Deformation Control and Countermeasures for Construction Surface When Using Pile-Beam-Arch Method in Confined Water-Rich Sand Cross-Stratification

Zhang Xue-gang1, Zhan-ping Song2, Chi-yu Yuan1, Jian-sheng Qu1 1．Shaanxi Railway Institute, Weinan Shaanxi, 714000；2. School of Civil Engineering, Xi'an University of Architecture and Technology, Xi'an 710055, PR

ABSTRACT

The of the subway is close to the core of the Beijing central business district. Together with the Shuangjing Station of , it forms a T-type transfer amid many surrounding buildings and underground pipelines. The Shuangjing Station is a two-layer double-column three-span subway station constructed by the pile-beam-arch method. In this study, a numerical simulation of the pipeline deformation that occurred during the construction of the pilot tunnel of the Shuangjing subway station was performed using the ANSYS software. “Eight-pilot-tunnel” excavation was used to consider the multitunnel effect. The deformation characteristics and control mode of the existing pipeline atop the station, caused by the different pilot tunnel constructions, were analyzed. Based on the results of the analyses, an implementation of the principle of pilot tunnel excavation is recommended for such projects. This involves an “up-to-down and crosswise” construction sequence. To avoid mutual interference between the pilot tunnels, longitudinal and horizontal distances of not less than 10 m and a vertical distance of 15 m between adjacent tunnels are recommended. Each small pilot tunnel should be rapidly excavated using the bench method to reduce ground disturbance. This construction technique requires strengthening and enclosing of the primary support as soon as possible. This is done to limit settlement and control the deformation of the existing pipeline during the construction. The proposed construction method has good economic and social benefits and can be used as reference for similar future projects.

KEYWORDS: Subway station; Pile-beam-arch (PBA) tunneling method; Pilot tunnel excavation; Multitunnel effect; Construction scheme; Numerical simulation

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Vol. 19 [2014], Bund. Y 8728

INTRODUCTION

China has experienced accelerated urbanization in recent years, and the development of infrastructure for alleviating traffic congestion, such as the construction of subways, has flourished. The construction of subway tunnels in developed metropolis that contain many high-rise buildings inevitably involves underground excavation, with the attendant impact on the surroundings.[1-2] It is particularly important during the tunneling process to effectively control surface settlement and other deformations caused by excavation, and to ensure the intactness of existing underground pipelines. Numerous studies have been conducted in this field by experts from home and abroad, and this has led to a number of findings[3-6]. The pile- beam-arch (PBA) method is often used for underground excavation over a large area[7]. It involves the creation of pilot tunnels to make piles, beams, and arches for forming force transmission structures, and to carry out the excavation of the inner tunnel under the arch cover of the underground excavation. Although this construction method has been successfully implemented several times, it still has some deficiencies and shortcomings such as the requirement of multiple pilot tunnels and diverse processes, long blasting times, and extended ground disturbance. This has prompted the present study on the control of the surface deformation caused by PBA construction. In this study, the PBA process that was used to construct the Shuangjing Station of the Beijing Metro Line 7 through a sandy soil artesian aquifer was simulated. The surface settlement and pipeline deformation caused by the different excavation steps were analyzed, and countermeasures for future projects are proposed.

PROJECT OVERVIEW

The Shuangjing Station of the Beijing Metro Line 7 is close to the center of the central business district, which is characterized by heavy traffic. The station forms a T-type transfer with the Shuangjing Station of Line 10. It has two underground two-column three-span floors and was constructed by underground excavation (PBA method). The station has a principal length of 229.6 m and a standard section 23.1 m wide and 16.25 m high. The baseboard of the structure has a buried depth of about 30.3 m, and the roof is covered by earth 13.89 m thick. The profile contains eight upper pilot tunnels in two rows. The excavation of the station was based on previously excavated pilot tunnels. The pilot tunnels in the main part of the station were created through a very complex stratum that included fine sand, coarse sand, sand and gravel, and silty soil. The upper pilot tunnels were created through a phreatic aquifer, and the lower pilot tunnels through an artesian aquifer. Above the structure of the station was a dense network of 14 important municipal pipelines. A schematic of the location of the main structure relative to the pipelines and the numbering of the pipelines (1 to 14) are shown in Figure 1. Vol. 19 [2014], Bund. Y 8729

