THE EFFECTIVE STRESS CONCEPT IN SATURATED SAND CLAY BUFFER

Uncertainty has existed about the applicability of the effective stress concept to dense, compacted, plastic clays. To examine this, a series of tests has been performed on mixtures of sand and bentonite. Specimens were made by combining equal weights of the dry components with water and compacting the mixture to 85 or 95% ASTM Modified maximum dry density. The tests investigated the proposition that the behaviour of the mixture can be expressed in terms of effective stresses defined as the tensor difference between externally applied total stresses and pore-water pressures measured outside the cell. The proposition cannot be examined directly, but must be studied through a series of corollaries, that is, statements of behaviour that would be expected if the proposition were true. The testing considered the behaviour of specimens in consolidation and shear. Effective consolidation pressures were from 0.3 to 3.0 MPa, with back pressures from 0.2 to 7.0 MPa. Within acceptable bounds of experimental error, the tests show that effective stress can be used to describe consolidation and shear behaviour. However, because part of the effective stress in the clay is derived from net interparticle repulsive (unit) forces seated in diffuse double layers around aggregations of bentonite particles, the applicability of the concept has at this stage been restricted to conditions of constant volume (or possibly constant straining rate), constant chemistry, and constant temperature.