Earthworks in or upon Sidelong Ground.—Some Insecure Conditions.—Precautionary Measures.—Embankments upon Soft Ground.—Embankments Composed of Soft Earth.—The Promotion of Stability and Consolidation.

With regard to earthworks in or upon sidelong ground, the configuration of the surface conduces to a movement of the hill-slope of a cutting, and the centre of gravity of an embankment is not in the middle, and in a narrow embankment may be outside the central portion, the tendency, therefore, is for the slope of an embankment on the upper or hill-side to assume a steeper inclination than that on the lower, as the earth is tipped against the inclined ground until the point is reached where a perpendicular erected upon the surface of the ground cuts the top of the inside slope. This slope, if protected from weathering, might be allowed to stand at the inclination at which it remains when tipped, and be simply trimmed and covered; but the toe of the slope on the lower or valley side is especially likely to slip, and therefore, every precaution should be taken to prevent movement. A retaining wall is a certain protection, benching the ground, weighting the foot of the slope, dividing it into benches about 10 to 15 feet apart, varying in width from 5 feet and increasing according to the depth; the benches having sufficient inclination to prevent an accumulation of water; counterforting the slopes in dry and mild weather with simple earth counterforts 5 to 12 feet in width at intervals, and in length about twice their height with a reasonable slope, the height being about half that of the embankment; a dry retaining wall may also be erected in temperate climates and in a comparatively unexposed situation: are all precautionary measures that can be adopted. But in the case of a cutting in drift or permeable soil upon rock or an impermeable surface liable to become wet, which not infrequently occurs, the conditions of equilibrium may easily be destroyed, and the slope upon the higher side may not stand at a less inclination than that of the hill. Obviously this slope cannot be obtained either in cuttings or embankments, therefore, provided the drift is of considerable depth and treacherous, means must be taken to prevent movement, and drains should be inserted so as to make the earth capable of standing at a steeper inclination than it naturally assumes; and in order to drain the ground at the seat of an embankment, a trench on the valley side filled with stones or supported by other means acting as a wall and drain can be made. Every precaution should be taken to prevent any accumulation of water in earthworks in sidelong ground, and to gently control them, and to promote this object it is well that lateral outlets be opposite to each other so as to afford through drainage and an unobstructed flow.

As drift soil upon the inclined surface of a rock may be held by friction, the least change may impair it sufficiently to cause movement, therefore it is advisable to have a narrow gullet or heading excavated in short lengths, followed pari passu with protective works, whether a retaining wall or not, so that the earth is weakened as little as possible by exposure and deprivation of its usual support; and there are many places in which, unless the system of excavating in short lengths be adopted, failure and slips must result.

In sidelong ground the formation is frequently partly in cutting and partly in embankment, if so, and the drain is impermeable and can be thoroughly relied upon, it may be advisable to make the formation incline towards the hill, so as to drain it, lessen, and perhaps prevent percolation into the semi-embankment or under its seat, for the soil being loosened by the process of excavation and deposition is in the mass more porous than in a cutting.

The chief aim of all drainage operations for the preservation of earthworks in sidelong ground is to prevent the hill waters guidelessly flowing into, upon, or under them, their accumulation, or any obstruction to their easy discharge; and to attain this object it is necessary to know the location of the sources of the supply, the quantity, and other particulars, vide Chap. IV. A proper system of drains must therefore be made upon the hill-side to gather and gently guide the waters, to lead them by channels across the formation and discharge them outside it, care being taken to protect and line the surface of the drains so that no leakage occurs or any erosion of the face. In countries where the rainfall is excessive or sudden at certain places or entirely, it may be sufficient to protect the slopes so as to allow of a flow over them, and particularly upon the valley slope of an embankment, and in a lesser degree upon the hill-slope of a cutting.

When the configuration of the country allows, it is advisable to avoid cuttings, and particularly embankments, in or upon drift soil lying upon an inclined surface of rock or impermeable soil, because it can so easily be disturbed, become unstable, and of the difficulty of restoring it to a state of equilibrium; in fact, this may be impossible, as vibration may cause it to move. In such a case the only means of preventing a slip is by erecting a wall at the toe of the slope, and as it is necessary that the foundations be in the rock beneath the surface earth, the simple operation of excavating may impel the drift down the hill. Also should an embankment be required upon drift soil its weight may be sufficient to cause the mass to slide and a diversion of a railway may be imperative, for the cost of a retaining wall, because of its height and the great pressure it would have to sustain, or be built to sustain, would prohibit the adoption of such a remedy. Therefore, an embankment is more to be feared than a cutting in such earth. Making benchings in the rock below the drift to resist movement of an embankment will most probably be impracticable, and the soil may be in such a delicate condition that a shock or very little additional weight may make it slip, and when once it has moved, its stability may not be permanently restored. Such a case may be considered as one of the worst that can be encountered, and certainly the simple removal of detached masses of rock that may slide upon the formation is easier to effect than to treat drift soil upon an inclined rock bed.

