Notes upon the Preservation of the Foot of a Slope.—Various Methods of Covering and Supporting a Slope.—Protection from Snow-Drifts.—The Formation Width of Cuttings and Embankments.—The Deleterious Effects of Vibration.

The protection of the toe of a slope is of importance, as it is usually the most vulnerable and the weakest part. When the earth is of the same character, the quantity of water is usually greater at the lowest level than above it, and the stability of the soil in its vicinity is therefore lessened. In clay soils this softening action at the base may cause a slip and probably can only be prevented by reducing the percolation by surface drainage, which has been referred to in Chapter IV.; the matter under consideration being the counteraction of movement in the toe by other means than draining, although combined with it, such as by—

1. An impervious retaining wall with a pervious backing of ashes, gravel or sand, and ample weep-holes, at the foot of a slope, which, by preference, should be of Portland cement concrete, a more homogeneous material than brickwork or masonry, as it has no joints, and is particularly to be preferred for retaining walls in clay soils, as it approaches in a greater degree the condition of air-tightness and that of equal resistance.

2. By a pervious wall or counterfort of gravel, burnt ballast, hard chalk, rubble, strutted timber framework, or a covering of other firm material.

In both the preceding cases, to ensure stability and to prevent any protective works being pushed forward, the foundations must be below the formation or ground level. To lessen sliding action the foundation should incline at right angles to the face, and should have a batter either for the whole width or for a distance not less than about one-third to one-half of the bottom thickness in the case of low retaining walls. A wall having a steep batter upon each face, and therefore a wide base as compared with its area, causes the centre of gravity to be at a greater distance from the exposed face, and therefore the resistance to overturning is increased; but care must be taken that it does not slide forward. A foundation for such a wall upon clay or tenacious soil is to be preferred, provided there is no upheaval of the ground in front, but it should not be upon a thin stratum, or the latter may slide upon another.

In towns, or where land is of considerable value, the two sides of a cutting can be made to support each other, and one of the following principal methods may be adopted.

1. An invert under the line.

2. A heavy flat platform, arched on plan or solid throughout, under the permanent way or formation, to prevent a forward movement of the toe.

3. Overhead arches and iron struts at intervals to resist and reduce the pressure upon a wall.

In such a situation, should a tunnel or covered way be not required, any subsidence of the earth may cause the destruction of valuable property, and the erection of a retaining wall be imperatively necessary from a due consideration of prudential construction, and altogether regardless of the character or condition of the earth or its liability to become water-charged. In order to prevent movement of the earth different forms of support may be required. In some cases, owing to excessive pressure, a counterforted wall with an invert under the permanent way may be essential, or the invert may be flat and be arched on plan, thus supporting the toe of a retaining wall between the intervals, in combination with a vertical or inclined pilaster and a front counterfort system, and in addition flat overhead struts of iron or other material between the walls acting as an auxiliary support above the required traffic space may have to be adopted where the walls have to sustain considerable thrust, the thickness of the retaining wall being thus reduced; or a simple retaining wall may be sufficient. A non-jointed material as Portland cement concrete of equal character is to be preferred to a mass consisting of hard materials yet incapable of possessing a joint equal to their strength or durability.

With regard to the protection of the toe or lower portion, of a slope by means of a retaining wall, it may be the only effectual support in one case, and not succeed in an exposed country without a complete system of open or closed drains, and then its adoption may be superfluous. In the following few paragraphs an endeavour is made to indicate some situations in which retaining walls have or have not been completely successful when erected for such purpose.

First, it is most important that ample provision be made for draining the back of the walls, for if the egression of the surface waters be obstructed, they must accumulate and cause hydrostatic pressure, soften the lower portion of the earth, thereby failing to partly effect one of the objects of their erection, and cease to protect the surface; the probable result being that the wall is pushed forward, broken up, or overturned. The drainage must be regulated by the quantity and velocity of the flow, and ample weep-holes should be provided to prevent an accumulation of water at the back. In damp soil there should be one to about every three or four superficial yards, and an outlet at each wet place, or a wall may not stand. The wall should be backed with a filtering medium such as coarse gravel, hard ashes, ballast, and no retentive earth should be used. In clay earths a retaining wall should always have a dry porous backing, as it not only reduces any pressure due to a head of water, but also allows of the earth swelling without affecting the stability of the wall, as it probably would if the clay rested against the back of the wall.

