Microscopic characteristics of partially saturated dense sand and their link to macroscopic responses under triaxial compression conditions

Abstract

This paper presents a set of triaxial compression tests on partially saturated dense sands to clarify the microscopic characteristics and their link to the macroscopic responses. Constant suction tests (CS tests) and constant water content tests (CW tests) are conducted under low confining pressure to observe microscopic and macroscopic behaviors of the sands associated with dilative shear bands. X-ray micro-tomography and image analysis techniques are applied to investigate the continuity as a defined index to evaluate the morphology of the pore water, the number of liquid bridges and the principal curvature of the air–water interface. The number of liquid bridges decreases for both the CS and CW tests, particularly during the strain softening process, while it decreases greater in the CW test than in the CS test. The curvature of the air–water interface remains at almost the same value under the CS test, while it tends to decrease under the CW test. The tendency of decreasing curvature corresponds to that of decreasing suction in the CW test. The peak deviator stress is higher in the CS test than in the CW test when the pore water is initially discontinuous, whereas it is identical between the two tests when the pore water is initially continuous. The residual stress is lower in the CW test than in the CS test, independent of the initial water retention states. The macroscopic responses at the different initial water retention states are qualitatively identical between poorly graded sand and well-graded sand.