The structure of soils that experience large loss of strength or great increase in compressibility with
comparatively small changes in stress or deformations is said to be metastable (Peck et al., 1974).
Metastable soils include (Peck et al., 1974):

1. Extra-sensitive clays such as quick clays,
2. Loose saturated sands susceptible to liquefaction,
3. Unsaturated primarily granular soils in which a loose state is maintained by apparent cohesion, cohesion due to clays at the intergranular contacts or cohesion associated with the accumulation of soluble salts as a binder, and
4. Some saprolites either above or below the water table in which a high void ratio has been developed as a result of leaching that has left a network of resistant minerals capable of transmitting stresses around zones in which weaker minerals or voids exist.

There are two types of soils that exhibit volume changes under constant loads with changes in water content. The possibilities are indicated in Fig. 18.1 which represent the result of a pair of tests in a consolidation apparatus on identical undisturbed samples. Curve a represents the e-log p curve for a test started at the natural moisture content and to which no water is permitted access. Curves b and c, on the other hand, correspond to tests on samples to which water is allowed access under all loads until equilibrium is reached.

If the resulting e-log p curve, such as curve b, lies entirely below curve a, the soil is said to have collapsed. Under field conditions, at present overburden pressure p1, and void ratio e0, the addition of water at the commencement of the tests to sample 1, causes the void ratio to decrease to e1. The collapsible settlement Sc may be expressed as

where H = the thickness of the stratum in the field.

Soils exhibiting this behavior include true loess, clayey loose sands in which the clay serves
merely as a binder, loose sands cemented by soluble salts, and certain residual soils such as those
derived from granites under conditions of tropical weathering.

On the other hand, if the addition of water to the second sample leads to curve c, located entirely above a, the soil is said to have swelled. At a given applied pressure p1 the void ratio increases to e'1 and the corresponding rise of the ground is expressed as

Soils exhibiting this behavior to a marked degree are usually montmorillonitic clays with high plasticity indices.


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