investigation on the effects of twin tunnel excavations beneath a road underpass
TRANSCRIPT
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Investigation on the Effects of Twin Tunnel Excavations Beneath a Road Underpass
Eshagh Namazi
Researcher, Department of Geotechnics and Transportation, Faculty of Civil Engineering, Universiti Teknologi Malaysia (Formerly a Tunnel Engineer at
Bamrah Construction); email: [email protected]
Hisham Mohamad
Lecturer, Department of Geotechnics and Transportation, Faculty of Civil Engineering, Universiti Teknologi Malaysia ; email: [email protected]
Mohammad Ehsan Jorat
Researcher, Department of Geotechnics and Transportation, Faculty of Civil Engineering, Universiti Teknologi Malaysia; email:
Mohsen Hajihassani
Researcher, Department of Geotechnics and Transportation, Faculty of Civil Engineering, Universiti Teknologi Malaysia; email:
ABSTRACT Excavation of tunnels underneath cities often intrudes the existence of piled foundation and in severe cases, can cause damage to the overlying structures. As there are very limited published case studies concerning understanding of the interaction between piled structure and tunneling, there is a significant uncertainty regarding tunnel-pile interaction. In this paper, a case study of the effects of two subway tunnels on the contiguous pile walls which support a road underpass is investigated using three-dimensional Finite Element simulations. The interaction between the tunnels and piles is investigated with a special attention to the effect of tunnel face pressures. Through the numerical modelling and field data, it is shown with presence of the piles, the minimum pressure to support the tunnel face is less than minimum face pressure in the green field condition. Field experience indicates that excessive tunnel face pressure can cause temporary heave to the ground surface but also cause damage to the cutter head of tunnel boring machine.
KEYWORDS: Pile Walls, Numerical Modeling, Surface Settlement, Face Pressure
INTRODUCTION Construction of subway tunnels in the urban environment is a complex problem particularly when
tunnels are excavated very close to existing structures supported with pile foundation system. The design and execution of these tunnels requires assessment of the impact of the tunnel-induced ground movement on the stability and integrity of existing piled foundations (Mohamed and Mattar, 2009; Cheng et al. 2007; Mroueh and Shahrour, 2002; Jacobsz et al. 2001; Leung et al. 2000; Chen et al.
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1999; Vermeer and Bonnier, 1991). Several researchers have focused on the influence zone for 2D pile-soil-tunneling interaction based on case studies and numerical simulations (Lee and Bassett, 2007; Selemetas et al., 2006; and Kaalberg et al. 2006; Lee et al. 2007; Coutts and Wang, 2000). In these studies the effects of different parameters (e.g. distance of the pile from tunnel centre, position of the pile tip regarding to the horizontal tunnel axis, pile and tunnel diameter) on the interaction between pile and tunnel have been investigated to develop understanding of the interaction mechanism between tunnels and piles.
In the first part of this paper, a case study of the effects of tunneling on the contiguous piles is presented. The 3D-finite element (PLAXIS 3D TUNNEL Package) was performed to investigate the effect of tunnel advancement on the contiguous pile with special attention to the most important parameter of excavation called face pressure. In the second part, parametric study of the effect of tunnel face pressure on the interaction between tunnel and piles is carried out. The last part represents the longitudinal settlement measured at the ground surface where the high face pressure was used in the tunneling operation.
SITE DESCRIPTIONS The growth and expansion of Shiraz, a southern city of Iran, and increase in the number
of vehicles and population led to construction of subway in order to overcome the transportation problems. The South eastern part of Line І of that subway with length of approximately 14 km consisted of a twin tunnel. The running tunnels were excavated by Earth Pressure Balance (EPB) of 6.88 m external diameter with tail-skin grouting. The tunnel linings were made from pre-fabricated reinforced concrete segments forming an internal tunnel diameter of 6 m. A particular interest of Line Ι project between stations Zirgozar and Zand Cross was the constructions of tunnels below an existing Zand Underground Motorway (Zand Underpass). Figure 1 shows the longitudinal section of Shiraz tunnel between the stations, under the Zand Underpass. The distance between the stations is 1215 m whereas the underpass length is about 607 m. Next to the Zand Underpass, cars must pass downward and upward ramps of 6.5% with lengths of 135 m. The twin tunnel underneath, on the other hand, were excavated with a generally more gentle slope of 1.9% running from the stations to their deepest point of approximately 16 m along the Underpass. The spacing of the two tunnel centre-lines next to the Underpass is equivalent to two tunnel diameters.
