Some clays, when dry, appear to be stable and firm, though they are often drift or dried-up mud simply requiring the influence of water and the atmosphere to cause them to return to their original state; the brown and boulder-clays are of this nature and are distinctly perishing, and therefore treacherous clays.
As in clay and all retentive soils, water is always present, and as the impervious nature of clay prevents water gravitating and being drained, and as owing to wet seasons the permeable portion of the clay may gradually become overcharged and be reduced to a muddy condition, slips may occur years after a clay cutting has been excavated or an embankment deposited. In clay embankments the greater weight upon the centre may gradually press water towards the slopes and cause them to slip; hence the value of covering the formation in order to lessen percolation. Cracks and fissures may be so produced in clay embankments, and the danger is that they form sliding surfaces and cavities from the presence of water, and gradually soften the interior until it fails from the thrust of the earth and want of uniform support; therefore, clay earths are more difficult to treat than granular soils, as local weakness is the cause of failure, a mass may be sound, but a crack or fissure may soon become large and pass through an embankment to the seat or the slopes. Although the filtration of still water may not cause an embankment to slip as long as the water merely restores it to its normal absorbing power, when that is exceeded the percolating water will be dammed-up and cause hydrostatic pressure and a soddened surface, destroy cohesion, and disintegrate the particles.
When clay is contained within the walls of a building, injurious action and unequal settlement may arise, as the earth inside and outside may be in a different condition; consequently in soils that expand and contract there may be external or internal pressure according to the state of dampness or dryness, consequent upon the ground outside being subject to weather influences and the contained earth being comparatively free from such operation.
Shale, whether black or brown, may become decomposed by water and be softened by time, and is either loose or firm. If loose, it requires protection, and the surface should not be exposed to the weather. The presence of iron pyrites in shale has been found to be a cause of its becoming treacherous when water has access to it, as the mass becomes decomposed.
Marly soils are of various hues, red, blue, grey, and yellow, and consist principally of clay and lime, and are usually called clay-marls when clay preponderates, and marl-clays or chalk-marls when chalk is in excess. They are dry to the touch and will effervesce with acids, the presence of lime being thus made evident by the ebullition produced, and some idea of the relative quantity may be judged by comparing results with different lumps: they vary in character according as lime or clay predominates.
Chalk-marl may act as an almost impermeable stratum and arrest the percolation of water from more permeable soil, but it is liable to slip because of fissures being present in it, which are common to most calcareous earths; and when these crevices are bared in cuttings by the ground being excavated, the flow from the previously-confined or diverted springs is facilitated, and perhaps the harmless underground passage of water is prevented, and consequently the earth may become loosened and unstable owing to the changed condition.
Although marl may be so hard that it cannot be excavated by picks and bars, but requires blasting, some varieties crumble and become soft under weather influences, and the slopes need protection, or a constant trickling and wearing away of the surface will result. Grey marl generally weathers quickly, and if there is much clay in it, it often falls to pieces upon exposure to the air, and becomes broken and so split into disjointed pieces as to admit water, and as it sometimes contains a considerable quantity of sand it is soon reduced to an unstable state, for marl-clay and sand beds are always likely to slip. The variegated marls are treacherous, but red marl is usually stable, still it must be lined in tunnels; and so are those that are impermeable or closely approach that condition, but any marl in which the lime may separate from the mass is unstable. All clay or marl soils that soon work into “slurry” may be classed as treacherous, or any earth that quickly becomes in a liquid state; for instance, some of the hard “pan” soil met with in Canada melts away by the action of air, rain, snow and frost, and becomes unmanageable and like blue paint, and will quickly fill drains and run down the slopes and cover the formation of a cutting. Also the top black cotton soil found in India expands by aqueous action, and contracts in drying as clay, and is of doubtful stability.
As might be expected, the more argillaceous or clayey marls when exposed to weather influences expand, contract, and act almost as clays, becoming decomposed, disjointed and separated, and when superimposed upon rock, especially if it be inclined, are very likely to slip, and are treacherous and unstable soils, as they soon become in a muddy state and have slimy surfaces, which, when resting upon any dipping bed, cause them to be disposed to move upon the smallest disturbing element being present or becoming increased, such as water or vibration. When a comparatively permeable stratum overlies even hard marl, water will penetrate to the latter earth and the superimposed layer will rest upon an unstable bed, and therefore be likely to slip. Blue, or any marl when found mixed with layers of small gravel and sandy clay is very treacherous, as air and water cause it to swell and crumble, and it becomes in a soft state requiring a very flat slope. All clay marls that swell when wet, and crack and fissure when dry, are unstable, as water percolates to them, and should sand veins occur in marl, water may trickle to a considerable depth and cause the ground to be in a loose and doubtful condition below the formation or bottom of a cutting. Such soil when tipped into an embankment is worse to treat than in a cutting, as it will absorb and retain water so that it is practically impossible to drain or extract it.
Professor Ansted has classed clay soils as under:—
When combined with 30 or 40 per cent. of sand they are CLAY LOAMS.