Chalk.
As in most public works, with the except:on of tunnels, wells and mines, the chalk with which an engineer has to deal is surface chalk, or the top layers of that deposit known as the upper chalk, almost invariably containing much more water than the lower chalk, although it rises quicker in the lower beds, as it is under greater pressure, and which vary in hardness, purity, and solidity and may have frequent fissures and holes, with or without flints, and be anything from hard, compact chalk rock to mere marly calcareous earth; considerable judgment is required to successfully determine the slope of stability and the precautionary works that may be necessary to attain repose; for some of the upper beds soon weather, and being soft, friable, and fissured are permeable and liable to slip; in fact, the Oolitic series, as it consists of alternating bands of limestones and clays and occasionally sandstone, is frequently fissured and has loose joints and therefore requires to be carefully treated.
The range of the slope of permanent stability obviously depends upon the nature of the chalk, whether it is denuded or covered or mere loose-jointed strata, the effect ground and surface waters may have upon it, and also the position of the beds, and whether a cutting or embankment is on the side or the base of a hill, and consequently at the place where it is likely to be in a wet condition.
The Needle-rocks in the Isle of Wight and Beachy Head may be mentioned as familiar examples, showing that firm and comparatively pure chalk will stand practically perpendicular, even when much exposed, if pure and free from faults and homogeneous in texture; and in blocks with beds inclined away from a cutting it will permanently stand
Almost vertically TO ½ to 1, at a great height,
a slow regular crumbling of the surface or falling down of small fragments, which seldom produce serious movement in such material, being the only deleterious effect of weather influences.
As the chalk becomes broken and less evenly bedded,
about 1 to 1.
Loose, friable chalk in surface beds will often not permanently stand at a less slope than from
1 to 1 TO 1½ to 1,
according to the depth and degree of exposure. The most usual slopes being
½ to 1 TO 1 to 1.
Impure wet chalk and marly chalk will require a slope not less than
1½ to 1.
Much depends upon its freedom from faults, crevices, and pot-holes, as they hold water, and the surrounding soil may fall away, for water quickly passes in quantity through the fissures and crevices which are generally numerous in the upper chalk, especially at the bottom of a bed of flints which in consequence of their impermeability lessen the upward flow; but flint beds in soft chalk are an advantage, as they act as drains.
The affinity chalk has for water, which has been considered a reason for the absence of important rivers in that formation, as water does not flow away freely upon it, causes it to be readily affected by rain and disintegrated from the effects of the expansive and contracting action of frost and thaw; hence draining and covering the surface may be important, but care must be taken not to interfere with natural springs. This property of chalk, viz., its affinity for water, although a disturbing cause in earthwork in that formation is of value for covering or filling in open trenches, counterforts, or drains in other soils, as the chalk attracts water, and therefore dries the surface of other earths.
Anything that localizes the percolation or flow of water, or helps to make water seams, veins, fissures, and hollows, which are sometimes filled with sandy gravel, loam, and detritus readily admitting water, will tend to break the chalk into separate masses and cause it to become loose and unstable by the action of rain, frost and thaw, and vibration. Should flint beds occur in chalk, and they frequently do in the upper beds if they are horizontal or nearly so, much more water may be expected to flow along their bed, as it forms a water-passage, than when they are in inclined or vertical seams. As chalk absorbs much water, but does not readily exude it, although it may soon become dry upon the surface after rain, it is advisable to lessen percolation in order to prevent slips. It is known that the angle of friction of water in chalk will affect the flow and that the discharge varies greatly according to the character, fracture, and other conditions of the soil; for instance, it has been proved that a hydrostatic pressure due to a gradient of about 1 in 132 is required to enable water to pass through the chalk as found at Dover, whereas in the Hertfordshire beds much less is required, namely, that equal to a gradient of about 1 in 350 to 1 in 420. This is named as showing, even when unfissured, the varying perviousness and character of chalk, and that it cannot be treated as a material of even approximate consistency of texture. The power of capillary attraction of chalk has been proved to be great and the evaporation from the surface practically unlimited. These properties and its known affinity for water render it liable to constant change; also the particles of calcareous soils being affected by moisture and to a certain extent soluble, water will take up lime in them, and therefore they are treacherous earths and liable to slip and subside.
Professor Ansted has shown that a cubic foot of the upper chalk when dried will absorb 2½ gallons or 40 per cent. of its bulk of water; the lower chalk 2 gallons, or 33 per cent. of its bulk; and that the pores of a cubic foot of chalk are equal to 40 per cent. of the bulk, and are therefore equivalent to the area of a pipe about 9 inches in diameter.
Ordinary drainage will not remove the water, hence chalk is a difficult soil to treat successfully, and slips and subsidences may result in such large areas as the surface of cuttings and embankments simply through the difficulty of preventing it becoming saturated.
Another characteristic of chalk, which requires careful observation to prevent slips, is that water does not generally issue through a mass, or equally over a consider able area, but is discharged through fissures, and crevices, and flint beds; hence one of the chief means of preventing slips cannot be adopted, namely, to disallow a localization of the flow of any water; therefore, the disturbing element of water seams is in greater or less degree present in all chalk-earth that is not solid and homogeneous in texture. The flow from such water-veins or seams should not be interfered with, as any obstruction, and possibly diversion, which is likely to fail, will only result in the spring saturating the adjacent soil, and in its bursting out at another place. There is no safe remedy but to gently lead away the water, for where springs occur, either in chalk or rock, they will find the line of least resistance; consequently the waters of percolation will tend to flow to one place, and cause a spring.
