Borehole Shear Test for Stiff Soil

The Iowa Borehole Shear Test (BST) is a rapid, direct shear test performed on the walls of borehole in soil. The test measures in-situ soil shear strength by failing the soil along the circumference of the hole. Sharply ridged, diametrically opposed shear plates are expanded by gas pressure to apply a normal stress against sides of the hole. A shearing stress is then applied by pulling the plates axially along the hole while maintaining a constant normal stress until failure occurs. By repeating the test with different values of normal stress, a series of shearing stress values are obtained from which a Mohr-Coulomb failure envelope is drawn. Allowing time for drainage between successive points permits interpretation of results on a consolidated-drained basis.