Construction of shallow buried large-span metro stations using the small pipe roof-beam method

Abstract

In relation to the Shifu Road Station project on Line 4 of the Shenyang Metro in China, a small-pipe roof-beam method for constructing subway stations is presented. First, a numerical simulation was performed to optimize the supporting parameters of the proposed method and determine the design scheme. Subsequently, the deformation of the pipe roof and surface settlement during the construction process were investigated. Finally, the surface settlement attributed to the excavation was studied through field monitoring, and the proposed method was compared with other methods. The results show that an increase in the pipe-roof spacing has little effect on the surface settlement and pipe-roof deformation. The bearing capacity of the pipe roof can be efficiently utilized once the flexural stiffness reaches 2EI, and the flexural stiffness is not the dominant factor controlling the deformation. The essential stages in controlling surface settlement are the excavations of the transverse pilot tunnels and the soil between them. The final settlement value of the ground was 24.1 mm, resulting in a reduction in the construction period by at least five months while satisfying the control requirements.

Graphical abstract

Keywords

subway station / pipe-roof method / surface settlement / simulation / on-site monitoring

1 Introduction

Urban underground spaces have grown rapidly worldwide in recent years [1–3]. As subway stations are a critical part of underground spaces, their numbers have also increased significantly. As most subway stations are concentrated in busy areas of cities, there is an urgent need to develop a method to build stations safely and efficiently [4,5].

The pipe-roof method has been extensively applied in the engineering of underground spaces because it can reduce construction-induced strata deformations [6,7]. For example, the new tubular roof (NTR) method was adopted in the construction of the Gongbei Tunnel [8]. The steel tube slab (STS) method was adopted to construct the Dongbeidamalu and Aotizhongxin stations [9]. The Pinganli station was built using the tubular roof method [10]. Hence, it is evident that the pipe-roof method is reliable for constructing metro stations. At present, research results on the abovementioned methods are concentrated in two main areas. The first area involves an investigation of the flexural performance and failure modes of the supporting structure of the station. Jia et al. [11–13] evaluated the impact of critical factors, such as tube spacing and reinforcement rate, on the bending behavior of STS structure-derived optimal construction factors based on fuzzy mathematical theory. Li et al. [14] examined the effects of different loading methods on the ultimate bearing capacity and bending stiffness of a new STS slab-supporting structure. Bi et al. [15] investigated the deformation response of a bundled integrated structure under shear stress using a full-scale test. The second area involves investigating the deformation response of the structure and surface and excavation face stability during construction. Yang and Li [16] assessed the impact of construction on strata deformation using the pipe-roof preconstruction method and provided a modified Peck’s formula. Xie et al. [17] studied the effect of excavation on the stratum under the support of a large pipe-roof structure and proposed a model to calculate the stability of the working face. Zhang et al. [18] proposed an equation to calculate the vertical deformation of a pipe roof based on the elastic thin-plate theory. Wang et al. [19] explored the changes in the surface settlement resulting from the application of the STS method to the construction of metro stations. Lu et al. [20] investigated the deformation of surface and support structures arising from construction under the support of a steel-supported cutting pipe (SSCP) structure.

All the above methods, namely, the NTR, STS, and SSCP methods, involve forming a pipe-roof structure by strengthening the connections between steel pipes. Although these methods can significantly reduce the surface settlement owing to excavation, the connections between steel tubes are too complex and lead to construction challenges. Furthermore, current studies on the pipe-roof method have primarily focused on large-diameter pipe roofs. Based on previous construction experience, large-diameter steel tubes require a greater jacking force and cause more disturbance to the ground during the jacking process, thereby increasing the construction period and cost. Therefore, improvements in the existing pipe-roof method are required.

