Multistage Triaxial Testing to Estimate Effective Stress Relationships for Unsaturated Compacted Soils

Abstract

This paper presents an experimental methodology for using multistage, drained triaxial tests on compacted soils under unsaturated conditions to estimate soil-specific relationships between mean effective stress and matric suction. Tests were performed by applying a matric suction to a soil specimen in a triaxial cell using the axis translation technique with back-pressure, then shearing the specimen under drained conditions until reaching stress-path tangency. The specimen was then unloaded, a new suction was applied, and the shearing process was repeated. The points of maximum principal stress difference for the unsaturated specimen were plotted versus mean effective stress, defined using the degree of saturation as the effective stress parameter, and they were found to correspond well with the critical state line defined from triaxial tests on saturated specimens. The suction stress for the compacted soil tested in this study was observed to increase nonlinearly with suction, tending toward a constant value with increasing suction. Although there are potential changes in soil structure in the specimen during loading, unloading, and reloading, the results indicate that the multistage testing method may be useful for estimating soil-specific effective stress parameters for compacted soils in unsaturated conditions. Furthermore, the fact that differences in the soil-water retention curve of soil specimens subjected to different net confining pressures were observed for the soil tested in this study emphasizes the importance of using soil-specific tests to validate predictive relationships between suction stress and matric suction.

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“…A conventional direct shear test device was modified to allow suction control and multi-stage loading. Even though limitations of the direct shear test and multi-stage loading exist, the test method and results have been proven to be reliable for both saturated and unsaturated soils [11][12][13][14][15]. Particularly for unsaturated soils of lower permeability that generally require a long equilibrium period under controlled matric suction conditions, the MDST can be a practical choice because of having a shorter test time and, thus, has been used successfully [16][17][18].…”