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

by John Newman

In depositing embankments it is well to remember that experience has proved that materials uniform in size and homogeneous in character form the most compact and impenetrable masses. The great stability of breakwaters formed of materials of uniform size and the firmness of macadamized roads are proofs of this. The same rule applies to soils. It is the separation of the larger bodies from the smaller that causes a want of cohesiveness and weight-sustaining power.

To prevent embankments of little height as in “forming,” spreading, slipping or weathering, as they are usually constructed from side cutting and loose top soil, the sods over the site of the side cutting should be removed and a turf wall be made of them on each side of the formation, the excavation being deposited within them, when it will settle equally and become consolidated, the wall preventing it spreading and also saving expense in maintenance.

In an embankment of soft earth that weathers quickly, the tipped material should be allowed to take its natural slope, and then every effort should be made to prevent its equilibrium being destroyed.

Where heavy and sudden rainfall occurs the edges of the formation have been purposely tipped and maintained from 6 to 12 inches above the height of the centre, in order to prevent during construction gutters or eroded channels being formed upon the slopes, the central portion being drained and the water led away. However, aqueous action may not deleteriously affect earth when tipped into an embankment notwithstanding that the soil cannot be made in the same state as before excavation, and that deposited material is more open and subject to percolation than the solid unexcavated earth; on the other hand, although the percolation and degree of exposure to the atmosphere are greater, in some soils this may tend to drain and render them harder and more stable. In fact, special circumstances may alter the general behaviour of any earth, but the material forming an embankment should be regarded as in the same condition as if it had been exposed to meteorological influences, and it should be remembered that in railway, dock, or canal works the deposited earth is seldom uniform throughout, it being either soft at one place and hard at another, or intermixed in a manner unknown in nature, and that exceptionally difficult cases may occur which can hardly be treated by any particular method of procedure or even in the way which experience has proved to be effectual in several instances in similar soils under apparently like conditions.

The system of tipping has some influence upon the stability of an embankment and the prevention of slips and subsidences.

The first operation is the preparation of the ground upon which an embankment will be deposited. Solid hillocks or firm mounds that in any way tend to arrest motion should not be removed, but vegetable or bush growth should be destroyed, and where slips and subsidences are likely to occur, turf and all soft soapy matter should be stripped so that the deposited earth rests upon a sound stratum. In the dyke countries of North West Europe, before an embankment is deposited, care is taken to remove all trees and roots so as to effect a thorough connection between the ground and the earth forming the embankment.

It is well if the ground be ploughed, raked, or harrowed, so as to offer a rough and similar surface to the tipped material, and also to obviate any arrest of the percolation of water, which upon reaching a turf or smooth or less permeable surface, may form a water seam at the foot of an embankment. It may not be necessary to strip the turf for the full width of the base of an embankment, but only for some 20 feet from the toe of the slope on each side. As the earth is bared it should be covered in order not to expose it to the weather, for should the surface be coated with mud, water will accumulate, as it cannot drain away or evaporate as quickly as when the ground at the seat is laid bare; and also if the top soil be mere dust moisture will convert it into mud. When the surface of the ground is inclined it can be benched or bared, and it is open to question whether baring and harrowing the surface is not a better practice than benching, as it gives uniform support and increases friction and prevents a sliding surface, and is generally a quicker and cheaper method to adopt as the soil need not be so prepared for a greater depth than 6 to 9 inches. Benching may be the better system to adopt, in loose and non-cohesive earths which require support to prevent rolling, but it must be carefully made with the necessary slopes and inclination of cess to prevent a localization of water, or the ledges may slip and carry away the material deposited upon them. The ground at the seat of an embankment can also be prepared by ploughing over it and removing the disturbed earth with a scraper.

The drainage of the ground is most important in order that water cannot reach the seat of an embankment. Many systems are referred to in Chapter IV., and others are herein named. A complete system of drains may be necessary, but much depends upon the character and sound condition of the earth and whether the slopes and formation are protected. Simply covering the whole area of the seat, or that upon which the base of the slopes rests, with broken stone obtained from the cuttings may suffice, the stone being so deposited that water flows from the centre to the sides, with a rough drain here and there, or central, diagonal, and lateral drains may be required; but the pressure of the deposited earth frequently causes land springs to issue in the seat of an embankment, the existence of which was not conjectured, hence the effect of tipping should be watched.

Slips and subsidences are induced in embankments by the material being deposited when the soil is dry, and in every state of humidity from dampness to saturation; consequently the earth settles unequally, is denser in parts, and a stratification is caused as if it were composed of dissimilar earths. Should any soil be deposited in the same condition throughout, and be punned or rammed in layers, it becomes as near its original texture as is possible to quickly attain; but in depositing railway embankments, which cannot be punned or rammed, the perfect incorporation of any soil is not effected by the simple operation of tipping from a spurn head, and stratification of earth is disadvantageous, inasmuch as the layers may not be homogeneous, and in railway embankments are, with a few exceptions, in a different state owing to changes in the character of the soil, the effects of weather, mode of excavation, and in a lesser degree the height and length of the lead or distance from the cutting to the tip; the stratification, therefore, becomes varied and irregular, and depressions are formed which may hold water, and none should be permitted to accumulate in or upon a freshly tipped embankment, for then it is in its most permeable condition, and in retentive soil especially the existence of a wet place may cause it to become so deteriorated that it cannot be made stable without being drained, and when the earth is liable to become alternately very wet and dry, every means must be taken to lessen the deleterious effects.

Provided an embankment is tipped of one kind of earth, although it will be in a more open state than the solid unexcavated ground, the slopes given to it are flatter than the angle of repose, the weight brought upon it does not exceed the safe load, that it is deposited upon level ground in regular and equal particles and the soil of ordinary character, there is no reason that it should slip or subside unequally if it be not undermined by aqueous action. Under these conditions settlement would be equal, or nearly so, and as all deep embankments must necessarily subside from about 10 to 40 per centum, and in extreme cases one-half, the object to be gained is to cause the settlement to be even and regular towards the centre. Benching the ground so as to prevent a movement of the toe, does not aid equal settlement nor keep the portion of an embankment above the level of the ground from spreading, except from the resistance offered by the cohesion and friction of the soil; whereas, if the ground slopes inwards towards the centre the earth has a tendency to rest and come together, and therefore not to slip upon the slopes. This system is subsequently referred to.