Evaluating the effect of soil structure on the ground response during shield tunnelling in Shanghai soft clay

doi:10.1016/j.tust.2016.05.003

Tunnelling and Underground Space Technology

Volume 58, September 2016, Pages 120-132

https://doi.org/10.1016/j.tust.2016.05.003 Get rights and content

Highlights

•
We proposed a model incorporating soil structure that is suitable for tunnel analyses in Shanghai soft clay.
•
Effect of soil structure on tunnel induced ground response and long-term behaviour was evaluated.
•
The mechanism of the effect of soil structure on ground response was discussed.

Abstract

When evaluating tunnel-induced ground response in Shanghai soft clay, the soil structure and its degradation behaviour of natural Shanghai soft clay during shield tunnelling should be properly considered. In this paper, a constitutive model that considers the initial soil structure and its destructuration is formulated within the framework of critical-state soil mechanics. The model is successfully calibrated and used to simulate the undrained behaviour of natural Shanghai soft clay. Based on the proposed model, finite-element analyses are conducted to simulate the short- and long-term ground responses induced by tunnelling at Shanghai metro line 2. The comparisons between numerical results and field measurements reported in literature indicate that the soil structure and the tunnel-induced destructuration significantly affects the magnitude and shape of the short-term surface settlement trough and horizontal displacement in Shanghai soft clay. The pore pressure variations around the tunnel are also affected by soil structure, which will significantly influence the long-term ground consolidation settlement in Shanghai soft clay.

Introduction

Shanghai located on the south bank of the estuary of Yangtze River, is a metropolitan and economic centre in China. With a major boost in economy in China during last three decades, the population and expanding urban areas of Shanghai increased several times. Hence, there is an increasing demand for underground tunnels used as efficiently alternative transportation. At present, there are 14 metro lines with a total length of approximately 560 km in operation in Shanghai. By the end of 2020, 22 metro lines, which comprise a total length of 880 km, will be operational (Zhang and Huang, 2014). The Shanghai metro lines are typically constructed using the shield tunnelling technology within a depth of 50 m. Over the upper 50 m of the Shanghai region, there distributes a typically 20 m thick soft clay layer. It has been well documented that the natural Shanghai soft clay is often highly structured with high compressibility and low strength (Ng et al., 2013, Shen et al., 2014). Tunnelling disturbance may degrade the soil structure, change the pore water pressure response, and decrease the stiffness and strength of the soil (Xu et al., 2003). Consequently, the variation in mechanical properties of soft clay around the tunnel may cause excessively large settlement and unpredictable long-term displacement, which may lead to the failure of soft clay foundations and tunnels. Hence, the effect of the soil structure on ground response during shield tunnelling is of practical importance.

Currently, numerical methods (FEM or DEM), which provide the flexibility of simulating different geometry and excavation sequences, and enable the application of advanced soil models, become the most popular method to analyse the ground response of tunnelling (e.g., Lee and Rowe, 1990a, Lee and Rowe, 1990b, Addenbrooke, 1996, Grammatikopoulou, 2004, Wongsaroj et al., 2007, Jiang and Yin, 2012). Clough and Leca, 1989, Hejazi et al., 2008 noted that the soil constitutive model in numerical analyses significantly affected the simulation of tunnels. Previous studies primarily focused on soil features such as small-strain stiffness (e.g., Addenbrooke et al., 1997, Masin and Herle, 2005, Hejazi et al., 2008), stiffness anisotropy (Addenbrooke et al., 1997, Wongsaroj, 2005), recent stress history (Dasari, 1996) and elasto-plastic behaviour within the yield surface (Wongsaroj, 2005). All these features have been proved to significantly improve the prediction. It should be emphasized that although the constitutive modelling of natural soils, which incorporates the effect of the soil structure, has significantly progressed in recent years, there remains a lack of experience in evaluating the effect of the soil structure on tunnelling. Several studies analysed the effect of the soil structure on the tunnel-induced ground response in overconsolidated stiff clays (e.g., Dang and Meguid, 2008, González et al., 2012, Zhu et al., 2013). Note that these limited studies may lead to contradictory results, because it is difficult to distinguish the effect of OCR from the soil structure for stiff clays. More importantly, extensive research work on the effect of soil structure in natural soft clay during tunnelling appears to be sparse.