Based on the aforementioned problems, a small pipe roof-beam (SP-B) method (300–600 mm) is proposed and applied to sandy soil areas. This method forms an entire support system by erecting beams underneath the pipe roof, reducing the complexity of connections between the steel tubes and decreasing the construction period while satisfying the surface control requirements. To provide a reference for design and construction, this study first optimized the key parameters, such as the pipe-roof spacing, the flexural stiffness of the pipe roof, and the number of beams, to obtain the optimal construction parameters. Subsequently, a study on the deformation of the pipe roof and surface settlement was conducted during the construction, and key construction stages were proposed. Finally, the reasonableness of the proposed method was confirmed through field monitoring, and a comparison with other pipe-roof methods was conducted to validate the advantages of the proposed method. The conclusions of this study can serve as a reference for the future applications of this method.

2 Engineering background

The Shifu Road Station is located at the junction of Shifu Road and Nanjing North Street and is an interchange station between Lines 4 and 7 (Fig.1) [21]. The station is 47.5 m in length, 26.5 m in width, and 15.7 m in height, and the top slab is buried at a depth of approximately 4.2 m. The majority of stations traverse medium-coarse sand and gravelly sand (Fig.2) [21]. The flow of people and traffic above the stations is dense, and there are numerous municipal pipelines. In addition, the station is surrounded by several high-rise buildings, such as the Huayin Building and Shengjing Financial Square. Therefore, the requirements for controlling surface settlement are stringent. Moreover, as the Shifu Road Station is located in a prosperous area, the Shenyang Municipal Government has specified the requirement of a short construction period to avoid impacting later planning.

The pile–beam–arch (PBA) method has been widely employed to build metro stations [22]. Owing to the remarkable effectiveness of the pipe-roof structure in controlling surface settlement, numerous Chinese engineers have developed tubular roofs and STS methods. The PBA method (Fig.3(a)) and tubular roof method (Fig.3(b)) were adopted to construct metro stations, which first required the construction of the upper guide holes. However, the burial depth of the top slab was only 4.2 m. Using these methods, the ground may be disturbed during construction, which may cause greater surface settlement, resulting in a higher construction risk. The STS method (Fig.3(c)) was applied to construct the Aotizhongxin and Dongbeidamalu stations in Shenyang, which exhibited excellent surface settlement control. However, several problems are encountered during the actual construction, such as overly complex connections between pipes, difficulty in clearing soil between tubes, and long construction periods.

In summary, we adopted the SP-B method (Fig.3(d)) to build the station. In this method, the steel pipes had a diameter and wall thickness of 402 and 16 mm, respectively, and were spaced 450–540 mm apart. The pipes were then filled with C30 concrete. The beams had a cross-sectional size of 1600 mm × 1300 mm and were made of reinforced concrete; the number of beams was 3–7.

Considering the complexity of construction, the process using the SP-B method was classified into four main stages (Fig.4). In Stage 1, six longitudinal pilot tunnels were excavated, followed by the installation of bottom longitudinal beams, strip foundations, side piles, and crown beams. In Stage 2, transverse pilot tunnels were excavated, and columns and beams were constructed. In Stage 3, the soil between the transverse pilot tunnels was first excavated, and then the top slab was constructed. Subsequently, the remaining soil in the negative layer was excavated, and the upper sidewall and middle slab were constructed. In Stage 4, the soil in the negative second layer was excavated, and the lower sidewall and bottom slab were constructed.

3 Optimization analysis of pipe-roof and beam

Eighty numerical models were developed using FLAC3D to obtain reasonable support parameters (Fig.5). An optimization analysis was conducted to determine the design scheme for the pipe-roof spacing (450, 480, 510, and 540 mm), flexural stiffness (0.5EI, EI, 2EI, and 3EI, where EI represents the flexural stiffness), and the number of beams (3, 4, 5, 6, and 7).