Figure 1: Construction profile of Shuangjing Station of Beijing Metro Line 7

NUMERICAL SIMULATION OF PBA METHOD

ANSYS model

ANSYS finite element program was used to analyze and assess the measures for controlling surface settlement and pipeline deformation during PBA construction in a sandy soil artesian aquifer. Numerical analyses of the three following aspects were carried out: (a) the effects of the first excavation of the four upper and four lower pilot tunnels; (b) the effects of the different excavation sequences of the four upper and four lower small pilot tunnels; and (c) the effects of the main construction works of the station. The size of the 3D computation model is 121 × 50 × 60.5 m (length × width × height). The model is shown in Figure 2.

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Figure 2: 3D computation model

ANSYS parameters

All formation parameters Table 1 gives the physical and mechanical parameters of the typical sections of the rock where the station is located, as determined by geotechnical investigations.

Table 1: Physical and mechanical properties of the rock at the location of the station

Elastic Severe/ Poisson’s Cohesion Friction Surrounding rock modulus [kN·m-3] ratio [kPa] angle [°] [MPa] First layer 16.9 36 0.3 19 28 Second layer 20.1 86 0.27 9.3 26 Third layer 20.3 110 0.27 11.6 27 Fourth layer 20.6 125 0.27 9.6 31 Fifth layer 20.8 132 0.28 10 33

Simulation parameters of grouting reinforcement layer and preliminary bracing The details of the grouting reinforcement layer and the primary support parameters in the design of the station are given in Table 2.

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Table 2: Physical parameters of initial support and reinforcement

Severe/ Elastic modulus Poisson’s Cohesion Friction

[kN·m-3] [MPa] ratio [kPa] angle [°]

Primary 23 26 0.2 － － support Reinforced 23 0.495 0.25 0.3 50 area

Material parameters of station structure The main structural parameters of the station based on its design and the current “code for design of railway tunnel” are given in Table 3.

Table 3: Structural parameters of the station model

Severe Elastic modulus Cohesion Friction Poisson’s ratio [kN/m3] [GPa] [MPa] angle [°] Grouting reinforcement 22 0.495 0.3 0.3 50 Backfill C20 concrete 25 25.5 0.25 - - Initial lining 26 20 0.2 - - Second lining 27 32.5 0.25 - - side piling 23 20 0.2 - - Top beam 25 32.5 0.25 - - Steel pipe pile 25 33.5 0.25 - - Steel tie rod 79 200 0.25 - -

Effects of different excavation sequences of pilot tunnels

Simulated condition Based on the specific circumstances of the small pilot tunnels in the main structure of the Shuangjing Station, the following three schemes were simulated:

Scheme 1: Stagger skipping excavation of the upper pilot tunnels. In this scheme, the upper pilot tunnels were first excavated till the hole-through was completed, after which the lower pilot tunnels were excavated. The specific excavation order was ① - ③ - ④ - ② - ⑤ - ⑦ - ⑧ - ⑥. The numbers of the pilot tunnels are shown in Figure 3.

Scheme 2: Stagger skipping excavation of the lower pilot tunnels. In this scheme, the lower pilot tunnels were first excavated till the hole-through was completed, after which the Vol. 19 [2014], Bund. Y 8732 upper pilot tunnels were excavated. The specific excavation order was ⑤ - ⑦ - ⑧ - ⑥.- ① - ③ - ④ - ②.

Scheme 3: Stagger excavation of the upper pilot tunnels beginning with the outer pilot tunnel. The excavation order of this scheme was ① - ④- ② - ③ - ⑤ - ⑧ - ⑥ - ⑦.

