Influence of Flooding on Borehole Shear Test (BST) Results in Unsaturated Soil

This study was undertaken to begin evaluating the influence of soil suction on shear strength parameters determined from the Iowa Borehole Shear Test (BST) in unsaturated soil, namely the friction angle and cohesion intercept. A new technique, the Flooded Borehole Shear Test (FBST), was employed in this study. The BST, before and after flooding the borehole, was conducted for comparison of results. Suction was determined using the filter paper technique on undisturbed specimens in accordance with the ASTM D 5298 test method. Results showed that the friction angles determined from the FBST were much lower than those determined from the BST, as expected, apparently due to the reduction in suction due to flooding. The reduction in friction angle due to flooding generally followed a predictable pattern that was a function of the suction prior to flooding. However, the effect of suction on the cohesion intercept was much less pronounced. The cohesion intercepts from the FBST generally decreased slightly in comparison to the BST results, although cohesion intercepts were small in both cases. This paper discusses the practical implications associated with BST testing in unsaturated soil.