Post-Cyclic Behavior of a Gulf of Mexico Clay

Offshore soft clays commonly exhibit changes in monotonic behavior following cyclic loading events. To better understand the shear strength of soft clays after a cyclic event, the authors performed a series of strain-controlled undrained cyclic triaxial compression tests followed by undrained monotonic compression to failure on a Gulf of Mexico clay. Prior to cycling, specimen overconsolidation ratios ranged from 1 to 3. Cyclic strain amplitudes of 0.5%, 1%, 2%, and 3% were used for the initial loading phase which consisted of 300 cycles at a frequency of 0.5 Hz. The monotonic behavior after cyclic loading generally exhibited: (1) a dilative response (compared to contractive response for monotonic undrained compression tests); (2) a reduction in maximum shear modulus; and (3) a decrease in post-cyclic undrained shear strength of about 25% compared to the virgin undrained shear strength. Despite these differences in response, all post-cyclic stress paths reached the same effective stress failure envelope defined by monotonic undrained compression tests on virgin specimens.