Influence of prestress on failure response and reinforcement mechanism of anchorage layer in tunnels: Experimental insights from P-wave velocity and PIV analysis

Abstract

The reinforcement of tunnels in weak surrounding rock using anchor bolts establishes an anchorage layer that collaborates in bearing loads, constituting a widely employed reinforcement technique. However, the bearing capacity of the anchorage layer remains unquantified, its failure response inadequately investigated, and the reinforcement mechanism of anchor bolts has yet to be comprehensively elucidated through a synthesis of mesoscopic and macroscopic perspectives. This study undertakes a series of anchorage layer loading and failure experiments under varying prestress values, aiming to quantify the bearing capacity by isolating the anchorage layer and investigating its failure response. To elucidate the reinforcement mechanism of the anchorage layer, the experiments integrate stress testing, P-wave velocity analysis, and PIV measurement. These methods provide a comprehensive perspective, elucidating stress distribution, integrity, and mechanical parameters at the mesoscopic perspective, alongside deformation and bearing capacity at the macroscopic perspective. The experimental results indicate that the failure of the surrounding rock layer (SRL) begins at the tunnel shoulder, while the failure of the anchorage layer (AL) and prestressed anchorage layer (PAL) begins at the tunnel crown, and the failure modes of AL and PAL are consistent. In comparison with SRL, both AL and PAL demonstrate notable enhancements in stress distribution, integrity, mechanical parameters, ductility, and bearing capacity. Notably, increasing the value of prestress further amplifies these improvements. For instance, relative to SRL, PAL (6P) exhibits a remarkable 244% increase in radial stress, a 23.26% rise in integrity, improvements of 23.21% in elastic modulus and 15.46% in cohesion, an approximate 217.75% enhancement in failure displacement, and striking increments of around 400% and 325% in failure load and ultimate load, respectively. From the mesoscopic perspective, the installation of anchor bolts modulates the stress distribution within the surrounding rock, enhances its integrity, and strengthens its mechanical parameters. This is subsequently manifested from the macroscopic perspective, where the ductility and bearing capacity of the surrounding rock are significantly enhanced.