In ordinary cases where slips may be expected, soiling with mould or top dressing, and sowing the slopes may be sufficient, and turfing in others of small extent; but in retentive soluble soil, a filtering layer under the turf may be required, so as to prevent spreading, particularly near the toe of a slope; however, in mountainous or hilly districts, experience seems to indicate that it is advisable to allow earthwork to become consolidated by the natural effect of rain and atmosphere before finally trimming or covering the slopes: on the contrary it may be advisable, especially in treacherous ground, to trim and cover with mould, sow, turf, plant, or cover the slopes as soon as possible after they are excavated or deposited, in order to protect soluble soil from rain, frost, or snowstorms consequent upon the wet season being near. Of course, when experience shows that slips are improbable, the slopes can remain bare and simply be trimmed, and a covering of grass be left to time to effect, and this is now the frequent practice. Should settlement be expected or be unavoidable, the face protection can be so made that it will not be disturbed or broken. In such a case a short, straight slope with a flat inclined berm or cess, the length of the slope being divided into three or four short continuous straight portions, has been adopted with success. Should sowing or planting the slopes be considered as too slow in producing a covering, experiments can be made by mixing available soils, or with one earth; and by exposing the mixture or earth to the deteriorating weather influences it will have to resist, the best protective covering to be readily obtained can be discovered.
Respecting the planting of saplings or shrubs, laws have been enacted to compel the planting of saplings and trees upon certain lands as a protection against landslips, and it seems to be generally acknowledged that trees, particularly when in plantations, are a protection, as they not only absorb moisture and bind the earth with their roots, but also lessen any flow of water down the steep sides of a cutting or embankment; and instances have been recorded in which trees have been shown to preserve the alluvial banks of rivers, as when they were felled the sides were eroded, or weakened, probably by the increase of moisture and exposure, with the result that the channel became widened. The systematic planting of live slips of poplar or willow has been found to effectually protect soft banks of rivers, washed by the stream, against weathering and erosion. Also in treacherous clay marly-soils, in which slips of earth were numerous, acacia trees have afforded a good protection, as their widely spreading roots conduced to hold the soil together, and their foliage and branches gently regulated and lessened the effects of rain and prevented quick infiltration into the earth. However, care must be taken that the roots do not open or strain the surface of the ground by force of the wind or otherwise, and increase and localize percolation; but they tend to prevent cracking and fissuring of the surface in clay and argillaceous earth, and form a protection against the effects of the sun’s rays and drought. Quick-growing trees should be selected having large and deep roots and abundant foliage, especially in non-cohesive earths. Acacia and birch trees appear to give satisfactory results. Unless in exceptional situations, such as to protect a cutting from drifts of snow, or where from local experience they are proved to act upon the earth as a holdfast, it is questionable whether the indiscriminate and non-systematic planting of saplings or bushes is not more likely to aid disruption than promote stability, and, as a rule, other and less expensive means of protecting a slope are to be preferred unless a uniform covering by trees or shrubs is practicable, whether over a small or large unstable area. Isolated trees should not be allowed within the fences of a cutting, although if just outside the toe of the slope of an embankment the roots may serve as a buttress, and therefore be beneficial. Although in some dyke embankments in North Europe no trees or plants are allowed to grow upon them, so that any deterioration of the mass maybe clearly and quickly apparent; in Holland the defensive covering varies according to the character of the earth, fascines, wattling, sodding, a gravel coating, benching at the top of a slope and planting it with reeds to protect an embankment from erosion and wash caused by traffic; pitching, plank facing, and sheet planking at the toe, when the ground is very soft, have all been used with success. On silty land willows generally grow rapidly, and when planted a little distance from the toe of the land slope in enclosure embankments they are found to protect the ground. In some situations it may be necessary to protect an embankment against boring by burrowing animals or crustacea. Clay and clay loams are soils especially liable to be burrowed. Usually as the quantity of sand increases, the boring decreases; a coating of hard ashes may afford the required protection: however, in the case of crustacea, local experience alone can indicate the best protection, probably nothing less than stone pitching may suffice, but as a general rule in this country, no precautions are necessary; in warmer regions it may be otherwise.