Retaining or breast walls are particularly useful in loose soil having no cohesion, i.e., those of a sandy character or consisting of very small grains, and which upon saturation by water or by the action of its flow become in an unstable state or one of actual movement; in such, a case not only is it requisite to protect the surface of a slope, but support at the toe is indispensable. In cohesive soil, such as clay or clay marls, surface protection, combined with systematic and thorough drainage, may be all that is required to make a slope stable, and the erection of an impervious or solid high wall be unnecessary, always provided the earth cannot slide from being superimposed upon an inclined stratum. In fact, retaining walls in retentive soil have been found to induce a slip, because they neither drain the earth nor prevent the additional impregnation of water, and they have consequently been destroyed.

In countries where floods or very heavy and sudden rainfall, frost and snow quickly succeed, masonry or dry stone retaining walls at the foot of a slope soon become impaired, and require constant supervision and careful maintenance, and cannot be considered as economically or generally effectual; and should the earth settle, as they are comparatively solid they will not follow any subsidence of the surface; therefore cracks occur and water accumulates in the hollows, the slope has no longer uniform support, a localization of the egression of water is caused, owing to the fissures in the wall inducing a flow, and the wall becomes a cause of a slip instead of a protection against movement. Should such a wall bulge after it has been restored to the condition of being a continuous support, its forward movement may be arrested by the erection of counterforts with an inclined face in front from 10 to 30 feet apart according as weak places exist, having joints at right angles to the batter; and this is, perhaps, the cheapest and quickest remedy, but the slope must be carefully drained and the number of weep-holes be increased.

In order not to obstruct, but induce the through drainage of water, retaining walls to prevent slips have been erected, consisting of arches turned upon piers, the intervening space being dry walling which allows a free flow of water, the idea being to afford the necessary support without interfering with the drainage. In some cases, as it tends to condense the soil, weighting the toe of the slope may prevent movement, and on the side of a hill weight and mass, apart from slope protection or drainage, must be provided to arrest a slip or prevent further movement, either by means of a continuous wall, or by the addition of frequent counterforts, 5 or 6 feet in width, to an existing wall, if space permits, the back and front having a considerable batter: the centre of gravity of such a mass being low and the base large, overturning is improbable. Forward movement can be also guarded against by deep foundations, and as the earth will rest on the inside they can have a flatly inclined base sloping towards the cutting, as it will tend to prevent overturning, the depth of the foundations at the face being two or three times that at the back. Such a structure can hardly be called a wall, being really an extensive and massive concrete toe.

Perhaps generally the most economical and secure way of preventing movement is to erect a low Portland cement concrete wall at the toe of a slope with a considerable batter; or to cut a trench at formation level outside the line of the slope, and to shore it by means of old sleepers strutted at intervals when better and durable material cannot be readily obtained.

The chief object of a protection at the toe in clay soils is to prevent the bottom of a slope being softened by a lodgment of water, or fissured by heat or drought; this may be effected without a wall; but in the case of sand or fine granular earth cuttings liable to become quicksand, support at the base is absolutely necessary, and the best way to prevent movement may be to erect a breast retaining wall at the toe a few feet in height, say 3 to 5 feet, the backing being of dry porous material such as ashes, coarse gravel, or broken stone, increasing according to the depth or height of the earthwork and quantity of water, the thickness being more than sufficient to contain the drainage waters, and not less than 1 foot 6 inches, in order that it may act as a filter and drain the slope, &c., so as to lessen the percolation of water. Sheet-piling and a backing of rubble acting as a drain 2 or 3 feet square, or a strutted timber duct, both made out of old sleepers, have been used with this object in quicksand where water rapidly percolated to and accumulated at the base. Instability of the toe may also be prevented by covering it with layers of gravel, but one of the reasons that may cause a gravel counterfort to fail in loose porous soil is that sand or mould may be washed through the interstices in the stones forming the gravel; hence a more impermeable covering is to be recommended in such a case.

