The main objective of this work was to experimentally investigate the influence of key environmental factors, including compaction-induced matric suction and confining pressure, on small-strain stiffness properties of partially saturated sandy and clayey soils using the pressure plate, resonant column, and bender element testing techniques. To achieve this goal, a comprehensive series of resonant column (ASTM D 2325-68), bender element (ASTM C 778), pressure plate (ASTM D 4015-92), and filter paper (ASTM D 5298) tests were conducted on several identically prepared specimens of loose, poorly graded sand (SP) and compacted, high plasticity clay (CH) subject to different compaction-induced (post-compaction) suction and confining stress states prior to resonant column and bender element testing. Compaction-induced suctions in all specimens were assessed prior to testing via a full set of previously calibrated soil-water characteristic curves (SWCC). Compaction-induced matric suction was found to exert a significant effect on the small-strain stiffness response of partially saturated SP and CH soils both from bender element and resonant column testing techniques.

Multistage triaxial tests offer a convenient alternative to determine the mechanical response of saturated and unsaturated soils. However, the primary challenge is to accurately identify the point of termination of the initial shearing stages prior to the failure of the brittle specimens. Hence, in this paper, the previous methodology used to define the point of termination of the shearing stages were studied in detail. Based on the limitations of methods used in the previous studies, a new twofold criterion integrating the variation of initial tangent modulus and the volumetric strain response with shearing has been proposed. The new approach was validated by comparing the results obtained from multistage triaxial tests and those from single-stage triaxial tests at low suction state under drained conditions. Each multistage test was performed on an isotropically consolidated specimen of silty soil at constant matric suction and different net confining pressures. Subsequently, it was confirmed that the shear strength parameters from both types of tests were practically identical.