Research Paper
Influence of seepage and tunnel face opening on face support pressure of EPB shield

https://doi.org/10.1016/j.compgeo.2021.104198Get rights and content

Abstract

The hydraulic head on the tunnel face is smaller than that in the far-field ground when the earth pressure balance shield excavates in a saturated stratum. The seepage force could thus drag the soil to the tunnel face. However, the effect of the seepage on the tunnel face stability is not yet thoroughly understood. A series of centrifuge tests and numerical back-analyses by the Finite Difference Method were performed to investigate the influence of the ground anisotropic permeability and the tunnel face opening on the tunnel face stability under the seepage condition. The results show that the pore-water pressure in front of the tunnel face decreases with the increasing opening area on the tunnel face and the increasing kv/kh (the ratio of vertical hydraulic conductivity to horizontal hydraulic conductivity) as well. A higher support pressure is required for a higher kv/kh to keep the tunnel face stable. When the tunnel face is sealed, the minimum support pressure to keep stable is 1.92 times that in the condition with the fully permeable tunnel face. It indicates that seepage induced by the water ingress could benefit the tunnel face stability.

Introduction

The seepage induced by the underground waters in the environment impacts human construction or existing structures in the soil (Kuriqi et al., 2016, Ali et al., 2020). When the earth pressure balanced shield is excavating in a stratum with a high water level, for example, tunneling under a river, there is naturally a difference in the hydraulic head from the ground to the shield chamber (Anagnostou & Kovari, 1996). It results in seepage toward the tunnel face because the hydraulic head at the chamber is close to the atmospheric pressure, which is lower than the hydraulic head in the ground. The seepage force drags the soil to the tunnel face, which may deteriorate the tunnel face’s stability. The tunnel face collapse possibly lead to a sinkhole on the ground and burying of the shield. Hence, it is essential to derive deep insight into the tunnel face’s failure mechanism under seepage conditions and determine the minimum required face support pressure.

Many researchers have analyzed the stability of the tunnel face (Horn, 1961, Leca and Dormieux, 1990, Anagnostou and Kovari, 1996, Broere, 2001, Oblozinsky and Kuwano, 2004, Kirsch, 2010, Idinger et al., 2011, Chen et al., 2013, Lü et al., 2018, Zou et al., 2019). However, only a few considered the seepage induced by the water ingress on the tunnel face. Anagnostou & Kovari (1996) proposed a classical 3D wedge-prism limit equilibrium model for calculating the limit effective support pressure in the condition of steady-state seepage. By assuming two conical failure mechanisms in front of the tunnel face, Lee et al. (2003) established a typical upper-bound solution for limit effective support pressure, in which the seepage force was computed by FEM software. Perazzelli et al. (2014) also proposed a limit equilibrium model by considering the failure mechanism. Several upper-bound solutions (Liu et al., 2012, Wang et al., 2013, Lü et al., 2017) were suggested by adjusting the statically admissible stress field.

Besides the improvement of the theoretical solutions, a large number of numerical solutions have been developed. The Finite Element Method was mainly adopted for investigating particular cases on the failure of the tunnel face induced by the steady-state seepage (Pellet et al., 1993, Ströhle and Vermeer, 2010, Li et al., 2011). Compared to analytical and numerical methods, earlier experimental methods for studying the tunnel face’s failure induced by the steady-state seepage were subjected to some difficulties and limitations. A series of 1 g small-scale model tests were performed to investigate the effects of the hydraulic head difference between the ground and the chamber on the limit effective support pressure (Lee et al., 2003). However, the stress level of the 1 g model test is much lower than the realistic condition, which could yield unrealistic results. In comparison, centrifuge modeling can provide reduced dimensions and realistic ground stress levels (Thorpe, 2007). Hence, centrifuge modeling is a reliable alternative to exam the problem on the tunnel face failure, which has been widely adopted in dry sandy ground (Chambon and Corté, 1994, Oblozinsky and Kuwano, 2004, Thorpe, 2007, Idinger et al., 2011) or saturated ground without considering the steady-state seepage (Mair, 1979, Wong et al., 2012, Ng and Wong, 2013). However, centrifuge modeling on the tunnel face failure induced by the seepage is rarely reported. The previous researches only considered the ideal condition with the fully permeable tunnel face and isotropically permeable ground.