3.1 Numerical model and parameters

The dimensions of the model were set to 150 m × 47.5 m × 50 m to avoid the effect of boundary conditions on the calculation results. The model had a fixed bottom and a free surface at the top, and the surroundings were horizontal constraints. The model contained 651812 elements. Precipitation was applied before construction; therefore, the effects of groundwater were not considered. Tab.1 lists the primary parameters of these strata and their structures. All the strata were simplified to a uniform distribution with the thicknesses of 3.0, 2.8, 11.8, 21.1, and 11.3 m. The soil was treated as an elastic–plastic material, and the Mohr–Coulomb constitutive model was employed to simulate the stratum. The station structures were simulated using an elastic constitutive model.

The top slab, beam, crown beam, strip foundation, and bottom longitudinal beam were simulated using solid elements. As the flexural stiffness of the interlock was much lower than that of the steel pipe [17], the interlock was not considered in the model. Advanced small pipes and feet-lock bolts were represented by cable elements [23]. Beam elements were used to illustrate the pipe roofs, side piles, and central columns. The pipe roof was calculated using the principle of equivalent flexural stiffness [24]. The steel grid frame and shotcrete were simplified [25,26] and simulated using solid elements.

The surface settlement resulting from steel pipe jacking can be controlled by grouting the pipes to achieve zero surface settlement before excavation [16]. Therefore, the deformation laws of the surface and pipe roof resulting from excavation after jacking were analyzed in this study.

3.2 Analysis of pipe-roof spacing

The maximum deformation values of the surface and pipe roof with different numbers of beams exhibited a similar trend with a change in pipe-roof spacing. Consider the simulation results for beam 3 as an example. Fig.6 shows the maximum deformation values at different spacings. As shown in Fig.6, under different flexural stiffness conditions, with an increase in spacing, the maximum deformation values of both specimens gradually decreased; however, the decrease in amplitude was less significant. When the pipe-roof spacing increased from 450 to 480, 510, and 540 mm, the surface settlement decreased by 2.83%, 1.49%, 2.36%, and 1.81%, respectively, and the deformation of the pipe roof decreased by 2.72%, 1.44%, 2.19%, and 1.83%, respectively. The reduction in the deformation values for both specimens was within 3%. Thus, an increase in spacing has only a marginal influence on the deformation of the surface and support structure. This indicates that the pipe-roof spacing is not the dominant factor controlling the station stability. In practical engineering, the spacing can be selected according to the design of the interlock.

3.3 Analysis of flexural stiffness

According to our previous study, the pipe-roof spacing has little impact on the deformation of the ground and pipe roof. Therefore, the spacing was not considered in subsequent analyses. The pipe-roof spacing was selected as 450 mm in this section. Fig.7 shows the maximum deformation values of the surface and pipe roof at different flexural stiffness values. The maximum values first decreased rapidly and then stabilized with increasing flexural stiffness. When the flexural stiffness was 0.5EI, the maximum deformation values of both specimens exceeded the control values for different numbers of beams. Moreover, when the flexural stiffness was less than 2EI, the increased flexural stiffness significantly reduced the deformation values of both specimens. In addition, when the flexural stiffness reached 2EI, the variation range in the deformation values for both specimens decreased as the flexural stiffness continued to increase, and the deformation value tended to level out. This indicates that the pipe roof played a supporting role at this point and that the flexural stiffness was not a critical factor in controlling deformation. When the flexural stiffness was 0.5EI–2EI, the deformation values for both specimens changed in a similar manner to the flexural stiffness values for different numbers of beams. Considering seven beams as an example, the relationship between the deformation values and the flexural stiffness is shown in Fig.7(c). In the figure, both curves exhibit an exponential function relationship. By analyzing the pipe-roof spacing and flexural stiffness, it can be concluded that the deformation values for both specimens are mainly affected by the flexural stiffness of the pipe-roof structure itself. In actual construction, the flexural stiffness can be improved by changing the material of the support structure or increasing the wall thickness of the steel tube.

