Experimental study on the mechanical properties of simulated columnar jointed rock masses

Columnar jointed rock mass is a kind of structural rock mass commonly encountered in igneous rocks. Due to the effects of columnar joint networks, anisotropy is the typical mechanical property of columnar jointed rock mass, i.e. deformation and strength varying with loading direction. Correct understanding of the mechanical anisotropy of columnar jointed rock mass is a key problem that should be solved for demonstration and design of large scale rock mass projects such as dams and underground cavern excavations constructed in it. Plaster simulated columnar jointed rock mass specimens at dip angles varying from 0 degrees to 90 degrees with respect to the axial stress were tested under uniaxial compression conditions to investigate the mechanical anisotropy and failure modes. Based on analyses of experimental results, it was found that the strength and deformation of columnar jointed rock masses had pronounced 'U-shaped' anisotropy. In the anisotropic curves, the maximum and minimum values occurred at beta = 90 degrees and beta = 45 degrees, respectively. It was also shown that the lateral strain ratio was relatively high, especially when the dip angle was close to (45 degrees-phi(j)/ 2), where phi(j) was the joint friction angle. An empirical expression was adopted to predict the 'U-shaped' anisotropy of deformation and strength and the predicted anisotropic curves agreed reasonably well with experimental data. Furthermore, four types of failure modes were summarized based on experimental results and corresponding mechanisms were also discussed.