Development of a dynamic hollow cylinder apparatus for investigating the cyclic response of unsaturated soils subjected to stress anisotropy

The construction of most geo-structures like earthfill dams, roads, highways, embankments, and airport runways deals with the compaction of unsaturated soils under anisotropic cyclic/dynamic loadings. Many of these partially saturated geo-structures also experience three-dimensional dynamic loadings during service life. This paper presents a new hollow cylinder apparatus developed for the investigation of the cyclic response of un-saturated soils subjected to stress anisotropy. The device is unique as it enables, for the first time, studying the combined effects of stress anisotropy (effects of both parameters alpha and b) and the degree of saturation on the dynamic properties of unsaturated soils, the aspects that cannot be thoroughly captured using existing soil laboratory testing equipment. It allows independent control of the magnitudes and rotations of the principal stresses and measuring strains as well as control/measurement of pore water and pore air pressures in the un-saturated soil samples. The paper describes the main challenges and the adopted solutions for the development of the hollow cylinder apparatus, including the complexities associated with the non-uniformity of water content in the sample height and the measurement of the sample volume change. The relations for determining the effective stresses and strains in the unsaturated soil samples are also derived. The results of several dynamic tests on unsaturated sand over a wide range of shear strains are presented to demonstrate the capabilities of the apparatus.