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

by John Newman

31. Trenches filled with stones or other hard permeable material across the base of an embankment.

32. Covering the toe of the slope of an embankment with sods or making a counterfort of turf.

33. Running to spoil all saturated earth, and suspending operations for a day or two after heavy and continuous rain or a fall of snow, or frost.

34. Filling any large fissures as they appear.

35. Weighting the earth so as to condense it.

36. Increasing the area of the base of an embankment according to the bearing power of the soil.

37. By the exclusion of all boulders, roots, turf, branches of trees, or bushes in forming an embankment.

38. By aiding consolidation and preventing separation at the junction of two embankments.

39. Tipping an embankment in such a way as to promote consolidation.

In subsequent chapters many protective and remedial measures are specifically named. Here reference is made to the more general principles. The purpose for which a cutting has been excavated, or an embankment deposited must be taken into consideration in providing protective works, for the surfaces may only be temporarily bared, as in trenchwork for walls, or be partly covered with water, as in canals, and entirely unsubmerged upon one side as in canal, reservoir, and reclamation embankments; or be fully exposed to meteorological influences as in railway and road cuttings and embankments. There can be no stereotyped system of operations for treating a slip, but experience indicates that a frequently successful initiatory method is to divide the earth into small portions, and to proceed to equally consolidate them. However, in the case of deep cuttings, especially when excavated in the side of a hill, it may be necessary to drive a heading beneath the formation and to connect it with a shaft upon the higher side, so as to tap the water-bearing soil, and to convey the water away to prevent it reaching the slopes; this may be considered as a slip requiring an exceptional remedy. Should a cutting be in moving ground or permeable soil of doubtful stability, such a system of wells and covered galleries, which are generally successful even in the worst soil, may be required. The wells should, if possible, be sunk a few feet into an impermeable stratum, their diameter being the least a man can excavate, to any size required, and their distance apart, say, from 30 to 60 feet, according to the quantity of water to be collected. They should be connected by drifts. Smaller intermediate wells can be made between the main wells. In order to be effectual such works must be carefully and uniformly constructed, or an accumulation of water will arise. When a slip is known to have occurred, simply from want of drainage, a sufficient remedy may be the removal of the slipped earth and the insertion of drains. An advantage of the loose counterfort system of drainage as compared with rigid and fixed drains, is that open drains will follow a slight subsidence of the earth, and yet maintain their efficiency, but care must be taken that they do not become choked. In shifting or doubtful soil all works should be quickly finished, and in sidelong ground it is best to commence drainage operations on the valley side so as to tap the water, as if they are begun on the hill-side they may, until through drainage is effected, form channels for the accumulation of water, and cause a slip. The repairs of a slip can be commenced at several places simultaneously if at short distances apart, such as 20 feet or so, and, as a rule, it is preferable in an embankment that the work proceeds towards the centre, and not from the central portion to the slopes. It is advisable to make ditches or galleries in short lengths, not only to disturb the ground as little as possible, but also to ensure perfect supervision, as if the work is not carefully and uniformly constructed, localization of water will ensue.