A toe of rammed coarse gravel or broken stone may supply the required support in the case of coarse and fine sand, when the latter is alone movable. Experiments have shown that sand rammed in layers of about 5 inches and earth mould 2 inches in thickness give the best results. The less the weight is increased by ramming the more solid the original earth. Brushwood or fascine work weighted with gravel, stone or broken bricks, and stone pitching can also be used for sandy soils, but a filtering layer must cover the sand, or the latter will be eroded. Counterforts of well-rammed natural earth may be sufficient provided the soil is firm and is made compact and uniform in texture. In otherwise stable soil, when it is known that a sand vein is the cause of a slip because of water percolating through it, it should be raked out as deeply as practicable if not too large, and the space be filled with stones, coarse gravel, or dry material forming an open drain for the water to issue without a flow of sand, thereby preventing any accumulation. When a stratum of shifting sand overlies beds of conglomerate and gravelly or clayey sand it will be necessary to support the lower portion of its slope by a wall. The wettest places in a slope should be noted and the surface be turfed or covered, and should the depth of the unstable sand exceed 5 or 6 feet, narrow benchings can be made 2 or 3 feet in width at every 5 or 6 feet of vertical height to divide the flow of water and prevent it following a continuous course. It also sometimes happens that two treacherous earths have a stratum of stable soil between them such as rock; in that event it is advisable to leave a cess between the bottom of the slope of the upper stratum of unstable soil and the underlying rock, so as to allow of weighting or other works to resist movement of the toe.

With respect to the adoption of open surface rubble drains upon the slopes, in Chapter IV., page 78, special reference is made to them as drains, and in Chapter VI., pages 111, 112, an objection is examined; but they are generally effective if cut at right angles to the formation and not diagonally or transversely. When a slip is repaired by replacing the slipped earth when dried and rammed between such drains, it should be determined whether it would be better to cover the slope or increase the number of drainage channels. A simpler remedy is the insertion of a dry rubble lining on the surface of the original ground upon which a slip has occurred, and the restoration of the slope by means of firm earth or rammed and dried slipped soil; but the system of rammed earth counterforts, although it has succeeded when effected during the dry season, the earth being wetted sufficiently to make it cohesive by ramming, will not succeed in wet or winter weather, as the soil cannot then be consolidated by ramming operations, for it will be overcharged with water from the effects of rain, frost, or snow. Fair or summer weather is required to ensure success.

Should it be considered advisable, in addition to the erection of rammed earth counterforts to a height of from one-third to one-half of that of an embankment, a porous layer or wall of broken stone can be inserted between the slipped material, which is not touched, so that the waters can filter away through it and the slope be restored; and the counterfort have a channel at its base, in order to prevent a lodgment of water, to drain the slipped earth and the embankment, and to prevent the counterfort becoming saturated. This method may be insufficient in any but comparatively firm soil, and it may be necessary to make a trench at the toe of the slope having its base 2 or 3 feet below the seat of the slip or firm ground, it being filled with rock chippings, shingle, gravel, burnt ballast, broken bricks, ashes, coarse clean sand, or other material having equable frictional stability and particles that may for earthwork purposes be considered as insoluble in water, and therefore that form stable masses and yet act as drains; and to drain the slopes by trenches varying in width according to the position and depth of cutting, the depth of a trench may be little or have to be such that it extends to the solid ground through the slip, or about 50 to 75 per cent. of its depth, which must be determined in each case, may be sufficient. Such dry stone counterforts may be required at distances from centre to centre of 15 to 66 feet, and are sometimes placed in the same right line and even through an embankment, and are connected with a drain sufficiently deep to prevent any lodgment of water at the centre of the base of the embankment or near the foot of a slope.

With regard to covering a slope of a cutting or embankment, its principal use is to lessen deterioration from meteorological action, to keep the ground underneath in an equable condition, to reduce percolation and make it uniform, to diminish the danger arising from cracks and fissures caused by heat, evaporation, or drought, to prevent the erosion of the surface by a flow of water or the production of unctuous surfaces, and yet not interfere with effectual drainage.

It is known that percolation is decreased by vegetation, and that it is less through turf than bare ground, but varies greatly according to the kind of covering, the season of the year, the regular or irregular distribution of the rainfall, and other causes. If the surface has to be covered with vegetation it should be close, uniform, and vigorous throughout the whole year, and nothing fulfils these conditions as well as grass or turf. The protecting value of turfing earthwork may be judged from an inspection of fortifications in which steep slopes, deep ditches, and perfect maintenance are a necessity. As a consequence of the binding and shielding of the earth by the roots of grasses and plants, and their protection of the surface, the slopes stand at a steeper inclination than they otherwise would, and the system of clinching the exposed faces of the parapets of earthwork with a layer of sods laid header and stretcher is found to be a great support.

