Modeling the relation between suction, effective stress and shear strength in partially saturated granular media

Abstract

Decades of geotechnical research firmly established that the mechanical properties (shear strength and deformation characteristics) of soils are related to soil's "effective stress", i.e. the stress carried by the solid matrix. The remaining stress is carried by the pore fluid in the form of pore pressure. In unsaturated soils, the coexistence of water and air results in negative pore pressure, which are termed "soil suction". A clear link between soil suction and effective stress has not yet been established. This research develops a model for linking effective stress to total stress and soil suction for uniform, spherical particles, at low water contents. The model includes the analytical formulation of water geometry and forces at the micro-scale. The change in effective stress due to soil suction is estimated from this particle contact scale, taking account of the particle packing. A method for inferring the effective stress in an unsaturated soil from strength test results is also proposed. In order to examine the partially saturated mechanical behavior of granular materials, a triaxial test setup was modified to accommodate an MIT Tensiometer as its pedestal. Materials consisting of uniform spherical glass beads were tested, under both saturated and unsaturated conditions, at conditions that matched those of the model.

(cont.) In order to accommodate the unique difficulties of constituting a specimen made of glass beads, new test preparation procedures were developed. New data correction methods, which are applicable to a broad range of triaxial test measurements, were also proposed. With the experimental program, new behavioral features unique to the glass beads were observed. Coupling saturated and unsaturated shear strength via the proposed method also enabled the effects of suction on effective stress to be inferred. Experimentally obtained increases in effective stress due to suction were significantly larger than those estimated by the model. However, many observed aspects of unsaturated behavior were in parallel to those predicted by the model. Therefore, discrepancies between observations and the theory developed here might originate from the method used to link effective stress to shear strength, leaving merit to the main model for effective stress in partially saturated granular media.