The objective of this study is to investigate the effect of the soil structure of natural soft clay on the ground response induced by shield tunnelling. Firstly, based on the critical-state theory, an advanced soil model is proposed that evaluates the stress-strain relationship of natural clay and incorporates the effect of the soil structure through the sensitivity framework. Secondly, the performance of the proposed model is validated by comparing the simulations with laboratory experimental results for natural Shanghai soft clay. Finally, the finite element method is used to evaluate the effect of the soil structure on the short- and long-term tunnel-induced ground response at Shanghai metro line 2 by implementing the advanced soil model. For simplicity, all the analyses in this study are conducted in the plane strain conditions.

Section snippets

Description of the soil model

There are several different approaches to represent the soil structure within a constitutive modelling framework. One of the most popular methods is to introduce a quantitative parameter, which represents the difference between natural and reconstituted clay, and to modify the hardening rule by adding a destructuration law based on the Modified Cam Clay (MCC) model (e.g., Rouainia and Muir Wood, 2000, Gajo and Muir Wood, 2001, Liu and Carter, 2002; Baudet and Stallebrass, 2004, Masin, 2007, Liu

Model geometry and boundary conditions

A plane strain model is created with the ABAQUS finite-element program to simulate the excavation of Shanghai metro tunnel line 2 at the interception of Chengdu Road and Nanjing Road. Detailed descriptions of the site investigation were obtained from Lee et al. (1999). The layout of the instruments and various soil strata near the investigated site are summarized in Fig. 6 and Table 2, respectively. From top to bottom, there are five main different soil layers, which include a layer of typical

Results

The three analyses in this study incorporate different constitutive models during the numerical simulation. The Shanghai soft clay (layer 4) is assumed to possess the soil structure and is represented by a “proposed model with soil structure”. To evaluate the effect of the soil structure, the case (“proposed model with soil structure”) runs while assuming no soil structure. Finally, MCC model is performed as the reference. All three analyses are run until the volume loss was close to the

Discussions

The ground response and pore pressure change results from the disturbance of the stress field during the tunnel excavation. For the 2D FE simulation, the tunnel excavation is not explicitly modelled. In this study, the degree of stress relaxation during tunnel construction is adopted to represent the effect of tunnel excavation. To achieve the same volume loss, different degrees of stress relaxation need to be set in the numerical simulation with different constitutive models.

The relationship

Conclusions

Numerical analyses have been performed for the shield tunnelling in Shanghai soft clay using a constitutive model that incorporates the soil structure. The effect of the soil structure on the ground response and long-term behaviour has been evaluated. The main conclusions of this paper are summarized as follows:

(1)
The soil structure is the key factor that improves the computed surface settlement trough for Shanghai soft clay. The computed surface settlement trough becomes deeper and narrower and

Acknowledgements

This study is supported by the National Natural Science Foundation of China (Grant Nos. 41502263 and 41372309), the National Key Basic Research Program of China (973 Program) (Grant No. 2015CB057803) and the Natural Science Foundation of Jiangsu Province of China (Grant No. BK20150819). In addition, a grant from the Fundamental Research Funds for the Central Universities of China (2015B00914) in support of this study is also gratefully acknowledged. The first author would like to acknowledge