In earth possessing considerable cohesion, such as the clays, provided the covering is uniform, grass having comparatively deep roots is not a particular advantage, but the closeness of the growth of the plant and the unimpaired maintenance of a complete cover is of importance, and the earth should be disturbed as little as possible so as to prevent any particles comprising the soil becoming disintegrated and dissolved. When it is considered advisable to adopt a covering of grass, plants, shrubs, or trees, it should be remembered that some grasses, such as the couch-grass, are injurious to other plants. To equally cover a slope with sods of turf they should be cut to a regular form and depth of about 6 inches, and be firmly pressed into their places so as to produce an even smooth surface. A steep slope or any less than 1 to 1, if of considerable depth, will not give a sufficiently flat surface for grass to properly grow upon it.

In sandy and gravelly ground, as the particles may be considered insoluble in water and as possessing no cohesion, grass, if it will grow, having deep roots is to be preferred, even if passing through any earth that has been placed upon a slope, in order to counteract the want of cohesion by a greater hold in the soil and any peeling of the turf or tufts forming or becoming detached, rolling down and leaving an unbared surface. Bent-grass will usually grow on most sandy soils, and a shrub known on the north-east coast of England as the sea buckthorn also rapidly vegetates, but marine grasses must be used in ground impregnated with brackish or sea water. To prevent light sand from being scattered by the wind or washed away a species of rush (Bot. Ammophila arundinacea) has been used in Holland and England, and is found to flourish in dry soils, and as it has spreading roots which often grow to a length of 20 feet or more it binds the grains of sand. Experience in Holland has shown that grass upon sea embankments does not grow well or flourish at a steeper slope than about 6 to 1.

In countries where wind storms occur, or the soil is of so light a character that the passage of a train at considerable speed raises the earth and moves it as dust, a covering of fine grass indigenous to the country has been found to be a protection and necessary to economical maintenance. Planting the white basket willow, or withy, has been recommended as a means of preventing slips in loose soil, as the roots form a network and bind the earth and cover the surface with shrub growth. In damp ground the cuttings need not be planted as deeply as in dry earth, but of course they will not flourish in all soils. Osiers have also been adopted with good results in damp places.

Embankments of blown or drift sand, easily moved by the wind, have also been protected and maintained by branches of trees laid horizontally. They were placed in regular courses alternately with a stratum of sand, the ends projecting over and down the slopes, which were sown with indigenous plants that it is known will flourish upon it. Great difficulty had been experienced in maintaining the embankment until its surface was so shielded. Sandy dunes have also been prevented from slipping and being eroded and destroyed by the sea and wind by reducing them to the same line, level, and slope, by filling all openings or depressions, or, if curved, by trimming them to a regular exterior and the largest radius, and by a covering of fascine work in the exposed places, and in any less exposed situation by planting them with grasses that will grow upon such soil, the object being to remove all local obstruction to wind or wave force and cause equable resistance and protection. In France, the drifting of blown sea-sand into cuttings, which not only may interfere with the free traffic but also be a destructive agent to the rolling stock, has been prevented by planting pine trees upon the cess.

In embankments in loose soil and across valleys, especially if they are narrow and deep, repeated gusts of wind which have their maximum effect upon a flat surface at right angles to it, will produce a hammering action upon the windward side, and when the velocity is very great, upon the leeward portion there may even be a partial vacuum created by the wind rushing over the top of an embankment. An actual maximum momentary pressure of wind of 80 lbs. per square foot was registered at the Liverpool Observatory in 1869, equal to a hammering action of 0·55 lb. per square inch. It is here referred to in order to show that in exposed valleys the surface of all embankments of light soil should be covered so that the particles cannot be blown away at the top or from the slopes, for a force of 80 lbs. per square foot nearly equals the weight of a cubic foot of dry open sand which cannot be taken as weighing much more than 90 to 100 lbs.

If seed be sown upon a slope it is evident that it should be of uniform kind in order to create a general equable condition, and the growth of all rank vegetation should be prevented, for an unevenly protected or covered surface will promote a localization of disturbing agencies, and especially in treacherous ground when it has been covered, neither the surface nor the turf should be allowed to be broken.

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:—

The resistances at 2 to 1 and 3 to 1 are practically the same.

