Effects of important factors on surface settlement prediction for metro tunnel excavated by EPB

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Abstract

Due to population growth, people demand more and more transportation services, there can be no doubt that the requirement for tunnels will also grow. Mechanized excavations using EPB have been successfully applied, especially in urban environments where there is less surface space available, over the past twenty years. Because of excavation speed and less hazardous working environments, use of the Earth Pressure Balance Machine (EPB) is a more popular method in metro, railway and road tunnels in urban areas. Control of surface settlement is very important when tunnels are excavated in urban areas or beneath important structures. This research mainly discusses effects of important properties like tunnel depth, overburden pressure, tunnel dimension and face pressure using empirical, theoretical and numerical methods used to control surface settlement. Results of these methods are compared with observation data.

Highlights

► Effective factors in surface settlement for metro tunnel excavated by EPB were investigated using different methods. ► Tunnel depth, overburden pressure, tunnel dimension and face pressure were investigated to control surface settlement. ► Results obtained from empirical, theoretical and numerical methods are compared with observational data. ► The most significant factor in ground surface settlement was found to be tunnel diameter in all methods. ► Inadequacies of empirical methods used for prediction of surface settlement were put forward.

Introduction

Because of recent city developments with limited available land to build on, more and more public facilities are developed underground. Complex underground constructions may cause serious damage to existing structures therefore it is important to predict ground behavior and surface settlements during excavation. Surface settlements can be estimated by using empirical methods (Atkinson and Potts, 1977; Attewell et al., 1986, Mair, 1983, O’Reilly and New, 1982, Peck, 1969), analytical methods, (Bobet, 2001, Chou and Bobet, 2002, Loganathan and Poulos, 1998, Park, 2005, Sagaseta, 1987, Verruijt and Booker, 1996), and numerical methods (Ercelebi et al., 2011, Melis et al., 2002, Mroueh and Shahrour, 2002, Suwansawat and Einstein, 2006). Estimation of accurate surface settlement for control of ground movement in EPB excavation needs the investigation of important parameter such as tunnel depth, overburden pressure, tunnel dimension and face pressure. All of these make the ground behavior complex. This paper reports the results of a study carried out on the effects of effective parameter on estimation of surface settlement for a single tunnel excavation opened with EPBM technique using well-known empirical, analytical and numerical methods. The monitored data allowed comparison of measured and estimated parameters by using an analytical and three-dimensional (3D) finite difference model (FDM).

Section snippets

Empirical and analytical methods

Empirical and analytical methods are based on data obtained from in situ observation and measurements during tunnel excavation. Schmidt (1969) and Peck (1969) were first to show that the short-term transverse settlement trough, taking place after construction of a tunnel, in many cases can be well approximated by the Gaussian distribution:Sx=Smaxe-x22i2where Smax is the maximum surface settlement (above the tunnel axis), x is the horizontal distance from the tunnel centerline and i is the

Location and site geology

Tehran Metro Line 7 is almost 27 km in length with 25 stations. It starts from Shahrak-e-Mir-al-momenin in the east of Tehran and is extended parallel to Navvab Safavi Highway toward the north and reaches to Saadat Abad district in the north of Tehran. The Line 7 tunnel can be divided into sections: one running in east–west direction, and the other along north–south. Based on this, the drilling work for the two sections starts at Station N7 located at the intersection of Ghazvin Street and Navab

Greenfield settlements

Deformations start when the tunnel approaches the surface settlement measurement and building measurement points and steadily increase up to measurements points. Fig. 3 shows the location of Dena building. Building deformation measurements were done with metal meters (Fig. 3b) and optical points (Fig. 3c) installed at different points fixed on the Dena building (Fig. 3d). Topographic measurements of a network of benchmarks arranged on axis with, and transversally to, the route plan (seven

Three-dimensional modeling of tunnel

Tunnel construction has been simulated with a 3D modeling performed with code FLAC3D based on the finite difference method. It is assumed that the soil is homogeneous and isotropic, with an elastic-perfectly plastic constitutional link with a Mohr–Coulomb resistance criterion, while the lining rings are modeled with an elastic behavior. In all the FLAC3D outputs, units of measurement are Newton (N) for force and meter (m) for length. The model has a longitudinal dimension (in y direction) of 60 

Comparison of maximum surface settlement results from various methods

Fig. 7 indicates an example of FD modeling including vertical (z) displacement contour after 30 m excavation of tunnel between A–A and B–B sections.

Calculated maximum settlement values for sections A–A and B–B obtained from the empirical, analytical and numerical methods are compared with observation data and summarized in Table 2. In this table, settlement values are presented at above the tunnel centreline.

The results show that the calculated maximum surface settlements from the FD model and

Investigation of the effects of important factors on surface settlement

There are several parameters that are effective on surface settlement and recognizing effects of these parameters can be very helpful for measurement of surface settlement and present a better empirical method for surface settlement measurements. According to experimental investigations the important factors causing settlements in EPB excavation are as follows:

  • Tunnel diameter.

  • Face pressure.

  • Surface surcharge (in shallow tunnels).

Conclusion

Because of the effect of different parameters, the prediction and control of surface settlement is one of the most critical problems in metro tunneling excavation. There are a few empirical relationships to predict maximum surface settlement value. In these equations, there are several important factors which are effective in maximum surface settlement values. Investigation of the effect of each factor can be helpful to understand and apply these relationships or proposed a new formula for

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