The Deposition of an Embankment.—Preparation of the Ground upon which an Embankment has to be Deposited.—Methods of Procedure.—Consideration of Some of the Different Systems.—The Effect of the Height of a Tip and the Length of a Lead.—The Steam Navvy and Embankments.

With regard to the deposition of an embankment, one of the chief objects to be accomplished is to make it homogeneous and prevent the formation of solid layers, which may become detached along the line of stratification from less compact strata. Careless and intermittent tipping conducted in dry and wet weather and with different kinds of earth, probably in various conditions, will cause slips and subsidences; as also any local disturbance of the soil, which is always more porous and absorbent of water than when in its natural unexcavated state, although it may be free from water-pressure which may exist in a cutting: and particularly so when fresh-tipped, as then the pressure and strain upon it is at the maximum, its tendency to unstableness gradually decreasing as it becomes consolidated. In hard granular soils this is almost certainly the case, but consequent upon percolation of water into a mass deposited in a dry state that expands; it may not be so in aluminous or other earth having particles affected by moisture, for an additional or an unequal load after a settlement may cause further movement and irregular density. In countries where the ground becomes caked or parched during the dry season, the change to a wet or saturated condition affects them much more than in variable climates, as they are then generally in a moist state.

It is easy to decree that no embankment shall be made of any earth of a treacherous character, if the circumstances are known to conduce to instability, and to rigidly specify what shall be done and what shall not be done, but the exigencies of an undertaking may cause such stipulations to be impracticable. For instance, it may be found that no other earth may be available except that which is proscribed. No one with a knowledge of earthwork would desire to erect an embankment of considerable height of yellow or sandy clay, or excavate a deep cutting in such soil unless a considerable allowance be made to provide against contingencies; nevertheless, it has to be done, and will have to be done. The precisianism and delicate refinement of a specification replete with good intentions has, therefore, very frequently to be toned down in order to allow of the execution of work.

It is obvious the care that is bestowed upon a reservoir embankment, such as the damping, punning, rolling, mixing of the material, and raising it in layers, is not necessary in a railway or ordinary embankment; however, certain precautions should be observed and may be effected at a small cost, and it is well to remember that errors of construction will cause undue strain upon particular parts. A fruitful cause of a slip or subsidence in an embankment is the variation of the character and condition of the tipped earth. It seldom happens that the soil of a cutting of considerable extent is the same throughout in character and condition; and the earth in an embankment tipped at one end may be different to that deposited at the other, and therefore the point of contact of the two earths will probably be troublesome. When a seam of unstable soil occurs in a clay cutting, it should not be tipped but be run to spoil. Such layers are usually of small extent, yet in an embankment they may cause portions to run in rainy or frosty weather.

Side cuttings are a protection against slips in embankments when they are sufficiently far from the toe of the slope, as they form drainage channels, but the excavation being top soil is loose and porous, therefore, when the lower portion of an embankment is made from side cutting and the upper from a cutting, the firm material may be at the top and the more open earth at the base; consequently, if an embankment in soil of a doubtful character must be partly constructed from side cutting and partly from cutting, especial care is required in forming it so as to prevent slips and subsidence, and also, in excavating any ground for such purposes, it is advisable not to interfere with any ditches, or join them, as by concentration a stream may be created. In countries where land is of little value, contractor’s plant expensive, not to be obtained in the district, and carriage costly, embankments in most cases can be more cheaply and quickly deposited from side cutting than cutting, the excavation from the latter being run to spoil instead of tipped by waggons into the embankment, therefore, their erection by such means becomes imperative for reasons of economy.

In depositing embankments it is well to remember that experience has proved that materials uniform in size and homogeneous in character form the most compact and impenetrable masses. The great stability of breakwaters formed of materials of uniform size and the firmness of macadamized roads are proofs of this. The same rule applies to soils. It is the separation of the larger bodies from the smaller that causes a want of cohesiveness and weight-sustaining power.

To prevent embankments of little height as in “forming,” spreading, slipping or weathering, as they are usually constructed from side cutting and loose top soil, the sods over the site of the side cutting should be removed and a turf wall be made of them on each side of the formation, the excavation being deposited within them, when it will settle equally and become consolidated, the wall preventing it spreading and also saving expense in maintenance.

In an embankment of soft earth that weathers quickly, the tipped material should be allowed to take its natural slope, and then every effort should be made to prevent its equilibrium being destroyed.