In all the excavation schemes, a distance of 8 m was left between adjacent pilot tunnels. The step method was used for the excavation of each pilot tunnel, with a gap of 3 m between successive steps. The excavation footage cycle of the pilot tunnel was 1 m. The primary supporting of the excavated soil mass was done in a further step.

Figure 3: Numbering scheme of the tunnels

Numerical simulation results and analysis Four typical construction steps were selected to clarify the impact of the pilot tunnel excavation on the ground deformation and pipeline displacement: (1) the excavation of Pilot Tunnel 1 (or Pilot Tunnel 5) by 25 m up to the middle of the model; (2) execution of the hole- through of the upper pilot tunnels (lower pilot tunnels); (3) excavation of Pilot Tunnel 5 (or Pilot Tunnel 1) by 25 m up to the middle of the model; (4) execution of the hole-through of all the pilot tunnels.

Surface movement and deformation

To reduce the boundary effect on the calculated results, the Y = 25 m section was used to analyze the surface deformation. By numerical simulation, the surface subsidence of the four typical construction steps and three schemes were analyzed as presented in Table 4.

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Table 5: Maximum settlement caused by different excavation steps

Scheme 1 Scheme 2 Scheme 3 Serial Excavation step number Sedimentation Sedimentation Sedimentation Percentage Percentage Percentage volume [mm] [mm] [mm]

Pilot Tunnel 1 (or Pilot 1 Tunnel 5) was excavated -1.98 10.1% -2.39 12.9% -2.03 11.2% up to the middle

Upper pilot tunnels 2 (lower pilot tunnels) -10.16 45.9% -7.78 29.1% -10.32 45.8% were transfixed

Pilot Tunnel 5 (or Pilot 3 Tunnel 1) was excavated -14.57 24.8% -1.22 18.6% -14.79 24.7% up to the middle

All the pilot tunnels 4 -17.82 18.2% -18.49 39.3% -18.09 18.3% were transfixed

Vertical deformation of pipelines

The simulation results indicated that the maximum differential settlement occurred in Line 3, and Line 10 had a large maximum settlement. The three excavation schemes for Line 3 and Line 10 and the corresponding bottom vertical displacement curve of the pipelines are shown in Figures 4–9.

Figure 4: When upper pilot tunnels were Figure 5: When upper pilot tunnels were excavated first (Scheme 1), and bottom vertical excavated first (Scheme 1), and bottom displacement curves for Line 3 vertical displacement curves for Line 10 Vol. 19 [2014], Bund. Y 8734

Figure 6: When lower pilot tunnels were Figure 7: When lower pilot tunnels were excavated first (Scheme 2), and bottom excavated first (Scheme 2), and bottom vertical displacement curves for Line 3 vertical displacement curves for Line 10

Figure 8: Bottom vertical displacement curves Figure 9: Bottom vertical displacement curves of of Line 3 for Scheme 3 Line 10 for Scheme 3

As can be seen from Table 4, the three different construction schemes produced basically the same degree of surface settlement, and the maximum value of the surface settlement when the lower pilot tunnels were first excavated was slightly greater than that when the lower pilot tunnels were first excavated. The maximum value of the surface settlement caused by the stagger skipping excavation scheme was slightly larger than that caused by the stagger excavation scheme when the outer pilot tunnels were first excavated. In addition, for the two different construction schemes, the maximum value of the surface settlement caused by the construction of the upper pilot tunnels accounted for about 56% of the total settlement, and that caused by the construction of the lower pilot tunnels accounted for about 44%. This indicates that the effects of the two construction schemes on the surface deformation were basically the same.