Figure 1: longitudinal section of the tunnels under the Zand Underpass
The Underpass was formed by two contiguous pile walls and a roof slab which was connected to the walls by pin connection. The roof slab was 0.8m thick. The contiguous pile walls formed of many piles with diameter of 1.2m and spacing of 0.1m. The filling between
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the piles comprised of cement and bentonite mixture. Figure 2 shows the geometry of the structure and the position of the pile walls regarding to the tunnels. The mechanical properties of the Underpass partitions are given in table 1.
Figure 2: Description of geological conditions of the site
GROUND CONDITION The site investigation includes three boreholes close to the area (Bamrah Construction,
2004). The sequence of strata identified from these boreholes is summarized in Figure 3. The ground profile consisted of made ground at the top, and the next clayey sand overlying the intermittent layers of clayey sand and inorganic silt. The tunnels were excavated in the clay and inorganic silt. Table 1 summarizes the characteristics of the soil used for the analyses. The water table was taken as approximately 8m below the ground surface, i.e. within the inorganic silt.
Table 1: Mechanical properties of encountered materials (After Bamrah Construction, 2004)
GROUP γunsat(kN/m3) γsat(kN/m3) E(MPa) υ C (kPa) Φ(o) Ψ(o) Rint
MADE GROUND
16 19 51.5 0.3 20 25 0 0.8
CLAYEY SAND
17.7 22.8 88.3 0.25 24.5 29 0 0.6
INORGANIC SILT
16.9 20.9 30 0.25 10 36 0 0.7
SEGMENT EA=30000(MN/m) EI=225(MN/m/m) ROOF SLAB E=23(GPa) Thickness = 0.8 (m)
CONTIGUOUS PILE E=19.23(GPa) Diameter =1.4 (m)
1.2 m
5.2 m
3.6 m
13.6 m
8 m
7 m
4 m
3.2 m
6.7 m
3 m
1.8 m
Made ground
Clayey sand
Inorganic Silt
Inorganic Silt
Clayey sand
Clayey sand R=3.44 m
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NUMERICAL MODELING In order to investigate the tunnel advancement on the contiguous piles, 3D-Numerical
modelling was performed using a commercial Finite Element program, i.e. PLAXIS 3D Tunnel (Brinkgreve and Broere, 2004). This software provides the flexible features to model the details of tunnel construction in soils. The finite element mesh used in numerical modelling is presented in Figure 3. The model is 100 m wide, 30 m deep and 70 m long. The geometrical boundaries considered here was found to be far enough from the tunnels axis in order to minimise the influence of boundaries on the tunnelling model. The model includes 4130 elements and 12389 nodes. The soils were modelled using 15-noded wedge elements, whereas 8-node plate elements represented the tunnel lining (Figure 3). To simulate the soil-structure interaction, a 16-node interface element was used.
The water table is assumed to produce the hydrostatic initial pore water pressure. An elastic-plastic soil model using the Mohr-coulomb failure criterion is adapted in this study. Because the soils are prevalently fine-grained and relatively low permeable, the analyses were carried out in undrained condition.
In general, the process of tunnel construction under the Underpass was modelled in two steps (for more information for simulation of tunnelling, see Potts and Zdravkovic´, 2001). First, the initial conditions were set up for the model before excavation of the tunnels. It was achieved by specifying the distribution of effective vertical and horizontal stress (using coefficient of earth pressure at rest, K0=0.5) and pore water pressure. The initial conditions were completed with simulating the underpass structure. In this stage, the vehicles loads were calculated and applied to the model. After establishing the initial conditions, the analyses continued with modeling excavation of the first tunnel. The tunnel excavation process was done through a step-by-step method in 16 phases. In each phase, the excavation process consists of: (i) excavation of the soil, (ii) application of pore water pressure, (iii) support pressure at the tunnel face to prevent active failure at the face, (iv) installation of the tunnel lining and finally (v) the grouting of the gap between the soil and the newly installed lining. The second tunnel excavation was modeled after the completion of the first tunnel in which the same manner of step-by-step method is applied.
