NOTES UPON SLIPS OF EARTH, SUBSIDENCES, AND MOVEMENT IN
FOUNDATIONS CAUSED BY “BOILS,” OR AN UPWARD RUSH OF WATER IN
LOOSE EARTHS—CONSIDERATION OF SOME PRECAUTIONARY AND REMEDIAL
OPERATIONS 216–225
EARTHWORK SLIPS AND SUBSIDENCES UPON PUBLIC WORKS.
CHAPTER I.
INTRODUCTION—GENERAL CONSIDERATIONS—ENUMERATION OF THE PRIMARY CAUSES
OF SLIPS AND SUBSIDENCES IN CUTTINGS AND EMBANKMENTS, AND EARTHWORKS
CONSTRUCTED TO CONTAIN OR EXCLUDE WATER—SOME DOMINANT PRINCIPLES TO
BE REMEMBERED IN DETERMINING THE LOCATION OF EARTHWORKS.
Earthslips and subsidences may be caused by the terrible power of an
earthquake or other dreaded subterranean destroying force, upheaving,
cracking, and shattering the earth’s crust and dealing death and havoc
in its awe-inspiring course. They may also originate from the untiring
efforts of the meanest rodents or the most minute crustaceous animals
burrowing passages for aqueous action, the chief agent of the
instability of the surface soils of the earth.
The ceaseless mutability of the created elements has been thus
magnificently described:—
“For know, whatever was created needs
To be sustain’d and fed; of elements
The grosser feeds the purer, earth the sea,
Earth and the sea feed air, the air those fires
Ethereal, and as lowest first the Moon;
Whence in her visage round those spots, unpurg’d
Vapours not yet into her substance turn’d.
Nor doth the Moon no nourishment exhale
From her moist continent to higher orbs.
The Sun, that light imparts to all, receives
From all his alimental recompense
In humid exhalations, and at even
Sups with the ocean.”
It is the constant absence of peace and rest in the earth that produces
instability, however great, however small, for the disastrous landslips
that have occurred in Switzerland and other parts of Europe, in India,
and recently at Quebec, and in all regions of the world were caused by
the same disintegrating operations as those which generate an earth-slip
of comparative insignificance. In proceeding to a practical
consideration of earthslips and subsidences, it may be well to call
attention to the complexity of the subject, the character and conditions
of earth and the impairing elements being so very variable and numerous
that it is impossible to determine any rules even for a particular soil;
and, moreover, it is necessary to separately consider slips in cuttings
and those in embankments, as movement is somewhat differently created,
and it does not necessarily follow because earth stands well in a
cutting that it will do so in an embankment, or _vice versâ_.
It may be said every kind of earth will slip or weather under certain
conditions, even the hardest rock if superimposed upon an unstable
stratum; therefore, some of the main questions to be considered are:—
I. THE PROBABILITY OF THE OCCURRENCE OF A SLIP.
II. THE EFFECT OF A SLIP.
III. SHOULD EVERY PRECAUTION BE TAKEN TO PREVENT A SLIP WHEN A
CUTTING IS BEING EXCAVATED OR AN EMBANKMENT BEING DEPOSITED; OR IS
IT BETTER TO REPAIR A SLIP AS IT HAPPENS?
It is obvious in railway cuttings and embankments a mere crumbling of
the surface may be disregarded, but in dock, canal, or any works
containing or expelling water, the smallest movement, crack, or aperture
must immediately receive due attention.
In order to effectually remedy a disease it is necessary to ascertain
its character. Many of the primary causes of slips in cuttings and
embankments are, therefore, here enumerated; but, of course, they are
not named in their order of importance, which cannot be established.
HEADS OF THE CHIEF CAUSES OF SLIPS AND SUBSIDENCES IN CUTTINGS.
1. The want of uniformity of the earth, particularly as regards
percolation, cohesive power, and resistance to change by the action of
water or meteorological influences.
2. The temporary or permanent exposure of the earth to the effects of
the atmosphere, rain, frost, and snow.
3. The opening to the air and weather, &c., of thin seams of an unstable
character, which, when unsupported, gradually crumble away and cease to
support superimposed strata.
4. The tapping of springs.
5. The lower portion of a slope being impaired or undermined through an
infiltration and flow of water.
6. The erosion of the slope.
7. The earth having intermediate unstable seams.
8. The unprotected surface of a cutting in light soil being loosened and
blown away by a storm of wind, especially when it is accompanied by
rain.
9. The slopes being honey-combed and disturbed by rodents, particularly
in clay soils, clay marls, and clay loams; and upon submerged earthwork,
and in certain districts in the tropics by a mollusk which will
penetrate and even destroy rocks.
10. From one portion of a cutting being more exposed than another to
disintegrating meteorological influences.
11. By the discharge of water from land drains following the old
drainage course, and by the localisation of the surface or land water
flow.
12. The improper or imperfect drainage of land outside a railway fence,
causing land water to accumulate in and discharge itself through the
slope, thus disturbing the established equilibrium.
13. By an interference with the natural flow of any underground waters.
14. By allowing water to accumulate in the gullet, or upon the formation
during the process of the excavation of a cutting.
15. The local percolation of water through the slopes of a cutting from
the defective construction, wrong location, or permeability of the
surface of a drain upon the cess.
16. An accumulation of water caused by the unevenness of the slopes.
17. The acceleration and inducement of a flow through the slope of any
water contained in land outside a railway fence, and consequent
incitement to the land-water to exude.
18. Vibration.
19. Insufficient flatness of a slope either at the time of excavation or
after exposure to meteorological influences.
20. The strain upon the face from lumps of earth being allowed to remain
upon a slope during the construction of the works, or the gullet being
excavated for a considerable distance in advance; the cohesion of the
soil being thereby unduly and unequally strained.
21. By overweighting.
22. By unequal loading.
23. The establishment of spoil banks upon the cess, or the additional
loading of the ground near and outside a railway fence, in soft soils
having practically no cohesion or tenacity.
24. The excavation removed destroying the continuity of support,
especially in soils partaking of a semi-fluid character.
25. The want of uniformity of the covering of a slope causing unequal
percolation or exudation of water.
26. The neglect to fill up, or otherwise remedy, cracks or fissures in
the slopes or cess.
27. By artificial or irregular consolidation either of the formation or
slopes superinducing movement and weathering in any portion not so
compacted.
28. From an accumulation of water behind a retaining wall at the foot of
a slope, resulting in the stability of the wall being overcome by
pressure.
29. By unequal pressure upon the foundations of a retaining wall at the
foot of a slope caused by lateral over-pressure tilting it, or by its
unequal settlement.
30. In sidelong ground, by the removal of support against the action of
sliding, which, without artificial aid, may not be arrested until the
slope of a cutting on the higher side approaches the steepest
inclination of the face of the hill.
31. By blasting laminated rock dipping at a considerable angle towards a
cutting in the side of a hill; the result sometimes being that a cavity
is made depriving the upper beds of support and causing them to
overhang, and a mass extending to the top surface of the hill to slip
along the unsupported stratum.
The first ten “heads of the chief causes of slips in cuttings” might be
classed as NATURAL, _i.e._ produced or effected by nature and,
therefore, beyond the power of man to entirely prevent; the remaining
heads as ARTIFICIAL, and therefore, in some degree to be prevented,
unless obviously the result of the unavoidable exigencies of
construction.
HEADS OF THE CHIEF CAUSES OF SLIPS AND SUBSIDENCES IN EMBANKMENTS.
1. The percolation of surface water into the toe and under an embankment
upon the original surface of the ground, and also downwards through the
formation.
2. Unequal percolation of water through the formation or the slopes.
3. The surface of the ground upon which an embankment is tipped
inclining in one direction, or falling on each side from the centre.
4. The effects of rain, frost, snow, and the atmosphere on the deposited
earth.
5. By a crumbling of the lower portion of a slope.
6. By a hurricane or extreme wind force, especially when accompanied by
rain and the location is that of a narrow, steep valley, blowing away or
dissipating the top and the surface of an uncovered embankment.
7. By a slope becoming honey-combed by rodents, and in some few
countries by an embankment in a river or the sea being bored and
disturbed by crustacea.
8. Insufficient slope for permanent stability at the time of or after
deposition.
9. The want of adhesion between the surface of the ground upon which an
embankment is tipped and the deposited material.
10. By the accumulation at the foot of an embankment of boulders or
lumps having no cohesion, and no adhesion to the surface of the ground
upon which they are deposited.
11. The weight being too great upon the ground upon which an embankment
rests.
12. By unequal loading.
13. An accumulation of water, caused by the unevenness of the surface of
an embankment, or by the ground not being prepared so as to prevent a
lodgment of water.
14. By obstructing the established discharge of land-water or by
attempting to divert the natural flow of underground waters.
15. The localisation of the surface drainage.
16. Water percolating into a benching trench, made to receive the toe of
a slope, thereby impairing the cohesion of the soil and reducing its
weight-carrying capacity and stability.
17. Vibration.
18. The different nature and state of the earth tipped into an
embankment and the consequent localisation of water in the more pervious
soil, causing unequal settlement, subsidence and movement.
19. An embankment being tipped of material in a different state of
dryness, moistness, hardness, softness, or in a frozen condition.
20. The size and character of the earth, as excavated in the cuttings;
for instance, whether picked and shovelled soil, or lumps of excavation
simply barred away and deposited in small masses with earth approaching
a state of dirt or mud.
21. Overpressure upon the material forming an embankment.
22. The different conditions of the weather, when an embankment is
tipped, causing portions to become dry, wet, or frozen.
23. The lead or distance from a cutting to the place of deposition being
of considerable length.
24. The earth being loosened from vibration and concussion during
transit in the wagons, and in the process of deposition, thus causing it
to be non-homogeneous.
25. By the earth being tipped with greater impetus at one part than at
another place.
26. An embankment not being tipped to its full width as it progresses,
whether in one or more wagon roads.
27. First tipping the central portion of an embankment, and completing
the width and slope by side deposition after some time has elapsed.
28. Tipping the contents of earth wagons from a considerable height,
thereby loosening and separating the soil, causing the larger and
heavier material to be near the foot of a slope and in lumps, and an
embankment to have interstices and be temporarily or permanently
unstable.
29. Irregular consolidation, artificial or otherwise.
30. Unequal exposure, particularly in embankments upon sidelong ground.
31. Insufficient width of the formation, especially in high and exposed
embankments.
32. The junction of two embankments tipped from cuttings in different
kinds of earth.
33. No time being allowed for subsidence or consolidation before the
deposited earth is subject to varying loads and vibration.
34. By allowing water to collect upon the formation and to form channels
down the slopes.
35. The neglect to fill, or otherwise remedy, cracks or fissures.
36. The want of uniformity of the covering of the slopes or top of an
embankment causing unequal percolation of water.
37. By a retaining wall at the toe of a slope preventing the discharge
of water that has percolated through the formation and the slope.
38. By an abnormal increase of the load upon the foundation of a wall
caused by lateral thrust and tilting forward, or fracture of the
footings or the concrete bed.
The first seven heads of “the chief causes of slips in embankments”
might be classed as NATURAL, _i.e._, produced by nature, and, therefore,
beyond the power of man to entirely prevent; the remainder as
ARTIFICIAL, and, therefore, more or less to be prevented.
In the case of earthworks made to contain or exclude water for the
purposes of docks, canals, waterworks, reclamation of land, irrigation
or drainage, &c., may be added, without reference to their
construction:—
1. Leakage along a discharge culvert, sluice, or tunnel in an
embankment, or through the earthwork.
2. Erosion of the land slope or backing of a retaining wall by waves, or
spray falling thereon and necessarily passing over the top of an
embankment.
3. From abrasion and damage, caused by vessels or barges rubbing or
colliding against a slope.
4. From erosion caused by wave action produced by the passage of boats
or ships propelled by machinery; or by wind waves.
5. Variation in the water level, causing unequal pressure.
6. In the case of a reclamation embankment, it being closed from the
ends, and not by raising in layers, from the ground level.
7. Variation of the submerged area, and consequent change in the degree
of exposure to deteriorating influences.
* * * * *
To sum up, the principal causes of slips in earthwork may be stated to
be air, water, frost and thaw, over-pressure, and vibration; the chief
agent both in cuttings and embankments being water, which forces forward
the surface of the slopes and destroys the cohesion of the soil, and
impairs its frictional resistance until the earth is unable to sustain
the weight upon it; vibration aiding and completing the movement, as it
not only tends to loosen the soil, but may disturb the equilibrium of
earthwork which is almost moving, and only requires a slight shock to
set it in motion: in fact, vibration is frequently the complementary
agency that causes a slip, and is obviously felt most in loose soils;
but if there should be fissures in earth of a tenacious character, or
boulders in clay to disconnect it, the effect of vibration will be more
serious in the latter case, as whole masses of earth may become
detached, instead of an equal settlement proceeding as with soils, such
as sand and gravel, which may become consolidated by shaking, owing to
wedging of the particles, should the slopes be sufficiently flat to
prevent lateral movement; but it may detach portions of the slopes in
soil having little or no cohesion, and thus initiate a slip.
