Comparison of shallow tunneling method with pile and rib method for construction of subway station in soft ground

Abstract

In the present study, a comparison between the new shallow tunneling method (STM) and the traditional pile and rib method (PRM) was conducted to excavate and construct subway stations in the geological conditions of Tehran. First, by selecting Station Z6 located in the Tehran Subway Line 6 as a case study, the construction process was analyzed by PRM. The maximum ground settlement of 29.84 mm obtained from this method was related to the station axis, and it was within the allowable settlement limit of 30 mm. The acceptable agreement between the results of numerical modeling and instrumentation data indicated the confirmation and accuracy of the excavation and construction process of Station Z6 by PRM. In the next stage, based on the numerical model validated by instrumentation data, the value of the ground surface settlement was investigated during the station excavation and construction by STM. The results obtained from STM showed a significant reduction in the ground surface settlement compared to PRM. The maximum settlement obtained from STM was 6.09 mm as related to the front of the excavation face. Also, the sensitivity analysis results denoted that in addition to controlling the surface settlement by STM, it is possible to optimize some critical geometric parameters of the support system during the station excavation and construction.

Keywords

shallow tunneling method / pile and rib method / ground surface settlement / subway station construction / numerical modeling

1 Introduction

The design and construction of tunnels require appropriate techniques and technologies during the implementation of all phases of a relevant project. The cost and time of construction of the tunnel are strongly influenced by choice of excavation method; so that choosing the suitable excavation method for large tunnels in soft ground and especially in urban areas is of particular importance. Insufficient attention to this issue can lead to deformation and surface settlement and, as a result, severe damages to the adjacent surface and subsurface structures [1,2]. Choosing the right method for excavating urban large tunnels and underground subway stations in soft ground is a key factor for successfully completing the project. Therefore, it is vital to provide an optimal method for constructing underground spaces, as it does not comprise the shortcomings of the currently utilized methods. Also it is acceptable economically and beneficial for construction speed and compatibility with the environment.

In recent years, the shallow tunneling method (STM) has been introduced as a new method to safely excavate and construct underground spaces with low overburden and complex geological conditions. STM is a concept of soft ground tunneling rather than a set of excavation and support techniques. This method is different from the New Austrian Tunneling Method (NATM) in terms of design concepts, although it also adopts some of the most common techniques such as sequential excavation, ground reinforcement, shotcreting, and monitoring similar to the NATM [3,4]. The depth of the critical overburden, which distinguishes between shallowly buried conditions and deeply buried conditions, is determined by the question of whether the arching effect can be sufficiently established or not. Under deeply buried conditions, it is assumed that the height of the arching zone cannot be changed. However, in shallowly buried conditions, the failure zones easily spread to the ground. Expansion of the failure zones and deformation of the ground surface leads to surface settlement and consequently damage to adjacent structures. Investigating the behavior of the ground due to tunnel excavation provides an overview of ground deformation and provides an insight on how to prevent possible damages to the existing structures. Additionally, it makes basic measures against each of these damages before starting the excavation and construction of the tunnel [5–10].

The problem of ground surface settlement caused by the excavation of underground spaces is critically important. So that it has always been studied by various researchers to estimate its value before adopting appropriate methods and starting the excavation and construction operations. In the field of predicting ground stresses and deformations caused by excavating underground spaces, much research has been done, and various methods have been proposed, some of which are based on computational principles and some are based on empirical observations [11–15]. Various methods have been recommended to predict the value of the ground settlement, referred to as experimental, analytical, and numerical methods. In comparison, numerical methods have proven to be an important and powerful tool for solving complex engineering problems in geotechnics, tunneling, fracture mechanics, slope stability, wellbore stability, etc. [16–25]. Numerical methods using computational codes (software packages) make it possible to model the tunnel execution process, taking into account the construction on the surface and adjacent structures. Numerical modeling is used as a factor and means to predict the ground behavior during the tunnel excavation process. In other words, one of the advantages of using numerical methods is the general prediction of ground deformation as well as its low cost [26–29].

Subway Station Z6 is one of the stations in the northern part of Tehran Subway Line 6. The excavation and construction method used for this station is the traditional method of pile and rib. One of the main problems of excavating the underground spaces in urban areas, despite maintaining the stability of the underground structure, is the control of ground settlement to prevent damage to neighboring underground and surface structures. Therefore, it is necessary to utilize the new methods presented today in the tunneling industry to build large underground spaces such as subway stations and to ensure excellent safety performance during excavation and construction operations. In the present study, the value of ground settlement for constructing Station Z6 with two methods of shallow tunneling (STM) and pile and rib (PRM) was numerically analyzed using Plaxis 3D Tunnel finite element software.