It is always well to remember that in sidelong ground a hill may be reposing at the steepest slope of stability, and when the earth is rock it may dip in the direction of a cutting or be imposed upon a perishable stratum, such as porous or inferior shale. Support upon the valley side is then absolutely necessary to restore its normal equilibrium, and every care should be taken to prevent water reaching the embankment or trickling under its base, and to control all water that may permeate through fissures in the rock face, especially should it be loosely bedded. Earthworks in such positions require to be regularly watched.

In forming an embankment upon sidelong ground every means should be adopted to increase the friction between the tipped material and the soil; consequently all turf or herbage should be removed in order that the embankment may be upon the bare earth, care being taken that no loose top mould or turf is deposited, but only the firm soil. It may be advisable to only tip earth excavated from the solid hill-side, and not top soil such as that obtained from side cutting, and unless in rock or firm soil to avoid partly side cutting and embankment.

Should an embankment have to be tipped upon rock, care must be taken to prevent the surface water flowing under its base, and to counteract sliding movement the ground should be benched and a drain made on the higher side; also in a deep cutting the slope or face should be stepped, or have a cess at about mid-height, or where desirable, to lessen the effect of the trickling of the surface water and to prevent a flow in a direct line, which might make the velocity of any leakage waters dangerous, especially after a rapid thaw. On a mid-cess a catchwater drain should be cut to intercept the surface water, which may not percolate but flow upon the slope, and, unless diverted and conducted to an outlet, may saturate or erode the bottom.

Before a cutting in drift soil upon the side of a hill can be drained, it may be necessary to sink a shaft upon the higher side some distance from the slope and construct a complete series of drains under the formation. Also should an embankment in sidelong ground be near to a river, the exposed river-bank face must be protected, and in any case, when a pier of a bridge has to be placed close to the edge of a rock, the face of the earth should be preserved by a substantial Portland cement concrete covering in order to prevent disintegration culminating in a slip or subsidence.

In setting out a line of railway or a canal care should be taken to avoid, if practicable, either cuttings or embankments in soil resting upon inclined rock, as slips are almost sure to occur. However, should it be unavoidable, their depth and height should be reduced to a minimum and, if possible, the formation be raised until the bare rock crops out, and this can frequently be effected within the limits of deviation. Also deep cuttings in clay or earth, having particles soon affected by weather, should be avoided, but they may be necessary; and should any have to be located upon a sharp curve, it is advisable to excavate the projecting spur on the convex side of the curve more than that required to obtain the formation width, as it is peculiarly exposed, and therefore more likely to slip. When cuttings are upon the escarpment of a clay hill the slope upon the valley side need not be so flat as that against the hill. An inclination of 3 or 4 to 1 may be necessary upon the hill-side, whereas a 1½ to 1 slope may be sufficient upon the other. When a slip may involve the destruction or injury of any adjacent buildings, the best plan is to erect a retaining wall at the foot of a slope, not less than about one-fourth of the height of an embankment or the depth of a cutting, ample provision being made for drainage and the expansion of the earth. It is a sure protection and support, and the consequences of a slip may be most serious in such a position. The protection of the slope and a breast wall may, however, be alone necessary upon the valley-side.

The construction of embankments upon soft soil is here referred to so far as regards lateral movement, or where vertical settlement causes such action by drawing asunder. Whenever the surface of the ground bulges or upheaves upon earth being tipped upon it or pressure applied, it shows that the disturbing harbingers of instability are in action and that its sustaining power has been reached, its surface ruptured, and any more soil placed thereon will only sink and displace the earth until it meets the solid ground beneath, or the material becomes sufficiently compressed and dense for it to sustain the load; such a condition of unrest generally proceeds from over-pressure and will loosen the soil, increase percolation, and aid disintegration. When it is known that soft, peaty, muddy, or silty earth overlies level solid ground, and it is certain it cannot escape, and can only be condensed by weighting until it will permanently bear the strain required, it can be built upon; although should there be more pressure at one place than another, or the character of the soil vary, in certain places be swampy and in others dry, the surrounding ground will probably rise where the weight is the least; therefore, increased bearing area of an embankment maybe necessary until the sustaining power of the earth is not exceeded, or slips and subsidences will occur, for the weight of an embankment in such a situation cannot be diminished by retaining walls, as stable foundations are not to be obtained. It is obvious the width of the formation cannot be reduced and therefore the only way of lessening the load upon the base per square foot, assuming the embankment to act as a mass, and of distributing its weight, is by flattening the slope and making a platform under the seat of an embankment. All earths, however, that are not hard rock subside upon being weighted, and in the case of tenacious clay soils, upon a considerable load being applied over a small area, a trifling contiguous uplifting of the ground will generally be noticed. If such rise of the soil does not exceed about one-sixth of the settlement of the clay under the load it may be disregarded as not likely to cause a slip or serious lateral movement.

