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

by John Newman

As any perforation of the surface must be prevented, in certain districts the covering should be capable of resisting the attacks of rodents and crustacea, and, therefore, a stone pitched or concrete covered slope is necessary, or the puddle towards the surface must be well incorporated with small stones or ashes or other tough material. In European countries the effects of the burrowing of rats may be insignificant, but in warmer climates, as, for instance, on the coast of Coromandel and in some parts of Bengal, where rats may measure as much as 2 feet in length, their attacks are not to be disregarded with impunity.

The causes of failure of water-containing embankments afford an indication of the direction in which especial care should be exercised in their construction. Assuming a reservoir embankment to be of the necessary form and bulk, and to be properly constructed, the principal causes of failure are as follows:—

1. Leakage along the line of a culvert or pipe passing through the lower portion of an embankment.

2. Leakage under the seat of an embankment.

3. Water overflowing the top and eroding the land slope and so destroying the equilibrium.

4. Bursting of springs over the site. Vide Chapter XII. on “boils” in loose soils.

With regard to the first, the most frequent cause of the failure of a water enclosure embankment; the culvert or outlet passages have been rendered unnecessary by conducting the waters in a tunnel passage under the seat of the embankment and without interfering with it; but this method is expensive. One of the causes of failure is the weight of the embankment owing to unequal settlement producing a breach in a culvert, the probable result of insecure foundations or want of a firm concrete base to the culvert to evenly distribute the weight; or it being placed upon clay puddle, which should never be done; or the embankment being constructed without due care. All culverts should be sufficiently large for a man to easily pass through and should be equally watertight within and without, for a leakage from the culvert to the bank is equally dangerous, and means should be provided so that it can be closed in short lengths. Reports on the temporary failures of reservoir embankments almost invariably state, failure occurred from water penetrating between the puddle and the culvert, from water percolating between the rock foundation and the central puddle wall, or the embankment gave way as water issued through interstices in it caused by settlement of the masonry outlet passage.

Careless construction has often been shown by inspection to be the reason of so many embankments yielding along the line of the culvert; but when the latter is properly designed and built it seldom causes a temporary failure of a well-made reservoir embankment. Respecting the second cause of failure, it generally proceeds from want of care in thoroughly binding an embankment to the solid ground and protecting the toe; by “boils” in the foundations; or by a porous earth seam, such as fine sand, existing under the stratum upon which the embankment is erected becoming a quicksand on flowing water reaching it.

In connection with the third, it may be said that it is not a frequent cause of failure, as provision is almost always made to prevent it, but it may occur in a reservoir embankment from extraordinary circumstances, or in a much less degree from sufficiently high waves being generated upon a lake or impounding reservoir that the top of the bank may be loosened, and the water dash over it and erode the land slope, and convert it into a kind of tail-race; tarpaulins have been temporarily laid upon the surface on an emergency to protect a soft place, also sand bags and planking to prevent an overflow. In an embankment erected for such a purpose the top width is more severely strained, consequent upon the greater exposure, than in a reservoir embankment, and the height must be sufficient to prevent water passing over it.

With reference to the fourth cause of failure, “boils” in loose soil and the bursting of springs are referred to in Chapter XII.