The Application of the Borehole Torsional Shear Test to Unsaturated Soils

Hirofumi TOYOTA

The borehole torsional shear apparatus has been developed to examine and evaluate the strength of real ground. It is also possible to conduct the test using undisturbed sample in a sampling tube. The constant volume and pressure tests have been confirmed on normally consolidated clay and dry sand from previous study. The aim of this study is to investigate the application of the test to harder samples, such as unsaturated soils or overconsolidated undisturbed soils, comparing with the results of the ring shear tests. As a result, equivalent results with the ring shear test were obtained from the borehole torsional shear test (BTST). It was clarified that three types of strength, which are peak, fully softened and residual strength, were able to be measured by the BTST.
The main results of the study were summarized as follows:
1.The results of the BTST were in good agreement with those of the ring shear test concerning peak and residual strength.
2.The failure line of statically compacted unsaturated soil was equivalent to the conventional failure line of unsaturated soils.
3.Although the shear resistance angle of unsaturated soil with high water content was coincident with that of saturated soil in residual state, the shear resistance angle of unsaturated soil with low water content was slightly higher than that of saturated soil.
4.The cohesion of unsaturated soil in residual state was zero and independent on water content.
5.The peak, fully softened and residual strength of undisturbed sample were able to be obtained by the BTST.
6.It was shown that the BTST was available to the samples from soft to hard soils (N value is about 25).