Stiffness of intermediate unsaturated soil from simultaneous suction-controlled resonant column and bender element testing

Small-strain stiffness properties of unsaturated soils, particularly shear-wave velocity, shear modulus, and material damping, play a fundamental role in the analysis and design of geotechnical infrastructure resting on unsaturated ground, or made of compacted unsaturated soils, as they are subjected to static or dynamic loading. Most conventional soil testing devices, however, are not able to adequately capture this small-strain behavior and hence vastly underestimate the true soil stiffness. This paper introduces a suction-controlled, proximitor-based resonant column device also featuring an independent set of self-contained bender elements for simultaneous testing of soils using both techniques. The work is partly aimed at assessing the suitability of bender elements for soil testing under controlled suction states, as compared to more reliable and fully-standardized procedures such as resonant column or simple shear test methods. A series of resonant column and bender element tests were simultaneously conducted on statically compacted specimens of silty sand for suction states ranging from 50 to 400 kPa attained via the axis-translation technique. Particular attention was devoted to the influence of suction over the frequency response curves, damped free-vibration cycles, and cyclic hysteretic stress–strain loops. Results show the critical influence of matric suction on small-strain stiffness properties of compacted silty sand, as well as reasonably good agreement between resonant column and bender element test results.