Mechanical deformation characteristics of support system for boulder-cobble tunnel

In response to the engineering and technical problems such as the extreme difficulty in excavation and support construction, and the frequent collapse disasters at the arch in the Nianggaicun Tunnel of Lhasa-Nyingchi Section on the Sichuan-Xizang Line, a combination of excavation and support system containing "three-bench complementary cyclic excavation+section steel arch+shotcrete+double-layer dense reinforcement mesh+multiple groups of feet-lock bolts (pipes)+grouting behind the lining" was proposed. Two typical sections of boulder-cobble tunnel were selected to conduct the test research on the stress and deformation of the support system. The load action characteristics of the surrounding rock, the mechanical properties of the support system, and the deformation laws inside and outside the tunnel were analyzed. The load-bearing action mechanism of the new support system was revealed for the boulder-cobble tunnel, and corresponding new prevention and control principles were summarized and proposed. Analysis results show that the surrounding rock pressure is mainly a loose collapse load at the arch and distributes unevenly around the tunnel. The average load-sharing ratios of initial support and secondary lining are 67.65% and 32.35%, respectively. The feet-lock bolts are either tensile or compressive, and the maximum tensile and compressive forces reduce by 45.9% and 20.0% after optimization, respectively. The force of the secondary lining is overall small with enough structural safety reserve. The vault subsidence is less than 15 mm, and the horizontal convergence is 8-9 mm in the tunnel body. At the tunnel portal section, the deformation is asymmetric and significantly affected by the shallow bias and rainfall conditions. The maximum settlement of the arch part reaches 52.4 mm, and the horizontal convergences of the upper and lower benches are 11.4 and 15.6 mm, respectively. Under the similar unfavorable conditions, the inverted arch and secondary lining should be constructed as early as possible to ensure the construction safety. The support system design for the boulder-cobble tunnel follows the prevention and control principles of "less disturbance, strong arch foot, anti-overbreak, dense reinforcement mesh, and frequent grouting". It can timely control the expansion of the loose zone at the arch and mobilize the self-bearing capacity of the deep surrounding rock, achieving the purpose of improving the mechanical performance of the support structure and effectively avoiding the occurrence of arch collapse disasters.