until the

height is about 25 or 30 feet, and to form the remainder of the

opening of arching, as shown in Fig. 91.

This arrangement is not only less costly than an embankment of such

height, but has also the great advantage that any or all of the arches

are available for the passage of streams, rivers, roads, and

accommodation works.

The character of the work to be carried out in the construction of

bridges or viaducts over rivers or valleys must greatly depend upon

the description of materials at command. Where good building-stone is

plentiful, and the price of labour moderate, works of masonry should

be adopted as far as practicable. Brickwork is an excellent substitute

for masonry, provided that specially selected bricks are used for all

facework, or parts exposed to the weather. For water-washed piers and

abutments, the lower portion should be faced with good hard stone.

[Illustration: Fig. 92]

Bridges and viaducts consisting of arches of masonry or brickwork form

the most substantial and permanent works of construction for railway

purposes; once properly built, the expenditure on future maintenance

or repairs is merely nominal. For viaducts the span of the arching

must be regulated by the height of the viaduct. The greater the height

the larger the span. In one case 30-feet spans may be suitable,

whereas in another it may be more economical to introduce spans of 60

feet or more, and so reduce the number of lofty piers. From a cost

point of view there is, however, a limit to the span of arching, and,

except for special cases, where expenditure is of secondary

importance, large spans are very rarely adopted. Arches of large

spans, no doubt, have been built both in masonry and brickwork, and

have been a complete success in every way except expense.

Unfortunately, the quantity and weight of materials in arching, and

the corresponding cost, increase very rapidly as the span increases, 95

and for openings of more than 60 or 70 feet girder-work becomes much

cheaper than arching.

Figs. 92 and 93 are examples of viaducts having piers of masonry, with

girders to carry the roadway. In the one case the roadway is carried

on the bottom flange of the girders, and in the other on the top. The

latter arrangement affords greater facility for securely bracing the

girders together, while for the former it is claimed that the girders

form a massive parapet, which would serve as a protection in the event

of an engine or vehicles leaving the rails.

In the early days of railways, many large viaducts were constructed

having masonry piers, and timber trusses to carry the roadway. Much

ingenuity was displayed in designing the trusses, and in the

introduction of cast-iron joint-shoes and wrought-iron bracings. Many

of these wooden superstructures served well for several years, but

they were always exposed to the imminent risk of destruction from

fire, and however carefully the logs may have been selected, the decay

of the timber was only a question of time. The deterioration of one

piece was equivalent to the weakening of the entire truss, and the

renewal of any part was both difficult and costly. The shrinkage of

the timber, and the working at the joints, caused the trusses to

deflect considerably under a passing load, and although the actual

strength of the structure may not have been much impaired, the

creaking and depression had anything but a reassuring effect. Timber

superstructures for anything but small spans are rarely adopted now,

except for temporary works, or on lines abroad, where the transport on

girder-work would be very costly, and where good timber is very cheap

and abundant. Even in the latter case the wooden superstructure is

generally looked upon as a temporary expedient, to be replaced at no

very remote date with iron or steel girders, when the materials can be

conveyed over the entire completed line.

Figs. 94, 95, and 96 are sketches of three types of timber trusses as

constructed in viaducts of several spans.

[Illustration: Fig. 93]

There are many localities, especially abroad, where suitable stone is

most difficult to obtain, and very expensive to work and convey. In

such cases it is compulsory to use as little of it as possible, and to

resort to iron or steel both for the girders and a large portion of

the piers. The piers may be made of cast-iron, wrought-iron, or steel, 97

of suitable form and arrangement to ensure strength and stability. Not

only must the piers be strong enough to carry the weight that may be

brought upon them vertically, but they must have sufficient width of

base to ensure lateral steadiness. The design should admit of facility

of erection, with a minimum of scaffolding, and the pieces should be

of convenient length and weight for transport. The lower length of

river piers, or portion liable to be in contact with flood-water,

should be of solid masonry, to resist the action of the water, or of

any _débris_ brought down by the current. More than one fine

viaduct has been swept away for want of due attention to the latter

precaution.