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The various forces applied to soil during shield tunneling will inevitably disturb the adjacent soft soil, inducing increased soil settlement after tunneling and endangering the service performance of the tunnel. However, the disturbance degree and distribution of the soil under the tunnel have not previously been measured in situ. The contributions of different tunneling parameters to disturbance are also not well understood. In this paper, cone penetration tests were carried out before and after shield tunneling to evaluate the disturbance degree and distribution of soil under the tunnel. The shear strain of soil was also obtained via back-analysis of tunneling with the finite element method. It was found that the shear strain was linearly related to soil disturbance degree. Furthermore, face pressure, contraction, and grouting pressure were parametrically studied to determine their relative contributions to soil disturbance. The results revealed that face pressure contributed the most to the soil disturbance. This study will be useful for evaluations of soil disturbance induced by shield tunneling and for controlling soil disturbance and post-construction tunnel settlement during tunnel construction.
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Construction-induced ground movement in soft clays is one of the most important topics for geotechnical engineering practice, especially in the rapid economic development zone in East China with numerous infrastructures constructed over this region (Bian et al., 2017a; Wu et al., 2017; Ye et al., 2018; Jin et al., 2019). The soil modulus provided a simple first-order estimation of the settlement of soft clay in the finite element method (Bian et al., 2016a; Di et al., 2016) or analytical method (Osman and Bolton, 2005). Specifically, the undrained secant deformation modulus Eu was considered as a suitable design parameter, particularly in predicting the initial settlement of soft clay ground (Casey et al., 2016).
The undrained secant deformation modulus E_u is an important parameter to quantitatively estimate the undrained settlement of soft clays. A series of isotropically consolidated undrained triaxial compression shear tests were carried out on four reconstituted clays and two naturally sedimented clays to evaluate the effects of the initial water content w₀, liquid limit w_L, and soil structure on E_u. The test results showed that the variation in E_u with deviator stress was significantly affected by w₀ and w_L for reconstituted clays. The normalized modulus E_u/(p′)^0.9 linearly decreased with increasing w₀/w_L. This linear relation at a lower w_L lies above that at a higher w_L. These observations suggest that the soil with lower plasticity and lower initial water content possessed higher values of E_u. As the deviator stress increased, the effect of w₀ and w_L on E_u gradually vanished. It is also evidenced that the value of E_u for naturally sedimented clays depended on the soil structure. The soil structure clearly enhanced the E_u in the preyield regime in comparison with the corresponding reconstituted clay. Once the soil structure was damaged with increasing consolidation stress in the postyield regime, a significant reduction in E_u was observed with a lower value than that of reconstituted clays. The difference in E_u/S_u between naturally sedimented and reconstituted clays gradually decreased with increasing deviator stress. This implies that the effect of the soil structure on E_u diminished at higher deviator stress levels.
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Based on the aforementioned research, it can be concluded that the soft clay beneath the breakwaters will be subjected to a combined disturbance induced by wave loading and shield construction, which may severely degrade the mechanical properties of the soft clay, eventually leading to the collapse of the breakwater (Fig. 1). Accordingly, appropriate constitutive models have been developed to replicate the observed typical response of soft clay to monotonic loading, cyclic loading, or their combination (Addenbrooke et al., 1997; Zhang et al., 1999; Mroueh and Shahrour, 2003; Liao et al., 2009; Gomes, 2013; Bian et al., 2016; Meng et al., 2018; Tian and Yao, 2018; Zhu et al., 2019a). To the best of the authors’ knowledge, however, previous constitutive models are limited as they separately investigate the degradation of post-cyclic stiffness and strength behavior of soft clay.
This study aims to investigate the mechanical behavior of soft clay under both shield construction and long-term dynamic wave loading in the practical construction of breakwaters to protect coastal power plants. Continuous rotation of the principal stress axis due to cyclic wave loading is reproduced by altering the control loads (that is, axial load, torque, inner cell pressure, and outer cell pressure) in a hollow cylindrical shear apparatus (HCA). In the HCA, the confining pressure is progressively reduced to replicate the stress release induced by ground loss during shield construction. The HCA test results show that both long-term wave loading and shield construction significantly affected the dynamic stiffness of soft clay. Under the combined effects of wave loading and shield construction, the post-cyclic shear strength and stiffness of the soft clay are reduced more significantly compared to the isolated effect of wave loading, especially at lower confining pressure. The degradation of the dynamic secant modulus, E_d, and the initial tangential modulus, E_i, is indicated to be synchronous at the end of the cyclic loading test, which implies the unique disturbance of the combined action of shield construction and dynamic wave loading on the microstructure of soft clay considered in this study. Based on the experimental observations, a simple and effective constitutive model is also developed to reproduce the degradation of stiffness and shear strength of soft clay induced by shield construction and dynamic wave loading.
Effect of the constitutive material model employed on predictions of the behaviour of earth pressure balance (EPB) shield-driven tunnels
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In the work of these researchers, the excavation was simulated simply by reducing the radial pressure along the inside boundary of the tunnel to zero. However, recently Xia et al. [48] developed a 2D model of Shanghai Metro line 2 by using a MCC model. The predicted results matched the measured data more closely than did the results of Mair et al. [24,25], however the predicted trough settlement differed by approximately 20% from the measured data.
Earth pressure balance (EPB) shield-driven tunnelling involves a complex soil-structure interaction problem, where performance is heavily influenced by stress history and its development during construction. Numerical modelling must therefore focus on selecting appropriate constitutive models for soils and structures, simulating construction procedures and sequences, and modelling the soil/structure interface. Reliable numerical models to predict expected settlements, lining pressures and other design parameters are essential for safe tunnel design.
This paper discusses these factors in detail by utilizing a well-documented case study of twin tunnels in Shanghai. A 3D finite element model of the behaviour of reinforced concrete tunnel linings is developed using the new PLAXIS concrete model. Predictions of four different advanced soil constitutive models are compared with measured field results to assess the model effectiveness and suitability. The undrained behaviour of the saturated soft silty clay soil at the tunnelling site is studied during and after advancement of the shield tunnelling machine. The comparison matrix includes surface settlement troughs along transverse sections, and changes developing in earth and pore water pressures around the tunnel. The HSSmall soil model, which accounts for increased soil stiffness at small strains, was found to be the most suitable for addressing these problems.
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On the other hand, the surrounding soil undergoes loading and unloading processes during the whole synchronous grouting. Thus, the mechanical behavior of surrounding soil also affects the ground movement, such as the OCR (over consolidation ratio) value (Soga et al., 1999), the anisotropic behavior (Yin et al., 2010), the soil structure effect (Yin et al., 2011; Bian et al, 2016), the creep behavior (Qiao et al., 2016; Yin et al., 2017) and others. However, in this work, only the synchronous grouting process and its effects are highlighted.
Synchronous grouting is one of the most vital processes during the construction of shield tunnels, especially the one with a special non-circular profile. This paper presents a large-scale mock-up test aimed to study the behavior of the synchronous grouting in a quasi-rectangular shield tunnel and its influence on the ground movement. Both the test results and the literature data reveal that the grouting pressure changes with time according to a power function while the hardening of grout could be described by a logarithm function. Incorporating these two new formulations, a novel simulation framework is proposed to interpret the observation during the mock-up test. Moreover, the effects on the ground movement of the grouting pressure and the grout types are highlighted and discussed via sensitivity analysis. The results demonstrate that the grouting pressure dominates the upheaval of ground while its dissipation and the solidification of grout control the following settlement. The effect of the quasi-rectangular shape on the ground movement is also investigated.