2 Specifications of Station Z6

2.1 Project overview

Line 6 is one of the main lines of the Tehran Subway network with a length of approximately 36 km. This line extends from the south-west of Tehran in Shahre-Rey to Kan in the north-west of Tehran, passing through populated regions (Fig.1). Station Z6 is located in the northern part of Line 6, at chainage 29 + 901 m in the Kan area. As represented in Fig.1, this station is located in Koohsar Boulevard and at the intersection with Abshenasan Expressway. The design and construction of this station were performed as a two-floor underground using the PRM.

2.2 Engineering geological and geotechnical characteristics

Exploratory studies at the site of Station Z6 include drilling a borehole (BH-135) and excavating a test pit (TP-212), as shown in Fig.2. Two types of soil layers have been identified based on engineering geological and geotechnical investigations conducted at Station Z6. The first layer to a depth of about 28 m is composed of clay and silty sand with gravel (SC-SM and lenses of clay gravel (GC). The second layer, from a depth of 28 m to the bottom of the borehole, is composed of clay gravel with sand (GC). Based on the data obtained from BH-135, the station level is located above the water table. The longitudinal profile of engineering geological for Station Z6 is reported in Fig.2. Also, the physical and mechanical properties of the soil layers at the station site are presented in Tab.1.

3 Evaluation of construction method on ground surface settlement

3.1 Numerical modeling of pile and rib method (PRM)

To evaluate the ground settlement for Station Z6, the finite element method was employed for the numerical modeling. The purpose of the finite element analysis in modeling the soil-structure interaction was to better investigate the surface settlement during excavation and construction. As mentioned earlier, the method of excavation and construction for Station Z6 was the traditional PRM. The cross-section of the station is shown in Fig.3.

Here, the excavation and construction stages of the station were modeled first using Plaxis 3D tunnel software. Then, the surface settlement results were compared with the instrumentation data recorded during the station construction. In general, the numerical modeling of the excavation and construction process of this station was done as follows (Fig.4).

1) The model’s dimensions were about four times the radius of the underground space below the station and on its left and right sides. The dimensions were considered so that it was far from the effect of unrealistic boundary conditions.

2) The excavation stages of the station were modeled according to the excavation process in the PRM.

3) Boundary conditions. Standard Fixities were considered fixed and roller conditions on the bottom (U_x = U_y = 0) and sidewalls (U_x = 0) of the model, respectively. To model the traffic load on the street surface, a wide load of 20 kN/m² [30] was considered.

4) Soil parameters and failure criterion. In the model, heterogeneous, anisotropic ground conditions were assumed. The constitutive model was based on the hardening soil failure criterion (Tab.1).

5) Vertical in situ stress in the model was considered a gravity load, and the ratio of the horizontal stress to the vertical stress was equal to the coefficient of lateral earth pressure at rest, k=1-\mathrm{s}\mathrm{i}\mathrm{n}\phi.

6) Meshing. Fifteen-node triangular volumetric elements were used to mesh the model.

Fig.4 suggests the geometric characteristics of the numerical model created by Plaxis 3D tunnel software and its meshing condition. As illustrated in Fig.4, Station Z6 is located at a depth of about 26.30 m from the ground surface with an overburden of 11 m. The ratio of the overburden height (H) to the station diameter (D) is less than 1, and it can lead to ground deformation and settlement due to incomplete formation of the arching in the station overburden. Note that the minimum overburden required to form a complete arching in front of the excavation face is about 1 to 2 times the diameter of the underground opening.

The support structure of the station consists of piles with a diameter of 1.2 m in distances of 2.5 m and ribs with a width of 1.2 m at the height of 1.8 m (Fig.3). In addition, the lateral inhibition of the piles is provided by implementing the middle slab. The middle slab of the station is 80 cm thick. The final specifications used for the different components of the station support structure are presented in Tab.2. As indicated in Fig.5, the modeling of the excavation and construction of Station Z6 is done in 6 separate stages as follows:

• Stage 1: creating the initial conditions (effective stress);

• Stage 2: applying the traffic load;

• Stage 3: excavating and executing piles and ribs;

• Stage 4: excavating the upper floor of the station;

• Stage 5: executing the middle slab;

• Stage 6: excavating the lower floor of the station.