In the following chapters the chief causes of slips in cuttings and
embankments are considered, together with others bearing upon a solution
of the subject, which is so interwoven that it is impracticable to
preserve a successive order, but an endeavour has been made to
separately indicate the cause and some remedies that may be adopted: but
before proceeding to particularise, it may be well to name a few
dominant principles of the alignment of public works, which if duly
regarded may tend to prevent slips of serious importance.
Consequent upon financial and other causes, an engineer is usually
required to so quickly prepare the necessary parliamentary plans and
sections of public works, more particularly for railways, that it is
beyond the power of the most experienced to set out in a few hours the
best line of railway, &c., across a country, giving due consideration to
the parliamentary, constructional, economical working, district and
through traffic, and financial requirements of the undertaking. There
are, however, a few points which he may be able to regard respecting the
stability of earthworks, some of which are now enumerated.
1. Avoid cuttings or embankments in drift soil in or upon the side of a
hill.
2. Avoid all damming back or flow of the natural drainage waters, or
heaping of snow by the erection of an embankment, especially in
mountainous, hilly, or sidelong ground, and in an undrained district.
3. When any excavation is in the side of a hill, observe the natural
configuration of the ground in the wettest part, and remember that the
slope of a cutting may not stand unless at the same inclination, or the
toe of the slope is supported by a massive retaining wall and extensive
draining, and that any disturbance may cause it to require a flatter
inclination.
4. Avoid river or stream diversions in earth of a very porous character;
and should an embankment have to be erected near to a deep river having
a steep bank, locate the line a sufficient distance from the edge that
the slope of the river bank may be flattened when required, as an
extraordinary flood may cause it to be unstable and to fall in, and in
order to restore it to a condition of stability, it may be necessary to
widen the river and reduce the angle of inclination of its banks.
5. In treacherous earth do not locate a railway close to a road, except
at a station, as if the line be placed above the road, a slip upon the
railway may result in a slip upon the road; and should the line be below
a road, the extra weight or vibration may cause the road to follow the
railway and act, as it were, in unison with it.
6. In treacherous soil, where practicable, have stations nearly upon the
surface of the natural ground.
7. Remember that upon one side of even the narrowest gorge or valley the
earth may be much more solid than upon the other.
8. In exposed situations in a hilly country ascertain upon which side
snow remains the longer, note which receives the greater amount of
sunshine, and is the wetter and more covered with trees and vegetation.
In a hill or mountain, the side to leeward of the prevailing winds
almost invariably receives the greater rainfall.
9. In mountainous or hilly districts it may be advantageous to place a
railway or road at a high level upon the sunny side of a hill or valley,
as obviously it dries quicker; snow does not accumulate with the same
facility, unless it happens to be exposed to the direction of the
prevailing storms; the sunny side may be practically clear of snow, the
shady almost impassable; but there is one drawback to the sunny side,
namely—the more frequent occurrence of snow-slips, which may or may not
be serious in extent. On the whole, experience seems to show that the
wooded side of a valley is the best to select—sometimes one side is bare
and the other wooded—unless there are special reasons to the contrary.
In the winter season in certain districts abroad, for instance, in parts
of Afghanistan and the adjacent mountain passes, the days are sometimes
as hot as the summer of European countries, but at night the thermometer
may fall below freezing-point, hence the value of tree-protection. It is
obvious that in such climates the soil is peculiarly liable to
disintegrating forces, and likely to slip, unless of a solid character,
and that upon one side earthworks may be stable, and upon the other
treacherous.
10. In many countries it may be advisable to adopt a valley in
preference to a hill-side line, especially if the district is free from
floods, or if the waters flowing down the side of the hills are
considerable and suddenly appear, and there is a river and ample means
available of controlling the hill-side torrents and conducting them to
the river. If not, and the line must be on the mountain side, it may be
advisable, in exposed places where water rapidly accumulates and becomes
a torrent, to adopt the system of short tunnels round the hill spurs, in
preference to deep cuttings, drainage, and slope protection works, and
to place the railway or road at the highest level, so as to be free from
the influence of floods.
11. Bear in mind that in high mountainous districts the drift deposit is
generally torrential alluvium.
12. Avoid as much as possible high embanked approaches to a river
bridge, especially when a deep river, which frequently changes its
course, is in a flat country.
13. Consider if the simple erection of an embankment may in time cause
its destruction, by the arrest or attempted diversion of the usual flow
of the land waters.
14. In treacherous soils, on the side of a cliff facing the sea,
determine whether it is preferable to erect timber trestles at its base
instead of a solid embankment, or to place the line in a tunnel. The
trestles can either be erected upon sills, resting on the ground and on
short piles well secured from movement, or on piles driven some distance
into the ground. The system may also be adopted if the ground be of a
yielding character.
15. It has been noticed in some mountainous districts that the clouds
break against the highest main ranges, discharge themselves on the
smaller ranges, and generally do not reach the inner ranges which rise
on the high table-lands; therefore, consider whether by locating a
railway or road near the latter, there is much less probability of a
slip in the earthworks and less provision required for surface and flood
waters.
16. The configuration of a district through which a railway or road must
be aligned, may be such that its location becomes one more based upon
placing it on soil which is less bad or treacherous, than upon firm or
stable ground. This is especially the case in hilly countries contiguous
to the sea or large rivers. It may be optional to construct the works
upon low or valley ground upon the side of a mountain or hill, or close
to the sea shore or a river bank, which may require continuous defence
works to protect it from waves and erosion; or on table-land which,
however, if impervious and retentive of moisture may act as a catchment
reservoir between hills, and cause the ground to be always in a damp
state.
The character of the soil, the magnitude, and especially the average
height of the embankments, or the depth of the cuttings, the easy
drainage and discharge of the rainfall, and an economically constructed,
maintained, and worked line are the chief conditions to inseparably bear
in mind in determining the location.
CHAPTER II.
THE PROBABILITY OF A SLIP—TIME OF THE MOST FREQUENT OCCURRENCE—SOME
CONDITIONS UNDER WHICH SLIPS AND SUBSIDENCES IN CUTTINGS AND
EMBANKMENTS MAY BE EXPECTED IN DIFFERENT EARTHS, SUCH AS ROCK,
CHALK, SAND, GRAVEL, CLAY, &C., &C.,—NOTES ON THE SLOPES OF REPOSE.
It is of importance to know when serious slips are most likely to happen
and under what conditions they are probable, for the process of
disintegration may commence immediately the earth is excavated, and be
very gradual, although the soil may remain stable for many months, or
even a year or two, because the earth has not had time to be affected to
the point of instability.
The history of recorded slips appears to indicate that the most serious
movements of earth and those most difficult to remedy occur in the
following soils.
DRIFT EARTH upon rock in sidelong ground.
CHALK SOILS, as witness the slips in the early part of 1877 in the
cuttings near Folkestone, and the more recent on the Calais-Boulogne
Railway, and that in an embankment of chalk at Binham’s Wood, near
Balcombe, in October 1853, when in a length of about 200 yards some
70,000 cubic yards of earth slipped towards a valley. Probably this is
one of the most extensive recorded slips of a railway embankment of
chalk; however, in this case the traffic was not stopped, but only
delayed.
CLAY SOILS, especially the yellow clay; illustrated by the notable slip
at New Cross, near London, when some 90,000 cubic yards of yellow clay
moved upon the smooth surface of a shaly clay bed and covered the
formation: also the brown, and boulder clay, and the lias clays, as
witness the well-known recorded slips in the Midland counties of
England, in which either aluminous or calcareous material may
preponderate.
There are few, if any, earths in which the cohesion, weight-sustaining
power and ability to resist the action of water and meteorological
influences are practically the same at all depths, the different
conditions, arrangement, and character in which they are found being
almost infinite, and there are earths which may become consolidated and
watertight if in a constantly moist and protected state, that when dry
and exposed will shrink, fissure, and soon become unstable.
Consequent upon cohesion, a cutting may stand for some time almost
vertically; nevertheless stability cannot be considered as solely
regulated by the cohesion of earth, for an embankment of gravel, sand,
or broken rock with a proper slope and protected from erosion will
usually safely bear more load than an embankment of clay, although the
former material may be said to have no cohesion; but the lateral thrust
of dry, firm sand is known to be small, provided the sand is not
disturbed; also no earth can be said to be immovable under every
condition, but consideration of the soils particularly liable to
disturbance or mutation is, under ordinary circumstances, the main
question to be determined.
With few exceptions the exterior or faces of cuttings and embankments
will, at certain times, become impaired or soddened by the infiltration
of water. In cuttings there is the additional danger, owing to the
geological formation, of the excavation reaching the depth at which
water is generally found in the locality, and it is therefore advisable
to ascertain this level, and also to decide whether the ground must be
excavated below it, as necessarily there will be a downward flow, and
the slopes and formation will consequently have to sustain a pressure
due to the difference between the normal level of the water-bearing
stratum in the neighbourhood and that of any depth beneath it. In such a
case, in addition to the usual softening and loosening aqueous action,
there is the particular insecurity of the formation and slopes being
undermined and eroded by springs: also in pervious soil in a drained
district a cutting will be found to be comparatively dry to about the
level of the bottom of the existing contiguous drains, but below that
depth water will be present, probably in considerable quantity. Land
drains also frequently cause slips, as they localise the flow of the
surface or underground waters, and when in excavating a cutting they are
intercepted, the discharge should be led away from the slopes; but
difficulty may be encountered in effecting this, as water will usually
follow its original course, and it may be impossible to entirely divert
the direction of the flow, and the only thing to do may be to gently
conduct the water down the slope by means of pipes, rubble, burnt brick,
gravel surface drains, or timber ducts.
An important question to determine is, when are slips in earthwork most
likely to occur.
In Europe they are most frequent in the autumn and winter months; but no
rule can be established, nor is it reasonable to conclude, because any
earthwork has remained stable during the usual period when slips may be
expected, that, therefore, none will happen, for the heaviest rain may
descend at an unusual season, and as moisture is the chief cause of the
instability of all earths, it is rather to the quantity of the rainfall
at any time than to the fixed seasons that attention should be directed.
Spring being the driest season in England and autumn the wettest,
October and November being the months of heaviest rainfall, slips are
more probable in the latter than the former season; but the first heavy
and continuous rainfall after a period of estival drought is that
particularly to be feared, or the first rainy weather after a dry period
irrespective of the season of the year; but serious slips may not occur
for many days or until the expiration of even a month or two after such
rainfall, as the ground waters require time before they percolate to or
reach the site of a cutting, and, therefore, all danger may be thought
to be past when it is steadily approaching. The autumnal rains have to
replace the moisture that has evaporated during the summer, and this may
not, and usually does not if it slowly proceeds, produce instability in
earthwork; but immediately the rainfall approaches or becomes in excess
of the power of the natural absorption of the soil, the rain must flow
away, for the earth being fully charged cannot contain it, the surface
becomes wet or the mass soaked, according to the degree of the
permeability of the soil, and the quantity of rain necessary to produce
saturation; therefore, the state of the earth that induces a slip is
that most desirable to know; this cannot be absolutely established in
every case, for it depends upon so many influences, and obviously varies
according to the character of the earth, the varieties and conditions of
which are practically infinite.
In countries that have dry and wet seasons, which cause the earth to
become parched and then to be rapidly saturated, mere surface waters to
become streams, and rivers torrents, slips are probable soon after the
commencement of the rainy season.
When frost follows rain or a fall of snow, and the latter has descended
upon a frozen surface and a thaw sets in, particularly if it be
accompanied by a warm wind causing it to be very rapid in action, the
earth is severely tried, for the frozen water in the ground becomes
suddenly liberated, while the surface is in a state of saturation.
Probably the worst event that can occur for causing floods is when a
sudden and rapid thaw follows a heavy snowfall upon frozen earth, as
then the snow will melt, and water cannot gently percolate the earth, as
the surface will be in a more or less frozen and impermeable condition,
and the snow-water consequently must flow away.
Should any excessive or violent rainfall succeed a period during which
the heat of the sun has caused fissures in the surface of the earth,
allowing water to enter, the state of the soil is favourable to
movement. Land has also become unstable in mountainous countries because
a district has been deforested, or tree-protection much reduced,
movement of the earth usually happening after the first heavy rains or
thaw succeeding frost or snow.
