Undrained creep behavior of a compacted clay under low confining pressure

An experimental investigation is made for evaluating the long-term creep behavior of a compacted clay under low confining pressure (from 12 to 50 kPa). Specimens compacted at the natural dry density and natural water content were first subjected to consolidated undrained shear tests to determine the shear strength. Undrained triaxial creep tests were thereafter conducted to study the axial strain versus time relationships under different deviatoric stress levels which were determined referencing the shear strength. Mercury intrusion porosimetry tests were performed to track the evolution of the microstructure during the undrained creep. Testing results showed that (i) the diameter of the dominant pores within the clay increases under low confining pressure but decreases under higher confining pressure after undrained creep, (ii) all axial strain versus time relationships show attenuated or transitional characteristics and no creep rupture occurs, and (iii) the proportion of total progressive axial strain in the total axial strain increases with the increasing deviatoric stress level and decreasing confining pressure. Three approaches were used to predict the measured axial strain versus time relationships under low confining pressures. Good agreements have been achieved between the predictions and measurements obtained in this study, which confirms the validity of the proposed approaches.