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

by John Newman

With respect to the adoption of open surface rubble drains upon the slopes, in Chapter IV., page [78], special reference is made to them as drains, and in Chapter VI., pages [111], [112], an objection is examined; but they are generally effective if cut at right angles to the formation and not diagonally or transversely. When a slip is repaired by replacing the slipped earth when dried and rammed between such drains, it should be determined whether it would be better to cover the slope or increase the number of drainage channels. A simpler remedy is the insertion of a dry rubble lining on the surface of the original ground upon which a slip has occurred, and the restoration of the slope by means of firm earth or rammed and dried slipped soil; but the system of rammed earth counterforts, although it has succeeded when effected during the dry season, the earth being wetted sufficiently to make it cohesive by ramming, will not succeed in wet or winter weather, as the soil cannot then be consolidated by ramming operations, for it will be overcharged with water from the effects of rain, frost, or snow. Fair or summer weather is required to ensure success.

Should it be considered advisable, in addition to the erection of rammed earth counterforts to a height of from one-third to one-half of that of an embankment, a porous layer or wall of broken stone can be inserted between the slipped material, which is not touched, so that the waters can filter away through it and the slope be restored; and the counterfort have a channel at its base, in order to prevent a lodgment of water, to drain the slipped earth and the embankment, and to prevent the counterfort becoming saturated. This method may be insufficient in any but comparatively firm soil, and it may be necessary to make a trench at the toe of the slope having its base 2 or 3 feet below the seat of the slip or firm ground, it being filled with rock chippings, shingle, gravel, burnt ballast, broken bricks, ashes, coarse clean sand, or other material having equable frictional stability and particles that may for earthwork purposes be considered as insoluble in water, and therefore that form stable masses and yet act as drains; and to drain the slopes by trenches varying in width according to the position and depth of cutting, the depth of a trench may be little or have to be such that it extends to the solid ground through the slip, or about 50 to 75 per cent. of its depth, which must be determined in each case, may be sufficient. Such dry stone counterforts may be required at distances from centre to centre of 15 to 66 feet, and are sometimes placed in the same right line and even through an embankment, and are connected with a drain sufficiently deep to prevent any lodgment of water at the centre of the base of the embankment or near the foot of a slope.

With regard to covering a slope of a cutting or embankment, its principal use is to lessen deterioration from meteorological action, to keep the ground underneath in an equable condition, to reduce percolation and make it uniform, to diminish the danger arising from cracks and fissures caused by heat, evaporation, or drought, to prevent the erosion of the surface by a flow of water or the production of unctuous surfaces, and yet not interfere with effectual drainage.

It is known that percolation is decreased by vegetation, and that it is less through turf than bare ground, but varies greatly according to the kind of covering, the season of the year, the regular or irregular distribution of the rainfall, and other causes. If the surface has to be covered with vegetation it should be close, uniform, and vigorous throughout the whole year, and nothing fulfils these conditions as well as grass or turf. The protecting value of turfing earthwork may be judged from an inspection of fortifications in which steep slopes, deep ditches, and perfect maintenance are a necessity. As a consequence of the binding and shielding of the earth by the roots of grasses and plants, and their protection of the surface, the slopes stand at a steeper inclination than they otherwise would, and the system of clinching the exposed faces of the parapets of earthwork with a layer of sods laid header and stretcher is found to be a great support.

In earth possessing considerable cohesion, such as the clays, provided the covering is uniform, grass having comparatively deep roots is not a particular advantage, but the closeness of the growth of the plant and the unimpaired maintenance of a complete cover is of importance, and the earth should be disturbed as little as possible so as to prevent any particles comprising the soil becoming disintegrated and dissolved. When it is considered advisable to adopt a covering of grass, plants, shrubs, or trees, it should be remembered that some grasses, such as the couch-grass, are injurious to other plants. To equally cover a slope with sods of turf they should be cut to a regular form and depth of about 6 inches, and be firmly pressed into their places so as to produce an even smooth surface. A steep slope or any less than 1 to 1, if of considerable depth, will not give a sufficiently flat surface for grass to properly grow upon it.

In sandy and gravelly ground, as the particles may be considered insoluble in water and as possessing no cohesion, grass, if it will grow, having deep roots is to be preferred, even if passing through any earth that has been placed upon a slope, in order to counteract the want of cohesion by a greater hold in the soil and any peeling of the turf or tufts forming or becoming detached, rolling down and leaving an unbared surface. Bent-grass will usually grow on most sandy soils, and a shrub known on the north-east coast of England as the sea buckthorn also rapidly vegetates, but marine grasses must be used in ground impregnated with brackish or sea water. To prevent light sand from being scattered by the wind or washed away a species of rush (Bot. Ammophila arundinacea) has been used in Holland and England, and is found to flourish in dry soils, and as it has spreading roots which often grow to a length of 20 feet or more it binds the grains of sand. Experience in Holland has shown that grass upon sea embankments does not grow well or flourish at a steeper slope than about 6 to 1.

In countries where wind storms occur, or the soil is of so light a character that the passage of a train at considerable speed raises the earth and moves it as dust, a covering of fine grass indigenous to the country has been found to be a protection and necessary to economical maintenance. Planting the white basket willow, or withy, has been recommended as a means of preventing slips in loose soil, as the roots form a network and bind the earth and cover the surface with shrub growth. In damp ground the cuttings need not be planted as deeply as in dry earth, but of course they will not flourish in all soils. Osiers have also been adopted with good results in damp places.

Embankments of blown or drift sand, easily moved by the wind, have also been protected and maintained by branches of trees laid horizontally. They were placed in regular courses alternately with a stratum of sand, the ends projecting over and down the slopes, which were sown with indigenous plants that it is known will flourish upon it. Great difficulty had been experienced in maintaining the embankment until its surface was so shielded. Sandy dunes have also been prevented from slipping and being eroded and destroyed by the sea and wind by reducing them to the same line, level, and slope, by filling all openings or depressions, or, if curved, by trimming them to a regular exterior and the largest radius, and by a covering of fascine work in the exposed places, and in any less exposed situation by planting them with grasses that will grow upon such soil, the object being to remove all local obstruction to wind or wave force and cause equable resistance and protection. In France, the drifting of blown sea-sand into cuttings, which not only may interfere with the free traffic but also be a destructive agent to the rolling stock, has been prevented by planting pine trees upon the cess.

In embankments in loose soil and across valleys, especially if they are narrow and deep, repeated gusts of wind which have their maximum effect upon a flat surface at right angles to it, will produce a hammering action upon the windward side, and when the velocity is very great, upon the leeward portion there may even be a partial vacuum created by the wind rushing over the top of an embankment. An actual maximum momentary pressure of wind of 80 lbs. per square foot was registered at the Liverpool Observatory in 1869, equal to a hammering action of 0·55 lb. per square inch. It is here referred to in order to show that in exposed valleys the surface of all embankments of light soil should be covered so that the particles cannot be blown away at the top or from the slopes, for a force of 80 lbs. per square foot nearly equals the weight of a cubic foot of dry open sand which cannot be taken as weighing much more than 90 to 100 lbs.

If seed be sown upon a slope it is evident that it should be of uniform kind in order to create a general equable condition, and the growth of all rank vegetation should be prevented, for an unevenly protected or covered surface will promote a localization of disturbing agencies, and especially in treacherous ground when it has been covered, neither the surface nor the turf should be allowed to be broken.