When the foundation consists of a firm stratum of gravel, sand, or solid earth overlying soft soil such as quicksand or silt, i.e., mud and sand; or alluvial deposit, care must be taken that the latter is not forced up at a weak place by the additional weight of an embankment, and the firm stratum undermined, as then it will sink, fissure, and be unsafe. Any disturbance of the bed or water of an estuary or lake other than by the tides, currents, or wind is an indication that the earth beneath the firm soil is overloaded, and that subsidences and slips will ensue.

Before determining upon the necessary precautions that must be taken, the deposits should be thoroughly examined and tests made of the amount of soluble and insoluble material in a certain mass, as although the earth may have the appearance of being mud, it may contain a preponderating proportion of insoluble particles of sand, and its character may hardly be that of mud or silt. When a marsh rests upon clay it usually requires only reasonable care, but if upon mud or loose sand it may always be in a more or less unstable condition.

The chief means of consolidation in the construction of embankments of soft soil upon soft ground may be stated to be as follows:—

1. Draining, which must be deep, and should be effected before the earth embankment is deposited.

2. Weighting the ground outside the slopes with firm material so as to consolidate the soil by intermixture with it.

3. The formation of terraces of earth, and by allowing the ground to subside until movement is arrested, but there may be a porous seam of harder soil, such as gravel, under the soft ground, then care must be taken not to dam back the flow of the underground waters by the subsidence of the added earth or the whole embankment may slide, become softened and finally be swept away; therefore, through drainage must be maintained under an embankment.

4. Making the load as light as possible consistent with due consolidation.

5. Constructing an embankment in equal and regular layers of ashes or other dry, light, stable soil, and by drying the earth to be deposited by separation, wind, sun, or by burning.

To prevent slips in such situations, draining, although causing subsidence, should be executed as long as possible before the embankment is tipped, is more effectual and certain in its action than any system of lessening the load or of counterweighting the soil, which may thereby temporarily be brought to rest but afterwards give way; and any damming back of water may cause additional sinking, and change the condition of the earth from a state of dampness to that of saturation. It may be easy to calculate the weight per square foot that will have to be sustained by the ground from an embankment and the heaviest train, but to make the necessary allowance for the effects of vibration is not so readily computed. The ground can be weighted to ascertain the limit of its sustaining power. The test should extend for a period of several weeks to be thoroughly reliable, which time can seldom be afforded upon works; therefore a considerable excess of sustaining power over the load should be allowed.

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.

It is obvious the earth should be deposited in even layers, similarly to the general method of closing reclamation embankments from the bottom upwards; staging is, therefore, necessary for the waggons to run upon and discharge their contents, but piles should not be driven, as they disturb and upheave the earth and do not consolidate any soil. A successful system of depositing embankments of bog or peat earth is to thoroughly cut up, pun, and tread down the soil in order that it may not shrink in cakes. If an embankment subsides and must be raised, only light earth or material should be used. “Forming” or embankments of little height are usually made from the drain excavation, and when more earth is required the outer ditches are widened and deepened so that the fenced-in land is made as firm as possible upon which a railway or a road has to be maintained.

Fascine mattresses, to prevent slips and subsidence in an embankment in very soft ground, such as loose sand, are to be recommended, and experienced engineers have expressed their belief that they are the best means of procuring a firm foundation and securing it from scour in loose sandy and soft deposits; and their employment may become necessary when the earth is very wet, and the contour of the district renders effective drainage or consolidation of the soil difficult or impracticable. Should the ground after drainage be only damp, brushwood, or heather, so laid as to offer the firmest and most even bed may be sufficient; and if sand can be used with it, it is very advantageous; but the fascine or a more solid method of distributing weight may have to be adopted. Fascines should be laid over the whole base of an embankment and extend some little distance upon each side, beyond the toe of the slope, and be made stronger according to the load they have to support, which, of course, increases towards the centre of an embankment, where it is greatest, in order to prevent their breaking apart and becoming detached.

In peaty soils an embankment should be formed of light dry materials, be spread out, have equal bearing, and be of as little height as possible, and all hollows and depressions should be carefully filled. The worst state of peaty or bog soils upon which an embankment has to be tipped is when they are no better than a floating mass.

Where a stratum of sound firm soil, such as gravel, lies upon a soft bed of great depth, by increasing the bearing area upon the firm earth and not interfering with it, a stable foundation for an embankment may be obtained. In such a case a cutting should be avoided, for if care be taken not to impair or injure the firm top layer it may stand without serious subsidence, but under other conditions it would be unstable, and when a soft stratum is affected and its natural condition altered, it may be very difficult to restore it to a state of permanent equilibrium in consequence of it being in almost a constant state of mutation.