It has also been noticed that when an earth has become completely
saturated or water-charged, a sudden fall of the barometer to a low
pressure will liberate the pent up water which the soil cannot contain
and cause it to burst out, the equilibrium having been so delicate.
Under such circumstances slips are nearly sure to ensue, and to be
serious from their sudden action.
Extensive slips in earthwork seldom occur during the excavation, or a
short time after the completion of a cutting; on the other hand,
movement in an embankment frequently happens during deposition. In the
case of an embankment, time may cause the earth to become consolidated,
but in a cutting the disintegrating and disturbing forces, and the
combined action of air and water percolating until they force forward
the earth, are usually gradual in their operation, and often require a
year or two to cause a state of instability; in fact, the history of
slips, with a few exceptions, in soils whose condition is very readily
changed by water, indicates that serious movement in cuttings does not
generally occur until a cycle or two of the seasons has elapsed, during
which period meteorological influences, aided by vibration and other
deteriorating operations, are slowly and regularly proceeding, until at
length such a change in the general condition is caused that a slip
happens, apparently from some sudden agency, whereas the stability of
the earth has been gradually and surely wasting away for a long time;
hence the importance of continual careful observation in cuttings even
of moderate depth in doubtful soil.
In canals and works of a similar character constructed to contain water,
if any movement or slip of earthwork takes place, it usually occurs
within a short time of the water being admitted, and generally within a
few months and seldom after so long a period as a year, the ground in a
short time becoming consolidated, being exempt from severe vibration and
many of the disturbing agencies present in railway cuttings and
embankments.
In endeavouring to ascertain the probability of a slip occurring, not
only should the superficial strata be considered, but also the original
formation of the country; for instance, drift-soil, which is generally
met with upon the surface of sloping rocks, may consist of various
earths intermixed in endless variety, and in every conceivable shape,
and is not necessarily produced by a weathering of the rock upon which
it lies, for it may have been brought from a distance. In any case,
drift-soil is the result of decomposition and disintegration, and from
its nature is unreliable and ever subject to change, to slip, and to
subside, and so are most of the glacial deposits and moraine found in
mountainous countries; and whenever the contour of a district is
irregular and has numerous clefts, soft and marshy places, valleys and
hills, earthworks will require to be protected against slips; also,
should a cutting be at the base of a cliff or hill, it will probably
have to be excavated in drift deposit and, perhaps, in silt if below the
water-level of adjacent sea or river, and the ground dips towards the
natural outfall of the land-waters. Such drift-soil may be alternately
dry and charged with water from the rocks above, especially if they are
much fissured and water-bearing and permit easy percolation of water,
and must always be in a state of mutability.
ROCK.
With regard to slips in rock, or earth generally classed as rock, the
unstratified or igneous rocks, although they are sometimes traversed by
mineral veins and dykes, are the less likely to slip; but rocks liable
to surface decomposition and disintegration, such as some varieties of
basalt and clay-slate, which latter by atmospheric and aqueous action
will partly return to its original state of being fine mud, thrown down
from the metamorphic rocks, may change their condition and are likely to
slip; and also limestone rock, which however resists the eroding action
of water better than sandstone, may become separated by frost although
its surface soon dries.
Simply knowing the general character of a rock without ascertaining the
proportion, state of the different particles of which it is composed,
and whether any metamorphic action has taken place, is not necessarily a
reliable guide to its stability. In districts situated at a high level,
rocks are usually less permeable than in low-lying lands, and the
surface discharge is greater and quicker because of the increased
rainfall, and less absorption and retention of water.
It should be noticed whether there are dips in the surface of rock, as
they often contain unreliable material, such as pockets and pot-holes of
clay, sand, mud, silt and detritus; and movement may be expected if it
be carelessly tipped with the rock into an embankment. Rocks which
oppose vegetation are usually hard and weather-resisting, and the faults
and fissures local; but it is not so much the equal weathering of the
face of rocks that is to be feared, as the presence and interspersion of
seams, breaks and fissures, and it should always be borne in mind that
the condition of a rock varies considerably—it may be sound in one
place, and be fissured, disintegrated, and quickly weather in others—and
that all laminated and fissile earths are liable to slip because of the
percolation of water down veins and crevices.
The durability of a rock may be approximately known by a careful
examination, commencing at the surface of the ground and proceeding
until it is reached, the thickness and character of the different top
soils being noted, and particularly whether the degradation is uniform;
but rock, such as some sandstone, which allows water to ooze, permeate,
or force a passage, is of doubtful stability. Weathering may be possible
only upon the surface or may gradually extend downwards, and as it can
hardly be called a slip, the point to ascertain is not so much that it
is sound and weather-resisting, as to know that there is no chance of
any portion becoming detached or sliding, through the cohesion of the
joints being impaired or destroyed by water, frost, or other agencies;
for in the case of rocks which show irregularities of stratification,
much cleavage, or are separated by upheaval, or have synclinal and
anticlinal folds, masses become detached along the line of cleavage and
independently of the normal stratification, therefore fissures or
faults, weak veins between masses of rock and crevices, or inclined beds
through which water may flow and always be present; the direction and
inclination of the dip of the strata, effects of weather upon the veins,
and to know the weight upon sloping ground which the rock will bear
without sliding down a hill are the main considerations; for rocks may
be distorted, upraised, contorted and tilted at every angle, and even
horizontal beds may repose upon the upturned edges of other strata.
An inclined water-bearing stratum between rock loosely bedded and
inclined towards a cutting, unless drained and supported, will probably
cause a slip consequent upon the action of water or frost; on the other
hand, veins may alone hold the masses together, and, therefore, when
they are affected the cementing medium is destroyed; however, inclined
water seams are a frequent cause of slips, for where any water-bearing
earth meets a closer and consequently less pervious stratum, damp
surfaces are produced, and an unstable condition; consequently, mixtures
of rock, clay, and sand, are usually troublesome. Also in a cutting in
sidelong ground if a stratified rock dips parallel, or nearly so, to the
slope of a hill, slips are probable, as it may slide towards the
cutting. Similarly, in a cutting in drift or alluvial soil, or any that
will quickly weather, resting upon rock, especially should it have a
smooth bed inclining towards the formation, the superimposed earth will
usually be unstable, and even the act of penetrating the top stratum, or
the erection of a retaining wall or the weight of a small embankment
upon it, may cause it to move; and when motion has commenced it is
difficult to arrest it; and should water trickle upon the surface of the
rock, it may cause the upper stratum to slide; also when water flows or
remains upon rock having a superimposed bed of shale or clay, the top
stratum may not remain at rest even though the surface of the rock may
be nearly level and practically waterproof, and where rock beds overlie
shale which is liable to become softened by time and water and to
perish, particularly when the beds are twisted or contorted; as, for
instance, limestone or sandstone upon shale or marly-shale, the latter
becoming softened by the action of the atmosphere, water, or frost will
form a sliding medium upon which the rock may move, or should shale
overlie rock, as it frequently does, it may slide upon the hard surface.
All alternate beds of shale or any softer earth than the rock,
particularly broken shales when found mixed with sand and clay and the
lias shales, and rock should be regarded as treacherous and liable to
slip. Also some of the slate rocks, as they frequently have veins of
limestone, &c., and as the latter decomposes it mixes with the clay and
becomes of a marly character. Dark blue shale or indurated slaty clay is
sometimes difficult to excavate, but when exposed to atmospheric and
aqueous action it breaks into pieces and becomes little better than a
treacherous clay. The cohesion of shale becomes less as it approaches a
greasy clayey condition, and, therefore, one readily affected by water
or air, and it may then not stand at a steeper slope than 3 to 1. Rock
and shale, which may stand at a steep inclination provided the beds are
horizontal, it has been found, do not permanently repose when they dip
towards a cutting until as flat a slope as 2 to 1 is given; and where
clay and shale beds in cuttings are present, a slope of 1½ to 1 has been
insufficient, and they have not been stable till an inclination of 2 to
1 has been adopted.
As cuttings in rock are frequently in the side of a hill, the dip of the
strata should be ascertained, and in the case of an unstratified rock,
it should be known whether it is fissured or lies upon a solid and firm
bed considerably below the level of a cutting, so that it may be
prevented from movement. It should also be ascertained if the top
stratum is a mere crust, such as a capping of conglomerate resting upon
clay-rock, through which water may burst and cause it to separate, and
sometimes the rock may be more solid in the valleys than upon the
hill-side because of greater diluvial action, and induration caused by
exposure.
The crystalline rocks are the least easily destroyed and are generally
rough and jagged. The science of geology shows that limestones,
sandstones and clays, were originally heaps of mud deposited, removed or
arranged by water; and that boulders are transplanted masses from the
parent rock, and are worn and rounded by mechanical attrition.
Consideration of the manner in which rocks have been formed affords a
fair indication of their stability in earthwork; for instance, many clay
rocks are reduced to a pasty condition by the action of water and air,
but with different results according to their nature, some requiring
blasting to excavate them. On the other hand, there are sandstones which
although soft in the quarry become hardened when exposed to the
atmosphere. A dock cut in red sandstone when exposed to the atmosphere
may slip and fail, but if the rock be protected from the weather or
constantly covered with water it may be reliable.
As cuttings are near the surface and seldom at greater depths than 100
feet, it is hardly possible to know the angle at which a rock dips or
whether there are faults and fissures in it, unless an examination is
made upon the site, and this notwithstanding the geological character
may be thoroughly understood. Local conditions may cause peculiarities
which no law can determine, and although at considerable depths, deeper
than railway or any works with the exception of well-sinking and mining
are likely to reach, the nature of the earth is accurately ascertained;
the surface soil may be in almost every conceivable variety, and also so
dislocated, denuded and rearranged, that usually horizontal strata may
be nearly vertical. Its character may be accurately known, but the lie
of the surface beds or dip of the upper strata, or the order of
supraposition cannot be invariably absolutely established; for instance,
when crevasses, fissures and veins are frequent in earth upheaved and
disintegrated by volcanic action, earthquakes and other disturbing
causes, as in parts of South America, Japan, and other eastern
countries, Italy, the Tyrol, Spain, &c., &c., slips of earthwork are to
be expected, and the soil is likely to be much inclined, full of faults
and probably water-bearing seams.
Under the comprehensive name of rock is usually included any earth from
the hardest mass to be found to that which will crumble in the hand, as
soft sandstone rock. The chief absorbent rocks with which engineers have
to do are the limestones, sandstones, chalk, and clay rocks. Rock may
also be simple or present the appearance of being homogeneous, or it may
be a mass of different substances, be flat bedded, have open or close
joints, and be what is called—
Solid rock.
Hard rock.
Dense, or compact, rock.
Loose rock.
Rock in loose layers.
Loose rock with cavities, caverns, and pot-holes of various earths.
Fissured rock.
Friable rock.
Indurated earth liable to be disintegrated by atmospheric influences.
Decomposed rock.
Rotten decomposed rock.
Or any mass of earth cemented together by a substance, weather-resisting
or not, requiring blasting or that can be excavated by means of bars and
picks.
The slope of repose required may range from overhanging or vertical to
that of the earth of which it consists when disintegrated and dissolved;
therefore the angle of repose varies considerably, but the following
cardinal principles may be followed without fear under ordinary
circumstances and conditions.
Granite. Quartz, if not fissured, and when little mica is present in it,
and most of the igneous rocks,
Overhanging, vertical, TO ⅛ to 1.
Also porphyry, gneiss, trap, but their stability varies considerably.
Compact hard sandstone and limestone and other solid sedimentary rocks
producing stone sufficiently hard and weather-resisting that it can be
used in construction,
Perpendicular, TO ¼ to 1.
But, if non-weather-resisting,
½ to 1 TO 1 to 1,
The slope becoming flatter as the rock becomes softer and more easily
disintegrated.
Friable rock, consisting of hard particles,
½ to 1 TO ¾ to 1.
Loose rock,
¼ to 1 TO 1 to 1.
Soft shaly limestone and the argillaceous rocks may not be permanently
stable until the slope is,
1½ to 1 TO 2 to 1.
Schistose rock is troublesome in earthwork, being fissile in structure
and deleteriously affected by rain and the atmosphere. On the Panama
canal works in the Culebra cutting, maximum depth 333 feet 6 inches,
numerous slips occurred, displaced the roads, and overturned the
excavators.
Mica-schist is variable and frequently has numerous water-bearing
fissures. Its hardness depends upon the quantity of quartz it contains;
when the proportion of mica is greater than the quartz it is soft and
very fissured and contains veins, sometimes of clay, often yielding a
considerable flow of water. Should the percentage of quartz be large it
becomes hard, holds little water, and is of a gneissose character.
