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

by John Newman

Extensive slips in earthwork seldom occur during the excavation, or a short time after the completion of a cutting; on the other hand, movement in an embankment frequently happens during deposition. In the case of an embankment, time may cause the earth to become consolidated, but in a cutting the disintegrating and disturbing forces, and the combined action of air and water percolating until they force forward the earth, are usually gradual in their operation, and often require a year or two to cause a state of instability; in fact, the history of slips, with a few exceptions, in soils whose condition is very readily changed by water, indicates that serious movement in cuttings does not generally occur until a cycle or two of the seasons has elapsed, during which period meteorological influences, aided by vibration and other deteriorating operations, are slowly and regularly proceeding, until at length such a change in the general condition is caused that a slip happens, apparently from some sudden agency, whereas the stability of the earth has been gradually and surely wasting away for a long time; hence the importance of continual careful observation in cuttings even of moderate depth in doubtful soil.

In canals and works of a similar character constructed to contain water, if any movement or slip of earthwork takes place, it usually occurs within a short time of the water being admitted, and generally within a few months and seldom after so long a period as a year, the ground in a short time becoming consolidated, being exempt from severe vibration and many of the disturbing agencies present in railway cuttings and embankments.

In endeavouring to ascertain the probability of a slip occurring, not only should the superficial strata be considered, but also the original formation of the country; for instance, drift-soil, which is generally met with upon the surface of sloping rocks, may consist of various earths intermixed in endless variety, and in every conceivable shape, and is not necessarily produced by a weathering of the rock upon which it lies, for it may have been brought from a distance. In any case, drift-soil is the result of decomposition and disintegration, and from its nature is unreliable and ever subject to change, to slip, and to subside, and so are most of the glacial deposits and moraine found in mountainous countries; and whenever the contour of a district is irregular and has numerous clefts, soft and marshy places, valleys and hills, earthworks will require to be protected against slips; also, should a cutting be at the base of a cliff or hill, it will probably have to be excavated in drift deposit and, perhaps, in silt if below the water-level of adjacent sea or river, and the ground dips towards the natural outfall of the land-waters. Such drift-soil may be alternately dry and charged with water from the rocks above, especially if they are much fissured and water-bearing and permit easy percolation of water, and must always be in a state of mutability.

Rock.

With regard to slips in rock, or earth generally classed as rock, the unstratified or igneous rocks, although they are sometimes traversed by mineral veins and dykes, are the less likely to slip; but rocks liable to surface decomposition and disintegration, such as some varieties of basalt and clay-slate, which latter by atmospheric and aqueous action will partly return to its original state of being fine mud, thrown down from the metamorphic rocks, may change their condition and are likely to slip; and also limestone rock, which however resists the eroding action of water better than sandstone, may become separated by frost although its surface soon dries.

Simply knowing the general character of a rock without ascertaining the proportion, state of the different particles of which it is composed, and whether any metamorphic action has taken place, is not necessarily a reliable guide to its stability. In districts situated at a high level, rocks are usually less permeable than in low-lying lands, and the surface discharge is greater and quicker because of the increased rainfall, and less absorption and retention of water.

It should be noticed whether there are dips in the surface of rock, as they often contain unreliable material, such as pockets and pot-holes of clay, sand, mud, silt and detritus; and movement may be expected if it be carelessly tipped with the rock into an embankment. Rocks which oppose vegetation are usually hard and weather-resisting, and the faults and fissures local; but it is not so much the equal weathering of the face of rocks that is to be feared, as the presence and interspersion of seams, breaks and fissures, and it should always be borne in mind that the condition of a rock varies considerably—it may be sound in one place, and be fissured, disintegrated, and quickly weather in others—and that all laminated and fissile earths are liable to slip because of the percolation of water down veins and crevices.

