Mechanical properties of partially bonded rock anchors in squeezing large-deformation soft rock tunnels

Abstract

Partially bonded rock anchors are crucial structural tunnel support elements whose effectiveness depends on their interactions with the surrounding rock. This study combined laboratory experiments and numerical simulations to examine the mechanical behavior of partially bonded rock anchors under pull-out tests, focusing on the influence of bonded lengths. The axial strain of the anchor body and the vertical strain of the concrete block along the anchor axis were tested using distributed optical fibers. The results indicated that the anchor body stress remained relatively constant in the unbonded sections but gradually decreased as the depth into the bonded section increased. Simultaneously, the significant compressive stress range induced by the rock anchor in the concrete block decreased rapidly as the bonded length increased. Subsequently, the effects of different bonded lengths, unbonded section positions, and bonding combinations on the mechanical behavior and support effectiveness of partially bonded rock anchors in highly stressed soft rock tunnels undergoing substantial deformation were investigated using numerical simulations in ABAQUS. These findings suggest that rock anchors with longer bonded lengths have better control of the rock between the anchors. Simultaneously, the maximum axial stress of rock anchors tended to increase with longer bonded lengths. The rock displacement tended to be obviously larger when the unbonded section was set at the end of the rock anchors compared with when the unbonded section was set in the middle. The rock displacement was larger when the unbonded section was set at the outer end of the rock anchors, indicating that setting unbonded sections near the tunnel wall was detrimental to controlling rock deformation while improving structural safety. Setting one unbonded section near the tunnel wall and another unbonded section in the middle of the rock anchors resulted in excellent structural safety and rock deformation control. Furthermore, fully bonded rock anchors were compared with partially bonded rock anchors based on suitable bonding combinations in the deformation section of Muzhailing Tunnel. The results indicated that the fully bonded rock anchors experienced axial forces exceeding the yielding load in the early stages of support, with noticeable early yielding of the anchor plate. In contrast, the unbonded sections in the partially bonded rock anchors did not yield in either the bonded anchor body or the anchor plate. Partially bonded rock anchors exhibited better overall stability and reliability under high-deformation tunnel conditions.