It should be noted that the model displacements are reset to zero after the second stage. The results obtained from the numerical modeling of the different stages of excavation and construction of the station are presented in Fig.6. As can be observed in this Figure, with the advance of excavation stages in different parts of the station, the value of ground settlement has increased, so that the maximum settlement of 24.65 mm resulting from numerical modeling is related to the final stage of excavation of the station section. As suggested above, the main stages of the excavation and construction of the station include excavation and execution of piles and ribs, excavation of the upper part, execution of the middle slab, and excavation of the lower part. The obtained values of settlement in these main stages are 5.62, 14.28, 20.43, and 24.65 mm, respectively (Fig.6). In addition, Fig.7 shows the contours of vertical and horizontal displacement obtained from numerical modeling in the final stage of the station excavation. As denoted in Fig.7, the maximum vertical and horizontal displacements created in the model are related to the lower floor of the station. The maximum vertical displacement of 27.57 mm at the bottom of the station is in the form of uplift due to the non-implementation/construction of the invert (Fig.7).

As can be seen from Fig.8, to control the ground surface settlement during the excavation and construction of Station Z6, the values of 30, 35, and 40 mm were considered being the alert level, the alarm level, and the action level, respectively. These levels were defined based on the geological and geotechnical conditions, geometry of underground space, construction method, excavation requirements and construction method, structural requirements, and specific limitations of the project [2]. In other words, these three values ​​were determined based on the prediction of 50%, 80%, and 100% of the settlement obtained from the underground space excavation, respectively. Based on the numerical analysis performed for the different stages of excavation and construction of the station, the maximum settlement of 24.65 mm is less than the minimum allowable settlement of 30 mm. Also, the maximum value of settlement recorded by the settlement pins (28.30 mm) at the ground surface and along the station axis (CSLP.AB.P010.C) is less than the minimum allowable settlement value ​​(Fig.8).

The difference between the maximum settlement value ​​obtained from numerical modeling and the instrumentation data was about 3.65 mm. The good agreement between the settlement obtained from the station monitoring program and the numerical simulation indicated the accuracy of the modeling performed by PRM. Therefore, according to the results obtained from the numerical analysis and calibration of the numerical model, the process of the excavation and construction of Station Z6 were numerically investigated by the new STM.

3.2 Numerical modeling of shallow tunneling method

The new STM is used to excavate and construct large underground spaces such as subway stations in soft ground with the shallow overburden depth. The effect of limited arching and limited mobilization of ground strength are the two main mechanical characteristics of the STM. Face stability and dry tunneling conditions are two necessary preconditions in this method. Several essential auxiliary methods are used to fulfill these two preconditions, which include five primary rules. These five important principles used in the STM concept or philosophy include auxiliary methods, sequential excavation with short advance lengths, rigid support with quick installation, short ring closure time, and systematic deformation monitoring. According to the construction sequence, the STM is classified into five executive methods: MDA, SDA, DCA, DPCA, and PBAA. The PBAA (Pile Beam Arch Approach) method can better control the ground settlement in station construction compared to other methods.

In addition to the main cross-section, the PBAA method can be performed in different cross-sections depending on the different conditions (Fig.9). Therefore, in this study, the PBAA method was employed as a suitable method to simulate the excavation and construction stages of Station Z6. Based on the station's geological conditions and geometric dimensions, cross-section A (double-arch and double-span) was selected as the ultimate cross-section for the investigations.

Modeling of the excavation and construction of Station Z6 by STM (i.e., PBAA) was carried out based on the numerical modeling conditions of the PRM. The 3D numerical model to investigate the surface settlement is presented in Fig.10. This Figure represents the dimensions of the model geometry and boundary and loading conditions during the station excavation and construction. As mentioned earlier, the dimensions of the model were designed to minimize the impact of boundary effects. The side boundaries of the model were fixed along the x and z axes and the lower boundary of the model was fixed along the y-axis. A wide load of 20 kN/m² [30] was considered the traffic load on the street surface and the station axis.

The excavation and construction stages of Station Z6 by STM are shown in Fig.11. As can be seen from Fig.11, the excavation and construction of the station were modeled in six stages to investigate the deformation and settlement of the ground surface. In the first stage, the station’s side and middle drifts were simultaneously excavated to execute the side piles and middle columns. Then they were supported by a concrete layer of 30 cm thick along with two layers of wire mesh and the lattice girders at intervals of about 1 m (Fig.11). In the second stage, the location of the side piles, the middle columns, and the invert of the station were excavated and executed simultaneously (Fig.11). In the third stage, the arch of the upper part of the station was excavated and executed (Fig.11). In the fourth stage, the upper part of the station was excavated (Fig.11). In the fifth stage, by excavating the upper floor, the station's middle slab was constructed to provide lateral inhibition of the piles (Fig.11). In the final stage, the lower floor of the station was excavated up to the invert level (Fig.11).

