8A hybrid mechanism for local failure of tunnel face collapse in $c - \varphi$ soils

Abstract

This paper develops a novel two-dimensional failure mechanism for local stability of tunnel face in $c - \varphi$ soils. This mechanism is hybrid in which the failure is involved in complex movement. The failure blocks respectively with the translational and rotational movements are introduced. To fulfill the compatibility requirement, the interface between the failure blocks is constructed by employing a spatial discretizing technique. In the framework of upper bound analysis, the energy that dissipates along the discontinuities and within the failure block are fully accounted and the solution of support pressure is derived. Optimization is performed based on the solution to select the optimal mechanism, obtain the support pressure, and identify the failure mode. The proportion of translational block to rotational block is explored. The energy dissipates along the discontinuities are compared to that dissipates within the block. The parametric analysis is performed to explore the influence of soil shear strength on the stability. The result indicates that the frictional angle has more significant influence on the failure mechanism than the cohesion. The failure movements in different patterns identified. The proportion of the failure involved in the rotational movement is reduced significantly with increasing $\varphi^{\prime}$. For the proposed hybrid mechanism the rotational movement is dominant at low $\varphi^{\prime}$ and it is replaced by the rotational one at high $\varphi^{\prime}$. This finding supports previous findings regarding mechanism constructing states; for pure cohesive soils the rotational mechanism works well, whereas, for high frictional soils, the translational mechanism outperforms it. This paper ends with the validation of the current solution by comparing to other analytical solutions and numerical simulation.