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

by John Newman

The resistances at 2 to 1 and 3 to 1 are practically the same.

Should it be determined to pitch the lower part only of a slope, in order to prevent slips, the pitching must continue to such a height that a flow of water down the slope is impossible, or it may become detached. If the pitching is not rough squared on the joints, but of various forms, it is preferable that the face having the largest area be laid downwards, smaller stones being carefully wedged in between the interstices. Any defective execution is usually followed by a falling of the stones, and should this happen the surface will be broken and erosion and slips will ensue. In laying the stones they should be so placed that if a few become removed those above or upon the sides will not be disturbed, and in loose soil no pitching should be laid until an embankment has had time to settle and consolidate, which the necessities of rapid execution may prevent, for however even the pitching may be when first laid, it will settle and become more or less uneven in such soil as sand or estuary slake, and every effort should be made to leave no hole or exposed surface, but to cause a continuous close covering. Fascine mattresses may have to be used in such a situation, as they will follow the contour of the slope. Should the slope be subject to considerable wave action, smooth surfaces offering the least frictional resistance, obviously aid the travel of a wave, which is undesirable, and provided the embankment is sufficiently strong a rougher surface is to be desired. Projecting stakes tend to subdivide a wave.

Should pitching be required to be placed partly upon the face of a cutting and partly upon the slope of an embankment, as in the case of a canal upon sidelong ground, care must be taken that it does not settle unequally, and provision should be made by setting it upon a pervious layer, so that any damming back of drainage waters may be prevented. The weight of pitching upon a slope affords a counter-pressure to that of any water in the slopes which may be trying to emerge. Chalk rubble, also hard chalk, or gravel has been used in lieu of stone pitching for protecting slopes against erosion.

With the exception of retaining walls, the preceding may be considered as the principal means, used separately or in reasonable combination, for covering the surface of a slope in a cutting or an embankment.

A frequent cause of instability of the slopes in countries having severe winters is the melting of unequal masses of snow, the result of drifts, and also from the thawing of a considerable snowfall. The chief preventive measures against snowdrifts and consequent protection to the slopes, although not a covering, may be stated to be as follows:—

1. Locate the line in a naturally sheltered position, which it is most improbable can be done throughout its length.

2. Adopt tunnels, covered or sheltered galleries where the district is subject to avalanches or snow-slips.

3. Prevent drifts by permanent or portable screens such as earth mounds, trees, hedges, fences, &c.

4. Obstruct drifts by having deep trenches some distance from the top of the slopes.

5. Avoid cuttings as much as possible, and make the line upon an embankment of such a height as to be above the depth of the drift snow in similar situations.