As water is the chief disintegrating agent and cause of instability, it
may be well to mention that Professor Prestwich has stated that “hard
quartzites, slates and grits (Silurian), purple and grey shales, schists
and fissile sandstones with hard compact limestones and dolomites
(Devonian), rarely contain any _levels_ of water, and that it is only
encountered in fissures”; hence the importance of knowing the position
of the fissures, and taking the necessary precautions to promote
stability. Where rocks, especially if generally known as water-bearing,
crop out at a high angle, and are in well-defined beds, water may be
expected, as although it may not percolate vertically, it will along the
inclined beds. Sandstone and limestone bands in rock usually cause small
springs.
The stability of a sandstone for earthwork and purposes of construction
is dependent upon the material which cements or holds it together,
whether iron rust, lime, free silica, alumina, &c., &c., &c., its
quantity and condition, and the degree of hardness imparted when it was
formed, and the nature of the agglutinant; therefore the varieties and
degrees of fineness and hardness are numerous. Sandstones are generally
found to be laminated when hard; and bare of vegetation if pure and free
from marls; they contain and part with water in different proportions,
and sometimes have watertight bands crossing them, severing water
communication, which may cause earthworks to be of unequal stability.
The firmest and strongest are close-grained and fine in texture, the
weaker are coarse and gritty, and have a sandy appearance. They may be
white, yellow, green, black, red, grey, brown, or other colour, and
although of the same hue, their character may not be identical; for
instance, the red sandstone is hard and also very soft. Sandstone of a
greenish hue is generally hard, much fissured and full of water. When
firm, greensand may stand at a steep slope, the surface being protected;
but it varies considerably, and may at one place be close and yet be
gradually deteriorated until it is of the character of fine loose sand.
Should it be found upon excavating sandstone that it is not upon its
natural bed, but distorted, upheaved, or vertical, it will probably
split and become detached under the destructive action of air and water.
In tropical climates it has been found that sandstones generally
dissolve and become disintegrated when used in damp foundations.
Limestone also varies much in character, and is treacherous, whether it
is hard or soft, when pockets of clay or sand are present and beds of
clay are contiguous. Percolated water having carbonic acid in it may
also soften or dissolve it. The softer kinds if in fragments as ballast,
or when deposited in an embankment, often become quickly disintegrated
by frost and the weather, as do sandstones.
Should any rock strata be vertically inclined instead of horizontal,
although it may be known in the latter case they are generally
watertight, fissures in the upheaved beds may become channels for the
passage of the subsiding or rising waters, and may cause saturation of
the soil over a considerable area and induce a flow through the slopes
or the seat of an embankment. Upon such a site no reservoir, dock,
canal, or any earthen structure to hold water should be placed; but
although the nature of the ground may be fatal to the stability of such
an embankment, so long as the underground waters do not rise to the
level of the seat of a railway embankment or flood a cutting or burst
the slopes, they may not seriously affect the stability; for, unless the
head level of supply is great, more danger maybe expected from downward
percolation saturating the ground upon which the embankment is placed
than from an upward flow. The insecurity of erecting a reservoir or
similar work upon such a site, in which water is brought into a district
in greater quantity than its natural flow, is obvious, as the earth may
gradually become saturated from the constant leakage down the upheaved
fissures, until it becomes in an unstable condition and finally slips
and subsides.
CHALK.
As in most public works, with the except:on of tunnels, wells and mines,
the chalk with which an engineer has to deal is surface chalk, or the
top layers of that deposit known as the upper chalk, almost invariably
containing much more water than the lower chalk, although it rises
quicker in the lower beds, as it is under greater pressure, and which
vary in hardness, purity, and solidity and may have frequent fissures
and holes, with or without flints, and be anything from hard, compact
chalk rock to mere marly calcareous earth; considerable judgment is
required to successfully determine the slope of stability and the
precautionary works that may be necessary to attain repose; for some of
the upper beds soon weather, and being soft, friable, and fissured are
permeable and liable to slip; in fact, the Oolitic series, as it
consists of alternating bands of limestones and clays and occasionally
sandstone, is frequently fissured and has loose joints and therefore
requires to be carefully treated.
The range of the slope of permanent stability obviously depends upon the
nature of the chalk, whether it is denuded or covered or mere
loose-jointed strata, the effect ground and surface waters may have upon
it, and also the position of the beds, and whether a cutting or
embankment is on the side or the base of a hill, and consequently at the
place where it is likely to be in a wet condition.
The Needle-rocks in the Isle of Wight and Beachy Head may be mentioned
as familiar examples, showing that firm and comparatively pure chalk
will stand practically perpendicular, even when much exposed, if pure
and free from faults and homogeneous in texture; and in blocks with beds
inclined away from a cutting it will permanently stand
Almost vertically TO ½ to 1, at a great height,
a slow regular crumbling of the surface or falling down of small
fragments, which seldom produce serious movement in such material, being
the only deleterious effect of weather influences.
As the chalk becomes broken and less evenly bedded,
about 1 to 1.
Loose, friable chalk in surface beds will often not permanently stand at
a less slope than from
1 to 1 TO 1½ to 1,
according to the depth and degree of exposure. The most usual slopes
being
½ to 1 TO 1 to 1.
Impure wet chalk and marly chalk will require a slope not less than
1½ to 1.
Much depends upon its freedom from faults, crevices, and pot-holes, as
they hold water, and the surrounding soil may fall away, for water
quickly passes in quantity through the fissures and crevices which are
generally numerous in the upper chalk, especially at the bottom of a bed
of flints which in consequence of their impermeability lessen the upward
flow; but flint beds in soft chalk are an advantage, as they act as
drains.
The affinity chalk has for water, which has been considered a reason for
the absence of important rivers in that formation, as water does not
flow away freely upon it, causes it to be readily affected by rain and
disintegrated from the effects of the expansive and contracting action
of frost and thaw; hence draining and covering the surface may be
important, but care must be taken not to interfere with natural springs.
This property of chalk, viz., its affinity for water, although a
disturbing cause in earthwork in that formation is of value for covering
or filling in open trenches, counterforts, or drains in _other_ soils,
as the chalk attracts water, and therefore dries the surface of other
earths.
Anything that localizes the percolation or flow of water, or helps to
make water seams, veins, fissures, and hollows, which are sometimes
filled with sandy gravel, loam, and detritus readily admitting water,
will tend to break the chalk into separate masses and cause it to become
loose and unstable by the action of rain, frost and thaw, and vibration.
Should flint beds occur in chalk, and they frequently do in the upper
beds if they are horizontal or nearly so, much more water may be
expected to flow along their bed, as it forms a water-passage, than when
they are in inclined or vertical seams. As chalk absorbs much water, but
does not readily exude it, although it may soon become dry upon the
surface after rain, it is advisable to lessen percolation in order to
prevent slips. It is known that the angle of friction of water in chalk
will affect the flow and that the discharge varies greatly according to
the character, fracture, and other conditions of the soil; for instance,
it has been proved that a hydrostatic pressure due to a gradient of
about 1 in 132 is required to enable water to pass through the chalk as
found at Dover, whereas in the Hertfordshire beds much less is required,
namely, that equal to a gradient of about 1 in 350 to 1 in 420. This is
named as showing, even when unfissured, the varying perviousness and
character of chalk, and that it cannot be treated as a material of even
approximate consistency of texture. The power of capillary attraction of
chalk has been proved to be great and the evaporation from the surface
practically unlimited. These properties and its known affinity for water
render it liable to constant change; also the particles of calcareous
soils being affected by moisture and to a certain extent soluble, water
will take up lime in them, and therefore they are treacherous earths and
liable to slip and subside.
Professor Ansted has shown that a cubic foot of the upper chalk when
dried will absorb 2½ gallons or 40 per cent. of its bulk of water; the
lower chalk 2 gallons, or 33 per cent. of its bulk; and that the pores
of a cubic foot of chalk are equal to 40 per cent. of the bulk, and are
therefore equivalent to the area of a pipe about 9 inches in diameter.
Ordinary drainage will not remove the water, hence chalk is a difficult
soil to treat successfully, and slips and subsidences may result in such
large areas as the surface of cuttings and embankments simply through
the difficulty of preventing it becoming saturated.
Another characteristic of chalk, which requires careful observation to
prevent slips, is that water does not generally issue through a mass, or
equally over a consider able area, but is discharged through fissures,
and crevices, and flint beds; hence one of the chief means of preventing
slips cannot be adopted, namely, to disallow a localization of the flow
of any water; therefore, the disturbing element of water seams is in
greater or less degree present in all chalk-earth that is not solid and
homogeneous in texture. The flow from such water-veins or seams should
not be interfered with, as any obstruction, and possibly diversion,
which is likely to fail, will only result in the spring saturating the
adjacent soil, and in its bursting out at another place. There is no
safe remedy but to gently lead away the water, for where springs occur,
either in chalk or rock, they will find the line of least resistance;
consequently the waters of percolation will tend to flow to one place,
and cause a spring.
Chalk is found in regularly stratified and separated masses, sometimes
caused by beds of flints, and although the position of the layers may
indicate their successive ages, age can hardly be taken as an absolute
indication of the increased stability of chalk in earthwork. When
overlaid with clay it is usually harder than when bare, probably owing
to pressure, non-exposure to atmospheric influences, and to the absorbed
water being of a different character, which has been proved by analyses.
It is especially advisable in chalk soils to know the head level of
water in the district, and to note if the bottom of a cutting is below
the usual water-bearing line in the open wells, which may not
necessarily be at the same depth; their average level being ascertained,
an idea can be formed of the probability of springs bursting out, and
according as the rainfall is excessive or not, so usually will be the
flow.
If chalk beds incline across a valley, and have an impervious stratum of
clay upon them, it has been found that the most water issues at or about
the point where the impervious seam first overlies the chalk, _i.e._, at
the edge of the basin, and the greater its depth, the less the flow;
therefore, should a cutting be located at a place where this stratum is
thinnest, more water from springs may be expected than at any point
where the impervious layer is thicker.
It is also well to remember that the line of water-flow is not
necessarily a horizontal plane, for it frequently follows the contour of
the chalk, and that the causes of surface irregularities of subterranean
water are unknown; but rain-water accumulating in chalk principally
rises and issues most rapidly along the bed lines; consequently the flow
along these must be gently discharged, or slips will occur; but chalk
uniform in character and of solid and close texture, without flints and
fissures, usually is not water-bearing, and will stand almost
vertically. As a rule the cohesion of the upper beds, if they are
homogeneous, is greater than the lower beds, although the mass may be
softer.
Should the drainage or natural outlet of the land waters of a chalk
district be obstructed or dammed back, from the quantity of water being
in excess of that the fissures or water seams in a chalk hill can
discharge, and the pent up waters be unable to escape, hydrostatic
pressure, in addition to a weakened condition of the chalk through
excess of moisture, will be caused, and extensive slips may be expected
along the escarpment, the displacement being gradual, the ground
separating and fissuring until at length it is pushed out by hydrostatic
pressure. Such a slip usually occurs in large masses, resembling a
fallen cliff, for the disturbing agent is all-powerful, and the area
affected very considerable, and particularly so if the chalk is
superimposed upon different soil, or harder ground, as then the whole
mass will probably move forward.
Although some approximate inclinations have been previously given, the
varieties of chalk are so numerous that no absolute slopes of repose can
be named, for chalk or calcareous earth may be:—
Marble or crystalline limestone.
Ordinary limestone rock.
Hard, compact chalk rock.
Lower white chalk.
Upper white chalk.
Hard grey chalk.
Ordinary grey chalk.
Pure white chalk.
Friable white chalk.
Yellow, light and dark blue, soft chalk becoming of marly character.
Hard chalk marl.
Grey marly chalk.
Grey clayey chalk.
NOTE.—The preceding chalk marls contain so large a proportion of
argillaceous matter as to become almost clays so far as regards
treatment in earthworks. Many serious slips have occurred in chalk
soils, and their history indicates that the chief disturbing element was
water, whether held back over a large surface until the hydrostatic
pressure became too great for the slopes to withstand it, or from its
bursting out in springs, and so separating and disintegrating masses of
the earth.
Near the entrances to tunnels slips appear to be most frequent in
cuttings in chalk. This would seem to lead to the belief that in places
where it is known the chalk soil is likely to be troublesome from
land-water and springs, it would be advantageous to prolong tunnels
beyond the economic depth of a cutting, and even to continue them until
such a depth as 40 feet is reached, to so arrange the gradients that
they drain the interior, and to provide a complete system of pipes and
drains, even if side galleries have to be driven to tap the water,
before it reaches the lining, to relieve the sides, crown, and invert of
a tunnel so that no water can pour down the roof or walls unless under
control.
