Rate-Dependent Undrained Shear Behavior of Saturated Clay

Abstract

This paper presents the findings from 25 one-dimensionally consolidated-undrained triaxial compression tests on resedimented Boston Blue Clay using a computer-automated triaxial apparatus with lubricated end platens and a midheight pore-pressure measurement device. Specimens were consolidated to four overconsolidation ratios (OCR) and for each OCR, undrained shear was conducted using four axial strain rates. The findings reveal that the undrained strength rate sensitivity across the two fastest strain rates does not differ with OCR and is equal to about 9 percent. Across the three slower rates, though, increases in OCR cause a consistent decrease in the rate sensitivity that reaches zero at OCR = 8. For high OCR clay, increases in undrained strength are caused by lower shear-induced pore pressures because the effective stress envelope at the peak strength does not change with strain rate. For low OCR clay, increases in undrained strength are caused by lower shear-induced pore pressures and increases in the mobilized friction angle. The authors propose a simple procedure for modeling the rate dependent stress-strain curves of overconsolidated clay.