Fig. 97 illustrates a type of pier composed of cast-iron columns, well

braced and stayed with wrought-iron. The ends of the columns and all

contact surfaces should be properly turned and faced by machinery to

ensure true and perfect joints, and the socketed ends should be turned

and bored to fit closely. The latter is important, and if not

carefully carried out, a slight sliding movement of the flanges may

take place, and throw undue strain on the bolts.

Fig. 98 shows a very similar pier, constructed entirely of

wrought-iron or steel.

Each of the above-described piers has a liberal amount of taper or

batter, both in the front and transverse elevation.

The size and number of the columns, and the dimensions of the braces

or stays, will depend upon the height of the pier and the weights and

strains to be sustained.

Many important and lofty viaducts have been erected on this principle

of iron piers springing from masonry foundations, more particularly

across deep rugged ravines abroad, where iron piers offered the only

practical, substantial means of dealing with what appeared otherwise

an impossibility.

[Illustration: Fig. 94, 95, 96, 101]

[Illustration: Fig. 97, 98]

[Illustration: Fig. 99]

[Illustration: Fig. 100]

Fig. 99 is a sketch of the Kinsua Viaduct on the Erie Railway, one of

the highest railway viaducts in the United States. In the transverse

elevation the piers have a large amount of taper; but in the front

elevation they are vertical, and of width to correspond to one of the

small spans of the main girder. This arrangement of long and wide base

gives great stability to the pier. The spans of the girders, which are

of the ordinary lattice type, are not large, being 61 feet for the

clear spans, and 38 feet 6 inches for those over the piers. The

principal interest is in the great height and simplicity of the piers. 102

The rail-level over the top of the pier is 301 feet above the level of

the water in the Kinsua stream. The width of this pier on the top is

10 feet (for single line), and the width at the bottom 103 feet.

Fig. 100 is a sketch of the Loa Viaduct on the Antofagasta Railway,

Bolivia, stated to be the highest railway viaduct in the world. The

arrangement of spans and piers is very similar to the Kinsua Viaduct.

The main spans are 80 feet, and the pier spans 32 feet. The width of

the pier on the top is 10 feet 6 inches (for single line), and the

width at the bottom of the highest pier is 106 feet 8 inches.

In contrasting these light iron piers with what would have been

required if constructed of masonry, an idea may be formed of the

enormous amount of material, labour, and time, which would have been

expended to erect the work in stone.

Before the principle of lofty iron piers had been thoroughly

developed, many high piers had been built of timber both at home and

abroad. More particularly was this the case in the United States of

America, where the presence of magnificent timber close to hand

offered special inducements for the use of wood. Like a mammoth

scaffolding, each pier was constructed with a most liberal supply of

material, judiciously selected and carefully put together, but the

danger of destruction by fire was ever present from the beginning.

Probably more timber piers and bridges have been destroyed by fire

than have been removed on account of natural decay.

One of the most notable of these timber-pier constructions was that of

the Old Portage Viaduct, on the Erie Railway, U.S.A. Fig. 101 is a

sketch of one or two of the piers. This viaduct was more than 800 feet

long, and 234 feet high from the bed of the river to the rail-level.

The spans were 50 feet each. Masonry piers were carried up to about 25

feet above the ordinary water-level of the river, and upon these the

timber superstructure was erected. Each timber pier consisted of three

complete sets of framework, securely connected together, and also well

stayed and braced to the adjoining piers. This viaduct was destroyed

by fire in 1875, and was reconstructed with piers and girders of iron.

Railway bridges over or under public roads of primary or secondary

importance must be constructed to the widths and heights prescribed

for such works in the fixed regulations of the country in which they 103

have to be built. As a rule, these road-bridges are simple and

inexpensive in character, except in towns, or in cases where the line

crosses the roads very obliquely, or where the road is situated at the

top of a deep cutting, or bottom of a high embankment. Away from towns

and out in the open country, permission is generally obtained to

divert the roads to a moderate extent, so as to obtain a more

favourable angle and height for the bridge; but in towns, where the

roads become streets, sometimes of great width, with houses and shops

on each side, little or no diversion can be allowed.

A railway passing through a portion of a densely populated town must

deal with the streets as they exist, as any great alteration in their

course or continuity would involve a large destruction of property.

