Control of surface settlements with Earth pressure balance method (EPB) for Istanbul metro

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1 ATS Control of surface settlements with Earth pressure balance method (EPB) for Istanbul metro Hamid Chakeri 1, Bahtiyar Ünver 2, Alireza Talebinejad 3 1 Phd student, Hacettepe University, Dept. of Mining Engineering, Beytepe, Ankara, Turkey 2 Professor, Hacettepe University, Dept. of Mining Engineering, Beytepe, Ankara, Turkey 3 University of Tehran, Dept. of Mining Engineering, Tehran, Iran ABSTRACT Regarding urban development, metro tunnels are increasingly being excavated in soft ground conditions. In metro tunnel excavations, it is important to control surface settlements observed before and after excavation, which may cause damage to surface structures. Construction of tunnels induces a state of strain in the soil around the excavation that could cause damaging differential displacements in existing structures near the tunnel. This paper reports the results of a study carried out to estimate the values of the surface settlements induced by the excavation of twin tunnels, which have been excavated between the Otogar and Kirazlı stations of the Istanbul Metro line namely a length of 5.77 km. Authors of the paper have not carried out field studies at the tunnel; data related to tunnel were taken from Mahmutoğlu (2010), Ocak (2009) and Ercelebi et al (2010). In this paper only numerical modeling has been performed on obtained data. In particular, the purpose of this study has been to compare the vertical surface displacements monitored during the advancing excavation and the settlements estimated by using 3D Finite Difference model. For numerical analyses, FLAC3D program is used. Geology in the study area is composed of fill, stiff clay and dense sand and hard clay. Tunnels are excavated by using two Earth Pressure Balance Tunnel Boring Machines with a 6.5 m diameter as twin tubes with 14 m distance from center to center. Ground settlement results obtained by numerical modeling are in good agreement with field measurements. KEYWORDS Numerical modeling, Surface settlement, Face pressure, FLAC 3D. 1. INTRODUCTION Many tunnels must be built in weak materials such as sands and clays. These tunnels tend to be at shallow depths and near urban infrastructures; consequently the construction method must minimize surface settlements. Ground loss at the tunnel is consequentially translated into an equivalent surface depression especially in cohesive soil and tunneling in shallow ground (Attewell et al., 1986). Effects of settlement due to shallow and soft ground tunneling are hazardous to nearby buildings, infrastructures and existing services. Problems related to tunnel-induced settlements have interested many researchers in the last 40 years. The surface settlements can be estimated by using empirical or semiempirical methods Peck (1969), Atkinson (1977), Attewell (1982), Mair (1983), New (1991), Einstein (1981), Oteo (1979), analytical methods Sagaseta (1987), Verruijt (1996), Loganathan (1998), Bobet (2001), Chou (2002), Park (2005), and numerical methods Suwansawat (2006), Mroueh (2006), Melis (2002). Many innovations have been introduced to improve tunneling in soft ground. At present, the earth pressure balance (EPB) Shield Tunneling Method, which was first developed in Japan (Stack, 1982 and Maidl et al., 1996) has become one of the most popular methods for soft ground tunneling. With this tunneling technique, ground movement can be, in theory, controlled by balancing the pressure inside the earth pressure chamber relative to the outside ground pressure during excavation. To achieve ground movement control, there are many operational parameters involved such as face pressure, penetration rate, pitching angle, and grouting quality. Shield ground interaction is complex due to variety of these factors. This paper reports the results of a study carried out to 1

