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Abstract: Tunnel face stability is important for safe tunneling and the protection of the surrounding environment. upper bound analysis is a widely applied method to investigate tunnel face stability. In this paper, a tunnel face collapse of Guangzhou metro line 3 is presented. Accordingly, seepage is considered in the upper bound solutions for face stability in layered soils. Steady-state seepage is reached in the first 1200 s of each drilling step. In the crossed layer, the seepage flow is horizontal toward the tunnel face, whereas in the cover layer, the seepage vertically percolates into the crossed layer. By considering the seepage forces on the tunnel face and on the soil particles, the upper bound solution for the support pressure needed for face stability in layered soil with seepage is obtained. Under saturated conditions, the support pressure is influenced by the variation of the depth ratio due to the seepage effect. Moreover, the support pressure depends linearly on the groundwater level. This study provides estimations of the support pressure for face stability in tunnel design.

The paper presents an interesting analysis on the tunnel face stability derived in layered soils obeying Mohr-Coulomb failure criterion under seepage flow conditions by the kinematic approach of the limit analysis. The work is mainly conducted based on the multi-layered failure mechanism of Tang et al (2014), and the seepage force, which is regarded as a body force in the work calculation, is calculated with the elementary theory for semi-confined aquifers (Verruijt, 1970). Comparative calculations are presented in order to discuss the effect of some model parameters, mainly the tunnel diameter and the groundwater level. The Authors conclude that the seepage effect significantly affects the stability of a tunnel face; especially the required support pressure is proportional to the hydrostatic hydraulic head. In a word, the manuscript is very well-written with convincing results.

成层土中地下水渗流对盾构隧道开挖面稳定影响上限分析

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