JZUS - Journal of Zhejiang University SCIENCE

Journal of Zhejiang University SCIENCE A 2016 Vol.17 No.11 P.886-902

Upper bound analysis for estimation of the influence of seepage on tunnel face stability in layered soils

Author(s): Wei Liu, Bettina Albers, Yu Zhao, Xiao-wu Tang
Affiliation(s): School of Urban Rail Transportation, Soochow University, Suzhou 215131, China; more
Corresponding email(s): zhao_yu@zju.edu.cn
Key Words: Face stability, Upper bound analysis, Support pressure, Groundwater seepage, Layered soils

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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

目的：盾构隧道在成层土中掘进时，地下水渗流容易引起盾构开挖面失稳破坏。本文提出考虑地下水渗流的机动场模型，探讨成层土中地下水渗流规律，研究渗流对开挖面稳定性的影响，并提出渗流条件下成层土中盾构支护压力的计算方法。
创新点：1. 提出考虑地下水渗流盾构开挖面失稳机动场的模型；2. 建立成层土中地下水渗流模型； 3. 推导考虑地下水渗流的盾构开挖面极限支护压力上限解。
方法：1. 根据已有工程案例，对渗流条件下成层土中盾构开挖面失稳进行受力分析（图5），并提出开挖面失稳机动场模型（图6）；2. 通过上限分析，推导得到盾构开挖面失稳极限支护压力计算公式（公式29）；3. 对成层土中地下水渗流进行数值模拟，并采用理论模型（图15）对渗流规律进行表征；4. 研究极限支护压力对地下水渗流因素的敏感性。
结论：1. 地下水渗流在失稳土体内部产生渗流力作用，在盾构开挖面上也对支护压力产生抵消作用。2. 提出成层土中考虑地下水渗流的失稳机动场模型，并推导出极限支护压力上限解； 3. 在盾构土舱未进行渗透性改良的条件下，成层土中地下水渗流在1200 s内达到稳定，其中，穿越层渗流方向主要为水平向，而覆土层中主要为竖向渗流；4. 考虑渗流影响，本文上限解预测的支护压力值更为合理。

关键词：开挖面稳定；上限分析；支护压力；地下水渗流；成层土

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