Cyclic Behavior of a Reconstituted Gulf of Mexico Clay

Offshore soft clays can potentially undergo a loss in shear resistance when subjected to "non-standard" cyclic events such as storms and earthquakes. This behavior, termed cyclic softening, or cyclic degradation, is caused by the cyclic shear-induced generation of excess porewater pressure and a progressive de-structuration of the clay fabric. To better understand whether the reconstitution process affects the cyclic response of soft clays, six strain-controlled cyclic undrained triaxial tests were performed on a reconstituted Gulf of Mexico clay and compared to six similar tests performed on intact specimens loaded beyond the preconsolidation pressure. The single-Amplitude unidirectional cyclic shear strain and overconsolidation ratios (OCRs) investigated were 1% and 2% and 1, 2, and 3 respectively. All cyclic strains were applied at a frequency of 0.5 Hz. The tests illustrated that the normalized cyclic shear stress response was nearly identical for the reconstituted and intact specimens. Reconstitution did not affect the undrained shear modulus during the first cycle normalized by the preconsolidation pressure. However, the reconstituted specimens generated excess porewater pressure more slowly possibly due to the absence of silt seams, that would otherwise aid in porewater pressure equalization. The reconstituted specimens exhibited a slower decrease in secant shear modulus with cycling; the values of modulus degradation parameter, t, of the reconstituted specimens were on average 25% smaller than those of the intact specimens. However, the authors conclude that alternatives to "t" may better capture shear modulus degradation.