Triaxial testing of unsaturated agricultural soils

In this paper, triaxial testing procedures and equipment as they have been applied to unsaturated soils are surveyed. Modifications to conventional triaxial apparatus and procedures for testing unsaturated soils based on principles of "unsaturated soil mechanics" are described in some detail. The modifications accommodate the independent measurement or control of pore-air and pore-water pressures and the resolution of both air and water components of volume change. This makes it possible to study the influence of soil suction on soil strength and volume change behaviour. To illustrate this approach, an agricultural soil was tested in a modified triaxial apparatus using independent stress state variables to describe stresses in the specimen. Matric suction, mean net stress and deviatoric stress were monitored or controlled in a series of constant water content tests. Volume change and shear strength behaviour were evaluated based on unsaturated soil mechanics principles. For low stress regimes, matric suction remained essentially constant under constant water content testing; however, as loading was increased matric suction varied significantly. The relevance of using matric suction as an independent stress state variable in fundamental studies of agricultural soil behaviour, as opposed to a total stress approach, is discussed. (C) 1998 Silsoe Research Institute .