2 estimate the values of the surface settlements induced by a twin tunnels excavation using an EPBM technique. The monitored data allowed comparison of the measurements of in situ displacements during the advancing excavation and the estimated subsidence by using three-dimensional (3D) finite difference model (FDM) that could simulate, step by step, the different tunnel excavation and supporting phases. Input data used in numerical modeling have been obtained from Mahmutoğlu (2010), Ocak (2009) and Ercelebi et al (2010). 2. LOCATION AND SITE GEOLOGY The first construction phase of the Istanbul Metro line began in 1992 and opened to the public in This line is being gradually extended, and additions are being constructed in other locations. One of these metro lines is the twin line between Otogar and Kirazlı 1, 5.77 km. Metro line consists of a 3.87 km tunnel, 0.62 km cut and covers station, and 1.28 km at grade crossing. The excavation of this section began in May 2006 and was completed in June This metro line will integrate the Kirazlı 1 Basaksehir Olimpiyat Koyu Metro Project that is currently under construction namely a length of 15.8 km. At the same time, the Otogar and Kirazlı 1 Metro Line will integrate the Aksaray Ataturk Airport light metro line that is now under service (Fig. 1). Table 1: Technical Features of the EPBMs (Ocak, 2009) Herrenknecht Lovat Excavation diameter (m) External diameter (m) Internal diameter (m) Segment thickness (m) Average segment length (m) Shield outside diameter (m) TBM length (m) Face pressure (kpa) The study area includes the twin tunnels between km and of Istanbul Metro opened generally in Güngören formation of the Miocene age. An extensive site investigation was carried out to determine the ground condition. Previously prepared information database on geological conditions of the site was revised and updated. The subsurface soil was characterized by investigating geotechnical drilling and in situ data and laboratory test results. The geology of the study area is given in Fig. 2 (Mahmutoğlu, 2010). Figure 1: Location of tunnels The metro lines in the study area were excavated by a Herrenknecht EPBM in the left tube and a Lovat EPBM in the right tube. One tube excavation has followed around 100 m behind the other tube. Some of the technical features of the machines are summarized in Table 1. (Ocak, 2009) Figure 2: Main route and geological section of Esenler and Basaksehir Metro (Mahmutoğlu, 2010) 2

3 Characteristics of lithology of the ground around tunnels are presented in Table 2. The Güngören formation consists of fill, very stiff clay, dense sand and hard clay sequences. The overburden thickness above the tunnels varies between 9.7 to 16 m. Table 2: Characteristics of the geological layers around the tunnels (Mahmutoğlu, 2010 and Ocak, 2009) Fill Very stiff clay Dense sand Hard clay Unit weight (kn/m 3 ) Cohesion, c (kpa) S U (kpa) E (kpa) N Poisson ratio Angle of friction THREE-DIMENSIONAL MODELLING OF TWIN TUNNEL 4. SURFACE SUBSIDENCE MONITORING Topographic measurements were carried out in order to control surface settlements induced by the underground construction. Contractors fixed measurement points with intervals at approximately 3 10 m each at the building measurement point (BMP) and surface settlement measurement point (SMP) in each 3 5 m, depending on building situations on the surface. Figure 3 shows the locations of monitoring points and the numbers. Surface settlement measured BMP point is given in Table. 3. Figure 3: The locations of monitoring points and the numbers (Mahmutoğlu, 2010) Table 3: Measured surface settlement at the monitoring points on tunnel centerlines. (Mahmutoğlu 2010) On the centreline of left hand side tunnel (LHST) Monitoring points km RS max (mm) BMP THREE-DIMENSIONAL MODELLING OF TWIN TUNNEL Finite Difference method has been widely used for modeling underground excavations. In this paper, FLAC3D (Fast Lagrange Analysis of Continua in 3-Dimesions), which is based on finite difference method, was used to simulate the excavations and to analyse the stability of tunnels. The main steps in the modeling procedure are: Selection of the area to be modeled Selection of suitable model for ground behavior and determination of necessary input parameters Application of boundary and stress conditions Solving the models towards a balanced condition Application of changes to the model, i.e. further stages of excavation Re-solving the model and calculation of new state of stresses and deformations Figure 4 (a, b) shows the cross section of the finite difference mesh for the twin tunnels and the model grid of the twin parallel tunnels generated in FLAC3D which consists of three-dimensional, isoperimetric solid elements in the soil layer. The model width in the horizontal (x) direction is m (approximately 4D from the centerline to both sides, where D is the equivalent diameter of the tunnel, being 6.56 m). The model length in the longitudinal direction is 49 m (7.5D) and the model height is m (7.59D). The centerline of each tunnel is located at a depth of m. The space between the two tunnels is 8 m. In the simulation, the lining and grouting are modeled by shell elements. The model is fixed in all direction at the bottom. In addition, no horizontal displacements are allowed on the two x z planes (i.e., y = 0 and y = 49 m) at the boundaries of the model. The initial stress state of the model is obtained through gravitational load which corresponds to an earth pressure coefficient at rest of Live burdens exerted on tunnels as 20 kn/m 2 is applied for traffic burden effect. Tunnel construction process is modeled using a step-by-step approach, i.e. first Herrenknecht tunnel is excavated. After excavation of this tunnel Lovat tunnel is excavated. In each step, the excavation length increment is taken as 1.4 m and supported using lining and grouting. Face pressure (300 kpa) applied to tunnels faces in each step. 3

