The lias clays are treacherous chiefly owing to the presence of much calcareous matter, and therefore approach a marly state; heavy slips have occurred in the has formation in the midland counties of England, notwithstanding that a slope was adopted which experience had shown produced stability, namely 3 to 1. A slight variation in the composition of this soil or an unfavourable position will cause a slip in such treacherous earth.
Pure clay shrinks some 5 per cent. in drying, the contraction being less as sand is present in it, for when it is mixed with twice its weight of sand it is reduced to 3 per cent., and as impurities increase in clays the less impervious they become. Most clays have silicious earth in them, but if sand is present the clay is then more open, and water will permeate and drain more freely; but mixtures of clay and sand may assume a pulpy condition when impregnated with water, consequently it is always advisable to test such earth. The varieties of sandy clay are many, and all are usually more or less unstable. Among them may be named red clay with sand and mica, blue sandy clay, sandy green clay, stiff red sandy clay, the loamy clays of various hues, dark grey, red sandy, and black clayey loams.
If clay could be kept in a moist state fissures would seldom occur. The constant alternation of wetness and dryness creates the fissures, and water completes the disintegration. There are a few clays which are stable when kept in solid masses, as then only a small surface is affected by air and water, but if they are loosened and broken up, as in the process of excavation and deposition, they readily become in a muddy condition. Any mud or silt which may be soft and readily pressed when wet, but cakes and shrinks in to detached lumps when dry or upon being exposed to the atmosphere, is a treacherous soil, as it will return to its original state upon becoming wet. Clays or any soils that cake should always be regarded with suspicion, as although having the appearance of solidity and the possession of weather-resisting qualities in their natural position, when disturbed, quickly become worthless for earthwork purposes, and may stand in one situation almost as a soft clay, and when disturbed and wet assume a horizontal surface. Such ground may repose at a 4 to 1 TO 8 to 1 slope, because its crust has become caked or case-hardened, yet when it is broken it may become, upon being exposed, simply fluid mud. To prevent clay soil weathering upon the surface, a layer of gravel 1 foot or so in thickness has been placed upon it, the idea being that it is not only a protection, but the weight of the covering upon the clay will cause the water to be pressed out from the soil into the gravel through which it can percolate to the drains.
A crude test to indicate the probable character of a clay as regards its stability in earthwork is to burn a piece of it and notice the colour. If it becomes white or of a whitish tint, the clay is generally less likely to slip than when it is of a reddish or yellowish tinge. Another rough experiment can also be made. Get a piece of clay, place it in water, and note the time taken and the depth to which the surface has become saturated, and whether it is very slimy and will easily slide down a slightly inclined plane; its tenacity may then be approximately judged. Also by weighing, an idea of the amount of sand may be imagined; the more sand there is in clay the lighter it will be, all other conditions being identical. A comparison between two clays will enable some opinion to be formed of their relative stability in earthwork, though, of course, there are many other features to be considered. All impure clays, such as shaly clay, sandy clay, loamy clay, and marly clay require to be carefully treated, although they may be easier to manage than yellow or brown clay.
When two retentive clay beds overlap or overlie, and have no intermediate permeable stratum, they must be in a humid state, as is the case in the Fen country, unless they are constantly drained; but serious slips are not so likely to occur in them as when two masses of clay have an interposing seam of sand or silt liable to be eroded by water falling down fissures in the clay, which probably extend to considerable depths, with the result that two slimy surfaces are formed and the clay slides. Clay underlying gravel often contains numerous pockets and seams filled with running sand, and should there be a permanent head of water the discharge will be in large quantities and at a considerable velocity. A cutting in wet sandy clay is generally treacherous and difficult to manage.
As a clay bed near the surface of the ground is sometimes upheaved, if a permeable stratum such as gravel or sand overlies it, the drainage of water through or down the slopes will be arrested, and the earth at the back of the slope will be constantly wet and may ultimately become saturated through the damming back of the water; then a slip may be expected.
All upheaved, dislocated, and twisted superficial beds of clay, which will generally be of varying consistency and therefore settle unequally; over or underlying seams of sand or gravel, are likely to slip and subside, and their stability much depends upon whether or not the lie of the beds obstructs the permeation of water. If the dip of the clay-beds is towards the natural outfall, most probably an adjacent river, slips are probable because of the creation of sliding surfaces and the continuity of the beds being destroyed by a cutting and the consequent loss of support.
Should permeable soil lie between the top stratum and a bed of clay, water will accumulate upon the clay, make it slimy and cause a flow upon the bed; for example, when a thin bed of vegetable earth rests upon gravel, sand, or peat, and that upon clay, water will percolate, and perhaps air, through the top soils, and may cause them to slip upon the clay-bed. Also should a layer of clay overlie permeable strata, as clay upon sand, or clay upon gravel, unless it is sufficiently thick and solid to prevent infiltration, it may slide upon the permeable soil as its lower surface becomes wet. When clay-hills have veins, water may accumulate in them and flow, and if very dark yellow clay overlies light yellow calcareous clay, which may rest upon hard blue clay, it is obvious each stratum is somewhat differently affected by weather and air, and therefore movement is to be expected. The edges of clay-hills are always likely to slip, especially should they be in the form of spurs.
Boulder-clay is seldom reliable, because, although it may be hard and stand vertically in dry weather, in wet it swells, weathers quickly, becomes soft and cakes upon drying. The stability of such soil is governed not only by the nature of the clay, but by that of the boulders and their effect upon the earth in which they are embedded, and much depends upon the degree of changeableness upon exposure to air and moisture of all the particles of which they are composed; hence boulder-clay, although hard to excavate, may quickly dissolve. On the contrary, it may occasionally be so hard that it seems to be solid rock, and may even resist erosion and weathering as well as if it were rock; but care must be taken to prevent indurated mud being mistaken for solid clay-rock, and therefore it is advisable to test the soil with water.
Seams of silt, soft pasty soil, or soapy earth met with in clay, which have become decomposed by atmospheric and aqueous action, are to be feared, and the brown clay, especially when soft: red, or dark yellow clays that break into laminæ and crumble upon exposure to the air, and although tenacious in the flakes and when fresh-cut are loosely held together in bulk, often have thin veins of sand in them; and when water percolates it remains, and is very difficult to drain. It has also been found that when minute non-adhesive particles of mica are present in clay that it will become disintegrated by water, although it may be hard to excavate. Some of the gault clays, although stable when dry, become soapy when wet and are not easily managed, but the bluish grey gault is usually tenacious and almost impermeable. The gault clays have little sand in them but much calcareous matter, and, as a rule, they do not swell and bulge like the London clays.