Figure 3: Three dimensional finite element model of the tunnels under the Underpass
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PREDICTION OF SURFACE SETTLEMENT TROUGH Prior to assessing the influence of piles walls on the settlement-induced by tunnelling, the
green-field surface settlement trough obtained by numerical modelling is compared with empirical method and field data. The green-field settlement troughs are obtained in the same geology conditions as the Zand Underpass ones. Figure 4 compares the surface settlement profile after excavation both tunnels. There is an agreement between field data and numerical modelling in terms of magnitude and shape of profile except maximum settlement which is under predicted. Results calculated using empirical method which was expressed by Peck (1969) are also shown in Figure 4. The superposition principle is used to obtain the settlement trough due to excavation of both tunnels. The volume loss of 1.1% was used to calculate the settlement trough induced by each tunnel. Although the empirical method predicts maximum settlement more accurately than the numerical model, the settlement in the far field was over predicted.
Figure 4: Surface Settlement Trough in the Green-field conditions
Figure 5 shows the predicted surface settlement by numerical modelling where both of the tunnels have been completed, for green-field and actual conditions (with the presence of the Underpass). The existing of structure would normally modify the surface settlement owing to tunnelling excavation. In this case however, the two Finite Element (FE) ground surface settlement plots are almost identical to each other except at the point where the contiguous piles walls are located. Piles walls do not follow exactly green-field movement induced by tunnels at the piles location and soil movement surrounding the piles also altered due to presence of piles. This is due to the additional of displacement (settlement) caused by the piles.
The displacement-induced by pile can be divided to the displacement caused by pile loading and the displacement by presence of pile without load. The displacement induced by loading is in accord to the green-field ground movement and displacement caused by presence of pile is in resistant to the green-field ground movement. Position of the pile tip regarding to the horizontal tunnel axis determines which one is dominance: pile loading or presence of the piles. In this example, because the tunnels are excavated exactly beneath the piles, loading increases the vertical effective stress and consequent ground movement beneath the pile tip. But in the different situation where the tunnel is excavated adjacent to the piles, the existing of the piles decreases the ground displacement induced by excavation (Ng et al., 2005; Huang
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et al., 2009). Such phenomenon, as discussed in the previous section, is called the shielding effect.
Figure 5: Effect of contiguous piles on the surface settlement trough
Figure 6 shows the soil displacement pattern around the Zand Underpass after excavation of the tunnels in FE. In general, before excavation of the tunnels the pile-soil system was in equilibrium. The surrounding soil applies an “upward friction force” (i.e. positive friction) to resist the downward displacement of the pile. When the tunnels are excavated, however, the equilibrium is disturbed and the soil moves to the tunnels boundaries. In this situation, the displacement of surrounding soil is larger than displacement of the piles and the “downward fiction force” (i.e. negative friction) exerts an additional load on the piles. The piles transfer their loads to the soil before excavation of the tunnel but after excavation of the tunnel the piles carry the load induced by soil displacement.
Figure 6: Soil displacement pattern around the Zand Underpass in FE
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indicated by the field observation (Bamrah Construction, 2010). Replacement of the cutter head had to be made which lead to further delays and interruption to the tunnelling operation.
Figure 9: Field measurement of longitudinal surface settlement
CONCLUSION A 3D numerical analysis with assistance of field data has been presented to study the
effects of face pressure on the surface settlement induced by tunnelling under the piled walls. Numerical modelling results showed the minimum pressure to support the tunnel face is less than minimum face pressure in the green field condition. In fact, applying the green-field pressure to the tunnel face in the presence of piles is a conservative method. The parametric study illustrated that increasing the face pressure significantly does not help to reduce the final surface settlement significantly but only slight. Field observation of the Shiraz subway tunnels under the existing Zand underpass showed excessive tunnel face pressure causes temporary heave to the ground surface but also cause damage to the TBM cutter head.
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