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

by John Newman

In the case of treacherous soil which circumstances compelled to be partly used in a reclamation embankment of moderate height, but which it was found would gradually become firm by compression, rough sheet piles with a plank at top have been inserted, giving the outline of the finished slope of the material to be afterwards tipped upon it, the piles and planking not being removed, and, therefore, affording the required temporary support before the earth became consolidated and stable by compression and time.

In Chapter IX. the deposition of embankments is referred to as it affects slips and subsidences in earthwork, but in an enclosure embankment an additional precaution is particularly necessary, namely, that immediately tipping from the ends increases the velocity of the flow of the outgoing or incoming tidal water through the opening, and consequently augments its scouring action; its deposition from a spurn head should be abandoned, and the embankment be uniformly raised from the base. The employment of cofferdams, piling and planking for effecting a closure is now generally discarded in favour of the horizontal system of equally raising the height of an embankment from its base; and is even to be preferred to fascines, unless the latter are merely used to distribute the weight over the base, to protect the surface of a slope, or form a shield against scour either temporarily or permanently.

In previous chapters the protection of a slope is examined; here some reference is made to the particular preservation of the slopes of an estuary or reclamation embankment.

When the protection afforded is not uniform care must be taken that although it makes one part secure it does not weaken another. In the case of river-banks in a soil that is in a delicate state of equilibrium, it may occur that soon after one portion has been protected, another is being scoured, whereas previously it was stable; therefore, to prevent localization of the erosive action, whether on the foreshore of an estuary, reclamation, or a river-bank, and consequent slips and subsidences, the covering should extend over a considerable length. In a sheltered position simply sodding the slopes may be effectual. Some other means of protection are a hard chalk or gravel counterfort founded a few feet below the ground at the toe of an embankment, and a covering of similar chalk or gravel upon the slope, should the soil be favourable. When an embankment of earth in an estuary or river is of firm soil and only requires to be made proof against wave action, stones may be simply deposited evenly upon a slope and so that they will not be washed out, and pitching be not required, as the rough face will tend to break up the waves; but where a simple covering is adopted, whether close or comparatively loose, the slope should be straight, as a concave form causing a recoil of the waves will in time damage or separate the face shield. A coating of clay about 2 feet in thickness, upon a slope with stakes driven into it, and large bushy boughs of trees fixed thereon with the tops downwards, is frequently used in India as a protective cover to a crumbling bank of a river, and to training spurs erected to prevent erosion and slips. Mattresses, fascine or wattled work, besides being expensive, will slide down a slope unless well secured to it, and therefore a constant strain is produced as in all stake-held coverings; it has also been observed by the experienced that although so largely and successfully used in Holland and on its coast, there is very little ground swell on the Dutch shores, and that in a very exposed situation, or where heavy ground swells exist, they may not answer, and may become disintegrated by the much greater weight and force of the sea; and this, notwithstanding the surface breakers of thin water and little mass broken up by the wind may produce more visible agitation.

To prevent a river-bank slipping, and also to maintain a channel in a muddy river, half-tide longitudinal training walls made of wattled work or fascines have been used, so as to cause the deposition of the suspended matter in the tidal water and to gradually restore the impaired slope and secure it from crumbling into the river. The stones brought down by heavy floods have also been used to maintain a river channel and protect its banks from slipping, the interstices gradually becoming filled with mud deposited by the water when the floods subside.

When sudden and unexpected scour of the slope or bed near an estuary embankment upon soft soil has to be immediately arrested to prevent a slip, gunny bags filled with sand afford a ready means of repairing any cavities, the interstices between the bags usually being filled rapidly. Material should be added as required and any concentration of the erosive currents should be avoided.

The required height has to be determined of an embankment in an estuary or the sea to prevent any flow over it or waves falling upon the inner slope; 4 to 5 feet above the highest water mark appears to be adopted in the lower reaches of the Thames and unexposed estuaries in England. In Holland 10 to 15 feet, depending upon the degree of exposure. It is of paramount importance to prevent any waves washing over the top, as damage and, perhaps, a breach may be caused thereby. The height of the highest known wave must therefore be ascertained.

Should the shore be sandy and loose, a characteristic of estuarine accumulations, although its usual bed may be preserved in any storm, when an embankment is erected the rapid and ceaseless process of wasting of the sand and loose soil by the recoil of the waves from the face may in time lay bare the toe of a steep slope and undermine it; for it has been found where the foundation was sand and a rubble mound, which should be so constructed that its interstices become filled in order to make it more solid and stable, was placed upon it, and the superstructure upon the mound, that the sea being resisted by a vertical wall recoiled and made the soil a quicksand, although the sand would be stable at its natural slope in still water. Obviously the less the action of the waves is impeded, the less the looseness of the sand. Experiments have shown that a slope of 1 to 1 will reflect waves, on a flatter slope they are broken. In such situations light structures, offering little resistance to the action of the waves and not causing an impediment to the current, should be adopted in preference to a massive or solid erection; but when an embankment is necessary, it should have a long sloping mound or foreshore upon which the waves will gradually become lessened and dispersed, the desired object being to prevent deep water close to the work. When a railway or road follows the shore, instead of erecting a retaining wall to protect an embankment, a preferable plan to adopt may be to have open trestle-work offering the least possible obstruction, and when the formation is at the base of a cliff of variable and doubtful soil it is the best construction, as the cliff is not touched, and slips and subsidences are avoided. Should the deposition of an artificial beach be considered necessary for the preservation of the foot of a cliff in addition to the trestle road, experience seems to indicate that the contour affording the most protection is one in which the slope has a flat terrace or cess at not above three-fourths of the vertical height, another short slope, and a nearly horizontal space some distance from the foot of the cliff; but a storm will straighten the face, and it may be impossible to economically maintain it; however, should such a slope be assumed it should not be disturbed. Vide Chapter VI. for information respecting slopes. When a sea or an estuary retaining wall is necessary in order to prevent the slipping of an embankment consisting of loose soil, an inner dwarf wall at the edge of the formation upon the land side should be erected so as to hold the embankment in a box, and not allow any spray or water passing over the retaining wall to erode the inner portion.

Deep water is generally required close to the work in railway piers or jetties to enable vessels to get alongside; a heavy and monolithic wall must consequently be erected; however, in the case of loose soil, the vertical system simply should not be used unless the foundation is thoroughly protected and below the reach or effect of wave action, and no re-entering or right angles should exist, as they increase the action of the waves. When a rubble mound is cast in and a vertical structure placed thereon, great care must be taken that there are no holes, except the natural interstices between the stones, and that they have a firm foundation and sink equally, or the random mound may give way and the superstructure will then necessarily follow.