Research Paper
Numerical analysis of ground displacement and segmental stress and influence of yaw excavation loadings for a curved shield tunnel

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Abstract

This paper describes the key influences of yaw excavation loadings on ground displacement and segmental stress for a curved shield tunnel. The influences are investigated through finite element models, the reliabilities of which are validated through comparisons to field data and analytical solutions. Multiple case studies of different curvature tunnels and their comparison to straight-line tunnels are presented. Under the dual action of overcutting and construction loadings, the surface settlement of the curved tunnel is larger than that of the straight-line tunnel. The horizontal displacements at the inner and outer sides of the curved tunnel are asymmetric with respect to the tunnel axis. This asymmetry can increase significantly during yaw excavation of over one ring width. Yaw excavation loadings have a significant influence on the horizontal and vertical displacements of the ground within a span of shield length starting from the position of the hydraulic jacks until the back. The circumferential compressive stress, axial tensile stress, and axial compressive stress of newly installed segment of the curved tunnel are greater than those of the straight-line tunnel. Interestingly, the stress increments increase linearly with yaw severity. The results are of benefit to suggest improvements for practical construction procedures.

Introduction

With increasing urbanization and need for complex transport infrastructure, underground tunnels have become progressively commonplace. The tunnels need to work around with constraints of both existing structures and structures that are about to be constructed. Typical obstacles include underground pile foundation of high-rise buildings and underground traffic rails. The need to maneuver through such obstacles has necessitated that some sections of the tunnel be built with very tight curvatures, which can complicate the selection of the appropriate underground tunnel shielding. Unlike straight-line tunnels, the excavating trajectories of curved tunnels are formed by a series of discontinuous straight sections that together, approximate different types of curves. Each section can be excavated by an earth pressure balance tunnel boring machine (EPB-TBM). In order to approximate a curve during curved tunnel excavation, each subsequent section is positioned at an angle with respect to the previously section. The change in direction can introduce unintentional yawing. Therefore, if the loadings are not controlled properly in the tunneling process, the extent of yawing (and thus increasing the angles between sections) can become more severe. If the construction loadings are not controlled properly in the tunneling process, which can negatively impact the integrity of the surrounding earth or structures.

In order to keep shield tunneling on the desired track and minimize disturbances to the surrounding ground [1], [2], [3] and local structures [4], [5], [6], a wide range of theoretical analyses, numerical simulations, and field tests have been performed. The published theoretical research covers the analytical prediction of ground displacement due to ground loss [7], [8], [9], [10], [11], [12], [13] as well as due to loadings generated during excavation [14], [15], [16], [17]. A potential drawback of pure theoretical analysis is the difficulty in knowing exactly the magnitude and direction of loadings exerted onto the soil mass during construction (e.g. jacking forces). Such knowledge can only be obtained through field tests [18], [19], [20], [21]. On the other hand, finite element modelling (FEM) has emerged as among the most effective and accurate methods for predicting ground displacement caused by shield excavation [22]. The influence of different construction parameters [23], [24] on ground displacement and structural behaviors [25], [26] can be estimated through FEM. In addition, numerical simulations of ground disturbances caused by construction loadings (e.g. face pressure, friction force, grouting pressure, etc.) [27], [28] show that the friction force between skin of the shield and the surrounding soil is exerted on the soil indirectly via a contact pair between the shield skin and the excavation interface [21]. Therefore, the reliability of this numerical approach in simulating the friction force remains to be verified.

To realize yaw excavation along a curved trajectory [29], two key factors, which include overcutting and the control of hydraulic jacks, must be considered. These two key factors distinguish curved-line tunnels from straight-line tunnels. Concretely, the EPB-TBM shield needs to open the copy cutter to over-excavate the earth at the inner side of the curved tunnel [30]. The purpose of the latter factor is to generate a differential jacking force between the inner and outer sides [31] and to produce a yaw angle [32] between its original orientation and the axis of newly installed segment ring. For curved tunnels, the asymmetric boring around the axis of the tunnel will inevitably impact the tunnel structure as well as the surrounding ground. Therefore, it is necessary to investigate in further detail regarding the effects of different levels of yaw excavation on both the tunnel and its surroundings.