3.4 Analysis of the number of beams

When the flexural stiffness was 0.5EI, the deformation value of the surface settlement owing to excavation was superior to the control value. When the flexural stiffness was 3EI, the construction material was over conservative, leading to a lower material utilization efficiency. Therefore, EI and 2EI were selected for analysis in this section. Moreover, as the surface settlement trends when the stiffnesses were EI and 2EI were similar, the results for the stiffness of EI are shown as examples. The pipe-roof spacing was set to 450 mm. The numerical simulation did not consider the excavation sequence of the transverse pilot tunnels, which were excavated simultaneously. Fig.8 illustrates the horizontal and longitudinal surface settlement troughs for different beam numbers. It is apparent that the horizontal settlement troughs are similar in shape and conform to a Gaussian distribution.

When the number of beams was six or seven, the bottom of the settlement trough was relatively gentle. As the number of beams increased from 3 to 4, 5, 6, and 7, the maximum surface settlement values were −33.31, −29.18, −27.67, −25.98, and −25.10 mm, respectively, showing a gradual decreasing trend. Compared with the case where the number of beams was 7, the settlement values when the number of beams was 3, 4, 5, and 6 increased by 32.71%, 16.25%, 10.23%, and 3.51%, respectively. These results show that the surface settlement is significantly affected by the number of beams. The overutilization of beams can result in a lower material utilization efficiency, whereas too few beams can cause greater surface settlement. As such, with a control standard of 30 mm, the construction requirements are satisfied when the stiffness is EI and the number of beams is four, or when the stiffness is 2EI and the number of beams is three.

4 Analysis of construction process

Based on previous conclusions and considering the accuracy of the numerical simulations, the pipe-roof spacing was selected as 450 mm, the flexural stiffness was selected as EI, and the number of beams was selected as 7 in the actual design plan. The size of the transverse pilot tunnels is 3.3 m (width) × 4.1 m (height), and the spacing is 4.2 m. They were excavated in two batches: pilot tunnels H2, H4, and H6 formed the first batch, and the remaining pilot tunnels formed the second batch. The construction process of a metro station built using the SP-B method was analyzed, and the deformation patterns at different stages were studied.

4.1 Surface settlement

A monitoring point was set up at the center of the station, and the settlement process is shown in Fig.9(a). In Stage 1, the surface settlement grew slowly at first, then increased significantly, and finally stabilized, with a total settlement of 4.66 mm. At this stage, as the longitudinal pilot tunnels were excavated, the pipe roof began to be forced and gradually deformed under an overlying load. The surface settlement also began to increase gradually owing to pipe-roof deformation. This significant increase was attributed to the excavation of the Z2 pilot tunnel, which had the largest section dimensions and was closer to the monitoring point. In Stage 2, the settlement value increased to 11.57 mm at an increasing rate. At this stage, seven transverse pilot tunnels were constructed, and the formation of a “soil-station structure” support system was observed (Fig.9(b)). In Stage 3, the settlement value increased sharply and then leveled off, reaching a total settlement of 24.96 mm. At this stage, the soil between the pilot tunnels was removed, disrupting the support system formed in Stage 2 and leading to a more intense surface settlement. Simultaneously, extensive excavation resulted in soil loss; therefore, the surface settlement increased rapidly at this stage. During the excavation process, the station structures were gradually stressed, forming a new “pipe roof–beam–central column” support system (Fig.9(c)); hence, the settlement trended toward leveling off. In Stage 4, the surface settlement increased by 0.14 mm. The final surface subsidence was 25.1 mm, which was within the limit (30 mm) specified by the station. The settlement values in Stages 2 and 3 accounted for 27.5% and 53.3% of the total settlement, respectively. Consequently, the essential stages for controlling surface settlement are the construction of transverse pilot tunnels and the digging of the soil between them.

