Effects of anti-pull ties on the bearing behaviors of shallow tunnel-type anchorages in soft rock

Tunnel-type anchorages (TTAs) installed in human gathering areas are characterized by a shallow burial depth, and in many instances, they utilize soft rock as the bearing stratum. However, the stability control measures and the principle of shallow TTAs in soft rock have not been fully studied. Hence, a structure suitable for improving the stability of shallow TTAs in soft rock strata, named the anti-pull tie (APT), was added to the floor of the back face. Physical tests and numerical models were established to study the influence of the APT on the load transfer of TTAs, the mechanical response of the surrounding rock, the stress distribution of the interface, and the failure model. The mechanical characteristics of APTs were also studied. The results show that the ultimate bearing capacity of TTAs with an APT is increased by approximately 11.8%, as compared to the TTAs without an APT. Also, the bearing capacity of TTAs increases approximately linearly with increasing height, width, length, and quantity of APTs, and decreases approximately linearly with increasing distance from the back face and slope angle of the tie slope. The normal squeezing between the tie slope and the surrounding rock increases the shear resistance of the interface and expands the range of the surrounding rock participating in bearing sharing. Both tension and compression zones exist in the APT during loading. The tension zone extends from the tie toe to the tie bottom along the tie slope. The range of the tie body tension zone constantly expands to the deep part of the APT with an increasing load. The peak tensile stress value is located at the tie toe. The distribution of compressive stress in the tie body is the largest at the tie top, followed by the tie slope, and then the tie bottom.