Independent stress control and triaxial extension tests on sand

Tests were conducted over the complete range σ′2=σ′3 to σ′2=σ′1 in a comparatively simple form of generalized three-dimensional shear testing apparatus (σ′1≥σ′2≥σ′3) with two pairs of rigid platens. These tests were compared with the results from triaxial compression and triaxial extension tests. Good agreement was found with the former but important strength differences were found between ISC tests near b=1·0 (μ=+1·0) and triaxial extension tests, particularly as the initial porosity of the samples increased. Special tests conducted with varying stress paths to failure demonstrated that this was not responsible for the difference in strength. In addition, errors of sufficient magnitude to account for this difference could not be invoked from examination of possible errors from platen friction, sample sheath strength, average stress level, non-uniform deformation, anisotropy or the assumption that the applied lateral cell pressure was increased by platen friction restraint. It is concluded that the method...