Study on Shear Strength Characteristics of Columnar Jointed Basalt Based on in-situ Direct Shear Test at Baihetan Hydropower Station

Columnar jointed basalt (CJB) widely distributes in the dam site of the Baihetan Hydropower Station. The columnar joint structure and fracture development of CJB have significant influence on the mechanical properties of rock mass, and the mechanical properties of CJB are of great significance to the Baihetan Hydropower Project. Therefore, in-situ direct shear tests were carried out on ten test adit at different locations in the dam site area to study the shear behavior of CJB. In this study, 21 sets of in-situ direct shear tests were conducted for rock types of type II2, type III1 and type III2, with horizontal and vertical shear planes and two different specimen sizes of CJB. Shear strength parameters of CJB were obtained by linear fitting of in-situ direct shear test results based on the Mohr-Coulomb strength criterion. The results indicate that the shear strength parameters of CJB with horizontal shear plane increase as the increase of rock type grade. The shear strength parameters of CJB show obvious anisotropy and the friction coefficient of the horizontal shear plane is greater than the vertical shear plane. The friction coefficient in the horizontal direction of the shear plane is 1.27 times that in the vertical direction of the shear plane. With the increase of rock type grade, the difference of friction coefficient becomes larger. However, the cohesion changes little whether the shear plane is horizontal or vertical. In addition, the size effect of CJB in this area is significant. The shear strength parameters of large size (100 cm x 100 cm) specimens are lower than those of regular size (50 cm x 50 cm) specimens. The reduction of cohesion is greater than that of the friction coefficient. For rock type III2, the cohesion of large-size specimens is 0.637 of the regular-size specimens. The reduction percentage of the friction coefficient for type III2 is 1.66 times that of type III1. The reduction percentage of the cohesion for type III2 is 1.27 times that of type III1. The size effect decreases with the increase of rock type grade. The research results of this study can provide an important basis for the selection of rock mechanics parameters in the dam site area of Baihetan Hydropower Station and the stability analysis of the dam foundation and rocky slopes.