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