With careful laying out it is possible to obtain favourable crossings

for many of the streets, but a number of others must be crossed

obliquely, and these oblique crossings very frequently result in a

span twice the width, or even more, of what would be necessary to

cross the street on the square. Bridge-work in towns is more costly

than in the country, as a higher class of work is demanded, more

finish or dressed work in the masonry or brickwork, and more

ornamentation in the screens and parapets in connection with the iron

girder-work. The work itself has to be carried on in a confined

locality, with limited space for materials and appliances, and where

the thoroughfare must be kept open.

Where the height is sufficient, and suitable materials readily

obtained, it is preferable to adopt an arch bridge, as being of a much

more permanent character than girders.

Fig. 102 is an example of an ordinary over-line arch bridge to carry a

public road over a double line of railway in a cutting of moderate

depth.

Fig. 103 shows a somewhat similar over-line arch bridge, but its

height from rail to road-level being greater, side arches are

introduced in preference to long heavy wing walls.

Fig. 104 shows an over-line arch bridge in a rock cutting. In this

case, by increasing the span and forming the springing bed in the

solid rock, the masonry of abutments and wing walls may be reduced to

a minimum.

Fig. 105 is a sketch of an ordinary under-line arch bridge to carry a

railway over a public road in an embankment of moderate height.

[Illustration: Fig. 102, 128, 129, 130]

[Illustration: Fig. 103]

[Illustration: Fig. 104]

[Illustration: Fig. 105]

[Illustration: Fig. 106]

[Illustration: Fig. 107]

Fig. 106 shows a similar under-line bridge, but with curved instead of 110

straight wing walls.

Fig. 107 is an example of an under-line arch bridge in a rather high

embankment, and where side arches have been adopted instead of long

wing walls.

The above six types are equally applicable for private roads crossing

the railway, but, as previously mentioned, a lesser width and headway

will be accepted for under-line bridges for private or occupation

roads, than for public roads. For the over-line bridges, however, the

width and headway will be regulated by the number of lines and

standard height of the railway.

When these arch bridges have to be built on the skew to suit an

oblique crossing of the road, extra care will be necessary in setting

out the work, and marking on the centering the spiral courses of the

arching.

Arch bridges may be built of masonwork or brickwork, or a combination

of the two. If the available quarries do not yield good flat bedded

stones readily worked, it is better, where possible, to use strong

hard bricks for the arching, and utilize the stone for the remainder

of the work.

Although arching undoubtedly forms the most durable type of

bridgework, numbers of cases occur where the available height or space

between rail-level and road-level is too small, or the cost of masonry

and brickwork too great, to admit of anything but girder-work.

Detailed sketches of some of the many forms of girder bridges are

given in Figs. 132 to 153, illustrating various systems of roadways

and parapets. In some instances the main girders are made sufficiently

deep to serve as parapets, while in others a shallower girder has been

adopted, on top of which has been placed a light cast-iron parapet

composed either of close plate-work or of ornamental open railings.

The open ironwork parapet has a good appearance, but as a screen is

not so efficient as the close cast-iron plates.

In addition to the bridges required for the regular public roads, it

is usually necessary to construct a certain number of occupation or

private road bridges over and under the line to accommodate portions

of estates and large properties intersected or severed by the railway,

and which would be inadequately provided for by ordinary gate

crossings on the level. The position and description of these

occupation bridges is generally matter of private arrangement. The

bridges will be somewhat similar in character to the public road 111

bridges, but of much less width for the roadway. Those over the

railway must have the standard span and height adopted as a minimum

for the other over-line bridges, and those under the railway must have

the full width on the top for the lines of rails, but will have less

width between the abutments for the roadway.

Foundations.--So much depends upon the soundness and security of the

foundations of any bridge, viaduct, or large building, that it would

be almost impossible to devote too much care to the selection and

treatment. Unless the foundation be firm, the entire structure will be

exposed to the risk of failure, either in subsidence of masonry,

giving way of arches, or depression of girders. A small matter

overlooked during the construction of this part of the work will be

most difficult to correct or adjust afterwards.

The insistent weight of all structures built of masonry or brickwork

will cause the mass to settle to a certain extent, according as the

joints of mortar or cement become compressed by the number of

superincumbent courses. In a similar manner the gravel and clay of a

foundation will compress more or less according to its compactness and

the weight of the structure. No inconvenience will, however, arise if

the settlement or compression be uniform throughout the entire area.

