6. In the case of a reclamation embankment, it being closed from the ends, and not by raising in layers, from the ground level.
7. Variation of the submerged area, and consequent change in the degree of exposure to deteriorating influences.
To sum up, the principal causes of slips in earthwork may be stated to be air, water, frost and thaw, over-pressure, and vibration; the chief agent both in cuttings and embankments being water, which forces forward the surface of the slopes and destroys the cohesion of the soil, and impairs its frictional resistance until the earth is unable to sustain the weight upon it; vibration aiding and completing the movement, as it not only tends to loosen the soil, but may disturb the equilibrium of earthwork which is almost moving, and only requires a slight shock to set it in motion: in fact, vibration is frequently the complementary agency that causes a slip, and is obviously felt most in loose soils; but if there should be fissures in earth of a tenacious character, or boulders in clay to disconnect it, the effect of vibration will be more serious in the latter case, as whole masses of earth may become detached, instead of an equal settlement proceeding as with soils, such as sand and gravel, which may become consolidated by shaking, owing to wedging of the particles, should the slopes be sufficiently flat to prevent lateral movement; but it may detach portions of the slopes in soil having little or no cohesion, and thus initiate a slip.
In the following chapters the chief causes of slips in cuttings and embankments are considered, together with others bearing upon a solution of the subject, which is so interwoven that it is impracticable to preserve a successive order, but an endeavour has been made to separately indicate the cause and some remedies that may be adopted: but before proceeding to particularise, it may be well to name a few dominant principles of the alignment of public works, which if duly regarded may tend to prevent slips of serious importance.
Consequent upon financial and other causes, an engineer is usually required to so quickly prepare the necessary parliamentary plans and sections of public works, more particularly for railways, that it is beyond the power of the most experienced to set out in a few hours the best line of railway, &c., across a country, giving due consideration to the parliamentary, constructional, economical working, district and through traffic, and financial requirements of the undertaking. There are, however, a few points which he may be able to regard respecting the stability of earthworks, some of which are now enumerated.
1. Avoid cuttings or embankments in drift soil in or upon the side of a hill.
2. Avoid all damming back or flow of the natural drainage waters, or heaping of snow by the erection of an embankment, especially in mountainous, hilly, or sidelong ground, and in an undrained district.
3. When any excavation is in the side of a hill, observe the natural configuration of the ground in the wettest part, and remember that the slope of a cutting may not stand unless at the same inclination, or the toe of the slope is supported by a massive retaining wall and extensive draining, and that any disturbance may cause it to require a flatter inclination.
4. Avoid river or stream diversions in earth of a very porous character; and should an embankment have to be erected near to a deep river having a steep bank, locate the line a sufficient distance from the edge that the slope of the river bank may be flattened when required, as an extraordinary flood may cause it to be unstable and to fall in, and in order to restore it to a condition of stability, it may be necessary to widen the river and reduce the angle of inclination of its banks.