Finite element investigation of pre-supporting system with vertical piles and concrete ribs in shallow soft- ground tunnel Alireza Ramezani Engineering department- McNally Construction Inc., ON, Canada ABSTRACT In tunneling, supports are usually installed after excavation. Thus, the risk of tunnel collapse is highest immediately after excavation at the tunnel face. This is one of the main problems faced when constructing shallow tunnels in soft ground. In this paper, a pre-support tunneling method using a bored vertical pile and concrete ribs technique is proposed to improve tunnel stability and reduce tunnel deformation. By implementation of concrete ribs on top of the tunnel and vertical pile as a support of concrete ribs beside the tunnel, it is possible to start excavation with more safety. This paper deals with design of pre-support system for 250 m urban shallow soft ground tunnel with finite element method using PLAXIS V8.2 software. All construction steps from pile implementation to final lining construction are modeled to main factors controlling tunnel behaviors’ are analyzed. Finally the induced stress, deformations and the yielding degree developed around the tunnel are determined to investigate the interaction of the supports with the tunnel surroundings. The total displacements, safety factors and the yielding degree are considerably reduced by using pre-supporting system. 1 INTRUDUCTION In tunneling, supports are usually installed after excavation. Thus, the risk of tunnel collapse is highest immediately after excavation at the tunnel face (Sozio, 1998; Sekimoto et al., 2001). This is one of the main problems faced when constructing shallow tunnels in soft ground. In this paper, a pre-support tunneling method using a bored vertical pile and concrete ribs technique is proposed to improve tunnel stability and reduce tunnel deformation. By implementation of concrete ribs on top of the tunnel and vertical pile as a support of concrete ribs beside the tunnel, it is possible to start excavation with more safety. This paper deals with design of pre-support system for 250 m urban shallow soft ground tunnel with finite element method using PLAXIS V8.2 software. All construction steps from pile implementation to final lining construction are modeled to main factors controlling tunnel behaviors’ are analyzed. Finally the induced stress, deformations and the yielding degree developed around the tunnel are determined to investigate the interaction of the supports with the tunnel surroundings. The total displacements, safety factors and the yielding degree are considerably reduced by using pre-supporting system. Iran mall double-track tunnel is under construction in heavy urban area in Tehran, Iran that construct in SEM (NATM) method. Figure 1 shows the route of Iran mall tunnels and its specifications is shown in table 1. The

stress redistribution caused by tunnel excavation induces movement in the earth and ultimately at the ground surface. The need to control ground surface settlements in urban area is widely recognized and new construction methods are continuously developed. Settlement induced by underground excavation may cause serious damages to nearby structure and subsurface underground utilities. In this paper, two short shallow double-track tunnels “Iran mall” have been studied using new method of construction that is called Concrete-arch pre-supporting system (CAPS). Iran mal tunnels have some special characteristics such as shallow depth, soft surrounding soil, wide span and heavy traffic. This method is mainly based on construction of rib consists of concrete piles and arch beams around a proposed underground space prior to its construction (figure 3). This method has been utilized in several underground stations in Tehran Metro since 2002 (Sadaghiani, Gheysar, 2003, Sadaghiani, Ebrahimi, A. 2006) and can be used for any large span underground spaces in similar ground conditions. Numerical modeling is used to simulate all the construction stages and analyze the ground behavior. The results of the modeling show that CAPS reduces the ground surface settlement and enhances the ground stability. Dimension of arch beams and their spacing has also notable effect on the ground deformation.

GENERAL PLAN

TUNNEL 1

TUNNEL 2

Figure 2. General plan Table 1. Tunnels specification

Tunnel No. Length (m)

Span (m)

Overburden (m)

1 99.3 12.8 4.0

2 81.29 14.5 6.5

Figure 3. Concrete Arch Pre-supporting Structure (CAPS)

constructed prior to main excavation

2 SITE SPECIFICATION AND GEOLOGY

Geology of the tunnel location consists of Quaternary alluvium. This formation is consisted of sediments of silty/clayey sand to gravelly sand. Soil is mid-dense sand with some clay and silt and also has some cementation. The cohesion is in the range of 15-70 kPa and friction angle is about 30-38. Soil is usually dry to moist. The groundwater level is variable during year. The geotechnical investigation was conducted to determine the geotechnical properties and groundwater condition along the tunnel. Three 40 m deep boreholes were bored near the tunnels and various tests carried out on the samples acquired from these boreholes. 3 CONSTRUCTION METHOD Cut-and-cover method was not applicable in this location due to street traffic and relocation of urban subsurface utility lines in application of this method. Firstly, fore poling method was considered as temporary pre-support system for the working area. Although this method