Should it be determined to pitch the lower part only of a slope, in order to prevent slips, the pitching must continue to such a height that a flow of water down the slope is impossible, or it may become detached. If the pitching is not rough squared on the joints, but of various forms, it is preferable that the face having the largest area be laid downwards, smaller stones being carefully wedged in between the interstices. Any defective execution is usually followed by a falling of the stones, and should this happen the surface will be broken and erosion and slips will ensue. In laying the stones they should be so placed that if a few become removed those above or upon the sides will not be disturbed, and in loose soil no pitching should be laid until an embankment has had time to settle and consolidate, which the necessities of rapid execution may prevent, for however even the pitching may be when first laid, it will settle and become more or less uneven in such soil as sand or estuary slake, and every effort should be made to leave no hole or exposed surface, but to cause a continuous close covering. Fascine mattresses may have to be used in such a situation, as they will follow the contour of the slope. Should the slope be subject to considerable wave action, smooth surfaces offering the least frictional resistance, obviously aid the travel of a wave, which is undesirable, and provided the embankment is sufficiently strong a rougher surface is to be desired. Projecting stakes tend to subdivide a wave.

Should pitching be required to be placed partly upon the face of a cutting and partly upon the slope of an embankment, as in the case of a canal upon sidelong ground, care must be taken that it does not settle unequally, and provision should be made by setting it upon a pervious layer, so that any damming back of drainage waters may be prevented. The weight of pitching upon a slope affords a counter-pressure to that of any water in the slopes which may be trying to emerge. Chalk rubble, also hard chalk, or gravel has been used in lieu of stone pitching for protecting slopes against erosion.

With the exception of retaining walls, the preceding may be considered as the principal means, used separately or in reasonable combination, for covering the surface of a slope in a cutting or an embankment.

A frequent cause of instability of the slopes in countries having severe winters is the melting of unequal masses of snow, the result of drifts, and also from the thawing of a considerable snowfall. The chief preventive measures against snowdrifts and consequent protection to the slopes, although not a covering, may be stated to be as follows:—

1. Locate the line in a naturally sheltered position, which it is most improbable can be done throughout its length.

2. Adopt tunnels, covered or sheltered galleries where the district is subject to avalanches or snow-slips.

3. Prevent drifts by permanent or portable screens such as earth mounds, trees, hedges, fences, &c.

4. Obstruct drifts by having deep trenches some distance from the top of the slopes.

5. Avoid cuttings as much as possible, and make the line upon an embankment of such a height as to be above the depth of the drift snow in similar situations.

6. Construct the embankments from side cutting, instead of cutting, so that the trenches may catch the snow before it reaches the railway.

7. Make the slope much flatter than its angle of repose.

8. Increase the width of the formation so as to lessen the depth of a drift, and to enable a snow-plough to more readily discharge its excavation.

With regard to the preventive measures enumerated.

No. 1.—As a rule this can only be practised to a limited extent consequent upon traffic requirements and the configuration of the country.

No. 2.—Shelter-galleries are found to be generally effectual, but obviously are expensive; they may, however, be the only means to adopt, as any obstruction to a snow-slip is avoided. The slope of the top of a covered or shelter-gallery should not be steeper than that of the hill, so as to offer no impediment to the free passage of the snow.

No. 3.—Experience has proved that permanent screens are to be preferred to portable shields, and that ultimately they are the more economical. The excavated material, either from a cutting, drains or fence-ditches, is generally used for the earth mounds so as to save expense; their height, which is often from 7 to 10 feet, of course is governed by the greatest depth of the snowfall, degree of exposure and usual direction of storms, wind, and drift. Should these vary much, in an open air line it may be impracticable to entirely shut out or prevent drifts, but the protective works will lessen drifting and may reduce it to manageable proportions.

A simple earth dam is seldom sufficient, owing to the impelling power of the wind upon the snow-flakes and the considerable height at which it acts; therefore, fencing has been fixed in the top of the mound, or trees and shrubs planted, also old sleepers have been successful when simply closely driven into the surface of the original ground. A screen, consisting of one or more rows of trees or shrubs placed at a little distance from the top of the slope of a cutting, is that most generally preferred. It is made either double or single, as may be thought necessary. In Germany, hedges of fir trees similar to Christmas trees have been found to afford excellent protection. In France, pine trees planted upon the cess have effectually prevented serious snowdrifts.

In sloping ground, mounds or dams, screens, plantations of trees and hedges in very exposed places have been insufficient to prevent some drifting and accumulation of snow in a cutting, and a mound or fence has consequently been erected upon the plateau at the top of the hill a sufficiently safe distance from its face that snow-slips, which might be increased into avalanches, were prevented from reaching the slopes or formation. A drain should be cut upon the higher side of such a hill-dam or fence so as to carry away the snow-water upon a thaw, and prevent the earth becoming in an unstable condition.

