Optimization analysis on thread connection parameters of an improved hollow grouting bolt

For soft rock tunnel with large deformation, timely anchorage support is often needed. Especially in the tunnel with low strength surrounding rock, the hollow grouting resin bolt is usually chosen to apply for prestressed anchorage support due to the small disturbance of surrounding rock. The large deformation of surrounding rock after excavation will require higher requirements on a bolt, how to improve the strength and deformation ability of the bolt becomes the key to controlling the stability of surrounding rock. Considering the thread connection characteristics between resin and mortar section, the influences of thread pitch, connection length, and outer wall thickness on the strength and deformation ability of the connection segment were analyzed by numerical simulation, and the optimal combination of thread parameters was obtained. The results show that affected by the shear strength of thread and tensile strength of steel bar and hollow outer wall, the failure modes of connection segments can be divided into three types: hollow outer wall tensile failure, thread slippage failure, and rebar tensile failure. The connection segment is prone to thread slippage failure when its length is insufficient, while the weak position will transfer from thread to the hollow outer wall with the increase of connection length. Under the condition of different thread pitches, an optimal connection length should be existed to make full use of each part's strength, which effectively improves the overall strength of the bolt connection segment. With the increase in thread pitch, the overall mechanical performance of the connection segment is significantly improved. When the thread pitch is 1.5 mm, the shear strength of the thread is the worst, which should be avoided in design. When the diameter of the hollow outer wall is constant, the connection segment is prone to hollow outer wall tensile failure if the diameter of the steel bar is too large, and rebar tensile failure is instead while it is small. With the increase of hollow outer wall diameter, the failure mode of the connection segment transfers from hollow outer wall tensile failure to rebar tensile failure, but the strength of the connection segment does not improve greatly. Therefore, the diameter between the steel bar and the hollow outer wall should be reasonably matched in the design. For the bolt with a hollow outer wall diameter of 32 mm, rebar diameter of 25 mm, a thread pitch of 2 mm, and connection length of 16 mm, the tensile performance of the bolt is the best, which can meet the strength and deformability requirements of large deformation tunnel in soft rock. © 2022 China Coal Society. All rights reserved.