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

by John Newman

In draining the site of an embankment in peat soil, it is advisable first to ascertain the depth to which trenches can be cut without the sides falling in. It generally varies from about 1 foot 6 inches to 3 feet. As the ground becomes firmer the drains can be deepened if required. The width should not be less than 2 feet 6 inches to 3 feet. Cross drains can be cut at every 20 to 40 feet according to the state of the soil. Peat usually drains freely at the surface when its level is not at or below the water-bearing line of the country. A successful method of thoroughly draining the surface of peat moorland is to first make a drain about 1 foot 6 inches in width and the depth of an ordinary spade, and leave it for a time until the top soil to that depth becomes firmer, then repeat the operation until a depth of 3 feet is obtained, the sides being excavated to a perpendicular face, the top turf being carefully cut and stacked upon the surface to dry. Along the centre of the bottom of the trench a small cut is made about 1 foot in depth and 6 inches in width, leaving a 6-inch ledge upon each side, then the dry stacked turf 1 foot 6 inches in width previously excavated, is placed upon the ledge, the grass downwards and the layers reversed in rotation to the order in which they were excavated. This system in moss peat land is cheaper and better than tile or rough stone drains, as they often settle unequally. The underneath drain will not close up provided the excavated turf is carefully deposited, and water will not disintegrate the peat. These drains should be cut about 20 feet apart.

A more extensive system may be necessary, and a comparatively large area may be required to be drained in order to obtain a firm foundation and to prevent breaking away of the ground. In such event the first drain on each side parallel to an embankment should be at a distance from the centre line of the formation at least equal to the bottom width of an embankment, and as deep as the soil will allow, so as to drain the land within the fences. A cut should also be made outside the fencing at a distance of not less than 10 feet from the inside drain, to catch the surface waters and prevent their flowing upon the enclosed land. Cross drains may be required at intervals of from 15 to 50 feet according to the character and depth of the bog. The sides of the open drains may require to be supported. Branches of trees and rough fascine work laid in them have been used to an extent so as not to interfere with the flow; and when a spoil or fence bank has to be deposited it should be made a reasonable distance from the drains or they may become closed.

Every effort should be directed to make the drainage of the same degree throughout the area; it is therefore necessary to know whether the water can be discharged into a channel or adjacent river, or if the bottom of the drains will be above the general water-discharge level of the district so as to prevent the ground being in the condition of a sponge always full of water and becoming a floating mass. When the peat soil is of little depth and rests upon sand or clay, drainage can be effected by side drains reaching to the underlying stratum, and the drains can be open and be filled with gravel to prevent the sides falling or the bottom rising. Few serious slips will take place in bog-land when the precautionary works named in this chapter are more or less executed; subsidence is then the chief difficulty. It cannot be entirely prevented, but it may be lessened. It is frequently very considerable, depending principally upon the depth, situation, and character of the deposit and the proportion of water in it. Deep bog-lands upon being drained have subsided as much as 4 to 5 feet the first year, and 10 to 12 feet the second, and in the worst situations they will continue to subside until the earth is sufficiently compact to support the insistent weight, and they have sunk as much as 30 feet before the ground became consolidated. Burnt ballast, fine gravel, or sandy gravel, is to be preferred as ballast upon peat embankments, as affording an even coating and tending to consolidate the earth, whereas broken rock ballast breaks up the surface, as the pieces are not uniform in size and are fragmentary. The cost of maintenance on bog or marsh-land is sure to be heavy for two or three years, but then the road will generally have become firm. The chief points in maintenance are to cause equal loading and to promote thorough drainage. Baulks laid under the sleepers, or the longitudinal system of permanent way, is not advantageous in soft soil, and even should a longitudinal bearer be placed upon the cross sleepers the road is difficult to lift or pack, and less timber will be required by a reduction of the distance between the cross sleepers. The rail joints should also be stiffened so as to make the depth of the undulations consequent upon the passage of a train as regular as possible, and the number of the sleepers can be increased with this object.