With reference to fissures in a slope which tend to produce slips as they allow water to trickle down them, which must either be absorbed by the earth or find an outlet; a slippery surface is thus created and the tenacity and continuity of the soil impaired or destroyed; separation also takes place in non-homogeneous earths such as boulder clay and in embankments formed of rock and earth tipped together. It is practically impossible to fill or pun every fissure that may appear in a cutting or an embankment, but as there are generally places where slips are more probable than others, it may be advantageous in treacherous soils to adopt a regular system of filling the fissures, especially before the wet season commences.
Coverings can also be made of a thin coating of burnt ballast, hard chalk, or gravel, which will reduce the number of cracks or crevices, or a mattress of fascine work can be used at the toe of the slope in submerged work, but care should be taken that the mattresses overlap and that there are no open places between them, or, instead of being a protection they will then be a source of danger by conducting water between the joints; for this reason, as with any other material having loose openings, it is advisable to use them over a continuous surface so that percolation may be uniform and not simply for weak places or for the purpose of repairing a slip in an earth embankment. The chief aim in fascine work is to thoroughly bind the work together so that it is of equal strength in all directions, and a little time after construction should be allowed before deposition in order that the material may settle, as an even surface is important. The most durable material should be used in making a fascine covering, willow being the best, or it may require constant renewing. Alder, aspen, and the best available brushwood are also employed, and straw and ordinary matting for shallow embankments, say up to 8 feet in height, which lasts only six to twelve months. The best time for cutting should be locally ascertained, and when they commence to deteriorate after being hewn, generally from three months to a year, depending upon the season in which they are cut, &c., &c. In England, thorn switches have been used in lengths of 5 to 6 feet, tied up with tarred rope in bundles having a diameter of about 1 foot, every endeavour being made to bind and interweave them. Care must be taken that the mattresses are well loaded or they will float; the loading should commence at the centre, and be equally continued in all directions, as that has been found to be the best method; they should be made to sink evenly if they have to be lowered through water, and they must be prevented from curling up at the edges. The props which fix them must not be too close to the border, or near the centre, as then the extremities will be torn away or bent upwards. The system of fascine work may be very useful for protecting sandy and soft foundations in such situations, to prevent slips in the slopes of an embankment and in providing a firm bed, and also for making training banks, groynes, and spurs, for correcting and directing a current in a desired channel, and to secure freedom from slips in a river-bank. Depending upon the degree of looseness of the sand or sandy bed, loaded fascine mattresses will assume a slope of from 1 to 1 TO 5 to 1 if allowed to sink in the bed, and after they have settled and reached the angle of repose which the action of water will effect; the large number of structures standing upon them for many years in considerable depths of water proves, when they are properly made, that they are to be trusted in comparatively unexposed situations.
A covering of stone pitching may be necessary, but in unsubmerged work it has some disadvantages for, having joints, it allows unequal penetration of water through them and impedes the equal discharge of water from the earth, although its weight is a recommendation as it opposes and may balance a pressure of water behind the slope. However, unless it is bedded upon a layer of soil of equal permeability, such as gravel laid upon a nearly impermeable bed of clay, which should always be mixed with sand to prevent it fissuring and bursting by heat or water, so as to convey the water that has percolated and also that which exudes through the slope; it may cause a localization of the flow, and, except in peculiar cases and provided the slopes are covered, there is no occasion for pitching if merely used to prevent slips and not erosion, as obviously weight and a secure protection may be obtained by other means and at less expense. For the protection of the slopes of rivers or canals or submerged work the case is different, as then pitching prevents erosion and may be the only secure preservative in exposed situations, although generally the most expensive. The pitching should rest upon a bed of permeable material, and this layer should have a power of suction and distribution more than equal to the quantity of water that may penetrate the joints of the pitching; there should be a bed of impermeable material next to the soil, and in exceptional cases even two to carry off any water that has percolated, so as to obviate any lodgment of water, due provision being made for the land drainage discharge.
Sir James Brunlees, Past-President Inst. C.E., found by experiment that at a slope of 2 to 1 pitching has the greatest resistance to extraction, i.e., it requires a greater effort to extract a brick at that slope. Taking the slope of 2 to 1 as unity, the relative resistances were found to be as follows:—
1 to 1 slope 0·71
2 to 1 slope 1·00
3 to 1 slope 0·97
4 to 1 slope 0·66