Modeling the mechanical behavior of unsaturated expansive soils based on Bishop principle of effective stress

Modeling the mechanical behavior of expansive clays is of interest in understanding the performance of nuclear waste disposal designs that include clay materials as buffers around waste containers, backfill for underground openings, seals between adjacent openings, or as host-rock constituents. The buffers, backfill, and seals will be unsaturated during construction and will re-saturate at varying rates after cessation of disposal operations. The mechanical behavior of the clay materials during re-wetting could affect the pressure on waste containers, other engineered components, or the host rock and could influence fluid flow and radionuclide transport. The authors describe an approach to mechanical modeling of unsaturated soils using the moisture retention characteristic curve and the Bishop principle of effective stress to evaluate suction effects on stress. The approach incorporates swelling, thermal expansion, and soil hardening and stiffening due to suction or compaction and uses stress–strain relationships based on elastoplasticity. Suction contributions to soil stress, strength, and stiffness are evaluated using the same moisture retention characteristic curve that is a standard input for hydrological modeling. The model is illustrated through several numerical simulations of oedometer free-swell and confined-swell testing of a bentonite–sand mixture and a granular bentonite. Results are presented with and without the effect of physicochemical swelling to illustrate magnitude of the swelling influence on independent variables and different mechanical responses. The model prediction is highly dependent on the swelling/shrinkage behavior of bentonite, which could be a source of uncertainty in estimating potential pressure due to a bentonite buffer in a nuclear waste repository design.