Development and engineering application of a new electronically controlled borehole shear instrument

A new type of electronically controlled borehole shear test apparatus was developed to aim at the shortcomings of traditional borehole shear apparatus, such as difficulty in controlling the shear rate, large test error, and the value of test parameters relying on experience. The structure, testing principles and methods of the new apparatus were expounded. To test the stability of the new apparatus, the new apparatus was applied to the landslide site and compared with the test results of the traditional shearing apparatus. Some results are obtained as follows. The normal stress of the first stage can be equal to or slightly greater than the initial normal stress sigma(0). The relationship between normal stress increment Delta sigma and plastic stress sigma(f), the initial normal stress sigma(0) is (sigma(0)-sigma(f)) / Delta sigma=4.0-4.5, which can guide the value of Delta sigma. When the normal displacement vs. time curve tends to be the horizontal and the normal displacement rate tends to be 0, the consolidation is stable, and the soil consolidation time can be determined accordingly. According to the curves of shear stress vs. shear displacement, the stress state of the soil can be judged intuitively, thereby guiding the exerting of shear force. Compared with the traditional borehole shear test apparatus, the new instrument can obtain more reliable strength index values, while the cohesion value c measured by the traditional apparatus is very small or even negative. At the depth of 4.5 m, the numerical simulation of the shear plate being pressed into the soil and the shear process shows that the soil can be effectively consolidated when the normal stress is in the linear deformation stage of the normal displacement curve; the soil near the shear plate is dislocated, forming a strip-shaped plastic failure zone, and gradually developing into a plastic failure zone through an upper and lower when shear failure occurs.