Development and functional testing of a tension-shear-torsion test system for bolt-anchored fractured rock mass

Addressing the frequent occurrences of surrounding rock instability and the unclear mechanisms of bolt anchorage failure in complex stress environments necessitates urgent laboratory research on bolt loading response testing under simulated real working conditions. We have independently developed a full-scale tension-shear-torsion test system for bolt-anchored fractured rock masses, based on the principle of active loading tests where rock masses exert forces on bolts. This system effectively simulates and reproduces the complex stress states experienced by bolts under multiple loading paths in underground tunnels. A key innovation of this system is its design, which allows rock masses to crack, shear-slip, and rotate along their fracture planes, thus replicating the mechanical behavior of bolts in situ. Utilizing a servo-controlled, multi-degree-of-freedom coupled loading system, it applies composite loads of tension (±1 000 kN), shear (±2 000 kN), and torsion (±2 000 N·m) simultaneously. This approach effectively addresses the challenge of simulating multi-dimensional stress coupling on bolts at rock mass fracture planes in actual mining engineering. Functional tests and result analyses indicate that the system can stably achieve comprehensive loading and mechanical response acquisition of bolts under combined tension-shear-torsion loads. This capability accurately reflects the bearing characteristics and failure mechanisms of bolts at rock mass fracture planes. It was observed that the rock bolt experiences combined tension-shear loading when the rock mass shears along its fracture surface, with the anchoring shear stiffness of the test rock bolt being less than its tensile stiffness. Additionally, the conditions of the rock mass significantly influence the deformation capacity of the bolts. We propose an evaluation framework centered on total tension-shear resistance and total displacement for assessing bolt anchorage performance. Furthermore, we discuss application strategies for bolt selection based on test results and performance evaluations. The successful development of this test system provides an innovative experimental platform for uncovering the composite loading failure mechanisms of bolts in deep rock masses and establishing evaluation standards for bolt anchorage performance.