Where heavy and sudden rainfall occurs the edges of the formation have been purposely tipped and maintained from 6 to 12 inches above the height of the centre, in order to prevent during construction gutters or eroded channels being formed upon the slopes, the central portion being drained and the water led away. However, aqueous action may not deleteriously affect earth when tipped into an embankment notwithstanding that the soil cannot be made in the same state as before excavation, and that deposited material is more open and subject to percolation than the solid unexcavated earth; on the other hand, although the percolation and degree of exposure to the atmosphere are greater, in some soils this may tend to drain and render them harder and more stable. In fact, special circumstances may alter the general behaviour of any earth, but the material forming an embankment should be regarded as in the same condition as if it had been exposed to meteorological influences, and it should be remembered that in railway, dock, or canal works the deposited earth is seldom uniform throughout, it being either soft at one place and hard at another, or intermixed in a manner unknown in nature, and that exceptionally difficult cases may occur which can hardly be treated by any particular method of procedure or even in the way which experience has proved to be effectual in several instances in similar soils under apparently like conditions.

The system of tipping has some influence upon the stability of an embankment and the prevention of slips and subsidences.

The first operation is the preparation of the ground upon which an embankment will be deposited. Solid hillocks or firm mounds that in any way tend to arrest motion should not be removed, but vegetable or bush growth should be destroyed, and where slips and subsidences are likely to occur, turf and all soft soapy matter should be stripped so that the deposited earth rests upon a sound stratum. In the dyke countries of North West Europe, before an embankment is deposited, care is taken to remove all trees and roots so as to effect a thorough connection between the ground and the earth forming the embankment.

It is well if the ground be ploughed, raked, or harrowed, so as to offer a rough and similar surface to the tipped material, and also to obviate any arrest of the percolation of water, which upon reaching a turf or smooth or less permeable surface, may form a water seam at the foot of an embankment. It may not be necessary to strip the turf for the full width of the base of an embankment, but only for some 20 feet from the toe of the slope on each side. As the earth is bared it should be covered in order not to expose it to the weather, for should the surface be coated with mud, water will accumulate, as it cannot drain away or evaporate as quickly as when the ground at the seat is laid bare; and also if the top soil be mere dust moisture will convert it into mud. When the surface of the ground is inclined it can be benched or bared, and it is open to question whether baring and harrowing the surface is not a better practice than benching, as it gives uniform support and increases friction and prevents a sliding surface, and is generally a quicker and cheaper method to adopt as the soil need not be so prepared for a greater depth than 6 to 9 inches. Benching may be the better system to adopt, in loose and non-cohesive earths which require support to prevent rolling, but it must be carefully made with the necessary slopes and inclination of cess to prevent a localization of water, or the ledges may slip and carry away the material deposited upon them. The ground at the seat of an embankment can also be prepared by ploughing over it and removing the disturbed earth with a scraper.

The drainage of the ground is most important in order that water cannot reach the seat of an embankment. Many systems are referred to in Chapter IV., and others are herein named. A complete system of drains may be necessary, but much depends upon the character and sound condition of the earth and whether the slopes and formation are protected. Simply covering the whole area of the seat, or that upon which the base of the slopes rests, with broken stone obtained from the cuttings may suffice, the stone being so deposited that water flows from the centre to the sides, with a rough drain here and there, or central, diagonal, and lateral drains may be required; but the pressure of the deposited earth frequently causes land springs to issue in the seat of an embankment, the existence of which was not conjectured, hence the effect of tipping should be watched.

Slips and subsidences are induced in embankments by the material being deposited when the soil is dry, and in every state of humidity from dampness to saturation; consequently the earth settles unequally, is denser in parts, and a stratification is caused as if it were composed of dissimilar earths. Should any soil be deposited in the same condition throughout, and be punned or rammed in layers, it becomes as near its original texture as is possible to quickly attain; but in depositing railway embankments, which cannot be punned or rammed, the perfect incorporation of any soil is not effected by the simple operation of tipping from a spurn head, and stratification of earth is disadvantageous, inasmuch as the layers may not be homogeneous, and in railway embankments are, with a few exceptions, in a different state owing to changes in the character of the soil, the effects of weather, mode of excavation, and in a lesser degree the height and length of the lead or distance from the cutting to the tip; the stratification, therefore, becomes varied and irregular, and depressions are formed which may hold water, and none should be permitted to accumulate in or upon a freshly tipped embankment, for then it is in its most permeable condition, and in retentive soil especially the existence of a wet place may cause it to become so deteriorated that it cannot be made stable without being drained, and when the earth is liable to become alternately very wet and dry, every means must be taken to lessen the deleterious effects.