As can be seen from Figures 4–9, during the phases of the excavation of Pilot Tunnel 1 and Pilot Tunnel 5 up to the middle, the longitudinal deformation of the bottom pipeline varied significantly along its length, whereas the variation was small during the rest of the construction. The maximum Vol. 19 [2014], Bund. Y 8735 pipeline deformation occurred after executing the hole-through of all the pilot tunnels. In the three different construction schemes, the final deformation and the variation of the settlement of the pipeline above the small pilot tunnels were greater. The maximum deformation of the pipeline and the maximum variation of the settlement were greater in the scheme in which the lower pilot tunnels were first excavated than in the scheme in which the upper pilot tunnels were first excavated. The maximum deformation of the pipeline and the maximum variation of the settlement were also greater in the stagger excavation scheme in which the outer pilot tunnels were first excavated than in the stagger skipping excavation scheme.

Based on the above analysis, the construction scheme in which the upper pilot tunnels are first excavated is superior, and it is suggested that, in the actual construction, the upper pilot tunnels should be excavated first, and the lower pilot tunnels should be excavated after the construction of the upper tunnels has been completed. The stagger skipping excavation scheme is also better than the stagger pilot tunnel excavation scheme, and it is recommended for use in the construction of both the upper and lower pilot tunnels.

Effects of main station construction works

Construction steps Based on the specific circumstances of the Shuangjing Station, the order of the main construction works is as follows: (a) construction of the eight main pilot tunnels and the small pilot tunnels; (b) construction of the pile-bottom longitudinal beam of the lower pilot tunnels and construction of the structural envelope pile and pile-top longitudinal beam by manhole digging; (c) construction of the primary support of the inside-span pilot tunnel, and backfilling; (d) construction of the primary support between the small pilot tunnels; (e) excavation of the soil mass, and the second lining at the top; (f) soil excavation to the bottom of the middle plate of the structure, and construction of the middle plate and longitudinal beam; (g) excavation to the bottom of the pit, the bottom of which is sealed by profile steel, and pouring of the middle plate and longitudinal beam; (h) removal of the remaining primary lining at the middle of the lower pilot tunnels, and construction of the baseboard of the structure; and (i) construction of the lower sidewall after the baseboard has attained the design strength, to complete the construction of the main structure.

Numerical simulation results and analysis To clarify the effect of the station structure on the surface deformation and surrounding pipelines, four typical construction steps were considered: (1) completion of the construction of the small pilot tunnels; (2) completion of the construction of the side pile, middle pile, and longitudinal beam; (3) completion of the construction of the arch; and (4) completion of the construction of the main structure. Vol. 19 [2014], Bund. Y 8736

Surface movement and deformation

The results of the analyses and calculations of the trough profiles of the surface settlements caused by the tunneling are presented in Table 6 and Figure 10.

Table 6: Typical maximum surface settlements during construction

Serial Maximum surface Excavation step Percentage number settlement Completion of the construction of the small pilot 1 17.8 46.4% tunnels Completion of the construction of the side pile, 2 19.3 3.9% middle pile, and longitudinal beam 3 Completion of the construction of the arch 37.2 46.6% 4 Completion of the construction of the main structure 38.4 3.1%

Figure 10: Curve of surface settlement during construction

Vertical deformation of pipelines The results of the analyses of the deformations of the 14 root pipelines caused by their settlements during the typical construction steps are given in Table 7.

Table 7: Settlements of main pipelines

Pipeline settlement [mm] Excavation step p1 p2 p3 p4 p5 p6 p7 p8 p9 p10 p11 p12 p13 p14

Completion of the construction of the small 9.4 13.4 16.7 16.2 17.6 18.4 17.2 17.0 12.5 18.9 16.2 18.8 6.0 0.4 pilot tunnels

Completion of the construction of the side 9.5 13.7 17.2 16.5 18.1 20.1 19.8 20.0 13.7 20.7 17.8 20.8 6.4 0.5 pile, middle pile, and longitudinal beam

Completion of the construction of the arch 19.1 28.0 34.9 33.9 36.8 38.3 35.8 35.4 26.2 39.5 33.8 39.4 12.5 0.8 Vol. 19 [2014], Bund. Y 8737

