Research PaperEvaluation of twin tunnel-induced surface ground deformation by empirical and numerical analyses (NATM part of Eurasia tunnel, Turkey)
Introduction
Inner city traffic load augmented due to population increase causes significant transportation problems. This necessitates the construction of subway transportation systems, such as metro tunnels. Even though they have many advantages, tunnel construction particularly through weak materials may bring about undesired effects on existing structures due to ground deformation. Therefore, safe tunnel design and construction require stability, surface deformation, and effective supports; thus, the assessment of ground settlements and their effects on structures above the tunnel is essential for tunnel projects [1]. The surface settlement trough is the manifestation of the movements around the tunnel cavity. Such deformations may be controlled by means of extra lining systems, such as jet grouted columns, ground freezing, and pipe jacking [2], [3], [4]. Pre-support, pre-confinement, auxiliary methods, and pre-improvement [5], [6], [7] are extra means of primary pre-support used in tunneling when the support of the tunnel face is required [8]. Pre-support techniques for underground excavation can be divided into two fundamental aspects: (i) support employed in the surrounding area of the crown above the face and (ii) face support [9]. Pre-support techniques are very important for inner-city shallow tunnels especially while applying New Austrian Tunneling Method (NATM), which allows for some deformation to relieve the stress. Previous studies [10], [11], [12] concerning the assessment of the magnitude of maximum surface displacements caused by NATM-twin tunneling do not include the decreasing effect of the pre-support. Moreover, established empirical equations were mostly obtained by using data from tunnels passing through clayey soil.
The Asian part of the Eurasia tunnel has been excavated using NATM and supported by the forepoling and umbrella arch methods; nevertheless, surface deformations occurred as a result of the tunneling. The use of Herzog’s equation yielded higher maximum ground settlements than those measured in the field. The current study contains the comparison of numerical, empirical and direct measurements of surface settlement due to tunneling and proposes a modification factor for Herzog’s equation by considering the effect of the pre-support and distance between pipes in the pre-support systems.
In order achieve this, surface deformations at several points were measured; then, various boreholes were drilled. Several standard penetration and a few pressuremeter tests were conducted within the boreholes. Laboratory testing was performed on disturbed and undisturbed samples to obtain the geotechnical parameters of the units. Afterwards, finite element analysis (FEA) program Phase2 with the Mohr-Coulomb elastoplastic material model was used to model the rock mass behavior. An axisymmetric FEA with the plane-strain model was performed in this study. Tunneling was modelled by removing the elements inside the tunnel boundary. The construction sequence (top heading, bench and invert face excavation) was applied in the plane-strain analysis. Twelve cross-section lines were selected along the tunnel alignment to perform the numerical analysis to provide a sufficient number of field measurement points (at least five points), representing each of the twin tunnels at the location where the distance between the tunnels was within the range of active tunnel interaction and avoiding the existing pocket tunnel. Then, FEA was performed along these sections and results were obtained where there was a consistency with the field measurement data. The next step was to conduct numerical parameter study in which the distance between pipes in the pre-support systems was used as a variable. This was performed to ascertain the relation between the percent decrease in the maximum surface settlement and distance between the pipes in the forepole and umbrella arch systems. This correlation was then tested to determine whether it was statistically significant. Finally, this relation was accepted as modification factor (i.e., reduction ratio) in terms of pre-support effects to modify an existing prediction equation of the twin tunnel-induced surface settlements.
Section snippets
Site characteristics and geological setting
The Eurasia Tunnel Project is located between Kazlıçeşme and Göztepe along a 14.6 km route including a total 5.4 km twin-tunnel that crosses the Bosphorus beneath the seabed, which aimed to alleviate Istanbul’s transcontinental traffic load. The project also provided for the enhancement and broadening of current roads that lead to the tunnel on both sides of the Bosphorus strait for 9.2 km in total. This study focuses on the data referring to the Asia NATM tunnels that join the Asia Transition
Geotechnical assessment of the study area
Geotechnical parameters are normally obtained from laboratory and field tests. In this study, a total of eight boreholes (Fig. 1) totaling 415.80 m in depth were drilled. Since the internal friction angle of the granular soils is very difficult to determine under field loading, when drained triaxial compression test results on reconstituted coarse-grained soils or on undisturbed fine-grained soils were available, their mean values were used. If no such tests were available, the values
Numerical modeling of NATM part of the Eurasia tunnel
Today, with the rapid increase in the development of computational tools and capability of solving the complicated issues, the finite element method (FEMs) is widely used, which allows for several restrictions of the analytical and empirical methods being eliminated. In addition to the geomechanical properties of the material, depth, stress-strain condition, and geometry of the tunnel structure can have an influence on ground deformation depending on the excavation procedure. FEM considers this
Empirical methods
The form of the surface trough above an underground tunnel construction was initially analyzed by Martos [46], Peck [47], Attewell and Yeates [48]. They indicated that ground settlement could corresponded to a Gaussian or normal distribution curve. Peck [47] proposed a shape that resembled a transverse trough and appeared above single tunnels for ground movements, and this has been confirmed by many site investigations and centrifuge tests (Fig. 12).
A semi-empirical approach has been adopted
Prediction methods for twin tunnel-induced surface settlement
The superposition method is defined as a way of estimating subsidence above any twin tunnel structure. A surface settlement prediction is produced using one of the methods used for single tunnel-induced settlement. Assuming that the second tunnel is of similar size and depth, then a similar settlement trough is placed over the centerline of the second opening disregarding any effects of the lead tunnel. O’Reilly and New [53] provided a formula for twin tunnels by superposition (Eq. (4));
Modification of the Herzog equation
The pre-support effect is not considered in all formulas given in the literature; however, the results of the Herzog equation were far from the field measurement data due to the original Herzog model not considering the pre-support that relieves ground deformation. Therefore, the field measurements from the twin NATM tunnel part of the Eurasia tunnel with forepoling pre-support were utilized and assessed statistically with respect to the center-to-center distance between the pipes in the
Discussion and conclusion
Twin tunnel-induced surface settlement is mainly controlled by geomechanical factors (deformation modulus representing rock mass quality, Poisson’s ratio) and engineering factors (tunnel depth, tunnel diameter and distance between tunnels). Hence, the determination of rock mass parameters is a vital step to be undertaken during the numerical analysis of the tunnel structure.
The Trakya formation and volcanic rocks were observed at the investigation site. Borehole logs and descriptions, core
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 authors gratefully acknowledge Yapı Merkezi Holding for permitting the tunnel data to be published. The authors thank the anonymous reviewers for their highly valuable and constructive comments on the manuscript.
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