Development of a Borehole Shear Test Device for Rock and Concrete

ABSTRACT: This study presents the design, development, and laboratory validation of a prototype borehole-deployable shear testing device. The device was developed to readily integrate with Reclamation field explorations and mimics laboratory direct shear testing. Using radially staggered pneumatic pistons, the device applies a stress to the borehole wall while an in-line load cell measures the force required to advance the device through the borehole. The pressure applied from the pistons can be adjusted similar to the normal loading schedule of a laboratory direct shear test. The device was evaluated on cored block samples of granite, sandstone, and concrete, with accompanying laboratory direct shear tests performed on sawcut specimens from the core samples. Results indicate a strong correlation between shear strength parameters obtained from the borehole device and traditional laboratory direct shear. Linear regression fitting in normal/shear stress space indicated that the slopes from the borehole shear device were consistently lower for the borehole device, indicating that it may be best suited as empirical correlation tool rather than a direct measure of friction angle. Future research will seek to perform testing on additional rock types and explore new shear platen designs. The next stage of the research will seek to fully ruggedize the device for field evaluation.