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

by John Newman

There are few, if any, earths in which the cohesion, weight-sustaining power and ability to resist the action of water and meteorological influences are practically the same at all depths, the different conditions, arrangement, and character in which they are found being almost infinite, and there are earths which may become consolidated and watertight if in a constantly moist and protected state, that when dry and exposed will shrink, fissure, and soon become unstable.

Consequent upon cohesion, a cutting may stand for some time almost vertically; nevertheless stability cannot be considered as solely regulated by the cohesion of earth, for an embankment of gravel, sand, or broken rock with a proper slope and protected from erosion will usually safely bear more load than an embankment of clay, although the former material may be said to have no cohesion; but the lateral thrust of dry, firm sand is known to be small, provided the sand is not disturbed; also no earth can be said to be immovable under every condition, but consideration of the soils particularly liable to disturbance or mutation is, under ordinary circumstances, the main question to be determined.

With few exceptions the exterior or faces of cuttings and embankments will, at certain times, become impaired or soddened by the infiltration of water. In cuttings there is the additional danger, owing to the geological formation, of the excavation reaching the depth at which water is generally found in the locality, and it is therefore advisable to ascertain this level, and also to decide whether the ground must be excavated below it, as necessarily there will be a downward flow, and the slopes and formation will consequently have to sustain a pressure due to the difference between the normal level of the water-bearing stratum in the neighbourhood and that of any depth beneath it. In such a case, in addition to the usual softening and loosening aqueous action, there is the particular insecurity of the formation and slopes being undermined and eroded by springs: also in pervious soil in a drained district a cutting will be found to be comparatively dry to about the level of the bottom of the existing contiguous drains, but below that depth water will be present, probably in considerable quantity. Land drains also frequently cause slips, as they localise the flow of the surface or underground waters, and when in excavating a cutting they are intercepted, the discharge should be led away from the slopes; but difficulty may be encountered in effecting this, as water will usually follow its original course, and it may be impossible to entirely divert the direction of the flow, and the only thing to do may be to gently conduct the water down the slope by means of pipes, rubble, burnt brick, gravel surface drains, or timber ducts.

An important question to determine is, when are slips in earthwork most likely to occur.

In Europe they are most frequent in the autumn and winter months; but no rule can be established, nor is it reasonable to conclude, because any earthwork has remained stable during the usual period when slips may be expected, that, therefore, none will happen, for the heaviest rain may descend at an unusual season, and as moisture is the chief cause of the instability of all earths, it is rather to the quantity of the rainfall at any time than to the fixed seasons that attention should be directed.

Spring being the driest season in England and autumn the wettest, October and November being the months of heaviest rainfall, slips are more probable in the latter than the former season; but the first heavy and continuous rainfall after a period of estival drought is that particularly to be feared, or the first rainy weather after a dry period irrespective of the season of the year; but serious slips may not occur for many days or until the expiration of even a month or two after such rainfall, as the ground waters require time before they percolate to or reach the site of a cutting, and, therefore, all danger may be thought to be past when it is steadily approaching. The autumnal rains have to replace the moisture that has evaporated during the summer, and this may not, and usually does not if it slowly proceeds, produce instability in earthwork; but immediately the rainfall approaches or becomes in excess of the power of the natural absorption of the soil, the rain must flow away, for the earth being fully charged cannot contain it, the surface becomes wet or the mass soaked, according to the degree of the permeability of the soil, and the quantity of rain necessary to produce saturation; therefore, the state of the earth that induces a slip is that most desirable to know; this cannot be absolutely established in every case, for it depends upon so many influences, and obviously varies according to the character of the earth, the varieties and conditions of which are practically infinite.

In countries that have dry and wet seasons, which cause the earth to become parched and then to be rapidly saturated, mere surface waters to become streams, and rivers torrents, slips are probable soon after the commencement of the rainy season.

When frost follows rain or a fall of snow, and the latter has descended upon a frozen surface and a thaw sets in, particularly if it be accompanied by a warm wind causing it to be very rapid in action, the earth is severely tried, for the frozen water in the ground becomes suddenly liberated, while the surface is in a state of saturation. Probably the worst event that can occur for causing floods is when a sudden and rapid thaw follows a heavy snowfall upon frozen earth, as then the snow will melt, and water cannot gently percolate the earth, as the surface will be in a more or less frozen and impermeable condition, and the snow-water consequently must flow away.

Should any excessive or violent rainfall succeed a period during which the heat of the sun has caused fissures in the surface of the earth, allowing water to enter, the state of the soil is favourable to movement. Land has also become unstable in mountainous countries because a district has been deforested, or tree-protection much reduced, movement of the earth usually happening after the first heavy rains or thaw succeeding frost or snow.

It has also been noticed that when an earth has become completely saturated or water-charged, a sudden fall of the barometer to a low pressure will liberate the pent up water which the soil cannot contain and cause it to burst out, the equilibrium having been so delicate. Under such circumstances slips are nearly sure to ensue, and to be serious from their sudden action.