Chalk is found in regularly stratified and separated masses, sometimes caused by beds of flints, and although the position of the layers may indicate their successive ages, age can hardly be taken as an absolute indication of the increased stability of chalk in earthwork. When overlaid with clay it is usually harder than when bare, probably owing to pressure, non-exposure to atmospheric influences, and to the absorbed water being of a different character, which has been proved by analyses. It is especially advisable in chalk soils to know the head level of water in the district, and to note if the bottom of a cutting is below the usual water-bearing line in the open wells, which may not necessarily be at the same depth; their average level being ascertained, an idea can be formed of the probability of springs bursting out, and according as the rainfall is excessive or not, so usually will be the flow.
If chalk beds incline across a valley, and have an impervious stratum of clay upon them, it has been found that the most water issues at or about the point where the impervious seam first overlies the chalk, i.e., at the edge of the basin, and the greater its depth, the less the flow; therefore, should a cutting be located at a place where this stratum is thinnest, more water from springs may be expected than at any point where the impervious layer is thicker.
It is also well to remember that the line of water-flow is not necessarily a horizontal plane, for it frequently follows the contour of the chalk, and that the causes of surface irregularities of subterranean water are unknown; but rain-water accumulating in chalk principally rises and issues most rapidly along the bed lines; consequently the flow along these must be gently discharged, or slips will occur; but chalk uniform in character and of solid and close texture, without flints and fissures, usually is not water-bearing, and will stand almost vertically. As a rule the cohesion of the upper beds, if they are homogeneous, is greater than the lower beds, although the mass may be softer.
Should the drainage or natural outlet of the land waters of a chalk district be obstructed or dammed back, from the quantity of water being in excess of that the fissures or water seams in a chalk hill can discharge, and the pent up waters be unable to escape, hydrostatic pressure, in addition to a weakened condition of the chalk through excess of moisture, will be caused, and extensive slips may be expected along the escarpment, the displacement being gradual, the ground separating and fissuring until at length it is pushed out by hydrostatic pressure. Such a slip usually occurs in large masses, resembling a fallen cliff, for the disturbing agent is all-powerful, and the area affected very considerable, and particularly so if the chalk is superimposed upon different soil, or harder ground, as then the whole mass will probably move forward.
Although some approximate inclinations have been previously given, the varieties of chalk are so numerous that no absolute slopes of repose can be named, for chalk or calcareous earth may be:—
Marble or crystalline limestone.
Ordinary limestone rock.
Hard, compact chalk rock.
Lower white chalk.
Upper white chalk.
Hard grey chalk.
Ordinary grey chalk.
Pure white chalk.
Friable white chalk.
Yellow, light and dark blue, soft chalk becoming of marly character.
Hard chalk marl.
Grey marly chalk.
Grey clayey chalk.
Note.—The preceding chalk marls contain so large a proportion of argillaceous matter as to become almost clays so far as regards treatment in earthworks. Many serious slips have occurred in chalk soils, and their history indicates that the chief disturbing element was water, whether held back over a large surface until the hydrostatic pressure became too great for the slopes to withstand it, or from its bursting out in springs, and so separating and disintegrating masses of the earth.
Near the entrances to tunnels slips appear to be most frequent in cuttings in chalk. This would seem to lead to the belief that in places where it is known the chalk soil is likely to be troublesome from land-water and springs, it would be advantageous to prolong tunnels beyond the economic depth of a cutting, and even to continue them until such a depth as 40 feet is reached, to so arrange the gradients that they drain the interior, and to provide a complete system of pipes and drains, even if side galleries have to be driven to tap the water, before it reaches the lining, to relieve the sides, crown, and invert of a tunnel so that no water can pour down the roof or walls unless under control.
In a tunnel so situated and liable to water-pressure, the thrust of the soil will be very variable, and cannot be foreseen. At one place during construction, the walls and lining may be finished without movement of the earth, at another, the pressure maybe great and act unequally, either upon the side walls or the arch. As a rule, at the entrances to tunnels the pressure is greater upon the arch than the sides, for then the whole of the wedge-shaped mass within the boundary of the angle of repose of the soil is disturbed, and its cohesion impaired or destroyed, and therefore it presses upon the arch, this pressure tending to counteract the lateral pressure; but as the depth increases, the load from this wedged-shaped mass becomes less upon the arch, although generally greater than any lateral pressure, because the earth above is not disturbed or impaired, consequent upon the depth of the hill being greater and the cohesion and side-pressure of the earth tending to support it, but the lateral pressure is increased because the normal pressure of the soil due to the depth is augmented. It is this disturbance of such friable soil as chalk at tunnel entrances, causing the particles to be loose and separated, and in a state especially disposed to percolation of water, that probably causes the earth to be in a condition favourable to slips, and for them particularly to occur at or about the entrances to tunnels. When, therefore, the depth of open cutting at the entrances is reduced, any slip cannot be of the same magnitude as it would be if it happened at a greater depth. The circular or one closely approaching it would appear to be the best form for the lining, where variable or great pressure, vertical or lateral, is to be expected; for the pressure in a tunnel will always be unequal, and the surface of the earth must be supported.
In some experiments to join substances by pressure it was found that though great pressure forms chalk into hard blocks, the particles are not firmly united, and that they separate along the surfaces of contact of the original particles and not through them; these tests tend to show that masses of chalk are usually in a state not indisposed to separation. The same result occurred in similarly testing pulverised sandstone.