Abstract: A novel hollow cylinder torsional apparatus was developed for rock to investigate the variations of the principal stress and orientation induced by the excavation of rock mass. The improved apparatus can be used to study the strength, deformation and failure mode of rock under complex stress in the laboratory. This study introduced the basic structure, working principle and technical parameters of the apparatus in detail. According to the features of the hollow cylinder sample with top and bottom pressure heads, the technical problem of the torque application was solved by combining the loading axis of the axial force with the torque into one. A new self-balanced loading triaxial cell was employed to load and control four loadings independently, which were from the axial force, torque, inner and outer confining pressures. Through the software integration technology, a load control method for stress and flow was applied to perform the accurate simulation of the principal stress, the stress path of the principal axis rotation and stability control of the loading process. At last, a preliminary test was carried out using the hollow cylinder torsional apparatus with advantages: simple structure, multiple functions and easy operation. Experimental results verified the feasibility and practicability of the apparatus, and indicated that the system realised the rotation of the principal stress axes and the change of the principal stress.

Key words: rock mechanics, hollow cylinder, torsional shear, testing system, complex stress path