Behavior of a gulf of mexico clay under variable-strain cyclic loads

To better understand the influence of variable-strain time histories on the cyclic and post-cyclic behavior of offshore soft clays, nine undrained triaxial tests were performed on a Gulf of Mexico clay. The specimens were subjected to strain-controlled cyclic loading, immediately followed by undrained monotonic compression loading. The specimens were consolidated to overconsolidation ratios of 1, 2, and 3. Three combinations of cyclic strain amplitudes were investigated, namely “ramp up” (cyclic axial strain ε_c =0.5→1→1.5→2.0→2.5→3%), “ramp down” (ε_c = 3→2.5→2.0→1.5→1→ 0.5%) and “ramp up and down” (ε_c = 0.5→1.5→3→1.5→0.5%). The results illustrate that both the sequence of cyclic loads and specimen OCR affected the normalized cyclic shearing resistance and shear modulus degradation, but had a lesser effect on the cyclic load-induced normalized excess pore pressure. All the tests followed a dilative stress path during post-cyclic monotonic loading and reached the same Mohr-Coulomb failure envelope defined from monotonic undrained triaxial tests.