A Comparison of Cyclic Triaxial Tests Performed Using Constant and Varying Cell Pressure Carmine Polito Associate Professor of Civil Engineering 143 Gellersen Center Valparaiso University, Valparaiso, Indiana, USA e-mail: carmine.polito@valpo.edu ABSTRACT 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 standard 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 tests were run: one set with a constant cell pressure, and thus a constant total confining stress, and a second set with a varying cell pressure. These tests were then compared in terms of the resulting cyclic resistance curves. 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. It appears that cyclic triaxial tests performed with constant cell pressure, and thus a constant total confining stress, provide valid results for liquefaction analyses. KEYWORDS: Cyclic Triaxial Test, Liquefaction, Total Confining Stress, Dissipated Energy I NTRODUCTI ON For nearly half a century, cyclic triaxial tests have been used to evaluate the liquefaction susceptibility of sandy soils. There are numerous differences between the conditions acting on an element of soil subjected to an earthquake in the field and a specimen tested in a cyclic triaxial test. One of the main differences concerns the total stress acting on the soil during the cyclic loading. Despite the differences between the conditions applied to the soil in the laboratory and in the field, cyclic triaxial tests have been used in many valuable studies to evaluate the liquefaction susceptibility of specific soils and to perform parametric studies to ascertain the influence of various factors on liquefaction susceptibility. - 5515 -