Study on a new energy absorbing anchor bolt and its parameter optimization for large deformation tunnel in soft rock

Anchor bolt support is one of the effective measures to solve the problem of large deformation of surrounding rock in underground engineering such as high stress soft rock tunnels and roadways. A new type of constant resistance energy absorbing anchor bolt is proposed, which is composed of a cone block, a smooth anchor body, a sleeve, a limit ring, an anchor tray, and a fastening nut, to address the problem of poor deformation adaptability of traditional mortar anchor bolts. When the surrounding rock deforms, the cone blocks slide in the casing to adapt to the large deformation of the surrounding rock and provide constant resistance; when the deformation of the surrounding rock reaches the allowable value, the anchor bolt transforms into a rigid support of traditional anchor bolts, relying on the material of the bolt body to provide bearing capacity. Numerical experiments are conducted to study the working performance of the new anchor bolt under different cone angles, casing wall thicknesses, and friction coefficients using numerical and indoor tensile testing methods. The geometric parameters of the new anchor bolt are optimized, and the performance of the optimized anchor bolt is verified through indoor testing. The numerical analysis and indoor test results indicate that when the cone angle of the new anchor is small, the load displacement curve of the anchor can be divided into three stages linear elasticity, transient descent, and constant resistance; when the cone angle is large, the short descending segment of the curve disappears. The average constant resistance and energy absorption capacity of the new type of anchor bolt increase linearly with the increase of anchor bolt cone angle and casing wall thickness α=15°, casing wall thickness t=5 mm, friction coefficient μ=0.20, the anchor bolt has the best working performance, with an average constant resistance of 186.32 kN and an absorption energy of 33.4 kJ. Indoor experiments have verified the superior working performance of the new energy absorbing anchor bolt.