No. 4.—This system is generally used when an earth mound is also adopted and it has to be deposited from side cutting. The trench, of course, should be on the side of the mound farthest from the main cutting. As a snow-catcher it is successful, especially in shallow embankments, for it not only retains snow that would otherwise heap, but drains an embankment.

No. 5.—This depends upon the configuration of the country, but as cuttings act as traps to catch snow, if possible they should be avoided; and particularly when of little depth, as it is found they quickly become choked, and want as much protection as deeper excavations, and from being more exposed to the cold air, the snow in them becomes caked and frozen, and requires breaking up before it can be removed. Embankments of a height a little above the ground, and the greatest depth of the uniform snowfall of the country, are to be preferred.

No. 6.—Generally approved, as not only affording an advanced catch-trench, but because it acts as a permanent drain.

No. 7.—This is a controverted system. Some engineers object to it as facilitating the deposition of snowdrifts; others approve, upon the ground that by flattening the slopes the snow can drift freely and will fall equally upon the formation. The balance of experienced opinion seems to be rather against the adoption of flattened slopes, unless they are made very flat, such as from 4 to 1 TO 10 to 1, at which latter slope snow it is found does not usually accumulate but passes on depositing only its general depth: and additional means of protection are afforded, and appears to indicate that the unaided system is only well adapted for countries in which winds of great force are generated, such as the “blizzards” of North America.

No. 8.—It is found that increased width of the formation of a cutting is an advantage, as it retards choking, facilitates clearing operations, and provides room for shovelling snow from the permanent way, whether effected by a snow-plough or by manual labour.

With regard to the formation width, i.e., the width of the bottom of a cutting or the top of an embankment, and the prevention of slips in earthwork, ample breadth is necessary in cuttings for the purposes of drainage, although less in a rock cutting than in that of ordinary soil, and the extent of the top of an embankment has some influence in the promotion of stability. The required width of the formation must be principally regulated by the character of the earth, the amount and suddenness of the rainfall, the height of an embankment or depth of a cutting, the degree of exposure, the exigencies of the traffic, and the required drainage. In wet cuttings the formation should be wider than in dry earth, and the side ditches should be made larger, especially when there is a steep gradient in a long cutting, in order to keep the formation and the permanent way in as dry a state as possible and aid traction, for the coefficient of friction of the wheels of a locomotive will then be greater than when the rails are damp and greasy. In cold climates the width of the formation is often increased so as to lessen the depth of snowdrifts, and to enable a snow-plough or men to more readily deposit the excavated snow and clear the track, as has been referred to in the immediately preceding pages; and in severe climates in wet places ample width in cuttings is found necessary, as drains frequently have to be cut in them as deep as 4 to 5 feet to afford a free flow for and to prevent an accumulation of water. In temperate climates the width can be much reduced. When the formation is narrow the simple percolation of water through the slopes of an embankment, unaided by any aqueous action caused by fissures, may gradually saturate the mass, and as the wider the formation the larger the cross-sectional area of an embankment, it follows that increased resisting power to deterioration is obtained by widening the formation, as there is more earth to absorb the water.

A train upon the permanent way will make a force act downwards until it meets with sufficient resistance to cause reaction. The direction of the resolution of these forces is towards a slope, therefore, the farther the surface of a slope is from the line of action and reaction, the greater is its distance from the disturbing force and the lateral resistance it receives from the mass. In the case of a solid rock foundation and a homogeneous embankment in the same state of consolidation throughout its mass, the direction of the forces might be accurately delineated, but as such a uniformly homogeneous and equable condition seldom exists in railway embankments, it cannot be absolutely said that the forces act throughout upon certain lines; however, it is advisable to ascertain the probable direction of the forces.

The allowance for lateral settlement should be liberal and be regulated by the nature of the earth, the height of an embankment, and other local conditions of situation and rainfall that affect earth, many of which are named in other chapters. It will vary greatly, and may be anything, from 5 per cent. to 100 per cent. additional width of formation. An addition of from 5 to 10 per cent. of the height of an embankment is usually sufficient, but in clay sand and such soils an embankment may be slowly washed away by rain until it has shrunk to half its required width. As an embankment settles or weathers the width of the formation should be maintained without having to steepen the slope, widen the top, or erect a wall upon the formation in order to hold the added earth. The additional formation width of embankments is also a provision against the effects which the “lurching” of an engine may cause by its weight temporarily acting upon one rail, and the pressure to be in the 4 feet 8½ inches gauge at a distance of about 2 feet 6 inches from the centre of the permanent way. For some distance upon each side of the point of junction of two high tip heads the top width should be increased, vide Chap. IX., and in sidelong ground it is advisable to widen an embankment more on the upper side than the lower, as slips seldom occur upon the higher side; and in the case of railways, should the lower slope move, the rails can be placed towards the hill, and perhaps away from the slipped portion of the embankment.

