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

by John Newman

When a reservoir is emptied the weight of the water on its bottom is removed, but the load from the embankment is the same, and should the ground be soft the embankment may subside towards the reservoir and the bed be uplifted; hence it may be advisable not to draw off the water unless the bottom is weighted or movement prevented.

Due regard of the different causes of failure herein enumerated and care in construction, will reduce to a minimum the probability of a slip or a subsidence in an earth embankment erected to contain or expel water.

The temporary or permanent diversion of rivers or streams being so often necessary in public works, a few paragraphs are here devoted to it so far as regards earthslips and subsidences. If it be possible when the soil is very porous and incapable of retaining water, it is advisable not to divert a river.

Some of the most vulnerable places in a newly-formed river-bank are:

The ends that join it to the old bank or to the land, to which it should be thoroughly connected.

The toe of the slope and seat, which should be tied into the bed of the river or be well protected by making the slope flat towards the base.

Any abrupt bends or angles should always be avoided as they increase erosive action.

The wind and water line which requires especial protection.

Provided these points are remembered and the usual precautions taken in forming a river-bank to make it thoroughly sound and homogeneous, a slip or subsidence of serious moment is improbable.

In order to protect the sandy bed of a river and to prevent the banks slipping and subsiding, it may be necessary to guard against scour of the bed and consequently of the toe of the banks. Stone thrown in will settle and compress the bed by weighting and consolidation. By periodical depositions the sand becomes more protected and the quantity of stone required is reduced, but especial care should be taken to preserve the normal bed, to offer no obstruction, and not to cause whirlpools or to interfere with the current except to direct and train it, or the erosive action so created will cause movement. Stones simply cast in and allowed to sink and find a permanent bed until the regular surface of the bottom of a river is so reached, have been proved in many instances to be a sure protection in sandy soils provided eddies do not exist. The preservation of the slopes is particularly referred to in Chapter VII.