The objective of this study is to examine the effects of two crucial factors (i.e., the tunnel face opening ratio and permeability anisotropy) on the tunnel face support pressure. A series of centrifuge tests were carried out to investigate the tunnel face failure mechanism induced by the seepage under different water table levels. Besides, a series of numerical back-analyses was performed by the Finite Difference Method (FDM). Based on the numerical model, the influence of the ground anisotropic permeability on the pore-water pressure and the minimum required support pressure on the tunnel face were examined. Moreover, the impact of the tunnel face permeable condition (i.e., fully permeable, impermeable, and opened with holes) on the pore-water pressure and the minimum required support pressure on the tunnel face were studied. The main novelty of this study is to reveal the natural seepage influences on the tunnel face stability by considering the tunnel face opening ratio and permeability anisotropy.

Section snippets

Problem definition

Fig. 1 shows the problem studied in this paper. A tunnel was embedded horizontally in the ground with a specific cover depth, C. The water keeps flowing into the tunnel face because the water pressure in the tunnel is 0. For various Hw (i.e., the distance from the water table to tunnel axis), tunnel face failure could be triggered by retracting the tunnel faceplate. By changing the hydraulic conductivity of soil (horizontal hydraulic conductivity, kh, and vertical hydraulic conductivity, kv)

Test program

The centrifuge model tests were performed at the Geotechnical Centrifuge Facility of Zhejiang University (i.e., ZJU-400 (Chen et al., 2010)). The centrifuge model was designed for the tests under high hydraulic pressure and high centrifuge acceleration level (see Fig. 2). A prototype stress condition was recreated in the model by applying a centrifuge acceleration of 50g (g is the Earth’s gravity). Table 1 shows the relevant scaling laws (Schofield, 1980).

The adopted rectangular model container

Simulation program

A numerical model was established to simulate the failure of the tunnel face induced by the water ingress. The back-analyses of the centrifuge tests were performed based upon the centrifuge model scale. 15 cases were modeled using the Explicit Finite Difference Method utilizing FLAC3D. Many researchers have used this software to investigate various geotechnical problems related to tunneling construction (Chen et al., 2013, Li et al., 2009, Zhang et al., 2015, Pan and Dias, 2016, Cai et al., 2019

Pore-water pressure in front of the tunnel face

The pore-water pressure in front of the tunnel face was investigated in this section. The pore-water pressures at the tunnel centerline were recorded when the numerical model was in the steady-state seepage state. The calculated pore-water pressures were validated by comparing them to those from the centrifuge tests and theoretical solution. In the centrifuge tests, the horizontally linked pores in the sandy silt were collapsed during the model compacting. As a result, kv was higher than kh

Influence of tunnel face condition

The influence of the tunnel face condition on the limit support pressure is discussed in this section. The limit support pressures, slim, were collected from the numerical results with different tunnel face conditions. Three tunnel face conditions were as following.

1) The tunnel face was sealed, so no seepage in the soil (i.e., Case 7 to 9).

2) The tunnel face was with holes, and the opening ratio was 16.56% (i.e., Case 4 to 6).

3) The tunnel face was fully permeable (i.e., Case 13 to 15).

The

Comparison with other methods

A comparison between the normalized minimum support pressure, slim/γ'D, from the existing theoretical methods (Anagnostou and Kovari, 1996, Lee et al., 2003, Perazzelli et al., 2014, Lü et al., 2017, Alagha and Chapman, 2019) and that from the numerical model was made in this section. For these methods, kv/kh = 1. The tunnel face seepage conditions were as following, the theoretical methods (Anagnostou and Kovari, 1996, Lee et al., 2003, Perazzelli et al., 2014, Lü et al., 2017) assumed that

Conclusions

A series of numerical simulations, including back-analyses of the centrifuge modeling, by FLAC3D, were conducted to investigate the tunnel face stability below the water table. The influences of the water level, the seepage condition, and the tunnel face condition on the pore-water pressure in front of the tunnel face were investigated. An evaluation of existing theoretical methods for calculating the minimum support pressure on the tunnel face was also made. The following conclusions can be

CRediT authorship contribution statement

Xinsheng Yin: Conceptualization, Methodology, Software, Writing - original draft. Renpeng Chen: Writing - review & editing, Supervision, Funding acquisition. Fanyan Meng: Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The present work was carried out with supports of National Key Research and Development Program of China (2019YFB1705201), Young Scientists Fund of the National Natural Science Foundation of China (51808493), Research Fund of Hangzhou Science and Technology Bureau (20201203B125), Research Fund of Zhejiang Provincial Natural Science Foundation (LY21E080004), Research Fund of Department of Education of Zhejiang Province (Y201839147), and China Postdoctoral Science Foundation (BX20200126,

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