The sustaining power of peat moss, and peat which is sometimes found under a bed of gravel and upon a substratum of clay or marl; and bog-land varies greatly. As a rule high bog-land will bear the greater weight. The only reliable method of ascertaining its sustaining capabilities is by an actual test, and such experiments are the more requisite in unreliable soils such as mud and slake, alluvial deposits, peat and bog-land. It is known that some mossy peat and bog-land will not even resist the weight of a stone of ordinary size, but will upheave and shake upon very little pressure being applied. By draining and other consolidating operations such land may be rendered capable of sustaining a load if the weight be spread over a considerable area upon a platform of poles, timber, or fascine work, as it then as it were, floats upon the surface; but piling is not successful, as the piles disunite the particles, destroy the continuity of the layers and make a passage for water. The elasticity of bog-land is manifest even when a road-bed has become sufficiently firm and even-bearing for traffic, by the agitation of the water in any adjoining drains.

In low bog-land, peat-moss, or peat-land, it is important to ascertain whether the deposit extends to a considerable depth, as in Holland it has been found that some marshes simply rest upon a bed of water, being nothing but floating peat moss, although having as great a thickness as 20 feet. It has also been noticed that marsh-lands often rest upon peat mosses with a small layer or film of loam upon them, and that they are floating masses, although the water upon which they float may not be deep; its depth may be known by the insertion of a bar or boring-tool, for when it sinks suddenly it has probably reached the water, and until its downward movement stops it shows that water exists.

Peat bogs consist of decomposed mosses, grasses, aquatic plants, and mud. In mountainous districts when they are superimposed upon hard or non-weathering rock, such as quartz, they have little thickness; if upon clay rock, or any that decomposes under weather influences, they are usually of considerable depth. As a rule, the greater the specific gravity of peat, the firmer it is and also the darker. The top layers are generally the most fibrous and the driest, although spongy and containing vegetable matter, and are of a light brown colour, and of less specific gravity than the next deposit, which is of a darker brown tint, and is denser and more decomposed; the peat then becomes brown black or black in hue and approaches a coaly condition. Compact turf usually contains little water, but should it be of a mossy nature it is generally saturated. Wet bog, peat, or moorland may contain as much as 80 per cent. of water, and any soil having so large a proportion must necessarily subside and change upon being thoroughly and regularly drained, and have its bearing power increased in a ratio approximate to the percentage of water in it before draining and that after such operation has been effected, other conditions being similar. The depth and character of the top layers will indicate the best method of procedure, and whether it is advisable to excavate them and deposit an embankment upon the lower and firmer bed.

In Holland, where the peat is superimposed upon sandy soil, in order to prevent slips in embankments it is found advisable to excavate a trench about 15 to 20 feet in width and as wide as the formation width of a railway or road, until it reaches the subsoil, and to fill it with sand; and when an embankment has to be formed of very porous earth, to make a trench at the toe of each slope, and to fill it with sand so as to act as a counterfort. Peat and sand have been found to laterally spread the most, clay and sand less, and therefore the latter soil stands at a steeper slope.

On the South Austrian system of railways marsh-land has been made firm, and malaria-fever almost annihilated by depositing ashes over the earth. When they are mixed with the bog or swamp deposit, it is found that they cause it to become fertile.

Peaty soils and peat bogs, which latter it should always be remembered may be almost floating upon water and have a surface layer of vegetation and moss, must be differently treated to other soft soils, and thorough drainage may be economically impossible in deep deposits. In such a situation, as the peat is alone capable of bearing the pressure, it should not be disturbed or the particles disunited, for the continuity of the layers will then be destroyed, and probably cause an upward flow or other passage of water upon pressure being applied. This state is frequently met with where inland waters have existed, as in that case the peat moss covering has sunk, the depression depending upon the depth of the peat underneath it and the distance at which the firm ground is reached. It is, therefore, advisable to ascertain this, as then some idea of the probable subsidence may be known.

The system of a platform of poles, hurdles, or fascine-work is well known, and has been proved to be effectual in preventing any dangerous lateral or vertical movement. Sand, fine gravel, ashes, or other binding material, spread over the surface of peat consolidates it and assists in preventing movement, and it is well if such a covering extends for some distance in front and at the sides of an embankment to keep the ground from rising. Upon soft soil, a thick layer of clean sand, well wetted and consolidated, 5 feet or more in depth, has been used in lieu of piles for forming a bed upon which to deposit a Portland cement concrete foundation to receive heavy masonry piers of a bridge, and at much less expense and with greater expedition, and no slip or subsidence of the ground has occurred.