Experimental and Numerical Study of the Shear Strength of Unsaturated Sand

In the last few decades there have been extensive studies conducted into the mechanical and hydraulic behaviour of unsaturated soils. To date most attention has been focused on fine grained soils where the effects of partial saturation are expected to be more significant. Coarser grained soils have been under-represented in the literature. This study, therefore, focuses on the behaviour and modelling of an unsaturated sand and is divided into two main parts, governing experimental and numerical studies. The experimental work involved two key parametric studies: (i) a laboratory based study of the relationship between shear strength and suction using a specially designed direct shear box where the suction was controlled using the hanging column technique (HCT). (ii) a laboratory study into the bearing capacity of a strip footing placed on the surface and at depth. The numerical part in this study extended the upper bound discontinuity layout optimization (DLO) method to account for the effects of partial saturation on strength. The modified DLO method was then calibrated and incorporated into the commercial Limit- State:GEO software. The software was then utilised to perform parametric studies into the effect of suction profile, and soil strength for passive earth pressure and bearing capacity problems, comparing the latter results with the experimental part. The experimental drained direct shear and bearing capacity tests indicated an increase in the shear resistance of unsaturated samples compared to the fully dry or saturated cases. An existing shear strength equation showed poor agreement with the experimental data found in the literature as well as the direct shear results conducted in this study. A reformulated shear strength equation was then suggested, evaluated and showed good agreement with the experimental results. Furthermore, an existing SWCC equation was modified and evaluated. Although the direct shear test aimed to evaluate the existing shear strength equation for unsaturated soils, the results showed several additional interesting findings such as (i) increase of both φ and c parameters for unsaturated samples, (ii) high shear resistance for fully saturated samples compared to the fully dry case, (ii) wetting collapse behaviour at a high degree of saturation, (iii) oscillation of the shear resistance with shear displacement, (iv) drying behaviour during loading and shearing. Simple hypotheses of these behaviours are proposed throughout this thesis. The DLO analysis of the passive earth pressure and bearing capacity problems demonstrated a non-linear relationship for passive earth pressure and bearing capacity with water table depth. The passive resistance and bearing capacity was seen to initially increase with increasing suction in the soil and then this increase levelled off as the saturation of the soil fell. Changes in capacity were significant, e.g. up to factors of 10 for bearing capacity despite the small magnitude of suctions involved (0 - 6 kPa).