Influence of stress states on soil-water characteristics, conjunctive surface-subsurface flow modelling and stability analysis

The soil-water characteristic curve (SWCC) is a fundamental physical property describing the water retention ability of a soil at different suctions, which is essential for transient seepage analysis in unsaturated soils. In this thesis, the influence of stress state on stress-dependent soil-water characteristic curves (SDSWCCs) of both intact and recompacted specimens of completely decomposed granite (CDG) excavated behind the Hong Kong Sanatorium and Hospital was studied using a triaxial pressure plate system for isotropic and deviatoric stress states, and a stress-controllable volumetric pressure plate extractor for Ko stress state. Measured results were used as input parameters in conjunctive surface-subsurface flow (CSSF) programme, which is capable to determine interaction between surface flow, subsurface flow and infiltration automatically, to investigate the influence of stress state on conjunctive modelling and stability analysis. Parametric studies were also conducted using the CSSF programme to examine the influence of conjunctive modelling on slopes with different slope angles. A slope stability analysis programme CSLOPE, which uses the Morgenstern-Price method, was written to determine the corresponding change in factor of safety after the computation of pore-water pressure by the CSSF programme. It is found that regardless of the stress state (i.e., isotropic, Ko or deviatoric stress conditions), as the stress level increases, the measured SDSWCCs of both recompacted and intact specimens show a reduction in desorption rate, adsorption rate, hysteresis loop size and amount of air entrapment. The above responses for intact specimens are more significant than that for recompacted specimens. Under the same net mean stress but at a higher stress ratio, both intact and recompacted specimens tend to show higher water retention ability and lesser hysteretic responses. Comparatively, the effects of stress ratio on soil-water characteristics are more significant on recompacted specimen than intact specimen. For the recompacted specimens, when the applied stress is higher, the effects of stress ratio are also more noticeable. From the results of numerical analysis, it is found that the surface water plays an important role on pore-water pressure changes under rainfall. The increase in pore-water pressure is larger when surface water is considered. In other words, ignoring surface water may underestimate pore-water pressure increase due to rainfall infiltration. It is also concluded that the use of deviatoric SDSWCC predicts the greatest increase in pore-water pressure under a given rainfall infiltration. As the deviatoric stress state is also the most relevant stress state for slopes, it is suggested that the deviatoric SDSWCC should be used to analyse slope responses under rainfall. For the range of angles studied, it is concluded that the influence of conjunctive modelling on slope stability at a given depth of 1.5m is the more significant on a 15-degree slope than on a 45-degree slope