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 conve ntional triaxial apparatus and procedures for testing unsaturated soil s based on principles of ''unsaturated soil mechanics'' are described in some detail. The modifications accommodate the independent measurem ent or control of pore-air and pore-water pressures and the resolution of both air and water components of volume change. This makes it poss ible to study the influence of soil suction on soil strength and volum e change behaviour. To illustrate this approach, an agricultural soil was tested in a modified triaxial apparatus using independent stress s tate variables to describe stresses in the specimen. Matric suction, m ean net stress and deviatoric stress were monitored or controlled in a series of constant water content tests. Volume change and shear stren gth behaviour were evaluated based on unsaturated soil mechanics princ iples. For low stress regimes, matric suction remained essentially con stant under constant water content testing; however, as loading was in creased matric suction varied significantly. The relevance of using ma tric suction as an independent stress state variable in fundamental st udies of agricultural soil behaviour, as opposed to a total stress app roach, is discussed. (C) 1998 Silsoe Research Institute .