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

by John Newman

In pulling down walls that have been built with offsets at the back it will usually be noticed that the filling does not rest upon them, but that hollows occur as the earth subsides; for this reason, a straight batter at the back is to be preferred. No support from the weight of the earth, which is supposed to be upon them, should be relied upon, although it may temporarily exist.

The filling should be commenced at the wall, and the layers be so deposited that their slope approximates to that of being at right angles to the surface of the slope of the ground rather than parallel to it. When thought desirable and the natural ground is solid, it can be benched, and the backing be damped and rammed.

The backing of a wall may consist not only of an inverted triangular piece of earth, but extend for some distance to the rear, and may have a top surface at a considerable height above it; the wall will then have to sustain a severe thrust. The angle of repose of the earth should be ascertained and the filling be thoroughly drained, or it may be disintegrated by moisture, the whole mass gradually become unstable, and finally push the wall forward with great force.

CHAPTER XII.

Notes upon Slips of Earth, Subsidences and Movement in Foundations caused by “Boils” or an Upward Rush of Water in Loose Earths.—Consideration of some Precautionary and Remedial Operations.

With regard to “boils” in sandy soils and the general drainage of the site of dockworks, operations should be commenced as long as possible before the works of construction, so as to lessen the probability of the occurrence of slips, subsidences, and movement of earthwork. In order to reduce infiltration, it is well to make a trench round the area of any ground that has to be excavated, which may embrace the whole site. A system of drains and conduits should be established within it, and at the lowest level a sump at a convenient position, and to a depth a few feet below that of the work; it can then be made the chief pumping station of the dock; but all drawing away or flow of the earth must be prevented: and to obviate erosion of the drains, they may require to be rough lined with an impervious covering such as clay. The sump should consist of an iron cylinder with proper provision against a run of soil. In all loose earths the pumping station should be some distance from buildings or roads so as to avoid any settlement.

When a choice of sites exists and the position of a dock is not absolutely fixed, it may be possible to have the foundations of the whole work in one kind of earth; if otherwise, one portion may be stable and another unstable, always treacherous, and liable to slip and subside. In any case settlement is not likely to be equal, and therefore a foundation which is well able to sustain an evenly distributed load may yield from unequal strain and excess of lateral thrust. Consequent upon the situation of docks, the superficial beds upon which they have to be erected frequently vary in stability and reliability; and the location being altered in any direction may result most seriously; the earth upon one side of a comparatively narrow river being stable in character and on the opposite bank most treacherous. Difficulties often arise in foundations, especially in sandy soils, from making borings and trial pits too near the important parts of the work; they should be made as reasonably far away as is convenient. In boring, a sand flow may occur, when it may be necessary to fill the bore-hole and sustain the soil by a covering, or by consolidating the sand by means of Portland cement, and then, perhaps, reliable operations may be continued. Borings should be considered as unreliable if merely superficial; in any case of importance they should only be trusted for the place where they are made, and not as indicating the nature or condition of the soil over a considerable area. When pits cannot be sunk, it is desirable that in a suspectedly treacherous site the bore-holes should be at every 200 or 300 feet.

Excavating pits, using test-bars, and driving piles are some of the methods of determining the character of foundations, but care should be taken to ascertain in boring that boulders, or thin strata of hard gravel, are not mistaken for solid rock. In sand, mud, or soft clay they can be made by means of an iron pipe and the water-jet system. Experience has proved that boring with an auger is not so reliable as boring with a tube, such as is used for artesian wells. In the case of augers, when boulders are encountered, further boring is usually arrested in that place and another bore-hole has to be commenced. Trial pits, where practicable, should be preferred to boring, and they should be sunk to a depth considerably below the lowest level of the intended foundations, and then they may do for sump holes for pumping operations. In testing ground by borings, several should be made, as one hole might encounter a boulder or some hard soil, such as indurated clay, and the latter may adhere to the auger and arrest its progress; the specimens then brought up, being crushed and pressed together, will appear to be firmer than the actual condition of the ground.

Having briefly referred to the preliminary drainage, and some methods of ascertaining the nature of the ground, the former to lessen, and the latter to aid discovery of the character of any probable upward lushes of soil: “boils” in foundations are more specifically considered. It may be said that they generally proceed from an impervious top stratum being pierced, thereby tapping the water in a pervious bed which may be imposed upon another impervious layer; for the upper bed being excavated removes the weight upon the lower strata and induces a flow of the previously confined water.

The source of disturbance may be either from surface-water in the top soil or from deep underground springs, depending in great measure upon the extent of the catchment area and head-water level of the district, as its quantity and uplifting pressure will be principally governed by them. When the soil is in a delicate state of equilibrium it only requires a slight deteriorating alteration of the normal condition to initiate a movement. The disturbance of the ground may be merely superficial, nevertheless its effect may be sufficient to start a “boil,” although the chief cause potently exists at some considerable depth and distance.