Comparison of extension and compression triaxial tests for dense sand and sandstone

It is generally observed that for a given confining pressure the friction angle at peak strength of dense sand deduced from triaxial compression test is smaller than the one obtained from extension tests. In fact, this conclusion depends essentially on the way one takes into account the global confinement condition. In this paper, we show that when there is no significant grain crushing, the dependence of the maximum friction angle with the average of minor and major principal effective stress is similar for compression and extension tests. A similar result is obtained for sandstones when the failure mode involves a shear failure plane.