Structural characteristics of columnar jointed basalt in drainage tunnel of Baihetan hydropower station and its influence on the behavior of P-wave anisotropy

Researches about the structural characteristics of rock mass are the basis for understanding their failure modes, mechanism properties as well as evaluation of structural stability during the process of excavation. Natural columnar jointed basalt has complex structures and exhibits remarkable anisotropy and size effect. In this study, the observation window method was firstly applied to describe the natural columnar joints and pillar shapes at engineering scale and then the structural characteristics of columnar joints were analyzed. The roughness of columnar joints scanned from large-size rock core obtained at drainage tunnel #1-1 was secondly investigated based on the 3D scanning method. Some techniques, such as field statistical analysis and CT scanning were used to analyse the characteristics of internal implicit joints in columnar jointed basalt. Thirdly, the large scale columnar jointed basalt cores were tested by P-wave, which assisted to investigating the anisotropy of columnar jointed basalt. The experimental and analytical results indicate that, at the engineering scale, natural columnar jointed basalt is heterogeneous, exhibiting the features of mosaic structure and mainly quadrilateral in shape. At the large-size rock core scale, the roughness of columnar joints is low with the value of 5.558. At the basalt column scale, the internal implicit joints are not randomly distributed, instead, showing obvious tendency. The internal implicit joints and columnar joints are important factors contributing to the special failure mode of columnar jointed basalt. At the rock core scale, with increasing angle between the P-wave propagation direction and column, the anisotropy of columnar jointed basalt core gradually increases and the degree of anisotropy is proportional to the number of columnar joints. The internal implicit joints could also affect the P-wave anisotropy of basalt column and the results in this study can provide some references for projects related to columnar jointed rock mass.