THE MEASUREMENT OF STIFFNESS ANISOTROPY IN CLAYS WITH BENDER ELEMENT TESTS IN THE TRIAXIAL APPARATUS

The paper presents the results of a program of research investigating the effectiveness of bender elements when used in conjunction with the triaxial apparatus for measuring the anisotropy of small strain stiff ness of fine-grained soils. Tests were carried out on both intact and reconstituted samples of London clay and on kaolin up to high stresses . The paper shows that the transverse isotropy of small-strain stiffne ss that commonly occurs in many soils because of a one-dimensional loa ding history can be fully investigated in the conventional triaxial ap paratus and that London clay is an example of such a soil. The stress- induced component of anisotropy was found to be very small for axi-sym metric loading conditions common to both the apparatus and the in situ state of these soils. In contrast, the inherent or structural anisotr opy was much more significant and is shown to be a variable factor res ulting from the plastic strain history and is not related to its natur al structure. Consequently, inherent anisotropy is reversible, but the rate of change is very slow when a new regime of stresses is imposed. Inherent anisotropy of the very small strain stiffness also persists long after the plastic strains of the soil have become oriented toward the new stresses.