Mechanical Anisotropy of the Columnar Jointed Rock Mass Under Unloading Conditions

The mechanical properties of a columnar jointed rock mass (CJRM) under loading and unloading conditions differ appreciably. The CJRM at the Baihetan Hydropower Station has a distinct type of joint and a self-organized joint network of discontinuities. Under an unloading condition during excavation, the joints present a characteristic anisotropy phenomenon. To study the relationship between the strength and deformation of the CJRM, a series of triaxial unloading tests were conducted on the CJRM physical model with a servo-controlled testing machine. The results indicate that the deformation has an obvious correlation with the angle β between the column direction and horizontal. Poisson’s ratio of the CJRM varies with β between 0.07 and 0.42 during the loading of axial stress. Furthermore, the relation between the confining pressure reduction and deformation modulus reduction of the CJRM as a function of β was studied.