Completion of the construction of the main 20.3 28.9 36.0 34.9 38.0 39.6 36.9 36.5 27.0 40.8 34.9 40.6 12.9 0.9 structure

In Figure 12, the abscissa represents the distance between the monitoring point and the center line of the station, and the ordinate represents the surface settlement. As indicated in Table 6 and Figure 12, the surface settlement trough curve is basically consistent with the peck formula. The main impact range is about 35 m from the center line of the station, and the maximum surface settlement after excavation of the small pilot tunnels has been completed is 17.8 mm, which accounts for 46.4% of the final settlement. After completion of the arch, the maximum surface settlement reaches 37.2 mm, accounting for 46.6% of the final settlement. The two excavation steps thus account for 93.0% of the total settlement, and this makes them key steps in the control of the soil mass deformation.

As can be seen from Table 7, the pipeline settlements during the construction of the small pilot tunnels and the construction of the arch each account for 45% of the total settlement. Both steps are thus key to controlling the construction. The final settlement of electric pipeline 14 (2600 × 2900) after the main construction works have been completed is 0.9 mm, which is far less than the settlement allowed by the control code. The final settlement of heating pipeline 13 (5900 × 2650) is 12.9 mm, which exceeds the allowed sedimentation control value of 10 mm. The final settlements of the other 12 pipelines after completion of the main construction works are all more than 20 mm; they significantly surpass the value allowed by the settlement control code, and proper action is required to solve the problem.

CONCLUSION

In this study, a numerical simulation of the PBA construction process of the Shuangjing Station of Beijing Metro Line 7 was performed using the ANSYS software. Based on the calculated surface settlements and vertical deformations of the existing pipelines, the following conclusions and suggestions are made: (1) In the construction of the eight pilot tunnels, the upper pilot tunnels should be excavated first, and the lower pilot tunnels should be excavated after completion of the upper ones. For the construction of the four small upper and lower pilot tunnels, the stagger skipping excavation scheme is better than the stagger pilot tunnel excavation scheme when the outer pilot tunnels are first constructed. (2) The surface settlement trough curve is basically consistent with the peck formula, and the stratigraphic vertical displacement is very symmetrical around the center line of the station. The main impact range is about 35 m from the center line of the station. The maximum surface settlement after completion of the excavation of the small pilot tunnels accounts for 46.4% of the final settlement, and that after completion of the arch accounts for 46.6%. The two excavation steps together account for 93.0% of the total settlement and are therefore key steps in controlling the soil mass deformation. (3) The pipeline settlements during the construction of the small pilot tunnels and the construction of the arch each account for about 45% of the total settlement. Both steps are thus Vol. 19 [2014], Bund. Y 8738 key to controlling the construction. After completion of the main construction works, all the pipeline settlements exceeded the respective allowed values, and measures are necessary to solve the problem.

Regarding the “group-tunnel effect” of the construction, it is suggested that the excavation of the pilot tunnels should follow a reasonable sequence. During the excavation and installation of supports in the small pilot tunnels, the lower ones should be excavated first in a staggered order, and the staggering distance between the tunnels should not be less than 10 m. Furthermore, timely information feedback from the monitoring and measurement should be used to guide the design and construction by adjustment of the support parameters and construction methods. This should constitute the primary means for assuring security. Meanwhile, the upper and lower pilot tunnels should be staggered by 15–20 m during the excavation and installation of supports. Vertical and transverse gaps should be ensured between the pilot tunnels to avoid mutual interference. Finally, the excavation sequence of the small pilot tunnels should be rapidly executed to reduce disturbance of the stratum. The primary support should be strengthened and enclosed as soon as possible to limit the settlement and deformation of the pilot tunnels.

ACKNOWLEDGMENT

The authors deeply appreciate support from the education department of Shaanxi province natural science projects (2013JK0990), PR China.

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