Method of in-situ rotation shear test for an embankment slope using a sounding borehole

- Verification by model tests and field investigations on small embankment dam -

Abstract

Because of risk of small earth dams’collapses induced by earthquakes and heavy rainfalls, the safety diagnoses of the embankments are necessary urgently. The authors developed an in-situ shear test method (BST; the Borehole Shear Test; BST) for measuring the shear strength of embankment soil easily by using a sounding borehole. Test method is 1) to install a balloon with shear blades into 42mm test hole in the embankment ground, 2) to expand the balloon at specific pressure and consolidate soil around the test hole, 3) to shear soil around the test hole by rotating the balloon and measure the torque, and 4) calculate shear strength using the Coulomb criteria by plotting peak shear stress calculated from torque to normal stress on the test hole calculated from balloon pressure. We performed large scale model tests and field investigations, and compared the shear strengths by BST to those by tri-axial shear tests. As a result, both strengths well coincided under unsaturated conditions while variation in the data of soil materials with gravel were little-bit large. In saturated soil, strengths of permeable soils by BST were close to CD-test strength, while those of impermeable soils were to CU-test strength.