Use of triaxial stress state framework to evaluate the mechanical behaviour of an agricultural clay soil

Triaxial tests presented in this paper are part of the first phase of this research program. They were undertaken under saturated soil conditions to provide some critical state parameters (the critical state line (CSL) and normally consolidated line (NCL)) while studying the soil mechanical behaviour. Triaxial tests under unsaturated conditions were also run to study the effect of suction on the shear strength. The tests performed under saturated conditions were done using the conventional triaxial cell, while those performed under unsaturated conditions were conducted using a double wall triaxial cell, which allowed the application of the desired suction to soil samples during shearing. The findings show that the distance between the critical state line and the normally consolidated line depends on the soil type. In contrast to other investigations, which showed that the shear strength increase with increasing suction is non linear in the 0-500 kPa suction range, this work finds that the shear strength increase of Sainte-Rosalie clay is linear in the same range of suctions. This investigation also suggests that the wheeling of saturated top agricultural soil layers induces a high pore water pressure in the subsoil layers, which reduces the effective stress of the yielding zone.