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Evaluating the effect of soil structure on the ground response during shield tunnelling in Shanghai soft clay

Highlights

Abstract

Introduction

Section snippets

Description of the soil model

Model geometry and boundary conditions

Results

Discussions

Conclusions

Acknowledgements

Soils Found.

Tunn. Undergr. Sp. Technol.

Comput. Geotech.

Comput. Geotech.

Soils Found.

Comput. Geotech.

Tunn. Undergr. Sp. Technol.

Tunn. Undergr. Sp. Technol.

Tunn. Undergr. Sp. Technol.

Comput. Geotech.

Comput. Geotech.

Numerical analysis of tunnelling in stiff clay

The influence of pre-failure soil stiffness on numerical analysis of tunnel construction

Géotechnique

A constitutive model for structured clays

Géotechnique

An interpretation of structural degradation for three natural clays

Can. Geotech. J.

Clay sediments in depositional basin: the Geotechnical Cycle

Q. J. Eng. Geol. Hydrogeol.

Application of a multilaminate model to simulate the undrained response of structured clay to shield tunnelling

Can. Geotech. J.

Modelling the variation of soil stiffness during sequential construction

A new approach to anisotropic, bounding surface plasticity: general formulation and simulations of natural and reconstituted clay behaviour

Int. J. Numer. Anal. Meth. Geomech.

The statistical features of the engineering properties of Shanghai soft soil

Quantification of the effects of structure on the compression of a stiff clay

Can. Geotech. J.