Unsaturated soil interfaces exist where unsaturated soil is in contact with structures such as foundations, retaining walls, and buried pipes. The unsaturated soil interface can be defined as a layer of unsaturated soil through which stresses are transferred from soil to structure and vice versa. In this paper, the shearing behavior of unsaturated soil interfaces is examined using results of interface direct shear tests conducted on a low-plasticity fine-grained soil. A conventional direct shear test device was modified to conduct direct shear interface tests using matric suction control. Further, the results were used to define failure envelopes for unsaturated soil interfaces having smooth and rough counterfaces. Results of this study indicate that matric suction contributes to the peak shear strength of unsaturated interfaces; however, postpeak shear strength did not appear to vary with changes in matric suction. Variations in net normal stress affected both peak and postpeak shear strength. Failure envelopes developed using the soil-water characteristic curve (SWCC) appeared to capture the nonlinear influence of matric suction on shear strength of soil and interfaces.

SUMMARYA constitutive model based on the disturbed state concept is presented to describe the behavior of interfaces in unsaturated soil. The model is an extension of an existing model developed for a sand-steel interface. As opposed to the original model, the modified model incorporates two independent stress variables, which are the net normal stress and matric suction. The saturated and dry state of the interface can be modeled as a special case using the constitutive model presented in this paper. The modified model is capable of capturing the main features of unsaturated interfaces observed during laboratory testing, including increasing shear strength and strain softening with increasing suction and net normal stress and increasing dilatancy with increasing suction. Laboratory tests were carried out on unsaturated interfaces in a modified direct shear test apparatus. The observed behavior of interfaces between unsaturated soil and steel plates (rough and smooth) is presented in comparison with model predictions.

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This paper evaluates three lateral load tests performed on 406 mm (16-inch) diameter Auger Cast Piles (ACP). The lateral deflection values of the ACP measured in the field are compared with the lateral deflection values of the ACP predicted using a commercial computer program LPile. The predictions were made assuming the linear as well as the non-linear variation of the flexural stiffness of ACP. A sensitivity analyses was performed to determine the effect of various soil parameters on the lateral deflection of the ACP. The sensitivity analysis indicated that soil layers at a depth of about 0.3 time the length of the pile (i.e., 0.30L, where L = length of the pile) or greater has a negligible effect on the lateral deflection of the ACP. Based on the results of the lateral load test data of ACP, it is determined that the measured response of ACP to lateral load is somewhere in between the responses predicted using the linear and non-linear flexural stiffness of ACP. Further, reliable prediction of lateral displacement is not possible without knowing the accurate values of modulus of elasticity of pile material.

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