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

by John Newman

[1]. Vide Chap. VI., pp. [111], [112].

With respect to the material with which the trenches should be filled, a uniform substance having considerable power of absorption, and but few particles between the interstices, is to be preferred, as the trickling of water and vibration causes the smaller material to fall towards the bottom of the trench, which, therefore, may become partly choked, and free drainage be interrupted at the toe of the slope, the most vulnerable place. As sand is always present in unwashed gravel, it will gradually flow or fall to the base of the trench and will prevent equal drainage, but properly burnt clay, being a more uniform and fragmentary substance, is better than sandy gravel, as it is dry and porous; but much depends upon it being well burnt or it may weather. Ashes and chalk are excellent collectors of water and are usually of an even character, but ashes are better than chalk, as the latter material, unless very hard, is liable to become disintegrated by the action of the atmosphere, rain, and frost. The difficulty with respect to ashes is to obtain them in sufficient quantity, free from dust, and of the requisite size. When weight is required as well as drainage, burnt clay, gravel, or chalk is to be preferred to ashes. Burnt clay, although burned upon the site, is generally a more expensive material than gravel, but the amount of moisture in the clay will principally determine the quantity of fuel necessary to burn it, and therefore the cost. If shale is present, by thoroughly igniting it the expense of burning may be nearly reduced to that of lighting and turning over, as it will usually burn unaided. All trenches should drain into pipes placed below the formation or into open drains at a sufficient distance from the toe of a slope as not to deleteriously affect it, in order that the water may be controlled and gently conveyed to an outlet.

The chief objection to open drains is that all excavated trenches or inserted drains in the slopes destroy the cohesion of the earth and aid in detaching portions of the surface. If the cohesion and adhesion of the soil were the same under every condition, this would be a cogent reason against the system, but, as in earthwork, every degree of moisture from dampness to saturation may be attained, the cohesive power is a very variable quantity, apart from the effects of vibration; and also open drains undoubtedly do cause a slope to be drier, and moderate local humidity, and therefore increase the cohesion and general stability of the part drained; the only fear being that from inattention they may become choked; then they are dangerous, as they will permanently collect and retain water instead of temporarily retaining and gently guiding it. A careful consideration of the circumstances may much reduce this objection by indicating whether it is advisable to have only a few deep, or several small surface drains. Provided proper precautions are taken, experience indicates that filled-in trenches in the slopes are generally successful, and certainly are simpler and cheaper than a retaining wall at the toe of a slope. In pervious soil it may be economically impossible to drain the slopes unless they are divided into sections, and should the material in the trenches be well-packed and pressed down, it may even increase the friction between the separated portions.

In connection with the drainage of cuttings may be named that excavation should not be allowed to be cast upon the cess unless some distance from the edge of a slope, and only temporarily for purposes of ballasting and metalling; as such spoil-banks increase the load and localize the water to be drained.

A more complete system of drainage may be necessary than those previously named, consisting of a combination of wells, open, or holding filtering material; pipe-drains or filled-in trenches, wells with a pipe leading to a catchwater drain, &c., or other usual methods of land drainage. For instance, it may be discovered that water issues from a spring outside the fence or cutting; if so, in order to drain the slopes it may be necessary to sink a well to a stratum below the formation level so as to tap the spring, thereby preventing an exudation of water upon the slope; this is a better plan than drawing the water into the slope and then draining it.

When pervious earth overlies an impervious stratum, i.e., gravel or sand upon clay, rough-filled wells at intervals inside the fence extending 3 or 4 feet into the clay, with an outlet drain, may be required to prevent a flow of water upon the clay and a wetted surface upon which the gravel can slide; and it may be necessary to have a cess on the slope between the top of the clay and the bottom of the gravel or sand, with a catchwater drain upon it, particularly in a cutting in sidelong ground.

Should the soil be silty sand, or be charged with water, consequent upon the formation of the country, it may be impossible to drain a cutting without a complete system of wells, catchwater drains and pipes, and even then it may be difficult to separate the water from the earth. In building a drain-shaft it should be remembered that it may not only be subject to a compressive strain, but also to transverse strain and flexure from different pressures of the earth at various depths, especially when the soil is not the same throughout, and unequally damp.