With respect to the deteriorating influence of vibration as regards the slope of a cutting or embankment, it is well established that vibration will cause movement in a retaining wall which would otherwise be stable, and that soils possessing considerable powers of cohesion are not so easily affected by vibration as those of a looser character consisting of particles having more or less rounded surfaces; but the action of water may produce seams in such earths as clay, and create a smooth greasy sliding surface upon which any reposing mass may but require the least disturbing force, on the principle that the least impact is sufficient to impart motion to the largest body; such as a man walking upon it or the tread of an animal, to unbalance the delicate state of equilibrium. This action may be gradual, continuous, and increasing, as the earth will always be subject to changes of weather. On the contrary, vibration in soils having particles insoluble in water, provided water does not dissolve any cementing material between them, may cause them to equally settle and become firmer by being pressed together than if they were not subject to such operation, and should the particles wedge by shaking and the slopes be sufficiently flat, vibratory motion may, under these circumstances, tend to consolidate certain earths in an embankment; but not so in the slopes of a cutting which vibration must disturb by reason of it agitating and loosening the surface and making it less dense. For instance, in sandy soils the surface friction on a cylinder, when sinking operations are not being prosecuted and when the material is being raised from the interior, is different; the latter resistance being from 20 to 25 per cent. less than the former, consequent upon the disturbance, and although fine, soft drift sand usually presents greater frictional resistance than firm sand, it obviously cannot be taken as equal to that of firm sand, as it is quickly dissipated.

The conditions of earth being so very diverse no rules can be deduced, for even the effects of such stupendous force as that of earthquake vibrations vary according to the nature of the ground, however, the weakening effects of vibration are undoubtedly very considerable. It is known that the lateral thrust of earth is thereby much augmented, and, therefore, that the strain upon the frictional resistance and the cohesion of the earth is increased; and experiments have shown that when a wall is nearly strained to the point of overturning, slight vibration will quickly destroy the equilibrium, thus demonstrating that it adds to the lateral pressure. An analysis of some reliable experiments proves this increase to usually range from 10 to 60 per cent., but it is evident that the practical effect of any increase may be very much greater than a mere computation of the increment, for it may supply the additional strain, however small, necessary to initiate a movement, hence the danger.

As collateral testimony to the important effects of vibration may be mentioned:—

A comparison of the coefficients of friction during motion and those at the commencement of movement or of repose.

The existence of a nearly vertical face generally assumed, when unshaken by artificial means, in the bared top earth in a quarry, shallow well, gravel or sand pit, pond, and even a river-bank or a cliff, which experience has proved could not be maintained when subject to vibration.

The fact that timber piles, which are chiefly supported by the frictional resistance of their surfaces, will not sustain a rolling, i.e., a vibratory, load equal to that of a fixed load, and also that the method of driving in soils easily disturbed, such as sand, is also considered as reducing the safe load according to the percussive action and frequent vibration caused by a pile-driver, whether worked by steam, hand, or by means of an explosive substance. As further proof may be named that in pile-driving, especially in open soil, piles continually driven penetrate the earth considerably quicker and easier than if driven at intervals, as the latter system allows the soil to settle round them and the loosening and friction destroying effects of vibration are lessened.

The experiments made by Mr. J. A. Longridge, M. Inst. C.E., for Mr. G. R. Stephenson, Past-President Inst. C.E., in Morecambe Bay, England, showed that by vibration the bearing power of driven timber piles was reduced to one-fourth or one-fifth of that when subject to a steady non-vibratory load.

Its deleterious effect on the structure of such a solid substance as iron, &c., particularly when it is loaded beyond its elastic limit.

It is generally agreed that a substance is broken sooner when a load is intermittingly imposed than when the same load is permanently placed upon a structure.

