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.