Schistose rock is troublesome in earthwork, being fissile in structure and deleteriously affected by rain and the atmosphere. On the Panama canal works in the Culebra cutting, maximum depth 333 feet 6 inches, numerous slips occurred, displaced the roads, and overturned the excavators.
Mica-schist is variable and frequently has numerous water-bearing fissures. Its hardness depends upon the quantity of quartz it contains; when the proportion of mica is greater than the quartz it is soft and very fissured and contains veins, sometimes of clay, often yielding a considerable flow of water. Should the percentage of quartz be large it becomes hard, holds little water, and is of a gneissose character.
As water is the chief disintegrating agent and cause of instability, it may be well to mention that Professor Prestwich has stated that “hard quartzites, slates and grits (Silurian), purple and grey shales, schists and fissile sandstones with hard compact limestones and dolomites (Devonian), rarely contain any levels of water, and that it is only encountered in fissures”; hence the importance of knowing the position of the fissures, and taking the necessary precautions to promote stability. Where rocks, especially if generally known as water-bearing, crop out at a high angle, and are in well-defined beds, water may be expected, as although it may not percolate vertically, it will along the inclined beds. Sandstone and limestone bands in rock usually cause small springs.
The stability of a sandstone for earthwork and purposes of construction is dependent upon the material which cements or holds it together, whether iron rust, lime, free silica, alumina, &c., &c., &c., its quantity and condition, and the degree of hardness imparted when it was formed, and the nature of the agglutinant; therefore the varieties and degrees of fineness and hardness are numerous. Sandstones are generally found to be laminated when hard; and bare of vegetation if pure and free from marls; they contain and part with water in different proportions, and sometimes have watertight bands crossing them, severing water communication, which may cause earthworks to be of unequal stability. The firmest and strongest are close-grained and fine in texture, the weaker are coarse and gritty, and have a sandy appearance. They may be white, yellow, green, black, red, grey, brown, or other colour, and although of the same hue, their character may not be identical; for instance, the red sandstone is hard and also very soft. Sandstone of a greenish hue is generally hard, much fissured and full of water. When firm, greensand may stand at a steep slope, the surface being protected; but it varies considerably, and may at one place be close and yet be gradually deteriorated until it is of the character of fine loose sand.
Should it be found upon excavating sandstone that it is not upon its natural bed, but distorted, upheaved, or vertical, it will probably split and become detached under the destructive action of air and water. In tropical climates it has been found that sandstones generally dissolve and become disintegrated when used in damp foundations. Limestone also varies much in character, and is treacherous, whether it is hard or soft, when pockets of clay or sand are present and beds of clay are contiguous. Percolated water having carbonic acid in it may also soften or dissolve it. The softer kinds if in fragments as ballast, or when deposited in an embankment, often become quickly disintegrated by frost and the weather, as do sandstones.
Should any rock strata be vertically inclined instead of horizontal, although it may be known in the latter case they are generally watertight, fissures in the upheaved beds may become channels for the passage of the subsiding or rising waters, and may cause saturation of the soil over a considerable area and induce a flow through the slopes or the seat of an embankment. Upon such a site no reservoir, dock, canal, or any earthen structure to hold water should be placed; but although the nature of the ground may be fatal to the stability of such an embankment, so long as the underground waters do not rise to the level of the seat of a railway embankment or flood a cutting or burst the slopes, they may not seriously affect the stability; for, unless the head level of supply is great, more danger maybe expected from downward percolation saturating the ground upon which the embankment is placed than from an upward flow. The insecurity of erecting a reservoir or similar work upon such a site, in which water is brought into a district in greater quantity than its natural flow, is obvious, as the earth may gradually become saturated from the constant leakage down the upheaved fissures, until it becomes in an unstable condition and finally slips and subsides.
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