Comparison of Strain-rate Dependent Stress-Strain Behavior from K_o-consolidated Compression and Extension Tests on Natural Hong Kong Marine Deposits

Abstract

This article presents results from a series of K _o -consolidated compression and extension triaxial tests on specimens from undisturbed samples of Hong Kong Marine Deposits (HKMD). To investigate the strain-rate effects, a total of seven K _o -consolidated triaxial tests were conducted including four compression tests and three extension tests. After K _o -consolidation, the triaxial test specimens were sheared at step-changed axial strain rates under three different confining pressures of 50 kPa, 150 kPa, and 400 kPa, respectively. The step-changed strain rates were applied in the following order: +2%/h, +0.2%/h, +20%/h, −2%/h (unloading) and +2%/h (reloading) for the four compression tests and −2%/h, −0.2%/h, −20%/h, +2%/h (unloading) and −2%/h (reloading) for the three extension tests. The results are reported and analyzed in the paper. The results show that the strain rate effects, the stress-strain characteristics, and the effective stress paths of the specimens for tests in a compression state are different from those for tests in an extension stage. One order of magnitude increase in axial strain rate causes an average 8.6% increase in undrained shear strength for compression tests and a 12.1% increase for extension tests. It is also found that the failure mode of the specimens in compression is different from that in extension. The stress-strain behavior of specimens shows strain-softening and a clear shear band in compression tests, but strain-hardening without any clear shear band in extension tests for the same absolute value of axial strain.

Keywords:

• K _o -consolidation
• marine deposits
• shear banding
• strain-rate effects
• triaxial compression
• triaxial extension

The work presented in this article has received financial support from a RGC grant of the University Grants Committee of the Hong Kong SAR Government of China and a grant from The Hong Kong Polytechnic University. This financial support is gratefully acknowledged.

Notes

*If the curve has no peak, the deviator stress at axial strain 16% was used.