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

by John Newman

The construction of embankments upon soft soil is here referred to so far as regards lateral movement, or where vertical settlement causes such action by drawing asunder. Whenever the surface of the ground bulges or upheaves upon earth being tipped upon it or pressure applied, it shows that the disturbing harbingers of instability are in action and that its sustaining power has been reached, its surface ruptured, and any more soil placed thereon will only sink and displace the earth until it meets the solid ground beneath, or the material becomes sufficiently compressed and dense for it to sustain the load; such a condition of unrest generally proceeds from over-pressure and will loosen the soil, increase percolation, and aid disintegration. When it is known that soft, peaty, muddy, or silty earth overlies level solid ground, and it is certain it cannot escape, and can only be condensed by weighting until it will permanently bear the strain required, it can be built upon; although should there be more pressure at one place than another, or the character of the soil vary, in certain places be swampy and in others dry, the surrounding ground will probably rise where the weight is the least; therefore, increased bearing area of an embankment maybe necessary until the sustaining power of the earth is not exceeded, or slips and subsidences will occur, for the weight of an embankment in such a situation cannot be diminished by retaining walls, as stable foundations are not to be obtained. It is obvious the width of the formation cannot be reduced and therefore the only way of lessening the load upon the base per square foot, assuming the embankment to act as a mass, and of distributing its weight, is by flattening the slope and making a platform under the seat of an embankment. All earths, however, that are not hard rock subside upon being weighted, and in the case of tenacious clay soils, upon a considerable load being applied over a small area, a trifling contiguous uplifting of the ground will generally be noticed. If such rise of the soil does not exceed about one-sixth of the settlement of the clay under the load it may be disregarded as not likely to cause a slip or serious lateral movement.

When the foundation consists of a firm stratum of gravel, sand, or solid earth overlying soft soil such as quicksand or silt, i.e., mud and sand; or alluvial deposit, care must be taken that the latter is not forced up at a weak place by the additional weight of an embankment, and the firm stratum undermined, as then it will sink, fissure, and be unsafe. Any disturbance of the bed or water of an estuary or lake other than by the tides, currents, or wind is an indication that the earth beneath the firm soil is overloaded, and that subsidences and slips will ensue.

Before determining upon the necessary precautions that must be taken, the deposits should be thoroughly examined and tests made of the amount of soluble and insoluble material in a certain mass, as although the earth may have the appearance of being mud, it may contain a preponderating proportion of insoluble particles of sand, and its character may hardly be that of mud or silt. When a marsh rests upon clay it usually requires only reasonable care, but if upon mud or loose sand it may always be in a more or less unstable condition.

The chief means of consolidation in the construction of embankments of soft soil upon soft ground may be stated to be as follows:—

1. Draining, which must be deep, and should be effected before the earth embankment is deposited.

2. Weighting the ground outside the slopes with firm material so as to consolidate the soil by intermixture with it.

3. The formation of terraces of earth, and by allowing the ground to subside until movement is arrested, but there may be a porous seam of harder soil, such as gravel, under the soft ground, then care must be taken not to dam back the flow of the underground waters by the subsidence of the added earth or the whole embankment may slide, become softened and finally be swept away; therefore, through drainage must be maintained under an embankment.

4. Making the load as light as possible consistent with due consolidation.

5. Constructing an embankment in equal and regular layers of ashes or other dry, light, stable soil, and by drying the earth to be deposited by separation, wind, sun, or by burning.

To prevent slips in such situations, draining, although causing subsidence, should be executed as long as possible before the embankment is tipped, is more effectual and certain in its action than any system of lessening the load or of counterweighting the soil, which may thereby temporarily be brought to rest but afterwards give way; and any damming back of water may cause additional sinking, and change the condition of the earth from a state of dampness to that of saturation. It may be easy to calculate the weight per square foot that will have to be sustained by the ground from an embankment and the heaviest train, but to make the necessary allowance for the effects of vibration is not so readily computed. The ground can be weighted to ascertain the limit of its sustaining power. The test should extend for a period of several weeks to be thoroughly reliable, which time can seldom be afforded upon works; therefore a considerable excess of sustaining power over the load should be allowed.