The properties of the materials utilized for the pile, column and the station arch were similar to the geometric and mechanical properties of the materials used for PRM (Tab.2). In this study, the strength parameters of the pile and the column were also considered equal to each other. By considering the cracking factor of f_c = 0.35 for the primary support system of the side and middle drifts, the normal stiffness (EA) and the flexural rigidity (EI) are as follows:

(1)EA=2.13\times {10}^7\times 0.3=6.39\times {10}^6\mathrm{k}\mathrm{N},

(2)EI=0.35\times 2.13\times {10}^7\times 0.00225=1.677\times {10}^4\mathrm{k}\mathrm{N}\cdot {\mathrm{m}}^2.

The modeling results of different stages of excavation and construction of Station Z6 are indicated in Fig.12. As shown in Fig.12, the ground settlement values ​​are increased with the advance of excavation and construction of the different parts of the station. The maximum settlement of 6.9 mm obtained during six different stages of excavation and construction is related to the final stage of the station. According to the proper execution of the different parts of the station, including excavation and support of the side and middle drifts, as well as installation of the support systems such as side piles, middle columns, and upper arches, the settlement of the ground surface was decreased significantly compared with PRM.

The STM played an important role in reducing the deformation and settlement of the ground surface by employing the middle columns that led to the proper distribution of the loads on the double-arch roof of the station. Also, the excavation of the side and middle drifts to implement the desired piles and columns played a significant role in the gradual release of stresses around the station.

According to the 3D longitudinal profile presented in Fig.13, the value of ground surface settlement ahead of the face-in progress shows a significant increase compared with the excavation of the face position and its behind. As reported in Fig.13, the rate of ground settlement in front of the excavation face is greater than behind the face. The value of the settlement in front of the excavation face, at a distance of approximately less than half the width of the station, reached its maximum value of about 6.09 mm. This value at a distance of about one times the width of the station behind the face was reduced by 5.07 and 3.52 mm, respectively (Fig.14).

As shown in Fig.13 and Fig.14, from the starting point of the station to a certain distance before the excavation face a (about half the width of the station), the transverse settlement was increased by increasing the distance from the station axis. In other words, the transverse settlement of the ground surface in the direction of the station axis was reduced due to the proper construction of the station support system (piles, beams, arches, etc.) as a result of the ground strength against deformation increased.

During the enlargement to the full station size, the station’s plastic zone, especially at the face position, was raised. In addition, by increasing the deformation around the excavation face, the ground surface settlement was increased along the axis of the station compared with the surrounding ground (Fig.13, Fig.14, and Fig.14).

The vertical deformation obtained from the modeling process of the station displayed that the maximum value of displacement of 9.27 mm was related to the floor of the station due to heaving (Fig.15). Moreover, the maximum horizontal deformation in the direction of the x-axis (perpendicular to the station’s axis) corresponding to the lower floor of the station was 3.60 mm (Fig.15).

In addition to the horizontal displacement perpendicular to the station’s axis, the value of horizontal displacement along the station’s axis was also investigated. As shown in Fig.16, the maximum displacement created after the full station size was 25.61 mm. It was found that the maximum deformation observed in the model was related to the horizontal displacement in the direction of the station axis (z-axis) at the position of the excavation face, especially on the lower floor of the station (Fig.16). As mentioned earlier, the ground surface settlement at the face position and in front of the face was primarily due to significant deformations in the form of extrusion in the core face of the station. Fig.17 and Fig.18 show the relative shear stress and the plastic zone around the station after the execution and completion of the support system and excavation of the upper and lower floors, respectively. Relative shear stress due to the use of a support system rarely occurred around the station’s underground space and toward the ground surface (Fig.17). As displayed in Fig.17, the maximum relative shear stress happened at the excavation face position, indicating a high potential for deformation at the face and its surrounding space. This value of the shear stress was observed more on the lower floor than on the station’s upper floor. In addition, the plastic zone created at the excavation face was more than the ground around the station due to the lack of the appropriate ground reinforcement (Fig.18). With the development of the plastic zone at the core face and around it, the deformation of the ground increases, and when it reaches the ground surface, it appears as a settlement in front of the excavation face (i.e., Fig.13).

