Comparison of Cyclic Triaxial Tests with Constant and Variable Cell Pressure

Cyclic triaxial tests are often used to evaluate the behavior of soils under seismic loads. The stress conditions imposed on a soil specimen during a cyclic triaxial test, however, are very different than those acting on an element of soil during an earthquake. One major difference is that the element in the field is subjected to a change in total confining stress, whereas in a conventional cyclic triaxial test the total confining stress (as applied through the cell pressure) is held constant. This use of constant cell pressure is usually justified by the assumption that in a saturated specimen the change in total stress is offset by a change in pore pressure, thus resulting in no change in the effective confining stress or liquefaction susceptibility. A laboratory study using cyclic triaxial tests was conducted on several soils to assess the validity of this assumption. For each soil, two series of stress-controlled cyclic triaxial tests were run: one set with a constant cell pressure, and thus a constant total confining stress, and a second set with a variable total stress/cell pressure. These tests were then compared in terms of both the resulting cyclic resistance curves and the amount of energy dissipated to trigger liquefaction. It was found that the two conditions of confining stress yielded results that were not statistically different. Therefore, the assumption that the change in pore pressure caused by the variation in total stress is offset by the change in pore pressure and thus results in no change in effective stress or liquefaction susceptibility appears valid. Based on these findings, cyclic triaxial tests performed with constant cell pressure, and thus a constant total confining stress, provide valid results for liquefaction analyses.