Provided an embankment is tipped of one kind of earth, although it will be in a more open state than the solid unexcavated ground, the slopes given to it are flatter than the angle of repose, the weight brought upon it does not exceed the safe load, that it is deposited upon level ground in regular and equal particles and the soil of ordinary character, there is no reason that it should slip or subside unequally if it be not undermined by aqueous action. Under these conditions settlement would be equal, or nearly so, and as all deep embankments must necessarily subside from about 10 to 40 per centum, and in extreme cases one-half, the object to be gained is to cause the settlement to be even and regular towards the centre. Benching the ground so as to prevent a movement of the toe, does not aid equal settlement nor keep the portion of an embankment above the level of the ground from spreading, except from the resistance offered by the cohesion and friction of the soil; whereas, if the ground slopes inwards towards the centre the earth has a tendency to rest and come together, and therefore not to slip upon the slopes. This system is subsequently referred to.

All saturated or wet earth should, if practicable, be run to spoil, but it is not easily effected, as it cannot well be filled into one waggon of a set, for it may be present throughout the whole surface of a cutting, nor in variable weather can operations be conveniently suspended until the exposed faces of a cutting are dry, but means are generally available by which any serious deterioration may be lessened.

No turf, mould, mud, peat-moss, soft pasty earth, frozen soil, or snow, should be deposited in an embankment, but only firm earth; and no solid lumps of large size intermixed with shovelled material unless they are broken up and trimmed when tipped, as the absorption of water will be greater in the earth having the smaller particles; and should it happen that several waggon-loads of such material are tipped and are succeeded by others containing large lumps, an embankment cannot be a homogeneous mass, but will consist of more or less consolidated portions in a comparatively dry state, and others which are less impervious and therefore more subject to the effects of water and settlement. All snow or frozen soil or muddy earth should be cleared away from the spurn head, and if it can be avoided no material should be tipped in bad weather.

With respect to the loosening of the soil by the process of tipping, taking into consideration that railway embankments are almost invariably formed in masses of 2, 3, or 4 cubic yards, according to the capacity of a waggon, it is obvious almost the whole bulk has been disturbed, and that the condition of the solid earth in a cutting is not maintained, for the soil in addition to being unbound by the process of excavation, casting into waggons, and shaking during transition, is ejected with force down the slope of the tip, the impetus having to be dispersed. It is certain the size of earth waggons cannot conveniently be much increased, and that little would be gained if a cubic yard or two were added to their contents; and that the higher an embankment the greater the velocity of the soil down the slope of the tip and the loosening action which causes the earth to be lighter and to take a flatter slope than when deposited in layers from a moderate height, also the greater the length of the lead, the greater the vibration and agitation. In the case of certain soils the particles of which become soft or dissolved when in a wet state, such as sandy clay, loamy soil, and some varieties of clay, vide Chapter II., the effect of a long lead or even a short one will be that the more solid portion of the contents of a waggon will settle, leaving the loose or mud at the top, and when a waggon is tipped the loose top “slurry” will roll down the tip almost as a fluid and proceed beyond the spurn head, thus making a wet sliding surface, the bottom toil frequently remaining in the waggons and requiring to be excavated from it. Nothing can be done in such a case but wait until the earth has had time to dry, or preferably the wet mud should be run to spoil. In fact a long lead, especially when aided by deposition from a considerable height, will cause friable soil possessing particles readily impaired or dissolved in water to become dust when in a dry state, or mud if saturated.

The gradients and leads chiefly determine the manner in which earthwork can be economically executed, and are, in great measure, governed by the configuration and roads of the country; therefore, it will usually happen that the length of the lead cannot be reduced, and the only resource is to lessen the height of the tip, and, consequently, the momentum of the earth down the slope, and to reduce the inclination of the temporary road at the tip head to that sufficient to cause the contents of a waggon to be freely ejected and no more. This cannot be effected at the same cost as depositing an embankment to the full height, although in the case of an uphill lead a down gradient upon which the waggons will run freely by force of gravity can be made from the commencement of a cutting to the required width, the embankment being raised to its desired height without extra expense but assuming the case of an embankment 60 feet in height, and that in order to make it as dense and firm as possible it has to be tipped in three heights averaging 20 feet, it involves the laying, maintaining, and removal of three temporary roads instead of one, and cannot be erected without extra expenditure. The large majority of railway embankments have been deposited to the full height and width, and they stand; although the effect of tipping loose soil upon loose soil, which has not had time to consolidate, is at each addition to cause a movement of the surface. It is obvious the higher the tip the greater the disturbance, but it may be said in firm and hard granular earth, if always dry, this unbinding, although temporarily a disturbing element, may really tend to produce ultimate homogeneity and stability; on the other hand, in tipping dry clay from a considerable height it often separates and becomes loose and mere dust.