In a tunnel so situated and liable to water-pressure, the thrust of the
soil will be very variable, and cannot be foreseen. At one place during
construction, the walls and lining may be finished without movement of
the earth, at another, the pressure maybe great and act unequally,
either upon the side walls or the arch. As a rule, at the entrances to
tunnels the pressure is greater upon the arch than the sides, for then
the whole of the wedge-shaped mass within the boundary of the angle of
repose of the soil is disturbed, and its cohesion impaired or destroyed,
and therefore it presses upon the arch, this pressure tending to
counteract the lateral pressure; but as the depth increases, the load
from this wedged-shaped mass becomes less upon the arch, although
generally greater than any lateral pressure, because the earth above is
not disturbed or impaired, consequent upon the depth of the hill being
greater and the cohesion and side-pressure of the earth tending to
support it, but the lateral pressure is increased because the normal
pressure of the soil due to the depth is augmented. It is this
disturbance of such friable soil as chalk at tunnel entrances, causing
the particles to be loose and separated, and in a state especially
disposed to percolation of water, that probably causes the earth to be
in a condition favourable to slips, and for them particularly to occur
at or about the entrances to tunnels. When, therefore, the depth of open
cutting at the entrances is reduced, any slip cannot be of the same
magnitude as it would be if it happened at a greater depth. The circular
or one closely approaching it would appear to be the best form for the
lining, where variable or great pressure, vertical or lateral, is to be
expected; for the pressure in a tunnel will always be unequal, and the
surface of the earth must be supported.
In some experiments to join substances by pressure it was found that
though great pressure forms chalk into hard blocks, the particles are
not firmly united, and that they separate along the surfaces of contact
of the original particles and not through them; these tests tend to show
that masses of chalk are usually in a state not indisposed to
separation. The same result occurred in similarly testing pulverised
sandstone.
SAND AND GRAVEL.
In fine sand-cuttings springs may be expected, and the earth become in a
semi-fluid state if there is water at a higher level to filter through
it; also in the case of all porous and open soils. Any drawing away of
the sand must be prevented, as it will induce a slip, and cause the
earth to become running sand, especially dangerous near buildings, for
its egress must be prevented, or subsidence will ensue, and serious
erosion. The excavation in such cases should be in as short lengths as
practicable, so that the surfaces are not unsupported, and walls and
structures should be quickly erected. The sands that are met with in
estuaries are frequently in such a condition, that a slight obstruction
to the tidal flow will cause movement, the equilibrium being easily
destroyed. Should there be a break in the continuity of a clay stratum,
overlying light loose soil, the latter will probably boil up, and in
determining the depth of a cutting, care should be taken that this
impervious stratum is not broken or injured.
Marl, clay and sand beds are likely to slip when they are superimposed,
and there are some districts in which sandy soil is so charged with
water that, unless the drainage of the slopes and formation suffices to
drain for some distance the land outside a cutting, the sand will become
overcharged with moisture and will act as a fluid and slip, the lateral
support being removed by the act of excavation and its normal condition
altered. Being so delicately balanced the least additional disturbing
force, such as a spoil bank being tipped upon the surface, or the
inducement or acceleration of a flow of water, will set it in motion and
make it a quicksand. For instance, small sand islands have been removed
by making cuts in them from 15 to 20 feet in width, and by men shaking
bars, &c., inserted in the soil; the sand along the edge becomes loose,
falls, and the current sweeps it away. As an example of the
changeability of the condition of sand may be named that in sinking pits
by congelation in loose sand it has been found that the grains during
the freezing of the water, by means of tubes containing a freezing
mixture, were additionally separated about 5 to 7 per centum.
As the sand met with in public works is seldom in very deep beds, it has
not been subject to the steadying forces which many earths have
undergone, and it may have been constantly moving until its final
deposition, and therefore it is easily set in motion; and although sand
will subside less from a load after it is saturated with moisture, the
water in it trying to escape may cause it to slip upon an unsupported
surface such as a slope.
Many experiments have shown that the power of absorption of sand
decreases with the fineness of the grain, and that sand when thoroughly
wet will contain water equal to about one-third to two-fifths of its
bulk, and that almost all this can be drained; hence its varying
condition and instability. If a well be sunk in sandstone and regularly
pumped it will drain the rock around for some distance, the drainage
space being conical, its vertex the bottom of the well, and its base the
surface, varying in extent according to the nature of the soil and depth
of well, showing the porous nature of sand.
The interstices of silicious sea-sand, when not compressed, have been
ascertained by Mr. J. Watt Sandeman, M. Inst. C.E., to amount to about
40 per cent. of the volume of sand. For coarse or fine sand, or a
mixture of the two, the interstices did not vary much. When it was
compressed by a rammer in water, its bulk could be reduced to the extent
of 12½ per cent. The interstices of broken red sandstone, varying in
size to that which would pass through an 8-inch ring, were found to be
36 per cent. of the whole volume, but as the stones were in contact 10
per cent, must be added, and if under water 15 per cent.
Per cent. of
the
interstices.
Broken Welsh limestone to pass a 3-inch ring 50·9
Gravel, free of sand. Small pebbles to pieces gauged by a 33·6
25-inch ring
Welsh limestone and gravel as above mixed in equal 34·0
proportions
Mason’s shivers of Anglesey limestone, small gravel to 48·0
pieces gauged by a 4-inch ring
Runcorn red sandstone, large, varying in size from pieces 50·0
to pass a 4-inch ring to an 8-inch ring
Ditto, small, varying in size from sand to pieces gauged 34·0
by a 4-inch ring
The two preceding when mixed in equal proportions 36·0
The experiments clearly show the known great capability of subsidence in
sandy and open sandy gravel soils, their clear water space, and how
easily fine sand may, by a current of water, become running sand, and
their adaptability for ramming and consolidation by moisture. Tipped
sand when rammed will subside if saturated with water nearly as much as
it can be beaten down, which shows how greatly its bulk is affected by
water, and although its rapid consolidation is an advantage in
embankments, it is of importance that percolation should be equal.
The chief conditions of a safe foundation upon pure sand, namely, that
it cannot escape laterally or be undermined, are obviously not to be
attained in either cuttings or embankments, as the lateral support is
removed, and the slopes are liable to be undermined and unequally
charged with water, and the influence of water on sandy soils is the
principal cause of their instability, for in excavating cuttings the
face will frequently stand at a very steep slope if dry, but upon its
becoming saturated the sand may flow, and in the case of gravel and
sand, although the stones forming gravel do not change, the whole
subsides.
As gravel is found in various conditions, it may be well to classify it
as it is herein regarded.
Clean gravel is considered as that which nearly approaches the condition
of a pebbly beach. If an appreciable quantity of sand is present, it is
sandy gravel. If loam, or marl, or clay, it is loamy, marly, or clayey
gravel.
Gravel hills are large accumulations of water-worn rocks, and may have
boulders in them intermixed with the freshwater deposits of sands and
marls, and by means of natural cementing material between the particles
seem to be firmly set and to be so conglomerated as to appear to be in a
similar condition to weak concrete; but there is always a chance of the
matrix becoming dissolved, therefore it is advisable to test a mass by
the application of water and to expose it to the atmosphere before
relying upon its permanent stability, and with the view to determine
whether it is hard cemented gravel or not.
Gravel may be made more compact and will subside if water is pumped upon
it and allowed to filter through, and in making an artificial foundation
of gravel, it is not reliable without water percolation and
consolidation by ramming.
All earth consisting of particles having rounded surfaces is liable to
become loose, and upon weight being placed upon it the grains are
inclined to roll and become detached, but if they are angular fragments,
which seldom is the case, this tendency will be lessened, and the angle
of repose will be steeper.
With regard to the slopes necessary in sand and gravel, the more
angular, rough, hard, and clean the particles, the steeper the
inclination.
Earth that can be properly called gravel seldom requires a flatter slope
than 1½ to 1, and usually a less inclination is sufficient, but if loose
it will not stand vertically even for a depth of a few feet.
Solid indurated masses of gravel will stand perpendicularly and as rock.
If the gravel consists of quartz or sandstone boulders, or is very
coarse with stones of considerable size, or like a clean pebbly beach, 1
to 1 TO 1¼ to 1.
Ordinary clean gravel of uniform size at about 1 to 1.
Thoroughly compressed, hard, clean sand, about 1 to 1.
Looser sand and light gravel, 1¼ to 1 TO 1½ to 1.
Irregular beds of sand, gravel, clay, and fragments of rock, 1¼ to 1 TO
1½ to 1.
Sand mixed with vegetable matter, argillaceous or loamy sand, about 1½
to 1.
As the proportion of mould or clay in the sand becomes greater a flatter
slope is necessary according to the nature of the earth with which it is
incorporated, the degree of wetness, and also the exposure of the
surface.
Clay loams require slopes from 1½ to 1 TO 3 to 1, and shifting sand when
a current of water reaches it will become a quicksand, and not be stable
even when horizontal, but if drained and the toe is secured it will
usually stand at an inclination of from 3 to 1 TO 4 to 1. On the other
hand, an embankment of hard, clean, angular sand, rammed but left bare,
when exposed to tidal action with little wave disturbance, has reposed
at 2 to 1 TO 2½ to 1 slopes.
Loamy soil and vegetable mould will, for any height not exceeding about
5 feet, stand nearly vertically for a reasonable time.
CLAY.
With respect to cuttings and embankments in clay soils, perhaps no earth
is more affected by water and air, or more difficult to treat, as it
will expand if only exposed to the atmosphere and without contact with
water, 6 inches being no unusual dimension to allow for expansion in
tunnel-work. This property and its contraction upon drying alone make it
an earth particularly liable to slip and induce fissures and cracks
through which water can trickle, notwithstanding the surface of the clay
may be almost impermeable. If clay could always be kept dry or in its
natural condition it would be stable and free from slips; but this
cannot be effected, for water is held in suspension in clay for a
considerable period, its plastic nature preventing gravitation, and
evaporation is known to be a very slow process; and as the same clay
under different circumstances may stand nearly vertically or only at a
very flat slope, its liability to constant change makes it very
treacherous, and it should be classed as a most deceptive earth of a
dangerously unstable and unsafe description, for it may be so hard as to
nearly turn a pick, and yet water and air will rapidly cause its
disintegration, but if weather influences can be prevented from reaching
it when in such a hard compact state it will afford a firm foundation.
London clay in its natural condition usually contains about 10 per cent.
of water. The more permeable the clay the more likely are slips to occur
and the face to become soft, loose and disintegrated, slimy beds being
thus produced which are difficult to prevent or remove; therefore a
covering of close grass turf, or layer of burnt ballast, ashes, or
chalk, upon any damp place or fissure after it has been filled is
advantageous.
Solid blue clay, which generally requires the use of the pick, of the
clays is, perhaps, the most stable, being almost impermeable if free
from delaceration; but yellow and most other clays are unequal in
texture, faults and breaks are frequently numerous, and water
penetrating converts the surfaces and the mass into a muddy and
semi-fluid condition resting only when horizontal, which has been
painfully experienced in the crushing in, during construction, of some
tunnels in the London district. The trickling of water down fissures
forms a slimy and easy-sliding surface most difficult to treat or
prevent, and so long as the natural contour of the ground does not offer
resistance to movement, a slip may extend for a long distance, either in
deep or shallow cuttings, and there may be considerable hydrostatic
pressure.
The disruptions, variableness of character, and existence of fissures
cause any but the most homogeneous clays to be treacherous and likely to
slip, particularly the yellow and any laminated clays, as they allow
water to enter by the veins which are usually frequent in the mass.
Yellow clay has a greater tendency to crack upon drying than blue clay
and does so much more quickly, hence its dangerous nature, and although
a mass may be only damp, fissures will enable water to penetrate and
reduce it to a state of instability; it is also not infrequently in a
plastic state, having fissures and cavities full of water.
It should, however, not be forgotten that in the endeavour to prevent
the deleterious effects of aqueous action upon clay soils that they may
be over-drained, so that they become too dry, as then the clay will
shrink, crack, and fissure; the chief aim should be to keep it always in
a sufficiently moist state so as to obviate the formation of cracks and
fissures, and at the same time cause it to be dry enough to be firm, and
never in a pasty or pulpy condition; in other words, to maintain its
natural state if one of stability, and prevent any excess of water
penetrating it or reaching its surface.
The lias clays are treacherous chiefly owing to the presence of much
calcareous matter, and therefore approach a marly state; heavy slips
have occurred in the has formation in the midland counties of England,
notwithstanding that a slope was adopted which experience had shown
produced stability, namely 3 to 1. A slight variation in the composition
of this soil or an unfavourable position will cause a slip in such
treacherous earth.
