Study on the bearing characteristic and failure pattern of tunnel-type anchorage

The wedge-shape geometry of tunnel-type anchorage makes the bearing behavior of the system of rock and anchor change with the engineering load. The ultimate bearing capacity varies with the failure modes of the anchor-rock system. To figure out problems mentioned above, the possible failure modes, the bearing behavior and the estimation formula of bearing capacity of tunnel-type anchorage in different loading stages are discussed in this paper. Then the evolution law of the anchor-rock system progressive failure process is revealed with the methods of numerical simulation analysis, the author edited programming and the 2-D laboratory model test. Meanwhile, the influences of the anchor plug geometry parameter, such as wedged angle and burial depth, on the failure surface shape, rupture angle and ultimate bearing capacity are analyzed as well. Main conclusions are draw as follows. (1) The failure mode of anchor-rock system revealed by numerical model test is trumpet-shaped. And it is verified by the 2-D laboratory model test. (2) The bearing behavior is distinctly divided into three stages. In the initial stage, the additional interfacial pressure is not generated. Then the pressure increases linearly with the increase of engineering load in the mid-term. At the later stage, the pressure decreases rapidly because the damage of surrounding rock. The mechanical mechanics of tunnel-type anchorage' stage characteristic can be described as follows. The initial bearing capacity of tunnel-type anchorage only depends on the geometry parameters of anchor plug and it is generated by the gravity of anchor plug. But the ultimate bearing capacity depends on the influenced range of surrounding rock. So the ultimate bearing capacity is determined by the position, mode and rupture angle of failure surface. And the reasonability of the mechanical generalization model and stage division method is verified by the numerical model test results, which mainly conclude the interfacial pressure change law with engineering load and the plastic zone evolution process. Moreover, according to the wedged angle and burial depth sensitivity analysis results, we find that the failure mode tends to a circular table shape when the burial depth and wedged angle is large. But the failure mode tends to trumpet-shaped while the burial depth is between 35-45 meters and the wedged angle is 2-6 degree. While the burial depth is greater, the failure mode tends to interface failure. Notably, if the failure mode is trumpet-shaped, the narrow section's rupture angle of failure surface is two to three times the wedged angle while the wide section angle ranges from 20 to 25 degree. Besides, the distance between the turning point and the back anchorage face is about 1/2 times the burial depths. (3) The burial depths have no influence on the initial bearing capacity. But the ultimate bearing capacity increases with the increase of burial depth. It is also to be known that the initial bearing capacity decreases gradually with the increase of wedged angle. And the ultimate bearing capacity increases first and then decreases with the increase of wedged angles. The above phenomenon indicates that there is optimal wedged angle for the tunnel-type anchorage. © 2019, Science Press. All right reserved.