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.
CHAPTER IX.
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.