If not covered or protected at from 2 to 1 TO 3 to 1, but when in clay soil of a doubtful character and non-homogeneous, or boulder-clay, about 4 to 1.
2 to 1 TO 3 to 1 is the most general slope; but in marsh land, unless the surface is protected, they may not permanently stand at 5 to 1. Fascine-work may be the easiest and most convenient protection, or a gravel or turf-covering may suffice, and should new channels have to be created, the question to decide is, will it be cheaper to make the slopes sufficiently flat so that they can be left unprotected, or is it preferable to have a steep inclination, such as 1½ to 1, and to carefully cover and protect the surfaces, the probable cost of maintenance of the protecting cover being duly considered?
As a rule in homogeneous earth, such as clay, underground excavation is the most free from water, and therefore from slips; but the enormous force clay possesses upon swelling—and this disturbance of the soil may occur many times by the action of air and moisture—and its contraction upon drying, cause tunnels in clay to be risky undertakings. The chief precautions against failure are to allow plenty of space for the expansion of the clay, reduce the uncovered lengths to the least dimensions, have dry, yet hard and compressible, filling between the arch and the walls and the clay, use the strongest bricks, or material with great compressive strength for the lining, adopt a form giving as equal support as possible in every direction, i.e., one closely approaching the circular, leave plenty of weep-holes to prevent any accumulation of water, and thoroughly drain and gently conduct to the proper drainage channels all water as it exudes.
CHAPTER III.
The General Effect of a Slip in a Cutting or an Embankment.—Enumeration and Consideration of some Protective and Remedial Works.—Treatment of the Slipped Earth.
With regard to the effect of a slip the chief consideration is, will it be dangerous and prevent traffic or the unrestrained use of the work? A problem most difficult to solve. Upon railways experience seems to show that slips of earthwork in cuttings of a depth exceeding about 10 to 15 feet are more to be feared as likely to interfere and stop the traffic than slips in embankments, and particularly as, except on sidelong ground, slips in embankments seldom occur in which the whole of the formation is moved, or becomes too unstable for a slow train to pass over, and a temporary road can usually be maintained by timber trestles and baulks, or other usual means. When a slip happens in a cutting the fallen earth may cover the formation, and it is certain that it can move in no other direction. The permanent way may be entirely buried, and it may be undesirable to excavate the slipped earth until other remedial works have been completed, and therefore the traffic is stopped until they are effected. In cuttings of little depth where a 3 or 4 to 1 slope assumed by a cutting originally excavated to, say, 1½ to 1 slopes, will not touch the ballast, the serious consequences of a slip are reduced to a minimum, and may not interfere with the traffic, and can be remedied by the road-men on the section; but not so in the case of embankments. Consequently the depth of a cutting or height of an embankment must be regarded as a governing condition apart from other considerations hereinafter named.
The height or depth at which a slip in an embankment or cutting becomes disastrous cannot be ascertained by any rule, but knowledge of the flattest slope at which any particular earth has remained stable in the same state as the embankment or cutting under deliberation, will enable a close approximation to be determined of the point to which a slip in a cutting is likely to reach, and that to which an embankment will subside. For instance, a cutting 15 feet in depth, having originally slopes of 1½ to 1, can assume practically 2 to 1 slopes without covering the rails, and, similarly, a 10-feet cutting, a slightly flatter inclination than 2 to 1. Taking into consideration that for some little depth from the surface a cutting, except in loose or treacherous soil, will stand at a steeper slope than 2 to 1, perhaps, on the whole, apart from the formation of proper water-tables, an engineer is not justified in making expensive provision in cuttings, even in doubtful soil, unless upon an inclined bed towards the cutting, of a less depth than from 10 to 15 feet, as they can be more economically remedied after they occur, and meteorological influences have shown the location of the unstable portion. Nevertheless heavy slips have taken place even in cuttings of such a moderate depth as about 10 feet, particularly in clay earths, and it has been necessary to entirely suspend the traffic consequent upon a continuous mass of earth of shallow depth moving forward upon an unguentous stratum slightly inclined towards the formation, and completely blocking up the cutting; but this was an exceptional case.
There are many cuttings in all parts of the world under very different conditions of weather and climate, and in every conceivable variety of earth, in which nothing has been done to prevent movement, with the exception, perhaps, of a surface-drain inside the fence and a water-table, self-formed or otherwise, near the foot of the slope; and there are many cuttings and embankments without any precautionary works. Nevertheless they stand or require but little attention. This fact naturally leads to the question: when ought any works to be executed with the view of preventing movement in earthwork? Expenditure in precautionary and preservative works, where they are not required, and the serious consequences of a slip are reduced to a minimum, cannot be defended; nor can the absence of such preventive measures in treacherous earth or soil so placed as to induce instability, and where the effect of a slip may be temporarily or permanently disastrous, notwithstanding that economy in construction has now become the watch-word of railway-extension; as then the expense of restoration and maintenance will be very considerably increased and far exceed the comparatively small cost of initial protection, for public traffic may be stopped and injury caused to life and property.
The preceding and the following pages have been written in the hope that they may afford some assistance in arriving at a correct decision, with regard to the necessity of precautionary works, and with the view of calling to remembrance many of the principal points to be considered in order to remedy slips, a matter usually demanding prompt decision. With respect to the measures that should be adopted to prevent slips, and the works required when they have occurred, it would be a mistake to generalize from any successful application of one method of reparation, as it is necessary to consider each soil separately, and to discover the reason of a slip; for an attempt to arrest the forward movement of a large mass will be futile; the disturbing cause must be ascertained and removed, or so lessened and controlled as to obviate further motion or deterioration; sand or porous strata, which may emit water uniformly, must be treated differently to earth which sets free water in a particular place, as also the same soil if it should be in a dissimilar condition. The object of all such works is to support, maintain, and drain the earth and prevent any accumulation of water in the slopes and formation, so that movement is improbable; therefore, water flowing towards the slopes must be intercepted and led away before it has time to percolate; and the method of discharging it must be governed by the position and nature of the soil, the chief aim of draining operations being to cause the earth to be always in its most consolidated condition. Without obstruction to the drainage, a general preservative is to cover the surface, and protect it against the deleterious effects of rain, frost and thaw, particularly in the case of soils which disintegrate rapidly under the influence of weather.
Some of the means that can be adopted to prevent slips and subsidences in cuttings and embankments may be enumerated as follows.