Soil classification and strength

Readers of soil mechanics literature are troubled with several papers asserting that the meaning that the word plastic now has in twentieth-century solid mechanics does not apply to soil. It had another meaning for many centuries before it was given the modern meaning. People who saw a potter forming a little figure and thought of man being formed from clay, or asked how the identical full-grown forms of many plants could come from a handful of small seeds, thought that all manner of creation of forms involves a plastic principle that applies both in art and in life. The SOED quotes a line from Sir Thomas Browne (a scientist and scholar who lived from 1605 to 1682): ‘in what diminutives the plastick principle lodgeth is exemplified in seeds’. Today this might be written as ‘seeds are examples of the small dimensions needed to contain the plastic principle’. The SOED states that the Greek derivation of the word ‘plastic’ alleges that a principle, virtue or force in nature causes the growth of natural forms of living organisms. It covers the growth of big recognisable plants from small grains. The solid mechanics of metal forming involves stress. The (τ, x) plot in Figure 1.4(b) shows a cycle of (τ) loading and unloading that leaves a solid body with a plastic deformation (x), but the original definition of the word ‘plastic’ is wider than this. The plastic process in Figure 1.4(a) gives a dimension x to a body. Unlike a fluid flow, it covers any process in which a body acquires a form that persists when the process finishes. No stress need be involved. An example is finding the density of soil by digging a hole and pouring in loose sand to fill it. The ratio of the weights of the soil taken out and the sand put in gives the ratio of the densities of the soil and the sand. By the SOED definition, the ability of the aggregate of grains to adopt the irregular form of the hole is correctly called a plastic property. The ductility of soil paste that lets a potter form it into pottery is the result of the mechanics of water-saturated fine-grained soil aggregates. The plasticity index I_p of a soil, in the soil classification system of Casagrande, is the range of water contents over which soil paste shows this plastic behaviour and can be moulded into different forms.