Damage failure mode and parameter optimization of the connecting section of extendable grouting bolts

ObjectivesTo address the challenges of large deformation， prolonged deformation periods， and poor anchorage in high-stress soft-rock tunnels， extendable grouting bolts are commonly used as an improved support and reinforcement method. Enhancing the strength and deformation capacity of these bolts has become critical for controlling the significant deformation of surrounding rock.MethodsTo improve the strength of extendable grouting bolts， theoretical analysis and numerical simulations were employed to study the threaded connection characteristics and failure modes of the connecting segments. The effects of different thread types on the mechanical properties of the bolts were analyzed， and the connection strength of commonly used thread types was compared. The influence of connection length， thread pitch， and connector outer diameter on the strength and deformation capacity of the bolts was evaluated， leading to the identification of optimal parameter combinations.ResultsThe findings reveal three failure modes for the connecting segments： thread slip failure， connector pull-off failure， and bolt pull-off failure. These modes depend on the thread shear strength and the minimum tensile strength of the bolts and connectors. When the connection length is short， thread slip failure occurs； as the connection length increases， the weak point transitions from the thread cross-section to the bolt thread root， resulting in bolt pull-off failure. Increasing the thread pitch initially raises the peak load， which reaches a maximum at a pitch of 2.0 mm before declining. Similarly， increasing the connector outer diameter changes the failure mode from connector pull-off failure to thread slip failure， and eventually to bolt pull-off failure， with the connection strength peaking before declining.ConclusionsTaking a hollow grouting bolt with an outer diameter of 22 mm and an inner diameter of 12 mm as an example， the optimal tensile strength of the connecting section is achieved when the connection length is 20 mm， the thread pitch is 2.0 mm， and the connector outer dia-meter is 28 mm. Under these conditions， the peak load reaches 118.8 kN， meeting the support requirements of high-stress soft-rock tunnels.