Abstract
The surrounding rock often show obvious rheological characteristics when tunneling in high geostress environment, accompanied by large deformation, slow convergence and increasing support pressure. “Yielding-resistance combination” support inspired by the concept of NATM is widely employed in squeezing tunnels which has been successfully implemented to control the large deformation in many high geostress soft rock tunnels. However, some failure cases indicate that all kinds of “yielding-resistance combination” support strategies are not universally applicable to all squeezing tunnels. To seek an effective support strategy for squeezing tunnels, different kinds of “yielding – resistance combination” supports are summarized into three categories, i.e., “yield before resist” support, “yield – resist” support, and “control – yield – resist” support, which are simplified to different stress paths. A simplified two-stage calculation method, referring the first stage “yielding stage” and the second stage “resist stage”, is proposed to derive the time-dependent rock stresses, strains, displacements, displacement rates, and support pressures based on Mohr-Coulomb (MC) criterion and the generalized Zhang-Zhu (GZZ) strength criterion, respectively. Results show that the deformability of plastic rock will be underestimated if the stress path is not considered, the calculated rock displacement will be larger if the actual stress path is considered. The duration of the first stage t0, the deformation at the first stage Δu0 and the primary support pressure pi0, and the allowable displacement rate u̇c are major parameters for the design of tunnel support in squeezing rocks. The allowable displacement rate u̇c cannot be very high to ensure the stability of permanent support at the second stage. However, the construction period would be long if the allowable displacement rate u̇c is very low which would increase the construction cost. It is suggested that the duration of the first stage t0 be as short as possible if a relatively low value of u̇c is ensured.
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