Sand mixed with vegetable matter, argillaceous or loamy sand, about 1½ to 1.
As the proportion of mould or clay in the sand becomes greater a flatter slope is necessary according to the nature of the earth with which it is incorporated, the degree of wetness, and also the exposure of the surface.
Clay loams require slopes from 1½ to 1 TO 3 to 1, and shifting sand when a current of water reaches it will become a quicksand, and not be stable even when horizontal, but if drained and the toe is secured it will usually stand at an inclination of from 3 to 1 TO 4 to 1. On the other hand, an embankment of hard, clean, angular sand, rammed but left bare, when exposed to tidal action with little wave disturbance, has reposed at 2 to 1 TO 2½ to 1 slopes.
Loamy soil and vegetable mould will, for any height not exceeding about 5 feet, stand nearly vertically for a reasonable time.
Clay.
With respect to cuttings and embankments in clay soils, perhaps no earth is more affected by water and air, or more difficult to treat, as it will expand if only exposed to the atmosphere and without contact with water, 6 inches being no unusual dimension to allow for expansion in tunnel-work. This property and its contraction upon drying alone make it an earth particularly liable to slip and induce fissures and cracks through which water can trickle, notwithstanding the surface of the clay may be almost impermeable. If clay could always be kept dry or in its natural condition it would be stable and free from slips; but this cannot be effected, for water is held in suspension in clay for a considerable period, its plastic nature preventing gravitation, and evaporation is known to be a very slow process; and as the same clay under different circumstances may stand nearly vertically or only at a very flat slope, its liability to constant change makes it very treacherous, and it should be classed as a most deceptive earth of a dangerously unstable and unsafe description, for it may be so hard as to nearly turn a pick, and yet water and air will rapidly cause its disintegration, but if weather influences can be prevented from reaching it when in such a hard compact state it will afford a firm foundation.
London clay in its natural condition usually contains about 10 per cent. of water. The more permeable the clay the more likely are slips to occur and the face to become soft, loose and disintegrated, slimy beds being thus produced which are difficult to prevent or remove; therefore a covering of close grass turf, or layer of burnt ballast, ashes, or chalk, upon any damp place or fissure after it has been filled is advantageous.
Solid blue clay, which generally requires the use of the pick, of the clays is, perhaps, the most stable, being almost impermeable if free from delaceration; but yellow and most other clays are unequal in texture, faults and breaks are frequently numerous, and water penetrating converts the surfaces and the mass into a muddy and semi-fluid condition resting only when horizontal, which has been painfully experienced in the crushing in, during construction, of some tunnels in the London district. The trickling of water down fissures forms a slimy and easy-sliding surface most difficult to treat or prevent, and so long as the natural contour of the ground does not offer resistance to movement, a slip may extend for a long distance, either in deep or shallow cuttings, and there may be considerable hydrostatic pressure.
The disruptions, variableness of character, and existence of fissures cause any but the most homogeneous clays to be treacherous and likely to slip, particularly the yellow and any laminated clays, as they allow water to enter by the veins which are usually frequent in the mass. Yellow clay has a greater tendency to crack upon drying than blue clay and does so much more quickly, hence its dangerous nature, and although a mass may be only damp, fissures will enable water to penetrate and reduce it to a state of instability; it is also not infrequently in a plastic state, having fissures and cavities full of water.
It should, however, not be forgotten that in the endeavour to prevent the deleterious effects of aqueous action upon clay soils that they may be over-drained, so that they become too dry, as then the clay will shrink, crack, and fissure; the chief aim should be to keep it always in a sufficiently moist state so as to obviate the formation of cracks and fissures, and at the same time cause it to be dry enough to be firm, and never in a pasty or pulpy condition; in other words, to maintain its natural state if one of stability, and prevent any excess of water penetrating it or reaching its surface.