The following study uses FEM to investigate the influence of yaw excavation on ground displacement and segmental stress for a curved shield tunnel. Numerical results are compared with field data and analytical solutions. The study also explores different construction cases (straight tunnels and tunnels with different curvature radii.

Section snippets

Overview of physical model

The inter-city railway from the Zhuhai urban area to the airport (i.e. between the “Wanzai North” station and the “Wanzai” station) includes a curved shield tunnel (curvature radius is 450 m, as shown in Fig. 1a). Markers are set on the ground surface at 10 m intervals along the curved trajectory of the tunnel.

The geology of the Zhuhai area typically consists of composite strata with soft upper layers and hard lower layers. The average soil properties of the different layers are listed in Table

Ground vertical displacement

Fig. 6 shows the contours of the vertical displacement fields for both the straight-line and curved tunnel models after 50 excavation stages.

For the straight-line tunnel (Fig. 6a), the ground immediately above the tunnel crown settles about 30 mm at the shield tail, and the tunnel invert rises about 24 mm. Moreover, the settlement trough gradually widens as the elevation increases, and the ground settlement gradually stabilizes as the distance from the excavation face increases [39]. The ground

Analysis of yaw excavation loadings influence on ground displacement and segmental stress

Due to an angle between the direction of the jacking force vector and the central axis of the preceding segment, lateral forces that can greatly impact the surrounding ground are generated. However, since hydraulic jacks act directly on the structure of the segment, there are limited empirical data that can be used to verify the lateral forces (location, magnitude, and direction). In the following sections, a validated FEM is used to investigate such impact.

In order to highlight the effects of

Conclusions

This study presents in-depth numerical simulations of the whole excavation process for curved shield tunnels. The results of the simulations are in good agreement with field measurements. Furthermore, the influence of friction forces estimated by analytical methods are in good agreement with the numerical solutions. A series of numerical simulations were conducted to investigate the influence of yaw excavation loadings on ground displacements and segmental stresses. The following general

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 financial support for this work has been provided by National Natural Science of Foundation of China (Grants No. 51538001, 51778025 and 51978019), which are gratefully acknowledged.

References (42)

  • A. Lambrughi et al.

    Development and validation of a 3D numerical model for TBM-EPB mechanised excavations

    Comp Geotech

    (2012)
  • D. Dias et al.

    Movements caused by the excavation of tunnels using face pressurized shields—analysis of monitoring and numerical modeling results

    Eng Geol

    (2013)
  • G. Zheng et al.

    Advance speed-based parametric study of greenfield deformation induced by EPBM tunneling in soft ground

    Comp Geotech

    (2015)
  • M. Migliazza et al.

    Comparison of analytical method, 3D finite element model with experimental subsidence measurements resulting from the extension of the Milan underground

    Comp Geotech

    (2009)
  • R. Hasanpour et al.

    Parametric study of the impacts of various geological and machine parameters on thrust force requirements for operating a single shield TBM in squeezing ground

    Tunn Undergr Space Technol

    (2018)
  • P.F. Li et al.

    An upper-bound analytical model of blow-out for a shallow tunnel in sand considering the partial failure within the face

    Tunn Undergr Space Technol

    (2019)
  • H. Yu et al.

    Analytical solution for longitudinal bending stiffness of shield tunnels

    Tunn Undergr Space Technol

    (2019)
  • J. Ninić et al.

    A hybrid finite element and surrogate modelling approach for simulation and monitoring supported TBM steering

    Tunn Undergr Space Technol

    (2017)
  • P.F. Li et al.

    Face stability analysis of a shallow tunnel in the saturated and multilayered soils in short-term condition

    Comp Geotech

    (2019)
  • M. Kavvadas et al.

    Development of a 3D finite element model for shield EPB tunnelling

    Tunn Undergr Space Technol

    (2017)
  • T. Kasper et al.

    On the influence of face pressure, grouting pressure and TBM design in soft ground tunnelling

    Tunn Undergr Space Technol

    (2006)
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