Rate-dependent behavior of a saturated reconstituted clay under different over-consolidation ratios and sample variance

This paper intends to study the influence of the shearing rate on the undrained monotonic behavior of marine clay under different over-consolidation ratios and the effect of sample variance. A series of undrained triaxial tests with isotropic consolidation was performed. The tests were either conducted with a constant strain rate (single-stage tests) or with different strain rates applied in succession (multi-stage tests, step-change shearing rate tests), the latter with and without relaxation phases. The tests were conducted on saturated reconstituted Macau marine clay. It has been found that a higher overconsolidation ratio tends to produce a lower excess pore pressure, while faster shearing tends to lead to a higher undrained shear strength and a lower induced excess pore pressure. Besides, the influence of sample variance, which means the differences between the undrained shear strength and excess pore water pressure derived from tests with equal OCR and shearing rate, performed as either a single-stage or a multi-stage test, was found relatively small. It can be concluded that a test with different shearing rates applied in succession to the same specimen is sufficient to study the rate-dependent undrained behavior of saturated clay. Two different approaches for the description of the rate effects on the undrained shear strength, following Mitchell and Soga or Graham et al., respectively, have been extended by the influence of OCR. Based on comparison, the extended equation of Graham et al. seems more appropriate for an application in practice.