In ordinary average, dry, solid ground, a good foundation can usually

be obtained at a moderate depth. The removal of a few feet of the

surface layers will generally lead to a good hard stratum of natural

material sufficiently firm to carry the abutments and piers of railway

bridges and viaducts. Two or more footings are usually adopted so as

to distribute the weight over an increased area, as shown in Fig. 108.

Where the weight to be carried is considerable, it is better to

increase the number of the footings, and give them a smaller

projection, as in Fig. 109, rather than have a lesser number and

greater projection, as in Fig. 108. There is greater liability of

fracture of the material in the latter than in the former.

Care must be taken to distinguish between made ground and natural

ground. Hollows which have been filled in must not be relied upon to

sustain heavy weights; the material may have been consolidating for

years, but it is safer to cut through it and found upon the natural

stratum beneath.

[Illustration: Fig. 109, 108, 110, 111, 124, 113, 114, 115, 125]

Soils of a clayey nature must be dealt with very cautiously. If the 113

ground be fairly level, and the material firm, a solid foundation may

be obtained, but the excavated portion should be covered up as quickly

as possible to prevent any decomposing action taking place upon

exposure to the open air. The expansive nature of some clays must be

carefully kept in view, so as to guard against any disturbance in the

finished foundation. There are some descriptions of shale which when

first opened out appear to have the solidity of hard rock, and yet,

after a few days’ exposure to the atmosphere, are changed to the

consistency of soft mud.

Sand, being composed of such small particles, is almost incompressible,

and makes an excellent foundation so long as it can be retained in its

position. Little or no settlement will take place if the sand remains

undisturbed, but so soon as it comes under the influence of running

springs, or underground drainage, the fine particles of the sand will

be gradually but surely carried away with the water, and the entire

foundation be undermined. The opening out of a neighbouring

excavation, or the carrying out of some low-level drainage, would

endanger a construction which otherwise would be solid and permanent.

In many cases of soft ground, more particularly abroad, sand piles

have been adopted and have given very good results. The system is

carried out by first driving a large wooden pile down through the soft

material into the more solid stratum below. The timber pile is then

carefully withdrawn and the cavity filled with clean sand. The number

and distance apart of these sand piles will depend upon the nature of

the ground and description and weight of structure to be carried.

Clean, compact gravel is one of the best materials to build upon,

being almost incompressible and quite unaffected by exposure to the

atmosphere. It is easily excavated and levelled off to the surface

required.

A foundation of rock may be considered in the abstract as the most

solid base to be obtained, but it must be treated judiciously, and a

proper surface secured. The outer portion of many descriptions of rock

consists of blocks or layers of stone partially or entirely separated

from the main bed, and these, lying in a loose condition, are

deceptive and treacherous as a foundation base. The exposed rock

should be carefully examined, and all detached or outlying pieces or

layers removed before placing any foundation course. Special care must

be paid to all shelving rock, and a level seating cut into it for the 114

entire width of the foundation, as shown in Fig. 110.

A thick bed of concrete, as in Fig. 109, makes an excellent foundation

course. When firmly set it becomes one solid massive base from end to

end, and prevents the yielding or dropping of masonry at any

intermediate points.

There are many places in soft, wet ground where instead of attempting

to excavate all the soft material down to a harder stratum, it is

better to adopt timber pile foundations, as shown in Fig. 111. The

size of the piles and their distance from centre to centre must be

regulated by the description of material into which they have to be

driven and the weight they have to sustain. Double waling pieces

should be properly checked and bolted on to the heads of the piles,

and trimmed or levelled off to receive a double floor of thick planks.

The spaces round the heads of piles and walings should be filled in

and levelled up to under side of flooring, with cement concrete.

