Automatic transformation of soil borehole shear equipment and its applications

In view of the shortcomings of the existing borehole shear equipment, the existing borehole shear equipment was transformed automatically. In order to evaluate the applicability of the modified borehole shear equipment to measure the shear strength of the soil, taking the cohesive soil of the Optics Valley Innovation Base and the residual soil of the Pazhou Station of the Guangfo Ring Line as the objects, a large number of borehole shear tests were carried out using the modified borehole equipment. The shear strength parameters of undisturbed clay soil and residual soil (undisturbed, short-term immersion and 24 h immersion) were obtained. The differences between the results of borehole shear test and direct shear test were analyzed from various aspects. Taking the silty clay of Nanjing Lukou Airport Station as the object, the influence of normal pressure was emphatically analyzed. The results show that the linear correlation of the borehole shear test data of clay soil and residual soil is good. After short-time (2～3 h) immersion, the cohesion of residual soil decreases by 15.28%～ 40.45%, the internal friction angle of upper soil of borehole increases by 17.35%～30.90%, and the internal friction angle of lower soil of borehole decreases by 4.83%～8.75%. After 24 h immersion, the cohesion of residual soil decreased by 45.10%～69.33% and the internal friction angle decreased by 52.14%～69.87%. Through comparison, it is found that the cohesion values obtained by the borehole shear test is smaller than that obtained by the direct shear test, and the internal friction angle is larger. Through the analysis of the shear strength parameters of silty clay at Nanjing Lukou Airport Station with the change of normal pressure, it is found that when the normal pressure at all levels increased from 30 kPa to 50 kPa, the cohesion values increases by 2.88 times and the internal friction angle decreases by about 41.6%, which better explains the difference between the results of borehole shear test and the direct shear tests. The research results can provide reference for the application of soil and the selection of normal pressure. © 2023, Central South University Press. All rights reserved.