Pure clay shrinks some 5 per cent. in drying, the contraction being less
as sand is present in it, for when it is mixed with twice its weight of
sand it is reduced to 3 per cent., and as impurities increase in clays
the less impervious they become. Most clays have silicious earth in
them, but if sand is present the clay is then more open, and water will
permeate and drain more freely; but mixtures of clay and sand may assume
a pulpy condition when impregnated with water, consequently it is always
advisable to test such earth. The varieties of sandy clay are many, and
all are usually more or less unstable. Among them may be named red clay
with sand and mica, blue sandy clay, sandy green clay, stiff red sandy
clay, the loamy clays of various hues, dark grey, red sandy, and black
clayey loams.
If clay could be kept in a moist state fissures would seldom occur. The
constant alternation of wetness and dryness creates the fissures, and
water completes the disintegration. There are a few clays which are
stable when kept in solid masses, as then only a small surface is
affected by air and water, but if they are loosened and broken up, as in
the process of excavation and deposition, they readily become in a muddy
condition. Any mud or silt which may be soft and readily pressed when
wet, but cakes and shrinks in to detached lumps when dry or upon being
exposed to the atmosphere, is a treacherous soil, as it will return to
its original state upon becoming wet. Clays or any soils that cake
should always be regarded with suspicion, as although having the
appearance of solidity and the possession of weather-resisting qualities
in their natural position, when disturbed, quickly become worthless for
earthwork purposes, and may stand in one situation almost as a soft
clay, and when disturbed and wet assume a horizontal surface. Such
ground may repose at a 4 to 1 TO 8 to 1 slope, because its crust has
become caked or case-hardened, yet when it is broken it may become, upon
being exposed, simply fluid mud. To prevent clay soil weathering upon
the surface, a layer of gravel 1 foot or so in thickness has been placed
upon it, the idea being that it is not only a protection, but the weight
of the covering upon the clay will cause the water to be pressed out
from the soil into the gravel through which it can percolate to the
drains.
A crude test to indicate the probable character of a clay as regards its
stability in earthwork is to burn a piece of it and notice the colour.
If it becomes white or of a whitish tint, the clay is generally less
likely to slip than when it is of a reddish or yellowish tinge. Another
rough experiment can also be made. Get a piece of clay, place it in
water, and note the time taken and the depth to which the surface has
become saturated, and whether it is very slimy and will easily slide
down a slightly inclined plane; its tenacity may then be approximately
judged. Also by weighing, an idea of the amount of sand may be imagined;
the more sand there is in clay the lighter it will be, all other
conditions being identical. A comparison between two clays will enable
some opinion to be formed of their relative stability in earthwork,
though, of course, there are many other features to be considered. All
impure clays, such as shaly clay, sandy clay, loamy clay, and marly clay
require to be carefully treated, although they may be easier to manage
than yellow or brown clay.
When two retentive clay beds overlap or overlie, and have no
intermediate permeable stratum, they must be in a humid state, as is the
case in the Fen country, unless they are constantly drained; but serious
slips are not so likely to occur in them as when two masses of clay have
an interposing seam of sand or silt liable to be eroded by water falling
down fissures in the clay, which probably extend to considerable depths,
with the result that two slimy surfaces are formed and the clay slides.
Clay underlying gravel often contains numerous pockets and seams filled
with running sand, and should there be a permanent head of water the
discharge will be in large quantities and at a considerable velocity. A
cutting in wet sandy clay is generally treacherous and difficult to
manage.
As a clay bed near the surface of the ground is sometimes upheaved, if a
permeable stratum such as gravel or sand overlies it, the drainage of
water through or down the slopes will be arrested, and the earth at the
back of the slope will be constantly wet and may ultimately become
saturated through the damming back of the water; then a slip may be
expected.
All upheaved, dislocated, and twisted superficial beds of clay, which
will generally be of varying consistency and therefore settle unequally;
over or underlying seams of sand or gravel, are likely to slip and
subside, and their stability much depends upon whether or not the lie of
the beds obstructs the permeation of water. If the dip of the clay-beds
is towards the natural outfall, most probably an adjacent river, slips
are probable because of the creation of sliding surfaces and the
continuity of the beds being destroyed by a cutting and the consequent
loss of support.
Should permeable soil lie between the top stratum and a bed of clay,
water will accumulate upon the clay, make it slimy and cause a flow upon
the bed; for example, when a thin bed of vegetable earth rests upon
gravel, sand, or peat, and that upon clay, water will percolate, and
perhaps air, through the top soils, and may cause them to slip upon the
clay-bed. Also should a layer of clay overlie permeable strata, as clay
upon sand, or clay upon gravel, unless it is sufficiently thick and
solid to prevent infiltration, it may slide upon the permeable soil as
its lower surface becomes wet. When clay-hills have veins, water may
accumulate in them and flow, and if very dark yellow clay overlies light
yellow calcareous clay, which may rest upon hard blue clay, it is
obvious each stratum is somewhat differently affected by weather and
air, and therefore movement is to be expected. The edges of clay-hills
are always likely to slip, especially should they be in the form of
spurs.
Boulder-clay is seldom reliable, because, although it may be hard and
stand vertically in dry weather, in wet it swells, weathers quickly,
becomes soft and cakes upon drying. The stability of such soil is
governed not only by the nature of the clay, but by that of the boulders
and their effect upon the earth in which they are embedded, and much
depends upon the degree of changeableness upon exposure to air and
moisture of all the particles of which they are composed; hence
boulder-clay, although hard to excavate, may quickly dissolve. On the
contrary, it may occasionally be so hard that it seems to be solid rock,
and may even resist erosion and weathering as well as if it were rock;
but care must be taken to prevent indurated mud being mistaken for solid
clay-rock, and therefore it is advisable to test the soil with water.
Seams of silt, soft pasty soil, or soapy earth met with in clay, which
have become decomposed by atmospheric and aqueous action, are to be
feared, and the brown clay, especially when soft: red, or dark yellow
clays that break into laminæ and crumble upon exposure to the air, and
although tenacious in the flakes and when fresh-cut are loosely held
together in bulk, often have thin veins of sand in them; and when water
percolates it remains, and is very difficult to drain. It has also been
found that when minute non-adhesive particles of mica are present in
clay that it will become disintegrated by water, although it may be hard
to excavate. Some of the gault clays, although stable when dry, become
soapy when wet and are not easily managed, but the bluish grey gault is
usually tenacious and almost impermeable. The gault clays have little
sand in them but much calcareous matter, and, as a rule, they do not
swell and bulge like the London clays.
Some clays, when dry, appear to be stable and firm, though they are
often drift or dried-up mud simply requiring the influence of water and
the atmosphere to cause them to return to their original state; the
brown and boulder-clays are of this nature and are distinctly perishing,
and therefore treacherous clays.
As in clay and all retentive soils, water is always present, and as the
impervious nature of clay prevents water gravitating and being drained,
and as owing to wet seasons the permeable portion of the clay may
gradually become overcharged and be reduced to a muddy condition, slips
may occur years after a clay cutting has been excavated or an embankment
deposited. In clay embankments the greater weight upon the centre may
gradually press water towards the slopes and cause them to slip; hence
the value of covering the formation in order to lessen percolation.
Cracks and fissures may be so produced in clay embankments, and the
danger is that they form sliding surfaces and cavities from the presence
of water, and gradually soften the interior until it fails from the
thrust of the earth and want of uniform support; therefore, clay earths
are more difficult to treat than granular soils, as local weakness is
the cause of failure, a mass may be sound, but a crack or fissure may
soon become large and pass through an embankment to the seat or the
slopes. Although the filtration of still water may not cause an
embankment to slip as long as the water merely restores it to its normal
absorbing power, when that is exceeded the percolating water will be
dammed-up and cause hydrostatic pressure and a soddened surface, destroy
cohesion, and disintegrate the particles.
When clay is contained within the walls of a building, injurious action
and unequal settlement may arise, as the earth inside and outside may be
in a different condition; consequently in soils that expand and contract
there may be external or internal pressure according to the state of
dampness or dryness, consequent upon the ground outside being subject to
weather influences and the contained earth being comparatively free from
such operation.
Shale, whether black or brown, may become decomposed by water and be
softened by time, and is either loose or firm. If loose, it requires
protection, and the surface should not be exposed to the weather. The
presence of iron pyrites in shale has been found to be a cause of its
becoming treacherous when water has access to it, as the mass becomes
decomposed.
Marly soils are of various hues, red, blue, grey, and yellow, and
consist principally of clay and lime, and are usually called clay-marls
when clay preponderates, and marl-clays or chalk-marls when chalk is in
excess. They are dry to the touch and will effervesce with acids, the
presence of lime being thus made evident by the ebullition produced, and
some idea of the relative quantity may be judged by comparing results
with different lumps: they vary in character according as lime or clay
predominates.
Chalk-marl may act as an almost impermeable stratum and arrest the
percolation of water from more permeable soil, but it is liable to slip
because of fissures being present in it, which are common to most
calcareous earths; and when these crevices are bared in cuttings by the
ground being excavated, the flow from the previously-confined or
diverted springs is facilitated, and perhaps the harmless underground
passage of water is prevented, and consequently the earth may become
loosened and unstable owing to the changed condition.
Although marl may be so hard that it cannot be excavated by picks and
bars, but requires blasting, some varieties crumble and become soft
under weather influences, and the slopes need protection, or a constant
trickling and wearing away of the surface will result. Grey marl
generally weathers quickly, and if there is much clay in it, it often
falls to pieces upon exposure to the air, and becomes broken and so
split into disjointed pieces as to admit water, and as it sometimes
contains a considerable quantity of sand it is soon reduced to an
unstable state, for marl-clay and sand beds are always likely to slip.
The variegated marls are treacherous, but red marl is usually stable,
still it must be lined in tunnels; and so are those that are impermeable
or closely approach that condition, but any marl in which the lime may
separate from the mass is unstable. All clay or marl soils that soon
work into “slurry” may be classed as treacherous, or any earth that
quickly becomes in a liquid state; for instance, some of the hard “pan”
soil met with in Canada melts away by the action of air, rain, snow and
frost, and becomes unmanageable and like blue paint, and will quickly
fill drains and run down the slopes and cover the formation of a
cutting. Also the top black cotton soil found in India expands by
aqueous action, and contracts in drying as clay, and is of doubtful
stability.
As might be expected, the more argillaceous or clayey marls when exposed
to weather influences expand, contract, and act almost as clays,
becoming decomposed, disjointed and separated, and when superimposed
upon rock, especially if it be inclined, are very likely to slip, and
are treacherous and unstable soils, as they soon become in a muddy state
and have slimy surfaces, which, when resting upon any dipping bed, cause
them to be disposed to move upon the smallest disturbing element being
present or becoming increased, such as water or vibration. When a
comparatively permeable stratum overlies even hard marl, water will
penetrate to the latter earth and the superimposed layer will rest upon
an unstable bed, and therefore be likely to slip. Blue, or any marl when
found mixed with layers of small gravel and sandy clay is very
treacherous, as air and water cause it to swell and crumble, and it
becomes in a soft state requiring a very flat slope. All clay marls that
swell when wet, and crack and fissure when dry, are unstable, as water
percolates to them, and should sand veins occur in marl, water may
trickle to a considerable depth and cause the ground to be in a loose
and doubtful condition below the formation or bottom of a cutting. Such
soil when tipped into an embankment is worse to treat than in a cutting,
as it will absorb and retain water so that it is practically impossible
to drain or extract it.
Professor Ansted has classed clay soils as under:—
When combined with 30 or 40 per cent. of sand they are CLAY LOAMS.
When combined with 40 to 70 per cent. of sand they are TRUE LOAMS and
LOAMY SOILS.
But not until they have 90 per cent. of sand in them are they SANDY
SOILS.
When combined with from 5 to 20 per cent. of lime, the soils become
MARLY.
But not until they have more than 20 per cent. of lime in them are they
CALCAREOUS SOILS.
With respect to the slope of repose in clay earths, it may be anything
from 1 to 1 TO 12 to 1, and when the clay is mere dried mud and becomes
saturated it may not be at rest until it is horizontal.
In cuttings, strong compact clay, if not seriously impaired by air and
moisture, will stand at from,
1¼ to 1 TO 1 to 1.
Ordinary clay, such as the blue clay, with protected slopes and proper
drainage, and firm boulder clay,
about 1½ to 1
Plastic clay from,
2 to 1 TO 3 to 1.
Lias clays of doubtful character,
3 to 1 TO 4 to 1.
Superficial beds of London clay,
3 to 1,
and not less for any weak clay.
In embankments, much depends upon the height, mode of tipping, state of
the soil when deposited, the protection afforded and uniform character
of the mass, the larger it is the more difficult it will be to drain.
Although firm clay may for a short time stand at nearly a vertical slope
for any height of face not exceeding about 10 to 12 feet, few clays will
permanently stand at a less slope than,
1½ to 1, for moderate heights,
although they may for a time do so at 1 to 1 in unsubmerged work, but if
the slope is covered with water, as in a canal, and subject to wash or
wave action, however shallow the depth, it is seldom they repose at a
less inclination than 1½ to 1.
