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

by John Newman

The position of a river may also affect the percolation of water more upon one side of a valley than another, for a river seldom has its course in the centre of a vale, but is generally nearest to the steeper and higher side of a hill. The configuration of a country governs to a great extent the flow and quantity of the rainfall that sinks into the earth, as in a hilly country and in impervious soil the water is quickly discharged into an adjacent river, taking the easiest course. In a flat country and pervious soil rain percolates the earth and saturates the ground, or reappears in springs.

As by drainage the retentive power of the soil is not allowed free operation, water rapidly flows into the drains instead of being chiefly held by the earth and watercourses, and channels are sometimes created; and where the rainfall is heavy or occurs in a comparatively short time floods may be caused, although the soil when drained, and therefore in a drier state, absorbs more water than when undrained and in a damp condition, water will pass through it quicker, and the discharge is thus increased in volume and velocity.

Earth may be in a damp state, either from mere surface percolation and accumulation of water, or from springs which may never cease to flow; on the contrary, in rainless districts, from the almost perpetual daily drying power of the sun, the earth is sometimes found to be firmest and hardest upon the top, and for a few feet below it, than at greater depths. Separate masses of vegetation usually indicate damp places in a bare country.

As rain flows more quickly from non-absorbent soil, such as rock, and slowly permeable earth, as the clays, than from porous soil, the surface discharge is greater; and unless the water is guided, pools are likely to be formed and weak places created, especially if the ground dips towards a cutting. It is well to remember that a cutting being excavated upon the side of a hill or upon table-land may change the direction of the flow of the drainage waters, and an embankment may obstruct and interfere with them, and should the strata incline towards the excavation, it obviously favours a discharge of water into it.

If water permeates the soil or trickles down through fissures or veins, it will continue to do so until an impervious layer is reached, when it will be deflected and may become a current; therefore, whenever a permeable stratum overlies an impermeable, and the impermeable earth inclines, an increased flow may be expected, as also near the junction of tributary waters with streams or rivers.

In a drained district, water will not usually be encountered in large quantities until a depth is reached below the level of general drainage, i.e., about the invert of the nearest drains or sewers, especially in porous soils, but in the case of a pervious subsoil, such as sand or gravel, or if the tides rise in an adjacent river or the sea to the level of any foundations or above it, more water may percolate over the site or drainage area than has time to flow away between tides, the water will then rise, and systematic pumping becomes a necessity, unless the volume of the ingressing waters can be sufficiently reduced by the deposition of an impervious layer upon or in the river-bed or sea-shore, or by sheet-piling or other means, or the flow confined, which may be a risky operation in loose soil, depending upon its resistance to scour.

In clay soils, so far as slips are concerned, the action especially to be feared is the trickling of water down fissures which may extend to depths below the bottom in cuttings and create slimy surfaces, disconnecting masses of earth, and finally offering a ready means of movement, which swelling of the clay or vibration may complete. The experiments of Mr. Baldwin Latham, M. Inst. C.E., on the absorption and retention of water by clay soils, gave the following results:—The stiffest clays retained the greatest quantity of water. Clay soils can absorb and retain from 40 to 60 per cent. of water by weight. Marly clays hold less water than the pure clays. In the case of loamy soils, the percentage of water retained varied from 35 to 60 per cent. by weight, the mixture of sand and clay, therefore, limited the amount of water which it would naturally hold.

As in chalk soils fissures occur, the percolation of water and the effects of the atmosphere through the pores causes movement, and even crevices and breaks in rocks are not to be disregarded with impunity, as they are channels of disintegration.

Most earths when dry attract water, but if they are regularly irrigated, they require less moisture, depending upon the nature of the earth, and slope and relative level of the land. All soils when broken, as in embankments, absorb more water than when in an unbroken state, as in cuttings; for instance, it has been found by experiment that clayey and retentive earths will absorb about 7 per cent. more water, and light porous soils about 6 per centum. Of course, the increment varies. Even wet retentive soil, if handled, becomes considerably less impervious to moisture.

Mr. Evans, F.R.S., has proved by experiment that percolation through pure chalk is much greater than through ordinary top soil consisting of gravel, loam, and mould, both being covered with turf, and that in winter the average proportions of percolation are as about 1 for soil to 1·5 for chalk; in summer 1 for soil to 2·6 for chalk. The depth also to which chalk will allow a passage of water is some 60 per cent. more than ordinary top soil.