ensures the stability of the perimeter of the open span until the primary lining is installed, but because of the presence of some cobbles in the soil and also need to excavate the running tunnel in order to implement the first series of fore poles, this method was inappropriate as well. As mentioned in above section, a pre-supporting system, Concrete Arch Pre-supporting System (CAPS), consisted of reinforced concrete arch beams, each supported by two side piles which all are constructed by underground method prior to the excavating the main underground space. The main advantages of CAPS in large underground spaces are that by pre-supporting the soil around the underground space prior to excavation, it reduces soil deformation and thus increases the stability of the large excavation with low overburden in soft grounds. This system restricts the ground settlement, thus enhances the general stability. Another advantage is that it reduces the construction time due to the fact that the piles and arch beams in CAPS can be constructed simultaneously from several faces in short time. After CAPS construction, the tunnel construction is preceded by the multi-stage excavation and initial supporting of the main space. The main excavation proceeds in large sections followed by installing very light initial supporting system such as shotcrete and welded wire mesh over the excavated surface. Due to simultaneous sequential construction, the rate of construction advance increases dramatically. In CAPS, small adits in vertical direction as piles, in semi-horizontal and horizontal direction as arch beams and connecting galleries are hand excavated around the large underground space(H. Sadaghiani , Saleh Dadizadeh 2009). The excavated pile and arch beam adits are filled with reinforced concrete to make a rib shape underground structural frames prior to main large excavation. The adits can be excavated and concreted in parallel manner. CAPS supports the surrounding ground during the main excavation and requires minimal initial supporting system. For Iran mall double track tunnels, the actual proposed CAPS has following properties. The piles have 1.0 m diameter and 7 m depth, dimension of the horse-shoe cross section of arch adits is 1.0 x1.3 m and the longitudinal distance between every arch frame is 3.0 m. The procedure for performing this task is shown schematically in Figure 3.

STAGE "1" STAGE "2"

STAGE "3"STAGE "4"

STAGE "5"

STAGE "7" STAGE "8"

STAGE "6"

Figure 3. Construction sequences In each tunnel, after excavation of Pilot tunnel in the middle of the tunnel (NATM method), three longitudinal adits are hand excavated at the specified locations along the length of the tunnel (Figure 3. Stage 1). Then at specified intervals inside longitudinal adits, wells are excavated for side piles ((Figure. 3 stage2) and after installing reinforcement, they are filled with concrete (Figure. 3 stage 3). In the next step, an adit is excavated between side piles in an arch shape, similarly, after putting in reinforcement, they are also filled with concrete through the top gallery and connected to piles making a monolithic concrete arch frame (Figure. 3 stage 4). After

making a concrete arch frame, construction stages of main tunnel is carried out in several stages (stage 5 to 8). In order to control the stability and reduce displacements sprayed concrete (shotcrete) and a layer of wire mesh is used over the soil between the piles and arches after main excavation. Reinforced concrete lining is used as the final lining in the final stages. Each subsequent excavation stage is executed at 6 m distance (lag) behind the previous stage. Final lining of the base and wall section is executed at 6 m lag and final lining of the crown is executed at 12 m behind the lower excavation face. During construction, groundwater level is assumed to be lowered to the invert level. The proper distance between the stages are determined initially by numerical modeling of different conditions during the design phase. The appropriate sequence is initially used to start construction phase. Using field instrumentation, monitoring and site observation, the construction methods and stages can be modified and optimized. 4 NUMERICAL ANALYSES This section describes 2D structural analysis of the ground–support interaction structure using PLAXIS computer software, which has built-in features for modelling soil-structure interaction and stages of the construction. The construction sequence of pre-supporting system and main tunnel were modeled and analyses were performed step by step to simulate exact excavation and construction stages. In using a two dimensional analysis of the rock-support interaction it is necessary to simulate the three-dimensional tunnel advance by means of some deformation control process. To compute the tunnel deformation at the point of support installation, empirical relationship developed by Vlachopoulos and Diederichs is used (Vlachopoulos et. al. 2009). To use the Vlachopoulos and Diederichs method, firstly, the maximum tunnel wall displacement far from the tunnel face should be calculated and then, the radius of the plastic zone far from the tunnel face is calculated. The Hardening Soil model is chosen for the simulation of soil behavior. As for the Mohr-Coulomb model, limiting states of stress are described by means of the friction angle phi, cohesion (C) and the dilatancy angle f. However, soil stiffness is described much more accurately by using three different input stiffnesses: the triaxial stiffness E50, the triaxial unloading stiffness Eur and the oedometer loading stiffness Eoed. In contrast to the Mohr-Coulomb model, the Hardening Soil model also accounts for stress-dependency of stiffness moduli. This means that all stiffnesses increase with pressure. Geotechnical parameters are obtained from field and laboratory tests are shown in Table 2.