Should an embankment be more than about 30 feet in height
2 to 1, average.
Firm clay from,
1½ to 1 TO 2½ to 1, according to the depth.
Plastic-clay, consequent upon the percolation and pressure of water in
cuttings, may stand at a steeper slope in a surface protected
embankment, and if carefully tipped, it will repose at from
1½ to 1 TO 2 to 1,
but from hydrostatic pressure the same clay may require from 2 to 1 TO 3
to 1 slopes in cuttings, and even an inclination of 5 to 1 has been
found to be necessary.
The varying slope-system is referred to in Chapter VI., as also the
range of slopes.
It is always well to bear in mind that in the case of clay, loamy or
marly soils, or any possessing soluble particles, the quantity of water
in them governs the required flatness of the slope, and as it may be
variable, a clay that will repose at a certain angle may slip upon
receiving an additional amount of water, hence the importance of
drainage and protection. The cohesive strength of clay also varies
greatly; and as it is likely to be impaired, and, perhaps, destroyed by
water and other causes, it should be considered as of fluctuating value,
and therefore as generally unreliable.
The slopes of repose required in river-banks have a wide range according
to the degree of exposure and opposition to the free flow of water, and
the protection given to the surface. In canals, and drainage-channels,
or ordinary rivers, they range from 1½ to 1, when protected, to 5 to 1;
but there are numerous small, shallow, and sluggish streams with almost
vertical banks. As the earth of river-banks is frequently of a clayey
nature the slopes are referred to under the head of Clay; usually it is
mixed in character, and, therefore, the slope of stability cannot be
determined from that of any particular and unalloyed soil. Towards the
mouth the land often consists of detritus liable to be washed away by a
stronger flood than that by which it was deposited; it may also be
eroded by constant or increased wave action. Ordinary clayey, loamy,
tenacious, or not easily moved, silt and sand river-banks, in a current
that will not erode them at moderate depths, if the face is protected,
will stand at an inclination of about 1½ to 1, when the channel is
freshly cut.
If not covered or protected at from 2 to 1 TO 3 to 1, but when in clay
soil of a doubtful character and non-homogeneous, or boulder-clay, about
4 to 1.
2 to 1 TO 3 to 1 is the most general slope; but in marsh land, unless
the surface is protected, they may not permanently stand at 5 to 1.
Fascine-work may be the easiest and most convenient protection, or a
gravel or turf-covering may suffice, and should new channels have to be
created, the question to decide is, will it be cheaper to make the
slopes sufficiently flat so that they can be left unprotected, or is it
preferable to have a steep inclination, such as 1½ to 1, and to
carefully cover and protect the surfaces, the probable cost of
maintenance of the protecting cover being duly considered?
As a rule in homogeneous earth, such as clay, underground excavation is
the most free from water, and therefore from slips; but the enormous
force clay possesses upon swelling—and this disturbance of the soil may
occur many times by the action of air and moisture—and its contraction
upon drying, cause tunnels in clay to be risky undertakings. The chief
precautions against failure are to allow plenty of space for the
expansion of the clay, reduce the uncovered lengths to the least
dimensions, have dry, yet hard and compressible, filling between the
arch and the walls and the clay, use the strongest bricks, or material
with great compressive strength for the lining, adopt a form giving as
equal support as possible in every direction, _i.e._, one closely
approaching the circular, leave plenty of weep-holes to prevent any
accumulation of water, and thoroughly drain and gently conduct to the
proper drainage channels all water as it exudes.
CHAPTER III.
THE GENERAL EFFECT OF A SLIP IN A CUTTING OR AN
EMBANKMENT.—ENUMERATION AND CONSIDERATION OF SOME PROTECTIVE AND
REMEDIAL WORKS.—TREATMENT OF THE SLIPPED EARTH.
With regard to the effect of a slip the chief consideration is, will it
be dangerous and prevent traffic or the unrestrained use of the work? A
problem most difficult to solve. Upon railways experience seems to show
that slips of earthwork in cuttings of a depth exceeding about 10 to 15
feet are more to be feared as likely to interfere and stop the traffic
than slips in embankments, and particularly as, except on sidelong
ground, slips in embankments seldom occur in which the whole of the
formation is moved, or becomes too unstable for a slow train to pass
over, and a temporary road can usually be maintained by timber trestles
and baulks, or other usual means. When a slip happens in a cutting the
fallen earth may cover the formation, and it is certain that it can move
in no other direction. The permanent way may be entirely buried, and it
may be undesirable to excavate the slipped earth until other remedial
works have been completed, and therefore the traffic is stopped until
they are effected. In cuttings of little depth where a 3 or 4 to 1 slope
assumed by a cutting originally excavated to, say, 1½ to 1 slopes, will
not touch the ballast, the serious consequences of a slip are reduced to
a minimum, and may not interfere with the traffic, and can be remedied
by the road-men on the section; but not so in the case of embankments.
Consequently the depth of a cutting or height of an embankment must be
regarded as a governing condition apart from other considerations
hereinafter named.
The height or depth at which a slip in an embankment or cutting becomes
disastrous cannot be ascertained by any rule, but knowledge of the
flattest slope at which any particular earth has remained stable in the
same state as the embankment or cutting under deliberation, will enable
a close approximation to be determined of the point to which a slip in a
cutting is likely to reach, and that to which an embankment will
subside. For instance, a cutting 15 feet in depth, having originally
slopes of 1½ to 1, can assume practically 2 to 1 slopes without covering
the rails, and, similarly, a 10-feet cutting, a slightly flatter
inclination than 2 to 1. Taking into consideration that for some little
depth from the surface a cutting, except in loose or treacherous soil,
will stand at a steeper slope than 2 to 1, perhaps, on the whole, apart
from the formation of proper water-tables, an engineer is not justified
in making expensive provision in cuttings, even in doubtful soil, unless
upon an inclined bed towards the cutting, of a less depth than from 10
to 15 feet, as they can be more economically remedied after they occur,
and meteorological influences have shown the location of the unstable
portion. Nevertheless heavy slips have taken place even in cuttings of
such a moderate depth as about 10 feet, particularly in clay earths, and
it has been necessary to entirely suspend the traffic consequent upon a
continuous mass of earth of shallow depth moving forward upon an
unguentous stratum slightly inclined towards the formation, and
completely blocking up the cutting; but this was an exceptional case.
There are many cuttings in all parts of the world under very different
conditions of weather and climate, and in every conceivable variety of
earth, in which nothing has been done to prevent movement, with the
exception, perhaps, of a surface-drain inside the fence and a
water-table, self-formed or otherwise, near the foot of the slope; and
there are many cuttings and embankments without any precautionary works.
Nevertheless they stand or require but little attention. This fact
naturally leads to the question: when ought any works to be executed
with the view of preventing movement in earthwork? Expenditure in
precautionary and preservative works, where they are not required, and
the serious consequences of a slip are reduced to a minimum, cannot be
defended; nor can the absence of such preventive measures in treacherous
earth or soil so placed as to induce instability, and where the effect
of a slip may be temporarily or permanently disastrous, notwithstanding
that economy in construction has now become the watch-word of
railway-extension; as then the expense of restoration and maintenance
will be very considerably increased and far exceed the comparatively
small cost of initial protection, for public traffic may be stopped and
injury caused to life and property.
The preceding and the following pages have been written in the hope that
they may afford some assistance in arriving at a correct decision, with
regard to the necessity of precautionary works, and with the view of
calling to remembrance many of the principal points to be considered in
order to remedy slips, a matter usually demanding prompt decision. With
respect to the measures that should be adopted to prevent slips, and the
works required when they have occurred, it would be a mistake to
generalize from any successful application of one method of reparation,
as it is necessary to consider each soil separately, and to discover the
reason of a slip; for an attempt to arrest the forward movement of a
large mass will be futile; the disturbing cause must be ascertained and
removed, or so lessened and controlled as to obviate further motion or
deterioration; sand or porous strata, which may emit water uniformly,
must be treated differently to earth which sets free water in a
particular place, as also the same soil if it should be in a dissimilar
condition. The object of all such works is to support, maintain, and
drain the earth and prevent any accumulation of water in the slopes and
formation, so that movement is improbable; therefore, water flowing
towards the slopes must be intercepted and led away before it has time
to percolate; and the method of discharging it must be governed by the
position and nature of the soil, the chief aim of draining operations
being to cause the earth to be always in its most consolidated
condition. Without obstruction to the drainage, a general preservative
is to cover the surface, and protect it against the deleterious effects
of rain, frost and thaw, particularly in the case of soils which
disintegrate rapidly under the influence of weather.
Some of the means that can be adopted to prevent slips and subsidences
in cuttings and embankments may be enumerated as follows.
1. Systematic drainage of a cutting, and the natural ground and
deposited earth in an embankment, so as to augment its weight sustaining
power and general stability.
2. A side ditch near to the foot of each slope, and at the top of the
slope adjacent to the fence line.
3. Catchwater drains upon the slopes.
4. Wells, filled with broken filtering material, contiguous to the fence
line and connecting drains with them.
5. Open timber trenches, strutted at intervals, and acting as drains and
counterforts, at the toe of a slope.
6. A channel for the discharge of any water issuing from an intercepted
field drain.
7. Tapping springs in the slopes or formation of a cutting, or that
exist in the ground upon which an embankment has to be deposited.
8. The profile of the formation being made higher in the centre, so that
water will flow into the side drains or water-table and not remain, due
provision being made for its discharge.
9. Side drains being made before the excavation is commenced or
deposited.
10. No accumulation of water being allowed upon the ground upon which an
embankment has to be deposited.
11. Covering a slope with rammed earth, burnt ballast, chalk, gravel,
ashes, or other protective material.
12. Turfing a slope, or sowing it with grass seed.
13. Depositing layers of material, consolidated by ramming or otherwise,
upon a slope at right angles or diagonally to the line of the foot of a
slope.
14. By benching, or a terrace or cess upon a slope.
15. Covering the toe of a slope with stone pitching.
16. Dividing a slope by trenches filled with stones or absorbent
material.
17. Weighting a slope to counteract the pressure of the internal water,
and to restore the equilibrium.
18. A breast-high retaining wall at the toe of a slope.
19. Covering part or the whole of a slope with fascine mattresses or
brushwood, systematically laid in combination with gravel, stone, broken
bricks, ashes, &c.
20. Counterforts of gravel, chalk, burnt ballast, ashes, rubble, &c., at
the toe and upon a slope.
21. A dry wall at the toe of the slope of a stratum of unstable soil,
found above the formation level and superimposed upon rock or firm
earth.
22. Varying or increasing the flatness of a slope towards the bottom of
a cutting or embankment.
23. In cuttings, by retaining walls, with or without overhead struts.
24. Systematic planting upon the cess and slopes, trees, shrubs, or
bushes having deep wide-spreading roots.
25. The removal of any spoil bank that may have been tipped or cast out
near the top of the slope of a cutting.
26. Removing the turf and all loose or decaying matter from the natural
ground before the earth is deposited.
27. Clearing away all snow and frozen soil before tipping or excavating,
and taking care that no frozen earth is deposited.
28. Forming the ground upon which an embankment has to be tipped, at an
inclination downward from the toe of a slope, towards the centre, and
the construction of a dry drain along the centre line so as to cause
percolating water to flow away, or prevent it reaching the slopes.
29. Benching the ground upon which an embankment has to be deposited.
30. Covering the ground upon which an embankment is to be tipped with a
hard permeable layer.
31. Trenches filled with stones or other hard permeable material across
the base of an embankment.
32. Covering the toe of the slope of an embankment with sods or making a
counterfort of turf.
33. Running to spoil all saturated earth, and suspending operations for
a day or two after heavy and continuous rain or a fall of snow, or
frost.
34. Filling any large fissures as they appear.
35. Weighting the earth so as to condense it.
36. Increasing the area of the base of an embankment according to the
bearing power of the soil.
37. By the exclusion of all boulders, roots, turf, branches of trees, or
bushes in forming an embankment.
38. By aiding consolidation and preventing separation at the junction of
two embankments.