[Illustration: Fig. 112]

For bridges of moderate span, over soft ground or over shallow fresh

water, strong cast-iron screw piles can be adopted with great

advantage. Fig. 112 shows a very usual form of screw pile, made with

an external screw at the lower end and with a sharp cutting edge to

facilitate penetration into the ground. The upper portions are made in

suitable lengths, and all to one pattern and template, for convenience

in carrying out the work. The screwing into the ground is generally

effected by means of a capstan or cross-head fixed to the top of the

first working length of pile, and which is pulled or turned round by

ropes worked from stationary windlasses. In some cases long bars or

levers are attached in radiating positions to the capstan-head, and a

number of men are employed to walk round and round, pushing the

levers, and in this way screwing the pile into the ground. As the pile

goes down the capstan-head has to be removed, and additional lengths

bolted on, until the pile enters a solid stratum, or is considered

deep enough for the duty it has to perform. The last or top length has

generally to be cast to a special length to bring the work up to the

exact height to receive the girders. The core of excavated material

passes up into the interior of the pile, and in some cases becomes so

compressed or tight as to require the use of an internal augur to

remove a portion of it to enable the screwing to proceed. The pile

shown in Fig. 112 is one of a number which were successfully screwed

into the ground to depths varying from 42 to 48 feet. A toothed or 116

serrated edge, as in Fig. 113, is sometimes given to the lower edge

for screw piles which have to cut their way through a hard stratum.

All bolting flanges should be accurately turned and fitted to ensure

close, parallel surfaces when bolted together.

The joint shown at A, Fig. 112, is one the writer has used to a

large extent for the bolting flanges of cast-iron screw piles and

cylinders. It is very simple in form, readily coated with white lead

to ensure a water-tight joint, and as the upper length is practically

recessed, or let into the lower length, the exact continuity of the

different castings is secured.

Solid screw piles of wrought-iron or steel, similar to Fig. 114, are

used for some descriptions of work. These are generally made in long

lengths, in sizes varying from 4 to 8 inches in diameter, and with

screw blades of wrought-iron or cast-iron fixed in the most secure

manner to resist the strain produced when screwing into the ground.

The couplings for these solid piles must be very carefully made, all

contact surfaces truly faced and fitted, bolts turned, and bolt-holes

drilled.

Fig. 115 is a sketch of a hollow cylindrical water-jet pile, which has

been used successfully in cases of light sand. The lower end of the

pile is made externally in the form of a solid disc, terminating in a

conical point, having an aperture in the centre to correspond to the

water-jet. To the top of the pile is secured a tight-fitting cover

through which a tube passes from a force pump. Water at high pressure

is pumped into the tube, and as it forces its way out through the

conical point the sand is stirred up and loosened, and thus allows the

pile to descend. When the pile has been lowered to a sufficient depth

the pumps and tube are removed, and the sand settles down into its

former compact condition.

Great care must be used with the first two or three lengths of any

screw pile to ensure the pile taking a correct or true vertical

position. Each series of screw piles should be properly braced

together to obtain stability under moving loads.

[Illustration: Fig. 116]

[Illustration: Fig. 117]

Hollow cylinders of cast-iron, wrought-iron, or steel form most

efficient foundations or piers for large bridges over soft ground or

fresh water of considerable depth. Made open at the bottom, and

constructed of complete rings, or, if of large diameter, of rings

built up in segments and securely attached together with water-tight 119

joints, the cylinder is placed in its proper position on the ground or

lowered into the water preparatory to sinking. The lower length is

made with a sharp cutting edge to facilitate penetration. By

excavating and removing the material round the cutting edge and base

inside the lower length, the cylinder descends gradually either from

its own weight or by assisted weights, and length after length is

added until it is sunk to the depth required. The excavated material

is filled into buckets and hoisted to the surface by a winch fixed on

the top length. When sinking in water the working top of the cylinder

is always kept at a suitable height above the water for convenience in

removal of the earth or clay from the interior to barges or gangways

alongside.

Some strata are more favourable for cylinder sinking than others.

Material of a strong clayey nature admits but a small amount of water

into the excavation, and a moderate-sized pump will keep the working

fairly dry until considerable depth has been reached. Some other

materials are so open that the water cannot be kept down with ordinary

pumps, and the cylinders can then only be lowered by the pneumatic

process. This process has been carried out in two methods, one of them

on the _vacuum_ principle, and the other by air pressure, or, as it is

termed, the _plenum_ system. With the former method the cylinder is

placed in position, and an air-tight cap, through which a pipe passes,

is secured on the top. Powerful air-pumps are then set to work, and

the partial vacuum thus created in the interior causes the material

round the cutting edge and base to be loosened and drawn into the

cylinder, the cylinder at the same time going down or sinking by its

own weight, or assisted, if necessary, by added weights. The cap is

then taken off, and the material removed from the interior, the

operation of exhausting and emptying the interior being repeated until

the cylinder is sunk to its proper depth. This method has been found

to work well in strata which contained a large proportion of clay to

assist in excluding the air and water, but was not nearly so

successful when applied to material containing stones and large

boulders.

