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

by John Newman

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.