Table 2. Soil properties for finite element models

Description Value Unit

Unit density 20 (KN/m3)

friction angle (F) 30 degree

Cohesion (C) 25 KN/m2

Dilatancy angle (f) 5 degree

E50 1.2x105 KN/m

2

Eur 3.6x105 KN/m

2

Eoed 1.2x105 KN/m

2

Figure 4. Finite element model of tunnel The numerical models comprise of 70 m soil in vertical direction and 85 m laterally soil in horizontal direction. Boundary conditions are taken as vertical constraints on the sides of models and full fixity at the base. Figure 4 shows 2D numerical model and mesh generation. The geometry of rib elements and the cross section of the rib piles and ribs are shown in Figure 5. In 2D simulation, rib properties (stiffness and strength) should be divided by ribs horizontal spacing to include 3D effect over the distance between ribs (3m in this case).

RIB SECTION

RIP/PILE CONNECTION PILE SECTION

Figure 5. Connection of pile and rib detail For investigation of the finite element result and stability of the tunnels, there are many factors such as vertical and horizontal displacement of the surrounding material, vertical and horizontal stresses, relative shear stress, plastic point distribution around the tunnel and internal forces in the structural elements that should be determined. But in this paper only vertical displacements and plastic point distributions are presented as it is the most critical factor in urban tunnels.

Figure 6. Vertical displacement counters after Pilot tunnel excavation

Figure 7. Vertical displacement counters after topping excavation

Figure 8. Vertical displacement counters after benching excavation (final stage) After excavating Pilot tunnel, the maximum displacement is almost 13 mm at the crown of the tunnel, which is acceptable. In case of topping and benching excavation, the vertical displacement reaches to 22 mm and 24 mm respectively. Displacements increase as the excavation continues but the rate of increment is reduced as the excavation stages proceeds further. In order to determine the effect of exaction on the plastic point distribution, analyses were conducted and results are presented in Figures 9 to 11. It is inferred that plastic point distribution are highly effected by excavation possess and as the excavation continues more plastic points are distributed around the tunnel. More distribution is obvious around the piles after topping excavation that shows mobilization of the strength of surrounding soil.

Figure 9. Plastic point distribution after Pilot tunnel excavation

Figure 10. Plastic point distribution after topping excavation

Figure 11. Plastic point distribution after benching excavation (final stage) One of the other important factor is considerable ground settlements caused by tunnel excavation that also makes problems for surface structures. Figure 12. shows ground surface settlement caused by excavation at final stage. According to the results, the maximum obtained settlement is 24 mm and in an acceptable range.

Figure 12. Ground displacement profile at the end of the excavation. To study the response of each structural components, Interanal axial forces and bending moment due to excavation are compared with the capacity of each element. Figures 13 and 14 show axial force and bending moment at concrete pile and pilot tunnel lining.

Figure 13. Bending moment (right) and axial force (left) distribution along the concrete pile

Figure 14. Bending moment (right) and axial force (left) distribution at pilot tunnel lining The calculation process results in a set of moment versus axial thrust for each structural elements, which are shown in figures 15 and 16 for concrete pile and pilot tunnel. This analysis illustrates that, for the in situ stresses, rock mass properties, and excavation sequence and lining properties chosen, are satisfied.

Figure 15. Support capacity curve for concrete pile

Figure 16. Support capacity curve for Pilot tunnel lining 5 CONCLUSION 2D numerical modelling are performed in this research in

order to obtain the effects of CAPS method. In this

method a reinforced concrete underground arch frame is

constructed around the proposed underground structure

prior to the main excavation. This structure reduces the

ground deformation and stress concentration in the

ground and main structure. The work including CAPS and

overall ground excavation and construction of main

structure is conducted by logical stages.

Magnitude of ground settlement and displacement of underground space are closely related to the construction stages of main underground structure. Observing ground settlements relative to changes in arch beam height and longitudinal spacing of arch frames, it is clear that they play an important role in variation in displacements and ground stability. Significant portion of the ground settlement occurs while excavating the crown section of main underground space. If necessary, e.g., presence of weaker ground, heavy traffic load or less ground settlement requirement; proper considerations such as increase in the height of arches or reduction in their spacing should be made. Modeling construction stages and their effect on the displacement and ground settlement helps to understand progressive effect of construction stages on ground behavior. Numerical modeling can be used initially to determine an optimum method of construction stages prior to construction. After the construction commences, using field observation, instrumentation and measurements during construction along with numerical modeling, more appropriate and practical construction stages can be determined. 6 REFERENCES Sozio, L.E., 1998. General report: urban constraints on underground works. In: Proceedings, 24th World Tunnelling Congress, Tunnels and Metropolises, Balkema, Sao Paulo, Brazil, pp. 879–897

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