The _plenum_ process is based on the principle of the diving-bell, the

water being prevented from entering at the bottom by keeping the

cylinder full of compressed air. An air-chamber, or _air-lock_, with

perfectly air-tight joints, is securely fixed to the top or upper 120

working length of the cylinder, and no access can be obtained to the

interior of the cylinder without passing through this air-lock, which

has one lower door or valve opening into the cylinder, and an upper

door opening out into the open air. Temporary inside staging is formed

by putting planks across from flange to flange, and placing short

ladders on these landings for the use of workmen descending or

ascending. The excavated material is hoisted by a winch, generally

placed on the landing just under the air-lock. The air-pump is placed

in some convenient position outside, near at hand, the pressure-pipe

passing through the air-lock into the interior of the cylinder. Air is

forced into the cylinder to a pressure sufficient to drive out and

keep out the water from the interior, and allow the workmen free

access for excavating the material round the cutting edge and base of

cylinder. The amount of pressure required will depend upon the depth

of the working below the level of the water alongside. Men accustomed

to the process can work without much inconvenience under a pressure of

20 to 22 pounds per square inch, equal to a depth of 45 to 50 feet;

but when the pressure exceeds 25 pounds, the duty becomes very trying,

and is attended with considerable risk. Instances are recorded of men

working at depths of 105 and 110 feet, necessitating a pressure of

over 45 pounds per square inch; but it is very questionable whether

the men exposed to such a severe ordeal were not permanently affected,

if some of them did not actually succumb.

It will sometimes occur that, after sinking through soft porous strata

to a considerable depth, a layer of clayey material is penetrated

sufficiently retentive to keep out the water and permit of the removal

of the air-lock and the completion of the sinking as an open-top

cylinder.

When working on the _plenum_ system everything must pass through the

air-lock, both materials and men. The excavated material is hoisted up

to the level of the air-lock, the upper and lower doors of which must

be closed, and the pressure inside the air-lock brought to the same as

that inside the cylinder by means of a regulating valve. The lower

door is then opened to admit the excavated material, and then closed

again to cut off all communication with the interior of the cylinder.

The upper door is then opened, and the material hoisted out into the

open air. The same process has to be adopted for the egress of the

workmen, and the reverse arrangement for the ingress of men and 121

materials. The shape and dimensions of the air-lock may be varied

according to circumstances, but the principle will remain the same.

When the cylinder has been lowered to what is considered a sufficient

depth, it is usually loaded with a certain amount of dead weight in

the shape of old iron or other convenient material, and allowed to

remain loaded for some days to ascertain if it will sink any further.

Should this test be found satisfactory, the dead weight is removed,

and the interior of the cylinder pumped dry and carefully filled with

good cement concrete.

Cylinders for foundations are generally made circular in section, that

form being the most convenient for turning and facing the

flange-joints. They can, however, be made oval in section, or of any

section that may be found most suitable for the work required. Figs.

116 and 117 give the particulars of a double-line railway bridge

carried on cylinder piers across a river. The detail sketches explain

the form of cutting edge, flange joint, and method of bracing. This

bridge is one that was reconstructed and widened from a single-line to

a double-line bridge. Traffic was carried over on one line while the

second line was being erected, hence the reason why one strong central

girder was not adopted.

Cylinders of 7 feet diameter and upwards are sometimes filled with

concrete in the lower portion, on which is built either a circular

lining or a solid mass of masonry or brickwork up to the level of the

girder-blocks. In some cases the cylinders proper, together with their

concrete filling, terminate a little above the water-level, and upon

these foundations are erected strong cast-iron columns, plain or

ornamented in design, to carry the girders and roadway. The cylinder

itself is generally considered merely as a casing or medium for

obtaining a foundation, the weight of the superstructure being carried

on the internal filling or lining.