In an embankment of moderate height the day’s excavation from a cutting will increase its length several yards, and the material from each set of waggons will only be exposed to the weather for a short time. On the contrary, in a high embankment, the effect of the deposition of the contents of a set of waggons upon the tip head is hardly perceptible, and is, until the toe of the slope is approached, the superimposition of a thin layer of earth which a shower of rain can convert into mud or cause to be in a soft or disunited condition, especially in the case of soils having easily soluble particles. The deteriorating influence of high deposition may be judged from the closing of an embankment of considerable height, as it is often a tedious and somewhat anxious undertaking, as might be expected from excavated, shaken, and loosened earth being deposited upon soil in a similar condition, instead of upon the solid ground like the other portions of an embankment. Also water frequently percolates and trickles down the approaching slopes, and penetrates them and loosens the soil, whereas if one tip is proceeding, any surface waters may flow away upon the slope and the solid ground, and the earth has time to become in a similar state throughout. To lessen these disintegrating effects, if it can be done without interfering with the due progress of the works, it is well to allow one tip end to consolidate for some time, and to complete the closure of the embankment solely from the other, suitable means being adopted to effect a firm junction. Should closing from a single tip head be impracticable, the earth from the meeting point of the toe of the slopes of the tip heads should be similar in character and be deposited in the same condition; in any case additional care should be taken to adopt every reasonable precaution to secure equal consolidation. As evidence of the dissipation of earth at and near to the junction of two high tip heads may be stated that they require more material to close them than would appear to be necessary from a computation based upon measurements taken from cross sections. In non-granular earths few cases will occur in which high tipping is an advantage; one of the few is when loose rock and firm earth are deposited, then the rock having larger particles, and consequently being heavier, will roll to the foot of the tip before the smaller material, and so form a broken stone seat for the embankment. In granular earths, such as gravel or sand, the height of a tip head is not a matter requiring much careful attention, as the particles are not deleteriously affected by water. When they are of nearly equal size, an embankment will settle equally, but in gravelly sand the stone will separate to some extent during transit and the process of deposition will be at the base; a varying slope may then be assumed, and it may here be named that it is known when sand is differently deposited, although its appearance is unaltered, it will exert dissimilar thrusts. An embankment of little height may be deposited in bad weather without slipping or subsidence, but when a high embankment is similarly tipped the surface of the earth is in an unfit state, and operations should be suspended for a few days.

With regard to the systems of tipping and the prevention of slips, as a general rule an embankment should be deposited to the full width, for if it be not erected at one operation the earth may be of a different character in diverse conditions, and will be more exposed to the vicissitudes of weather; and in wide embankments, should the system be adopted of tipping two outer roads and one central road, the three tips should equally proceed in order that the earth may be in a similar state and be subject to equal exposure; or weathered surfaces will be created down which water will more easily percolate than through the solid mass. In the three tip system the two sides have a tendency to lean towards each other, and cause the greatest pressure to be upon the inner material during construction, and therefore the embankment is supposed to be more consolidated and less likely to slip; but, unless the conditions are exactly similar, the advantage of this arrangement is more fanciful than real, for should the inside slopes of the outer roads meet first, the earth deposited from the centre tip has not an equal distance to descend and, therefore, the mass has not been tipped from the same height, the looser material, temporarily or permanently, being upon the outer tips and the denser in the centre; whereas the slopes, being the most exposed, should be the more compact, and when they settle towards the centre without an outward movement at the foot, so much the better for the stability of the embankment. The pressure upon the seat is more regular when it is deposited to the full width, as weight is then not irregularly added, probably after some settlement. Instead of dividing the tips into two outer roads and one central road, and in that way endeavouring to obtain permanent stability, it is better to diminish the height the earth has to be cast, and to deposit it as far as practicable in lifts, and to the full width. In treacherous soils such as some of the clays, embankments that would not stand at a depth of 30 feet have been permanently stable when the seat was drained, and they were tipped in lifts of 15 to 20 feet in soft clay, and 20 to 25 feet in firmer clay, at double the height at which they previously slipped.

This method involves the expense of moving the temporary roads, but allows more time for subsidence, and the mass of an embankment is not so exposed to the weather; however, in a dry season, an embankment can often be tipped to the full height, whereas in wet or changeable weather it would not stand if so deposited, and two lifts may be required. Care should be taken that the width is always sufficient to receive the top lift. It is more in the direction of equally reducing the height the material has to be tipped, especially in treacherous soils, that solidity is to be attained, than in an attempt to consolidate earth by endeavouring to cause it to fall together by deposition from two or more separate parallel roads at the same level.