The experiments of Professor Stokes, 1849; M. Phillips, 1855; M. Renaudot, 1861; M. Bresse, 1866; and recently of Dr. Winkler and others, show that the increase of the intensity of strain consequent upon the dynamic effect of a suddenly-applied moving load may be as much as 33 per cent. more than that of the computed statical pressure.

The fact that in masonry piers of considerable height and small dimensions, as in piers of viaducts, the vibration caused by trains loosens the brickwork or masonry and necessitates frequent repairs.

The theory that the particles of all solid bodies may be in a state of continuous vibration and motion, though there may be no means of rendering their motion visible, has not been refuted by deductive reasoning; but, on the other hand, it is in accord with the theory that “motion communicates itself among material bodies, and is never lost; when it appears to be so, it in fact only passes from the moving body into other bodies which are at rest, or are endued with a less velocity, and at length it becomes insensible in consequence of its enormous diffusion. In fact, motion can only be destroyed by motion; resistances and friction disperse it, but do not destroy it.”

The laws of statics and dynamics are well established, and are fully described in many admirable works upon mechanical philosophy; so far as the subject of this book is concerned that which is required to be answered is the question. Have the deleterious effects of vibration upon earthwork in various conditions been determined so as to be of practical value?

Not by experiment upon a large scale, nor is it probable that they will be; but they have been deduced from experience, reasonable inference, and experiments on a small scale, as has been before mentioned.

The effect of vibration is usually more marked in cuttings than in embankments, although it may nearly approach when they are of little depth or height: because a train in a cutting is contained within the area excavated, whereas in an embankment it is without the area deposited. In a cutting vibration commences upon the formation level and the toe of the slopes, the latter the most vulnerable parts and those most strained. In an embankment it proceeds through the formation to the base and the toes of the slopes which are necessarily the most distant. On the other hand, the material in an embankment is generally in a looser and lighter condition, and therefore more inclined to move and to suffer from vibration. It may also be greater upon one side consequent upon the “lurching” of the engine and carriages.

Obviously, vibration is increased with the speed and weight of a train; probably a short, heavy train travelling at high speed causes a more deleterious effect than a long heavy train travelling at a slow speed: also the higher an embankment or the deeper a cutting the greater the area of the cross section. Assume a train weighs 100 tons, and the weight of the earth is 112 lbs a cubic foot, or 0·05 of a ton; it might be considered that the effects of vibration would be less as the areas increased. Consider the formation to be 18 feet in width and the slopes 1½ to 1, the respective areas of cross section would be as follows:—

Note.—For the purpose of a comparison of ratios it is not necessary to consider the length of the train.

A simple inspection of the above ratios would lead to a supposition that the effects of vibration would be no less than 3·210
0·165 = 19·6 times greater in a 10 feet than in a 60 feet embankment. Merely comparing the weight of a train with that of an embankment, and assuming that the results of vibration at the same rate of speed are so governed is incorrect, for the effect of vibration at the formation level is not regulated by the height of an embankment or the depth of a cutting. The weight of a train may bear a very small relation to that of the quantity of earth slipped, yet the soil may have gradually become in such a state of delicate equilibrium that at last the least vibration will destroy it, even a little of the top soil falling upon the slope; and it frequently occurs that a slip commences by the detachment of a few small lumps and increases until it becomes of serious dimensions; therefore, the area of a cutting or embankment cannot necessarily be considered as reducing vibration although the source of disturbance may be more distant; but, of course, the heavier the mass the greater the weight and speed required to cause the whole to vibrate.

Lighthouses and such exposed works being constantly subject to vibration, the experience gained through their behaviour may be considered as indicating the direction in which the stability of structures in analogous situations has to be sought. It is generally agreed that it favours weight and bulk, as they are unchangeable, and shows that although the form, execution, and the material may be perfect, a light fabric will gradually be deteriorated by constant tremor and vibratory motion, at length culminating in the loosening and separation of the parts.

Except from actual experiment in each case, it is impossible to determine the greatest weight of and the speed at which a train should be allowed to travel so as to prevent any deleterious effect from vibration, and the circumstances are so various that a practical rule cannot be deduced, except by assuming conditions from experience alone, which would so modify a formula as to make it show any desired result, and cause it to be regarded as too complaisant to be trusted. Perhaps the best test of the effects of vibration that any earthworks can receive is when a temporary railway is laid upon the formation or cess for the carriage of materials, and in a lesser degree a steam excavator, as the weight and vibratory action may show the weak places in a cutting, or so shake portions of an embankment that should the earth be unstable a slip will soon occur.