Earthwork Slips and Subsidences upon Public Works / Their Causes, Prevention, and Reparation

by John Newman

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.

CHAPTER IV.

Notes on the Percolation of Water.—Systems of Drainage of Cuttings and Embankments in Different Kinds of Earth and under Diverse Conditions.—The Construction of Culverts, Pipe-drains, Trenches, Ditches, and Catchwater Drains.

With respect to the percolation and drainage of water in cuttings and embankments, in cuttings the chief consideration is to gently extract and conduct the water so as to avoid any accumulation or localised flow beneath the original surface of the ground in order to prevent the surface water eroding the slopes or collecting or forming a course, saturating the ground outside them, and to ensure that the earth is not more charged with water than when in its normal condition; for, as soon as the state of absorption has reached that of dissolving or separating the particles, however fine, aqueous action is likely to produce slips, and a flow of water or vibration will supply the disturbing force necessary to commence a movement. In embankments one of the chief precautions is to obviate any flow of water upon or at a few feet beneath the land upon which an embankment has to be deposited, as it will disturb the feet of the slopes and the base, reduce the adhesion to and the friction of the tipped earth upon the ground and form a sliding surface.

An accumulation of water upon the formation must be prevented, and, as in the majority of cases a railway or road is not level, any collection of water at the commencement of an incline or at a change of gradient should be provided against, and especially any localisation of flow down the slopes from the formation; the main point being to keep a cutting or embankment in a uniform state so that settlement is equable. By carefully watching the effect of heavy rain upon the slopes and the formation, the places where water amasses can be traced, and means used to restore the surface to the same condition as the other portions of a cutting or embankment. As water is the chief cause of slips, the friction and cohesion of earth being impaired and, perhaps, destroyed by it, it is obvious that at the time percolation, which varies greatly with the seasons, is at its maximum, i.e. when the earth becomes in a damp or wet state, slips are to be most expected, and particularly soon after the commencement of the wet season. It is known that upon the thawing of a heavy fall of snow, and of quickly succeeding and separate falls of snow, percolation is great; also after heavy and continuous rains, especially if the strata dip towards a river, and in the case of springs whose yield depends more upon percolation than the amount of rainfall, a wet winter will cause an increased flow some time after, when the earth may become saturated.

If it could be determined at what depth in any earth in any state percolation, evaporation, and meteorological influences would cease, and also their effects, rules could be deduced for guidance in draining cuttings and embankments. The manner of the execution of ordinary cuttings and embankments is so dissimilar to that of filter-beds of waterworks, that the experiments made for such purposes are only of comparative value for the former works, for the condition as well as the character of the soil affects its permeability by water. For instance, in cuttings, with the exception of some surface disturbance during the process of excavation, the removal of vegetation or a covering, and the exposure of the ground to the atmosphere, &c., the normal state of the earth is not much altered; but in embankments different soils may be intermingled in a manner almost unknown in nature, the varieties of mixtures of earth being most numerous, and they may be in every condition of compactness, dryness, and dampness amounting almost to saturation, and in any case, therefore, percolation is temporarily or permanently increased consequent upon the earth having been disturbed and loosened.

The general principles of the percolation of water are here only briefly referred to, as they particularly concern slips in earthwork: but obviously the quantity of percolated water greatly affects the stability of a slope, for the surface water should not be guidelessly allowed to soak into or be absorbed by the ground at the top, and so proceed through and down the slopes, as then the pent-up water tends to press out the face which may be temporarily sun-dried. As in excavating cuttings the surface is bared and vegetation removed from the soil, water has easier access, and unless the ground when excavated is immediately covered as before, its normal state is not preserved. One point of considerable importance is to ascertain whether in any cutting or embankment percolation is uniform and regular; some infiltration will necessarily take place, as water will gravitate from the top to the bottom and will find the easiest course or line of least resistance, which may not be at the lowest level.

As the amount of the annual rainfall varies greatly according to the country, and, even in England, considerably in a small area, the earth will be more affected in one place than another; for instance, 48 to 50 inches is approximately the annual average rainfall upon the extreme S.S.W. coast of England, being greatest at the highest level nearest the sea and to leeward; it diminishes gradually from W. to E. to from 26 to 24 inches, the minimum of about 20 inches being in Essex. The differences of quantity must therefore be regarded; but such rainfall is as nothing compared with that of tropical lands, for the fall often continues many hours, and yet equals and perhaps exceeds an inch per hour. The heaviest fall and its usual time of appearance should be ascertained, as earthworks may have to be constructed in a peculiar district where the rainfall may be more than double that of the average wettest district, and it will often be much greater at the foot or the top of neighbouring hills than on a flat coast. Local information from reliable sources is the best guide when confirmed by general knowledge. In the tropics 100 to 200 inches in depth of rain instead of 20 to 30 inches has to be treated, and frequently half the total annual rainfall in England comes down in twenty-four hours. It is almost superfluous to name that the protective works which would be amply sufficient in one country may be useless in another, simply because of the variation in the amount of the rainfall and the capacity of different earths to resist or invite the percolation of water.

Obviously percolation will vary considerably with the seasons, and a succession of wet periods or a continuous downpour will increase the quantity of infiltrating water; but the effect of a fall may not be experienced until some time after it occurs, as in districts or rivers that are fed with water from the thawing of snow upon surrounding or distant hills water reaches the lower tracts of country in hot and sunny, and therefore generally dry weather, when evaporation is the greatest, and not in winter. Again, there is generally very little rainfall over flat deserts, but an excess upon mountain ranges which may surround a desert, and especially in tropical countries experience has proved that storms and rainfall are often local and extend over a small area, one district being more liable to such a visitation than another; they are also, as usual, of irregular duration and severity.