Many slips and failures of embankments have been caused by the central portion being deposited of one material, such as clay, and the slopes afterwards made of a different earth, as rock. The latter will then slip upon the greasy surface of the clay, the porous nature of the broken rock readily admitting water and air. In fact, a more flagrant example of effecting that which on no account should be done is not easy to imagine. In treacherous soil the system of tipping an embankment wider than that required from considerations of lateral and vertical settlement, and allowing it to stand at a steeper slope than its permanent angle of repose, and paring down the top unnecessary width is not to be commended, as the earth is then strained, and the lower portion is in the looser condition and the toe the weakest part: however, in the construction of a single line of railway with a narrow formation width, it is very convenient as affording room for roads for loaded and empty waggons, and in any soils other than aluminous and calcareous earths it may be done with impunity, provided all slimy and slippery surfaces are removed. No side tipping should be allowed in depositing a new embankment, and any addition of soil to a slope should be avoided as much as practicable, although in trimming some filling may be necessary, but the slopes should be rough trimmed by the bank-head men as the earth is deposited so as to prevent hollows and depressions, form a comparatively even and regular surface, cause the mass to be equally exposed to the weather, and prevent any lodgment of water. The form a tip head naturally assumes will afford some indication of the stability, for should it be regular and approach a semicircular shape it shows the earth is of a comparatively uniform character and is subsiding equally. When it is uneven and jagged, with streaks down the tip, it indicates unevenness of soil and condition.

Circumstances arise which necessitate a departure from any generally approved method, for frequently an embankment must be widened to provide for increased traffic, and tipping upon a consolidated surface must be effected. Chapter VIII. and this refer to such a condition of work. It sometimes occurs when a slip has taken place that only a narrow tip can be adopted, as the soil may not temporarily be able to bear the weight of a larger mass unless equally diffused; then there is no other reasonable course to pursue than to complete an embankment to the required width and slopes by dry side-filling, such as ashes, broken bricks, or other absorbent firm material regularly and carefully deposited. In order to prevent slips and subsidence and increase friction, when an embankment must be widened by tipping upon its side, the existing slope should be stripped of all turf or covering, but only so as to leave no uncovered surface exposed to the weather; and the bared earth should be made to present an even surface. When the slope is benched, care should be taken that the benchings do not localise water, and that it cannot percolate down the face of the old bank at its point of junction with the new; also in tipping an embankment upon sidelong ground or upon the slope of an old bank, the seat should be bared, particularly on the lower side, as the earth meets with no resistance from the ground except from friction; but upon the upper side, the material being tipped partly against the hill has a less distance to travel, and, therefore, the upper or lesser slope is not so loose as the lower or longer slope, which latter is the more likely to admit water, and requires a flatter inclination than the denser portion of the embankment. In the absence of other protective works, it is advisable to form on the lower side an earth wall or counterfort covered with turf at the toe of the slope in advance of the tip, and to make it of the hardest and largest material deposited and with a foundation upon bared soil, as it is the part most liable to injury. This is a simple precaution against slips, and will save its cost in lessening the expense of repairing and trimming the slopes.

As the steam navvy or excavator has now become necessary plant upon most large public works, and greatly accelerates the speed at which cuttings can be executed, always provided the excavation is not of a treacherous nature, and is in such quantity that if not so used it would have to be run to spoil, many narrow valleys in the near future will probably be wholly or nearly closed with an earthen embankment, thereby effecting an important saving of time and expense; or the number of spans of a viaduct will be lessened, and only made sufficient to cross a road or allow the required waterway for a river or the discharge of any drainage or surface waters. The tendency, therefore, will be to increase the height of embankments of earth; and in countries where the first cost of an iron, masonry, or brickwork viaduct is too great, and the use of timber rendered necessary, earth embankments, except in treacherous soil or upon soft ground, are to be preferred to wooden pile and trestle bridges or culverts, whose average life in America, which probably now possesses as many as all other countries, is from eight to ten years, and that of timber truss bridges nine to eleven years.

In addition to other recommendations, the steam navvy usually excavates the earth in small pieces of nearly uniform size, and as it will perform the work of many men it may be most useful in treacherous soil, as it may enable a cutting to be excavated in dry weather and be protected before the commencement of the wet season. The slopes should be excavated in such earth as soon as possible after the gullet has been removed so as to prevent a slip, and it may be necessary to close-sleeper or consolidate the ground in order that the weight of the machine can be supported upon soft ground.