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

by John Newman

Rain will percolate through chalk or any open soil until it meets an impervious stratum, or to that place which is in a state of saturation, when the water must either flow away or the level of saturation must gradually rise. This causes rivulets to burst out in places after heavy rain, when the water has had time to percolate, and the rainfall has exceeded the average, but such springs will cease when the local excess has terminated. However, in cuttings it is the flow from fissures that is to be feared, and their size may indicate the quantity of water that may be expected to issue from them. It seems to be generally agreed that the supply of water from chalk is derived from rain, which percolates through innumerable fissures, and that in all rocks, whether limestone, sandstone, granite, sand, or clay, it is by means of the fissures, seams, and veins that the supply of water is obtained from rain, and springs created.

With regard to the percolation of water through sand, it may always be expected to be very considerable, and the soil may under certain conditions become water-charged. Mr. Greaves, M. Inst. C.E., has shown by experiments that the average percolation through ordinary top soil is only about one-third of that of sand, but the evaporation from a surface of ordinary soil was about four times more than from a similar surface of sand, and also the amount of percolation in ordinary top earth was small on the whole, and, perhaps, the percolation through ordinary ground would be about 25 per cent. of the rainfall, but 80 per cent. in average sand.

Experiments have also shown that the absorbent capacity of sand decreases regularly according to the fineness of the grain, and that “some sandy soils will not absorb more than 20 per cent., but sandy soil containing peat, as moorland, as much as 80 per cent., both computed by weight.”

The quantity of water absorbed by loamy soil will vary considerably according as clay or sand preponderates in the mass. Earth may become so mixed with coarse or fine sand that, when saturated, it approaches the condition of a quicksand.

The general effect of percolation has been briefly described as follows. Upon water entering the pores of an earth it displaces the air or liquid previously present, forcing the former upwards into the atmosphere, and the latter downwards.

Having briefly referred to the percolation of water in cuttings and embankments, the drainage is now considered. It must be either precautionary, i.e., to prevent a slip, or remedial, i.e., to drain a slip.

The aim of any draining operations to prevent slips in earthwork is to search for the source of water discharge, to tap and gently conduct it away and prevent it reaching, accumulating, percolating, or being confined within the slope of a cutting, which it may then reduce to a pulpy condition; its free effluxion being most important, as also the lessening of the percolation of rain and surface-waters. The drainage of cuttings or embankments may consist of wells, culverts, closed or open channels, pipes, and tile drains of every reasonable and economical form, and may be placed in various positions. To describe them and the different systems of draining is to open up a subject requiring several volumes; here the endeavour is made to indicate whether elaborate or ordinary drainage is required, or mere water-tables and surface drains, and care in the process of excavation and deposition, protection of the slopes, and in giving them sufficient inclination to prevent movement. If possible and time allows, the drainage in treacherous soils should always be commenced before the main excavation, and, in any case, simultaneously.

Rock and solid impermeable earth may merely require to be surface drained, but all treacherous and porous soil, deep draining; and granular soils, which usually exude water from the whole mass, demand different treatment to those earths which discharge water at particular places; but it may be most difficult to drain a mixed soil, such as sandy loam and silt. With the exception of a counterfort and drain at the foot of a slope, and an impermeable catchwater drain upon the slopes and top drains, to prevent and lessen surface percolation, the best method to adopt in earth of this description may be to sink wells at intervals to intercept the flow or percolation of any ground waters; to attempt to drain or draw out the water in the soil will end in comparative failure. To reduce the volume of the percolating waters is the object to be attained, and then evaporation, vibration, which tends to shake down water, and time may gradually convert the earth to the desired drier condition. The wells can be filled with broken stone or coarse gravel to support them, and prevent their closing.

It is an advantage to prevent the percolation of water into soil that will not readily part with it, such as the clay earths, as it may be economically impossible to drain or restore the earth to its normal condition, and should the strata be upheaved, intermixed, and of a permeable and impermeable character, a scientific application of drainage can alone succeed. When the source of the water is ascertained, it can be seen whether a complete system of drainage is necessary over the whole of a cutting or only a portion of the slopes. Dampness and the egression of water may be merely local; if so, by boring a hole and inserting a drain into a slope to tap the flow it may be cured, the surface being made dry by a layer of ashes or other absorbent material. Pipes or tile drains may be sufficient when springs exist, or the flow of water is local, and in loose soils it is especially advisable to provide openings for cleaning any covered drains. Brickwork, masonry, concrete, pipes, or other rigid drains, may not be suitable for ground likely to unequally subside, as they will probably crack or leak, and loose-jointed pipes or over-lapping tile drains may be required. In treacherous clay soils surface longitudinal and transverse drains will most probably be insufficient, and deep draining of the mass be necessary, also the ground upon which earth is deposited will require to be drained and a layer of rubble stone placed upon it, a cutting or embankment being divided into small drainage areas by deep open dry stone trenches.

Water may not sufficiently percolate into a hill, either because of its surface being covered with vegetation, or the soil being of an impervious nature. It may then flow down the slope of a cutting which will probably be bare and unprotected. To prevent slips the discharge must be carefully controlled and led away, particularly when the formation is drift soil upon rock, or the earth will be liable to saturation and degradation; also to prevent a flow of water under its seat, and upon the natural ground, should an embankment be deposited upon it.