Rate effects on the undrained shear strength of compacted clay

Unconsolidated-undrained (UU) triaxial compression tests were performed on low-plasticity clay specimens compacted to the same void ratio but different initial degrees of saturation to evaluate the impact of axial strain rates ranging from 0.1 to 150%/min on the undrained shear strength. Although an effective stress analysis cannot be performed on the results, they are useful to evaluate the relative roles of initial hydraulic conditions (i.e., matric suction and degree of saturation) and compaction effects (i.e., potential changes in soil structure with compaction water content). This evaluation is relevant due to difficulty in measuring shear-induced pore water and air pressures in consolidated-undrained (CU) compression tests on unsaturated clay. In all tests, the undrained shear strength quantified as the maximum principal stress difference increased log-linearly with axial strain rate, with rates of increase ranging from 4.1 to 9.7% per log cycle of axial strain rate for specimens having initial degrees of saturation ranging from 0.99 to 0.59. The undrained shear strength, rate of increase in undrained shear strength with axial strain rate, and secant moduli all increased nonlinearly with decreasing initial degree of saturation, although compaction effects played an important role in these trends. The increase in undrained shear strength with axial strain rate can be attributed to a reduction in the magnitude of excess pore water pressure, with similar reductions in magnitude for all the degrees of saturation considered. A comparison between the measured undrained shear strength values and the drained shear strength values estimated using the suction stress concept was useful in delineating the impacts of initial hydraulic conditions and compaction effects on the trends in measured undrained shear strength.