The convergence created in front of the face was increased compared to that of behind the excavation face due to the enlargement of the station cross-section (the excavation area). Behind the excavation face, due to the presence of support systems, the value of deformation and settlement of the ground surface was significantly reduced (maximum 3.52 mm). In contrast, in front of the face, due to the lack of ground reinforcement systems, deformation and settlement of the ground surface showed a higher value (maximum 6.09 mm). In other words, the results showed that by using the support systems around the underground space before the excavation operation, it is possible to prevent the increase in deformation and settlement of the ground. Whereas, by increasing the area of the excavation face (full station size), the value of horizontal ground deformation (extrusion) along the excavation axis was increased and thus led to a more significant deformations around the station and at the ground surface. To control the deformations created at the station core face (at the face position and in front of the face), the auxiliary methods of ground reinforcement such as forepoling, face bolting, foot piling, and other techniques can be used in addition to the support systems (i.e., pile, column, arch, etc.).

The maximum settlement obtained from STM compared to PRM and the minimum allowable settlement of the ground surface showed a significant decrease of about 75% and 80%, respectively.

4 Sensitivity analysis of the support system

According to the minimum allowable settlement of 30 mm and a significant reduction in the settlement obtained by the STM method (6.09 mm), a sensitivity analysis was performed on some of the most important geometric parameters of the station support system. Doing so was to achieve an optimal support system to control the ground settlement. The analysis was performed on the longitudinal distance of the station’s side piles and middle columns and their geometric dimensions. The sensitivity analysis results and the modeling results obtained from two methods of the PRM and STM, are presented in Table 3. The results indicated that by changing some of the station’s executive components, including the geometric characteristics of the support systems, a significant technical optimization could be carried out during the execution and completion of the station.

As shown in Table 3, with the execution of the piles and columns at intervals of about 3.25 m, the diameter of the piles and columns in 100 cm, and the geometric dimensions of the upper arches of the station in 100 cm (width) at 130 cm (height), the value of settlement (29.30 cm) was within the allowable limit of the ground surface settlement (30 cm). In other words, by increasing the longitudinal distance between the piles and columns from 2.50 to 3.25 m, reducing the diameter of the piles and columns from 1.20 to 1 m, and also reducing the width and height of the arch beam from 1.20 to 1 m and 1.80 to 1.30 m, respectively, the ground surface settlement was within the allowable limit of the station design. In addition, it should be noted that the width and height of the middle slab were also reduced from 1.20 to 1 m and 0.80 to 0.60 m, respectively.

Therefore, it can be concluded that by using the new STM instead of the traditional PRM, it is possible to contribute significantly to the excavation and construction operations of subway stations in soft ground.

5 Conclusions

This paper compares the STM to PRM for constructing the subway station in the Tehran geological conditions. Based on the current study, the main findings can be summarized as below.

1) The maximum surface ground settlement (24.65 mm) obtained from PRM was within the minimum allowable limit (30 mm). This value of the ground settlement indicated a good agreement with the settlement recorded by the instrumentation (28.30 mm).

2) The excavation and construction of Station Z6 by the STM was modeled based on the numerical model validated in the PRM. The results represented that there is a large difference between the settlement obtained from STM and PRM. The maximum ground settlement obtained from STM was 6.09 mm as related to the front of the excavation face.

3) In the PRM, large deformation of about 27.57 mm was observed on the station floor in the form of uplift, while it was decreased to 9.27 mm in the STM, which was used as the invert.

4) The horizontal deformation or extrusion created in the station core face (due to the enlargement to the entire station size) was one of the important factors in developing the ground surface settlement, especially in front of the excavation face. The maximum extrusion of 25.61 mm was obtained during the excavation of the lower floor of the station.

5) The sensitivity analysis results showed that some critical geometric parameters of the support system could be optimized by using STM. Therefore, this method can be a good alternative for the PRM in the excavation and construction of subway stations in the geological conditions of Tehran.

6) The new STM improved performance in reducing and controlling the ground deformation and settlement compared to the PRM.

Notations

b: width

h: height

C: tunnel cover (overburden)

H: tunnel depth

D: tunnel diameter

{\gamma }_{\mathrm{m}}: unit weight

E: modulus of elasticity

E_s: secant modulus of elasticity

E_ur: unloading and reloading modulus of elasticity

v: Poisson’s ratio

c: cohesion

φ: internal friction angle

ψ: dilation angle

I: moment of inertia

EA: normal stiffness

EI: flexural rigidity

f_{\mathrm{c}}^{\prime }: compressive strength

f_{\mathrm{c}}: cracking factor

A: area

R: tunnel radius

x: distance to the face

m: power in stress-dependent stiffness relation

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