Small-Strain Stiffness of Unsaturated Soils Using a Suction-Controlled Resonant Column Device with Bender Elements

Small-strain stiffness properties of soils, such as shear wave velocity, shear modulus and material damping, play a key role in the adequate design/analysis of unsaturated earth structures subject to static/non-static loading. Most conventional geotechnical testing techniques, however, cannot capture this small-strain behavior and, hence, vastly underestimate the true soil stiffness, mainly due to inaccuracies in small-strain measurements. Bender element technique has proved to be a viable way to investigate soil stiffness at very small strains, albeit mostly under dry or saturated conditions. Hence, there is a great need for assessing the feasibility of bender element technique in unsaturated soils as compared to fully standardized procedures, such as the resonant column and simple shear test methods. This paper introduces a suction-controlled proximitor-based resonant column device featuring self-contained bender elements for simultaneous testing of soils using both techniques. A comprehensive series of RC and BE tests were conducted on samples of silty sand at 50, 100, 200 and 400-kPa matric suction states via the axis-translation technique. Results show a reasonable good agreement between RC and BE test results. Both techniques showed the critical influence of matric suction on soil's small-strain stiffness response.