There is no reason why the varying slope system should not be adopted in embankments of clay or soil having considerable powers of cohesion, as the expense of trimming the slopes is very little more than making them to a straight line. To trim a slope to an elliptical, parabolic, or cycloidal curve would be a needless refinement requiring a template; moreover, the surface is better when it consists of straight lines, provided that at the junction of any two inclinations the point of meeting is sufficiently rounded to prevent a lodgment of water. In cuttings in non-weathering tenacious soil the upper part might be left at a steep slope for 5 or 6 feet from the surface of the ground.
It is evident that the depth of a cutting or the height of an embankment affects the stability of the slopes, but some soils are so weak that an embankment of even little height will not be at rest until the toe of the slope is supported. If the slopes were not pressed out the earth in the central portion of an embankment would stand at any weight less than that which would crush it when in its weakest condition; therefore, as the load increases downwards it is a logical deduction that the slope should be flatter as it approaches the base. In slips the form generally assumed in soils having considerable cohesive power nearly approaches that of a parabolic curve, which shows that a straight slope is not the correct one in tenacious soil, and theory confirms it.
In the case of cuttings of considerable depth, apart from the question of the best form of slope, in order to lessen the velocity of the surface water and the extent of a slip, and cause supported weight upon the slopes, they can be divided by broad terraces or benchings, about 6 feet in width, and at vertical distances of 15 to 20 feet, upon which can be impermeable catchwater drains. It is important not to allow a flow in a straight line or nearly so, as the velocity of the water may erode the slope. If the nature of the earth and its resistance to erosion will allow, the benches or steps should be abrupt, in order to cause the greatest resistance and deviation from a direct discharge. The slopes of the catchwater drains may require to be covered in a flood district or one having a heavy rain or snowfall.—Vide Chapter IV.
In excavating a cutting in soil likely to slip, care should be taken that the surfaces are not strained by lumps being left upon them which are only retained by reason of the cohesion of the earth, as they will cause weak places less able to bear any pressure brought upon the slopes through the sides being deprived of continuity of support. Therefore, in such cases, the slopes should always be rough trimmed as quickly as possible after the gullet has been excavated. Also when the slope of a cutting is furrowed so that its surface consists of separate and unsupported masses of earth clinging to it, continuity of support is destroyed and the earth is more exposed to meteorological dissipation; and in non-cohesive soils such as sandy or gravelly earth, in which especially a straight and uncut surface is of importance, movement is incited. Therefore, when it is found necessary to insert open trenches in a slope, they should be at right angles to its foot, and the inserted material should be well packed so as to support the sides and to interfere as little as possible with the slope. Even in clay soils or any having considerable cohesion, trenches diagonally or transversely cut in the surface are generally inexpedient as disturbing and destroying the continuity of support of the surface and increasing its exposure to the action of the weather; and although they may be temporarily effectual as drains, such division of the inclined face cannot but induce a disunited condition which will at once be apparent should the trench become choked and it miscarry as a drain; and a temporary failure may so destroy the existing delicate equilibrium as to cause movement. Such trenches should be regarded with suspicion, as any stability caused by their draining or conducting away water may be effected at the cost of continuity of support, the deterioration of the resistance of the soil to weather, and the impairment of its frictional and cohesive properties. If placed upon a slope at right angles to the formation the preceding objections are removed.
It may become necessary to excavate or trim a slope to a steeper inclination than that which would otherwise be adopted and is considered to be its angle of repose, in order to widen a railway between the fences, or enlarge a station. Then, pre-eminently the question of the effects of a slip demand attention. The great majority of railway stations are located prior to the commencement of, or as the works progress, but additional accommodation which, for reasons of economy must proceed pari passu with the development of a district, is usually required some time after a railway has been opened for public traffic, and it may be imperatively necessary to confine the works within the boundaries of the land originally purchased. Consideration of the several chapters in this book will recall to the mind the chief points to be regarded. Fortunately, stations are seldom placed in deep cuttings or upon high embankments; but frequently when a station is opened, houses will be erected around it, thus causing any movement of the ground to be of serious moment and dangerous. Assuming the railway must be widened within the original fences, the position of the toe of the slope is fixed, and also that of the top of the slope. The questions then in the case of a cutting are principally:—
1. Will the slope be too precipitous for the earth to stand at one inclination?
2. Can it be made sufficiently steep for 5 or 6 feet from the top to obtain an inclination for the lower portion at which it will have permanent stability?
3. Can the earth be made to repose if the face is evenly protected under circumstances 1 and 2?
4. Is it necessary to erect a retaining wall to a distance a few feet from the original surface of the ground?
5. If a wall be necessary, what should be its lowest height consistent with the stability of the unsupported inclined earth above it?