4 Fig. 6 (a, b) show the contour of vertical displacement and settlement pattern along km (y=15m for FLAC3D model) at the end of the second tunnel construction. The FD model predicted the maximum surface settlement as mm for the right tube. For the left tube (opened after the right), FD prediction in y=15m was mm, while measured maximum settlement was 29 mm. There are good matching between numerical results and measured data. Figure 4: (a) Three-dimensional views of the finite difference mesh; (b) typical section 6. ANALYSIS OF TWIN TUNNEL EXCAVATION The contour of vertical stress around twin tunnels after excavation is shown in Figure 5. There are symmetrical statuses for vertical stress around twin tunnels. Figure 6: (a) The settlement pattern of the ground surface y=15 m (b) Contour of vertical displacement Figure 5: Contour of vertical stress Effects of different face pressures on surface settlement are illustrated in Figure 7. The EPB face pressures are adjusted at 250, 300, 350 kpa and open mode. As the applied pressure is increased amount of surface settlement 4

5 is decreased. It is shown that face pressure action is an important parameter for controlling of surface settlement and environmental effects of tunnel excavation. 4th International Conference on Ground Movements and Structures", invited review paper, Cardiff, Pentech Press, London, July 7,1991, Figure 7: The settlement pattern for face pressure variation Conclusions In this study, Twin tunnels of 6.5 m diameter are excavated by EPB tunnel boring machine. Settlement predictions were performed by using surface monitoring point and FD modeling methods and the measured results after tunneling were compared. The results of FD model were found to be in good agreement with the surface monitoring point measurements. Therefore use of finite difference method is useful for prediction of surface settlement and controlling of face pressure action. The result showed that face pressure action is an important parameter for controlling of surface settlements. REFERENCES [1] Mahmutoglu. Y., "Surface subsidence induced by twin subway tunnelling in soft ground conditions in Istanbul", Bull Eng Geol Environ, May 2010, Springer. [2] Ercelebi. S. G., Copur. H., Ocak. I., "Surface settlement predictions for Istanbul Metro tunnels excavated by EPB-TBM", Environ Earth Sci. March 2010, Springer. [3] Ocak. I., "Environmental effects of tunnel excavation in soft and shallow ground with EPBM: the case of Istanbul", Environ Earth Sci, January 2009, 59: Springer [4] Chakeri. H., Hasanpour. R., Hindistan. M. A. and Unver. B., "Analysis of interaction between tunnels in soft ground by 3D numerical modeling", Bull Eng Geol Environ J., 2010, Springer. [5] Itasca consulting Group, Inc. FLAC3D (Fast Lagrangian Analysis of Continua in 3Dimensions.), Version [6] Perri G., "Analysis of the effects of the new twin-tunnels excavation very close to a big diameter tunnel of Caracas Subway.", A, Editor. Tunneling and Ground Conditions. Rotterdam: Balkema, p [7] Attewell. PB, Yeates. J. and Selby. AR. "Soil movements induced by tunneling". New York: Chapman and Hall, [8] New, B.M. and O ReilIy, M.P "Tunneling induced ground movements; Predicting their magnitude and effects. Proceedings of the 5