39. Tipping an embankment in such a way as to promote consolidation.
In subsequent chapters many protective and remedial measures are
specifically named. Here reference is made to the more general
principles. The purpose for which a cutting has been excavated, or an
embankment deposited must be taken into consideration in providing
protective works, for the surfaces may only be temporarily bared, as in
trenchwork for walls, or be partly covered with water, as in canals, and
entirely unsubmerged upon one side as in canal, reservoir, and
reclamation embankments; or be fully exposed to meteorological
influences as in railway and road cuttings and embankments. There can be
no stereotyped system of operations for treating a slip, but experience
indicates that a frequently successful initiatory method is to divide
the earth into small portions, and to proceed to equally consolidate
them. However, in the case of deep cuttings, especially when excavated
in the side of a hill, it may be necessary to drive a heading beneath
the formation and to connect it with a shaft upon the higher side, so as
to tap the water-bearing soil, and to convey the water away to prevent
it reaching the slopes; this may be considered as a slip requiring an
exceptional remedy. Should a cutting be in moving ground or permeable
soil of doubtful stability, such a system of wells and covered
galleries, which are generally successful even in the worst soil, may be
required. The wells should, if possible, be sunk a few feet into an
impermeable stratum, their diameter being the least a man can excavate,
to any size required, and their distance apart, say, from 30 to 60 feet,
according to the quantity of water to be collected. They should be
connected by drifts. Smaller intermediate wells can be made between the
main wells. In order to be effectual such works must be carefully and
uniformly constructed, or an accumulation of water will arise. When a
slip is known to have occurred, simply from want of drainage, a
sufficient remedy may be the removal of the slipped earth and the
insertion of drains. An advantage of the loose counterfort system of
drainage as compared with rigid and fixed drains, is that open drains
will follow a slight subsidence of the earth, and yet maintain their
efficiency, but care must be taken that they do not become choked. In
shifting or doubtful soil all works should be quickly finished, and in
sidelong ground it is best to commence drainage operations on the valley
side so as to tap the water, as if they are begun on the hill-side they
may, until through drainage is effected, form channels for the
accumulation of water, and cause a slip. The repairs of a slip can be
commenced at several places simultaneously if at short distances apart,
such as 20 feet or so, and, as a rule, it is preferable in an embankment
that the work proceeds towards the centre, and not from the central
portion to the slopes. It is advisable to make ditches or galleries in
short lengths, not only to disturb the ground as little as possible, but
also to ensure perfect supervision, as if the work is not carefully and
uniformly constructed, localization of water will ensue.
The extent of a slip will to some extent govern the remedy. Simple open
stone-filled drains, 2 to 4 feet in width, and 1 foot to 2 feet in
depth, extending from the base to the top, may be sufficient for shallow
cuttings or embankments, such as 10 to 15 feet, and larger and deeper
trenches above those depths or heights, and complete drainage of and
around the slipped earth, and division of it by means of drains and
pipes.
In countries where there is an excessive rainfall in a short time, it
has been found necessary to catch as much of the surface flood waters
and torrential streams as possible, and to reduce their velocity before
passing through an embankment or down a cutting, and to provide a pond
or “tumbling bay” at the base of a waterfall for such purpose, or to
erect dams, when the force is not too great, so as to arrest and lessen
the velocity of the flow. Without such precautions, flood waters will
erode the earth, and the beds become gradually deeper; and walls at the
toe, culverts, and dry stone filling across the whole width of an
embankment, and stone covering upon the slopes where water issues or
flows may be required. Catchment reservoirs have also proved of use in
controlling the surface waters before they reach a cutting or
embankment, and in permitting them to be controlled.
Many of the chief causes of slips in embankments are enumerated in
Chapter I., &c. Some of the most important operations to prevent slips
in an embankment are to thoroughly drain its seat, prevent a flow upon
the surface of the original ground, percolation of drainage waters into
its lower part and filtration of rain-water at the crown, and to
generally protect the surface.
The stability of an embankment is not regulated by the cohesiveness of
the soil, as a sand or gravel embankment, or that formed of any material
whose particles are not deleteriously affected, will stand with a
sufficiently flat slope if protected against erosion, and be more stable
than a clay or any embankment in which the particles are soluble and
soon impaired by water, however great their original tenacity. If any
part of an embankment has become saturated, the internal water must be
tapped and drained; as the lower surface portion is almost certain to be
the wetter, the drains should be made at the base, trenches filled with
open porous material upon the slopes, and, perhaps, it may be necessary
to sink a few shallow wells. The slopes and formation should be covered
so that when the excess of moisture is extracted from the mass it cannot
be replaced. Porous earth counterforts can be adopted, 6 to 10 feet in
width, placed at intervals depending upon the nature and condition of
the soil and height of an embankment, or a continuous bank of similar
material at the toe to support the embankment during the temporary
weakening from drainage operations. One of the worst cases that may have
to be treated in embankments is in clay or marl soils, when the central
portion has been first tipped in a wet state in the winter months, and
after an interval the embankment completed to its required width in a
frozen condition, or nearly so, or one in which the earth becomes frozen
when deposited.
In Russia it has been found that rain-water percolating into such an
embankment cannot drain away, but accumulates and finally bursts the
slopes, and that water will exude from frozen soil when it thaws, a
considerable time elapsing before all the frozen earth has thawed; water
is thus, as it were, taken into the mass, which in all retentive soils
will be difficult to drain without turning over the earth; and it will
cause slimy surfaces and general instability. It will always be an
expensive operation to make such an embankment secure, and it cannot be
made as firm as one properly deposited. After subsidence has ceased, an
approved remedy in such a case is to cover the formation with an
impermeable layer, and to raise the embankment to the rail level with
sand, which is ultimately alone used for repairs, the slopes being
carefully trimmed and sown.
Although not considered in the usual acceptation of the word as a slip,
the trickling of the surface soil is mentioned as it is a movement which
may, if allowed to proceed, cause a slip, and frequently necessitates
attention because of the soil becoming in a liquid condition and flowing
upon land beyond that acquired for any works, and also because it
obstructs and chokes drains. It may be expected when a thaw occurs after
severe frost, or heavy rain succeeds drought, or subsequent to a rapid
change of weather in any earth of a clayey or calcareous nature, as, for
instance, in clay marl and argillaceous chalk cuttings or embankments,
and if the configuration of the ground should be favourable to its
passage to land outside that purchased, such issue must be prevented by
protecting the slopes by means of a covering, by draining, or by the
consolidation of the surface of the earth, which latter operation may be
difficult, or by the erection of a small mound near the fence line.
In ground containing salt or other solvents in appreciable quantity,
care should be taken that water does not reach it; if it should, the
soil immediately becomes damaged and subsides; also it is found that the
earth from which soda nitrate is manufactured in North Chili must be
kept perfectly dry to be secure as a foundation. All soils of a
salifiable character should be considered as likely to subside and slip.
In the salt-producing districts in England it is found that when the
brine, which is about 25 per cent. of the mass, is pumped up to the
surface to be made into white salt, the land will subside, as in effect
it is pumping up the underground supporting stratum or rock salt bed;
and when a river is contiguous or copious springs, the rock-salt will be
supplied with water to make it brine. The experience of those who have
had to maintain embankments in these districts indicates that so long as
subsidence is uniform embankments can be raised and maintained; but when
water penetrates into an old pit previously comparatively dry, unequal
and dangerous slips and subsidences may be expected, and on so large a
scale as to require much expenditure for restoration. If the settlement
is uniform, the easiest way is to simply raise an embankment; the rate
of sinking varies considerably, averaging, say, from 2 to 5 feet per
annum, and depending upon the amount of brine extracted and the
percolation of water, &c. It is obvious that embankments upon such land
require constant attention to prevent serious slips.
At the edge of a cliff or hill where loose rock exists and is joined by
clayey soil, but is sufficiently stable not to slip in a mass, it may be
necessary to have a cover shed over a railway or road to prevent
detached pieces of rock falling upon the surface, the slopes from being
injured, and larger masses sloping down. Covering a slope in such a case
is useless, but an open deep trench, specially constructed to catch
pieces of rock, may suffice.
Important questions to determine when a slip has occurred are:—
1. Should the whole or part of the earth that has slipped be removed?
2. How are the voids to be filled that have been caused by the slip?
3. Can the disturbed material be again used?
4. What protective measures should be adopted?
A thorough examination of the site of a slip and the slipped earth is
absolutely necessary before the most effectual and economical means of
restoration can be determined, for weakness or the presence of a
disturbing agent in the upper or lower portion of a cutting or
embankment may be the cause of movement. An embankment may be solid in
the mass and only portions may slip and subside, but then may become
unstable and require different treatment, or local restoration may alone
be necessary, and the slipped earth to be removed be small in quantity.
Extensive slips of the whole of an embankment usually occur from springs
in its seat or the existence of a flow of water upon the ground under
the base, producing a greasy surface. Should it happen that an
embankment of pervious soil is tipped upon impervious ground having a
depression resembling a basin, water will accumulate until it reaches
the level of discharge, and a serious slip may result. In such a case
the slipped earth must be removed and the water tapped and permanently
drained. The upper portion of an embankment may slip and the lower be
stable; if so, it is not so serious as when movement commences at the
toe and the slope bulges outwards and the embankment subsides; in the
former case, provided the lower portion is not affected or its drainage
obstructed, it may not be necessary to remove the slipped earth, but it
is advisable to drain it, and any localization or lodgment of water
between the slipped mass and the firm part of an embankment must be
prevented, or the toe of the slope will be made in an unstable
condition.
In cuttings in order to keep open the formation the whole of the slipped
earth may have to be excavated, but in embankments, so long as the soil
does not extend outside the fencing, its entire removal becomes
optional, and is unnecessary provided further movement be prevented, and
the soil drained; but in most treacherous earths, although a slip may be
arrested, it will generally be a place requiring constant watching, and
be one of doubtful stability. Earthwork slips require to be remedied as
soon as possible after they occur, not only to repair them and obviate
an interruption of traffic, but in order that the unslipped portions may
not be deleteriously affected and movement be induced.
When the earth is very soft, silty, and difficult to drain, the only
course may be to remove the slipped material, although it may not be
necessary to excavate all of it, as it may form a reservoir for the
accumulation of water, and is certain to be liable to disturbance from
the effects of weather; but in firmer soil a portion of the slipped
earth may be excavated, and be rammed in layers inclining at right
angles, or nearly so, to the surface of the slope, and a drain can be
inserted at about the bottom of the line of the slip to prevent any
water that may percolate from the unslipped mass flowing into the rammed
earth or any counterfort so constructed; but counterforts may afford
insufficient support in very treacherous soil, and it may be necessary
to remove either the whole or a considerable part of a slope and replace
it with the best available material watered and rammed: however, the
simple ramming of the earth and depositing it in inclined layers may not
be sufficient to ensure stability, and should it be found that the
slipped material is very soft and cannot be readily drained, it must be
excavated, and solid and firm earth put in its place.
When the base of a slip is level with the bottom of any side ditch that
may have existed before movement occurred, the drain should be below the
level of the ditch, or a flow of water may be induced at the seat which
will probably cause further unsettlement; and if a slip extends below
the bottom of a cutting it is necessary to remove the slipped earth as
far as the solid ground, and to fill the void with dry material of
sufficient weight to prevent the surface being uplifted, and to cause
solidity in order to avoid any movement of the toe of a slope.
When the slipped soil is clay or shale it can be burnt _in situ_ down to
the solid ground, or upon an incombustible bed, and be converted into a
kind of brick rubbish and then be restored to its original place; but
this may be an expensive method, and it may be cheaper to procure firm
earth, nevertheless, should no other material be available it may be the
only economical means of repairing a slip. Before deciding whether clay
or shale shall be burned _in situ_ it may be advisable to test the
amount of ballast that can be made by, say, 1 ton or more of coal, and
the cost including every item of expenditure: 10 or 12 cubic yards of
ballast may be obtained for every ton of coal burnt, but this quantity
may be so reduced that the cost of burning may prohibit the use of such
a method for replacing the slipped earth when made into firm soil. It
much depends upon the quantity of water in the material, and also upon
the nature of the earth; for instance, burning becomes more difficult as
the amount of silica in the clay becomes greater, and the ballast is not
so good as the quantity of lime increases in the clay; therefore pure
clay makes the best burnt ballast. Should it be decided to burn the
slipped earth, it is necessary that it be placed upon firm ground, and
that it rests upon an open layer of stone or material that will not
kindle in order to obtain the necessary draught. The thickness of the
layers must be regulated by the degree of wetness of the soil, from 1 to
2 feet being required for thorough burning, and should layers of a less
thickness than 1 foot be required, the process of making the slipped
earth into burnt ballast will usually be too expensive, but of course
much depends upon the price of the coal upon the site. The burnt ballast
may cost anything from 1_s._ to 2_s._ 6_d._ a cubic yard; when the
latter price is reached, it may be cheaper to procure sound earth. If
the slipped earth approaches the condition of carbonaceous shale, black
or dark brown in colour, it may kindle easily; the more argillaceous
shales will require a little coal to convert them to burnt ballast, the
quantity increasing as they gradually become of a clayey character. Blue
clay, when thoroughly burnt, generally makes better ballast than most
other clays, but as a drain the ballast is not equal to clean gravel.