The Strength and Deformational Behaviour of a Cohesionless Soil Under Generalised Stress Conditions

Abstract

Predictions of the stresses and displacements induced within a soil mass by interaction with an engineering structure are made, at present, with only a limited knowledge of the deformational behaviour of soils under generalised stress conditions, The majority of routine testing and laboratory research investigations are performed on cylindrical soil specimens in the conventional triaxial apparatus, which is limited to imposing axially-symmetrical stress conditions. In very few field problems are such conditions strictly relevant. Many problems approximate more closely to plane strain, and recent progress has been made in investigating the behaviour of cuboidal specimens constrained from deformation in one of the principal directions. However, the experimental difficulties are increased, and controversies remain regarding the influence exerted by the apparatus on the soil behaviour. The general field condition is one in which the stresses and strains are different in each of their three principal directions at a point. Very few satisfactory attempts have been made to overcome the complex problem of testing elements of soil under similar conditions. The design and development of a new apparatus described which allows generalised stress or strain conditions to be applied to cuboidal specimens. The methods used to apply the three boundary principal stresses ensure a high degree of stress, uniformity, and suitable lubrication encourages uniform strains, Comparisons between the results of tests carried out under simple stress conditions, on specimens of the same sand, in this and in more conventional apparatus, indicate negligible apparatus interference. A further series of tests in plane strain shows that the deformational behaviour of loose specimens is similar to that observed in triaxial compression, For dense specimens, however, considerable strength increases result from the restriction imposed on strains. The results of generalised stress tests suggest that the intermediate principal stress is of most significance when its magnitude is less than that observed at failure in plane strain.