Abstract: The hollow cylinder torsional testing system (RHCA) was used to conduct true triaxial test on gray sandstone. Controlled conditional compression tests (?2??3), constant ?3 and variable ?2 conditional compression tests were carried out respectively. ?2 is the intermediate principal stress, ?3 is the maximum principal stress. Strength and failure characteristics were analyzed in detail, and intermediate principal stress effects, minimum principal stress effects, stress Lord angle effect and applicability of strength criterion were further analyzed. The results indicate that rock hollow cylindrical apparatus can realize the simulation of true triaxial stress path and the end friction can be avoided. The pressure difference between the inner and outer confining pressure has a great influence on the strength of the rock, especially under high confining pressure, a large pressure difference makes the strength of the rock weaken rapidly, resulting in severe damage. The intermediate principal stress has a significant effect on the strength of the rock and has an interval effect, and the minimum principal stress has a strengthening effect on the strength of the rock. This feature has an important guiding significance for the efficient and timely support of the rock mass in actual engineering excavation. Further, according to the obtained test results, the strength criteria used to describe the mechanical properties of the rock at this stage were analyzed. It is considered that the calculated rock strength obtained by using the exponent criteria can better reflect the stress state of rock and has a relatively good applicability. It can be used to evaluate the strength of rock in practical engineering.

Key words: rock mechanics, hollow cylindrical specimen, true triaxial test, strength criterion