Experimental study on improving mechanical characteristics of initial support structure of deep buried large-span tunnels with long bolts or cables

To address the issues of the initial cracking at the arch foot and crown and the excessive deformation of steel frames in deep-buried, large-span soft rock tunnels, a support technology involving localized addition of long anchor bolts or cables is proposed to adjust and improve the structural stress. Based on the tunnel structural performance testing platform independently developed by the authors, the stress and deformation characteristics of the initial support structures under the same surrounding rock loads are compared and analyzed between the system anchor support and multiple types of long anchor bolts or anchor cables deployment schemes. The effects of long bolts/cables support with different circumferential spacings and layout ranges are studied. The research results show that: (1) Under the conventional support, the initial support of a large-span tunnel under surrounding rock loads tends to flatten, with the inner side of the arch crown and the outer side of the arch foot bearing the maximum bending moment of the structures being the first to crack. After the inner side of the inverted arch cracks, the model accelerates deformation, leading to the overall instability and damage of the structures. (2) Under the four conditions of adding long anchor bolts or anchor cables for support, the structural safety factors at the initial support arch crown are 4.59 times, 2.12 times, 1.96 times and 1.80 times those of the conventional support system, and the structural safety factors at the arch toe are 5.23 times, 2.80 times, 2.34 times and 2.37 times those of the conventional support system, respectively. (3) Placing long anchor bolts at a circumferential distance of 2 m within 120 ° of the arch has the best effects on improving the internal force of the initially supported structures. The combined effects of local negative bending moments generated by the strong axial tension at the support points offset the larger positive bending moments at the arch crown. (4) The overall supporting force of long anchor bolts and anchor cables decreases first and then increases from the arch crown to the arch shoulder side.