Influence of Initial Water Content on the Mechanical Behavior of Unsaturated Sandy Clay Soil

The mechanical behavior of a sandy clay soil was investigated by a series of constant water content triaxial tests on unsaturated samples with suction measurements. The tests were carried out in double-cell triaxial cells on compacted samples and also on samples wetted and dried from the as-compacted conditions. A series of tests on saturated samples was also performed to provide a reference state for the unsaturated tests. Because the specimens were at high degrees of saturation (generally >80%), calculations based on effective stress showed a reasonable interpretation of the data for the critical state parameters M and λ. However, the intercept of the critical state line in the ν-axis ($\Gamma$) differed and increased with an increase in water content (at compaction). The data were also analyzed using the Bishop stress ($p*$) approach, and a better fitting was achieved in the stress plane, where it was possible to define a unique critical state line, where M was 0.91. However, in the $\nu \text{-}p*$ plane, different critical state lines were still obtained for different compaction water contents. The mechanical behavior of the sandy clay soil was found to be governed by the initial conditions, the as-compacted conditions. Although the critical state stress ratio, M, itself was not affected by the initial water content, the differences in volumetric behavior led to differences for samples subjected to wetting and drying after compaction. Samples that were dried back to a particular water content had a higher strength than samples compacted at that water content; in contrast, a sample wetted to achieve that water content had a lower strength.