Void ratio redistribution in undrained triaxial extension tests on Ottawa sand

Water-pluviated samples of Ottawa sand were tested in monotonic, undrained triaxial extension tests. The specimens exhibited similar "limited strain softening" behavior, and they all experienced phase transformation from contraction to dilation at small axial strains. The tests were stopped at different stages and the samples were frozen to obtain void ratio distribution along the length of the specimens. It was shown that void ratio redistribution can start at very low axial strains in an undrained triaxial extension test. Before phase transformation, void ratio redistribution was very small, but after phase transformation void ratio redistribution started rapidly and continued until the end of the tests at ultimate state. The location of the ultimate state line in a void ratio - mean normal effective stress plot was shown to be affected by localized failure at large strains in undrained triaxial extension tests. The actual ultimate state line with respect to void ratios and effective stresses within the failure zone in the samples can be located above the average ultimate state line obtained from average measurements of void ratios and effective stresses of the entire specimens.