Anchorage mechanical behavior of interface-shear-stress-dispersing yield bolt for soft rock tunnel

Enhancing the anchorage force and deformability of the anchor bolt is key to controlling surrounding rock stability in soft rock tunnels. Based on the previously proposed interface-shear-stress-dispersing (ISSD) bolt, this study proposes an ISSD-yield bolt to more effectively address progressive anchorage interface damage. Based on the theoretical derivation and field pull-out tests, the anchorage mechanical behavior of the ISSD-yield bolt is obtained: (1) The ISSD-yield bolt comprises compressible layers in front of each anchor to accommodate anchor deformation. During the pull-out process, the compressible layers in front of the anchors are successively compressed to fully leverage the overall bonding strength of the grout-rock interface, thus increasing the load-bearing capacity and deformability of the bolt. The result of the field pull-out test verifies that the anchorage force and the deformation capacity of the ISSD-yield bolt improve by 1.4 times and 2.4 times, respectively. (2) Mechanical analysis of the ISSD-yield bolt yields an optimal method for determining the required number of anchors ( N b ) and compressible layer thickness ( d ). With the increase in the bonding strength and the decrease in the shear stiffness of the grout-rock interface, N b decreases, and d increases gradually. Under the anchor length of 4 m to 6 m, N b should be set to 2 to 6, and d should be set to 1 cm to 10 cm. The ISSD-yield bolt provides a feasible option for the support of soft rock tunnels with large deformation and poor bonding strength of the grout-rock interface.