When “boils” occur in sand-cuttings, perhaps the cheapest expedient is to place a shaft over the boil, weight the bottom sufficiently to prevent a movement of the sand, but to allow the water to escape, and make a discharge outlet after having ascertained its head level: vide Chapter XII. On no account should a spring be stopped, as such action will result in its diversion to some other place; but the water flowing from it should be guided and discharged. Weighting may arrest a slip in any sandy soil, also clay or any impervious material placed upon the sand, or sinking a well outside a cutting to a depth of some 5 feet below the bottom may effect a remedy by abstracting the surplus water, but care must be taken not to disturb the sand.

Slips in embankments frequently occur from the percolation of water through the formation to the slopes, and so to the toe, the lower portions become disintegrated by moisture and the effects of weather, and cause the upper parts to slide or move. To lessen percolation and to prevent an accumulation of water upon the formation, it is usual for its centre to be raised a few inches above the level of the top of the slope. This is undoubtedly a good practice, as it also tends to drain the ballast, but it may be nullified in time if the entire width of the formation be not covered with an impermeable layer or with ballast; for when an unprotected space remains between the toe of the slope of the ballast and the top of the slope of an embankment, water is liable to percolate through the cess and cause a slope to become wet and unstable; particularly so if the ballast is broken rock and has side walls instead of slopes, as then a depression will probably be made by the platelayers or signalmen walking upon the cess. In all cases where the material is treacherous or likely to slip, it is advisable to cover the top of embankments of considerable height throughout their width with ballast or some impervious soil, provided the permanent way is also properly drained. This is the simplest precaution to take respecting the preservation of the formation level or summit of an embankment. All grass, dirt, and refuse should be regularly removed from it and anything that obstructs free drainage. The nature of the ballast also affects the evenness of the surface of the formation, as if it consists of broken rock, the equal and regular packing of the sleepers is not so easily effected as with gravel ballast; the sleepers are frequently not uniformly supported throughout their length, the pressure upon the formation is localized, depressions are formed and water collected, and slips and subsidences in soils of a treacherous nature may be induced from this cause, as the equilibrium is soon disturbed. The formation should be so drained and constructed that water cannot percolate to or cause the surface to become soft and work up into or through the ballast, or a state of unsettlement will be produced by water soaking through the ballast to an embankment, and so saturating part of it and forcing out the upper portion of a slope. In all close granular ballast cross channels should be made to lead away the surface water. Transverse open tile-drains may be required leading to an impervious channel. Water has been known to percolate through a considerable depth of ballast when added to restore a sunken embankment, even through as much as 7 to 10 feet when two falling gradients induced a flow of the surface waters to one place. In certain situations it may be necessary should an embankment be of clay or treacherous soil when wet, to cover the formation with an impervious stratum to prevent percolation to the embankment, and to thoroughly and separately drain the ballast placed upon it.

At the base of an embankment a ditch should be cut upon the higher side, or both sides, as near as convenient to the fence. When in addition to the interception of any surface waters by an embankment the ground is very retentive of moisture, it may be necessary to drain the seat; with this object trenches can be excavated at intervals at right angles or obliquely to the centre line of the embankment, and be filled with some hard filtering substance, such as stone or gravel, so as to effect and control the discharge of the waters. Should this be too expensive a method to adopt, and always provided the surface waters are prevented from flowing or trickling into the base of an embankment, the ground might be excavated so as to equally incline downwards towards the centre, the level at that point being 1 foot to 2 feet below the toe of the slope on each side, according to the width of the base of the embankment; a small trench being cut in the centre and filled with stones, and covered at the top with brushwood or hurdles or other provision necessary to ensure it being permanently an effectual water channel, with occasional or other drains to lead the water to the nearest culvert or side ditch. When it is found that water passes over the surface of firm soil upon which an embankment is deposited, the water must be intercepted and led away; and should an embankment be of retentive earth, in order to tap the water that has flowed and percolated into it, and to restore the earth to its normal condition and prevent slips and subsidences, it may be necessary to sink shafts to a depth of a few feet below the seat of an embankment until the mass is drained.