Modification method of borehole shear mechanical parameter characterization and its experimental study

Abstract

The borehole shear test is an important method to quickly measure the shear strength of rock in the field, but its application is limited due to its large measurement error. By analyzing the principle of borehole shear and the research results, it is found that incomplete shear is the reason for the large error of rock borehole shear test, and a correction method of borehole shear method based on the unequal area of pressure and shear was proposed. Experimental equipment was designed to carry out the simulated borehole shear test, and the characterization rules of sandstone shear mechanical parameters under different shear plate parameters were studied, which proved the effectiveness and correction effect of the proposed method. Finally, the applicability of the method was verified by literature data. Specific results are as follows: ① The root cause of large error in rock borehore shear test is that the selection of compression area and shear area is distorted and unequal, the actual shear failure area is less than the area between shear teeth, and the corrected compression area and shear area increase linearly with the increase of normal force under different shear tooth lengths. Based on the relationship between the parameters of shear tooth, compression area and shear area, a correction method of borehole shear test is proposed. ② The shear stress-shear displacement curve of rock borehole shear can be divided into compaction stage, elastic deformation stage, plastic deformation stage and post-peak stage. With the increase of normal force, the positive stress, shear stress and shear stiffness increase under different shear plate parameters, and the displacement decreases when the peak point of shear stress is reached. There are obvious rules between borehole shear parameters and the characteristic value of shear mechanical parameters. With the increase of the shear tooth length, the cohesion increases linearly, and the internal friction angle increases as a quadratic function. With the increase of the shear plate area, the cohesion increases linearly and the internal friction angle decreases as a quadratic function. ③ The increase of the shear tooth length and the shear plate area can reduce part of the measurement deviation. The shear tooth length increases from 1.0 mm to 2.5 mm, the total deviation of cohesion and internal friction angle decreases from 62.96% to 19.91%, the total improvement increases from 30.8% to 74.1%. The shear plate area increases from 400 mm2 to 520 mm2, the total deviation of cohesion and internal friction angle decreases from 64.07% to 37.69%, and the total improvement increases from 39.56% to 56.05%. Both experimental and literature data validation show that the correction method improves the accuracy of cohesion and internal friction angle by about 50% and 6%, respectively.

Keywords

• in-situ test
• remediation method
• borehole shear test
• shear strength
• mechanical parameters