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 1s. to 2s. 6d. 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.

In considering whether it is only necessary to simply replace the dried earth in its original position, it should be determined whether the undisturbed portion of an embankment will support the weight when unaided by counterforts with a foundation in solid ground at the toe of the slope, trenches and drains upon the slopes, and perhaps a rough stone bed below the seat of the slip acting as an open drain over part or the whole of it. In any case provision must be made that there is no localization of water between the original embankment and the filling or the counterfort. In some soils, particularly those having seams of sand or silt, the slipped earth frequently becomes displaced in layers, and if allowed to remain, each bed will form a water seam upon which any stratum can slide, and then the earth may not be at rest until the slope is very flat. The removal of the whole mass is the cheapest remedy. The surface left bare by the slipped earth should be trimmed, and all fissures in it be filled so as to prevent any accumulation or lodgment of water, but the slipped earth should only be excavated in short lengths, as it may render support and keep part of the face covered, and it should be remembered that although the upper portion of the fallen earth may be the drier it may not be the most stable.

The system of removing the slipped earth, erecting rough rubble walls at intervals of 20 to 30 feet projecting as far as the face of the original slope, and then filling the intervening space with the material that has slipped, when turned over and punned, has been successfully adopted. It is advisable to cover with turf the replaced earth in the slope, unless some other protection is supplied. When any signs of movement afterwards take place a few additional counterforts, which, may also be made to act as drains, will generally restore the equilibrium. This system relies upon preventing movement in earth by separating the masses of the slipped soil, and draining and supporting them in detail. The foundations of the counterforts must be in the solid ground and not merely below the seat of the slip. It may be impossible to drain the site of a slip or the soil that has moved without dividing it into portions, the chief object being to thoroughly drain the site and the slipped earth, so that it is practically encircled with drains and any water prevented from collecting in or upon it.

When the land is of little value and a cutting is in a mountain or hill-side, it may be advisable to assist an extensive slip, provided it happens before any public works are opened for traffic; and to remove the earth by loosening it by the action of a stream of water until it slides away, as draining or supporting it may be insufficient. Under such circumstances it is the best course to adopt, especially if a stream of water can be readily diverted to it as the unstable soil is finally disengaged.