The durability of a rock may be approximately known by a careful examination, commencing at the surface of the ground and proceeding until it is reached, the thickness and character of the different top soils being noted, and particularly whether the degradation is uniform; but rock, such as some sandstone, which allows water to ooze, permeate, or force a passage, is of doubtful stability. Weathering may be possible only upon the surface or may gradually extend downwards, and as it can hardly be called a slip, the point to ascertain is not so much that it is sound and weather-resisting, as to know that there is no chance of any portion becoming detached or sliding, through the cohesion of the joints being impaired or destroyed by water, frost, or other agencies; for in the case of rocks which show irregularities of stratification, much cleavage, or are separated by upheaval, or have synclinal and anticlinal folds, masses become detached along the line of cleavage and independently of the normal stratification, therefore fissures or faults, weak veins between masses of rock and crevices, or inclined beds through which water may flow and always be present; the direction and inclination of the dip of the strata, effects of weather upon the veins, and to know the weight upon sloping ground which the rock will bear without sliding down a hill are the main considerations; for rocks may be distorted, upraised, contorted and tilted at every angle, and even horizontal beds may repose upon the upturned edges of other strata.

An inclined water-bearing stratum between rock loosely bedded and inclined towards a cutting, unless drained and supported, will probably cause a slip consequent upon the action of water or frost; on the other hand, veins may alone hold the masses together, and, therefore, when they are affected the cementing medium is destroyed; however, inclined water seams are a frequent cause of slips, for where any water-bearing earth meets a closer and consequently less pervious stratum, damp surfaces are produced, and an unstable condition; consequently, mixtures of rock, clay, and sand, are usually troublesome. Also in a cutting in sidelong ground if a stratified rock dips parallel, or nearly so, to the slope of a hill, slips are probable, as it may slide towards the cutting. Similarly, in a cutting in drift or alluvial soil, or any that will quickly weather, resting upon rock, especially should it have a smooth bed inclining towards the formation, the superimposed earth will usually be unstable, and even the act of penetrating the top stratum, or the erection of a retaining wall or the weight of a small embankment upon it, may cause it to move; and when motion has commenced it is difficult to arrest it; and should water trickle upon the surface of the rock, it may cause the upper stratum to slide; also when water flows or remains upon rock having a superimposed bed of shale or clay, the top stratum may not remain at rest even though the surface of the rock may be nearly level and practically waterproof, and where rock beds overlie shale which is liable to become softened by time and water and to perish, particularly when the beds are twisted or contorted; as, for instance, limestone or sandstone upon shale or marly-shale, the latter becoming softened by the action of the atmosphere, water, or frost will form a sliding medium upon which the rock may move, or should shale overlie rock, as it frequently does, it may slide upon the hard surface. All alternate beds of shale or any softer earth than the rock, particularly broken shales when found mixed with sand and clay and the lias shales, and rock should be regarded as treacherous and liable to slip. Also some of the slate rocks, as they frequently have veins of limestone, &c., and as the latter decomposes it mixes with the clay and becomes of a marly character. Dark blue shale or indurated slaty clay is sometimes difficult to excavate, but when exposed to atmospheric and aqueous action it breaks into pieces and becomes little better than a treacherous clay. The cohesion of shale becomes less as it approaches a greasy clayey condition, and, therefore, one readily affected by water or air, and it may then not stand at a steeper slope than 3 to 1. Rock and shale, which may stand at a steep inclination provided the beds are horizontal, it has been found, do not permanently repose when they dip towards a cutting until as flat a slope as 2 to 1 is given; and where clay and shale beds in cuttings are present, a slope of 1½ to 1 has been insufficient, and they have not been stable till an inclination of 2 to 1 has been adopted.

As cuttings in rock are frequently in the side of a hill, the dip of the strata should be ascertained, and in the case of an unstratified rock, it should be known whether it is fissured or lies upon a solid and firm bed considerably below the level of a cutting, so that it may be prevented from movement. It should also be ascertained if the top stratum is a mere crust, such as a capping of conglomerate resting upon clay-rock, through which water may burst and cause it to separate, and sometimes the rock may be more solid in the valleys than upon the hill-side because of greater diluvial action, and induration caused by exposure.