Fig.10 shows the surface settlements of the horizontal and longitudinal monitoring sections at each stage. For the horizontal monitoring section, the settlement trough at Stage 1 was shaped as a “double groove”, showing a similar shape to the settlement trough resulting from the twin-tunnel excavation [27]. The settlement trough at Stage 2 was shaped as a “single groove,” and the middle part of the settlement trough was close to being horizontal. The settlement troughs at Stages 3 and 4 were the same and followed a Gaussian distribution. The distance between the surface affected by the excavation and the center of the station was within 1.7 multiples of the station span. For the longitudinal monitoring section, the settlement troughs at Stages 1, 3 and 4 were shaped as a “single groove”. In Stage 2, there was a slight bulge at the bottom of the curve. This was primarily attributed to the construction sequence of the transverse pilot tunnels.

4.2 Comparison of pipe-roof deformation and surface settlement

Fig.11 shows the deformation contours of the stratum and pipe roofs at each stage. In Stage 1, the deformation patterns of the pipe roof and the surface with the excavation were similar, exhibiting a “double groove” in the horizontal direction, and the maximum values were both in the center of the Z3 guide tunnel. In Stage 2, the deformation pattern of the surface was irregular, and large settlements occurred above the H3 and H5 pilot tunnels. The deformation pattern of the pipe roof was more significant than that of the surface, and the deformation values above the H3 and H5 guide tunnels were significantly larger than those at the other locations, which is analogous to the situation with the surface settlement values. This is attributed to the transverse pilot tunnel being excavated in two batches. In the simulation, the H2, H4, and H6 pilot tunnels were first excavated. The excavation volumes of the three pilot tunnels were small, and the soil beneath the pipe-roof structure demonstrated a greater supporting action. Therefore, the pipe-roof deformation above the first excavated pilot tunnel was small. Subsequently, the pilot tunnels H1, H3, H5, and H7 were excavated, at which point the strength of the soil support weakened. Therefore, the deformation value above the later excavated pilot tunnels was greater. However, the H1 and H7 pilot tunnels were restrained by both sides of the station; therefore, the pipe-roof deformation above the H3 and H5 pilot tunnels was larger than the pipe-roof deformation on other locations.

In Stage 3, the surface subsidence had a “basin” shape, with decreasing values of subsidence from the center to the periphery. The deformation of the pipe roof above the transverse pilot tunnel increased. Compared with the deformation at Stage 3, the deformation values of the surface and pipe roof at Stage 4 showed some, but not evident, changes. The maximum pipe-roof deformations at Stages 1, 2, 3, and 4 were 10.28, 15.42, 28.16, and 28.5 mm, respectively, which are also within the limit (30 mm) specified by the station. During the excavation, the maximum settlement value was always less than the maximum deformation value of the pipe roof. The soil arch formed above the supporting structure during excavation bore part of the overlying load; therefore, the surface settlement was smaller.

5 On-site monitoring

5.1 Analysis of the monitoring data

Surface settlement is an important indicator of station safety during construction. Monitoring points were arranged to validate the reliability of the station construction using the SP-B method. The numerical simulation results were considered when the monitoring scheme was established. As numerous pipelines above the station cannot be moved, the positions of the pipelines were considered when arranging the monitoring points. Six monitoring sections were arranged at every 7–10 m. There were 16–17 monitoring points installed in each section, with an interval of 3–5 m between them. To monitor the surface settlement more comprehensively, five new monitoring sections were added to the original six monitoring sections, and three monitoring points were installed in each section with a spacing of approximately 10 m between them. In total, 115 monitoring points were installed (Fig.12).

The surface settlement patterns of different sections after excavation are shown in Fig.13. For the horizontal sections, the curves of the surface settlement were groove-shaped. The settlement troughs in the DBC1 and DBC11 sections were similar in shape, and the bottom was relatively flat, whereas the settlement curves for the DBC5 and DBC7 sections were steeper, and the settlement values in the center of these two sections were significantly greater than those around them. The settlement curves for sections DBC3 and DBC9 were intermediate. Because of the constraining effect of the station boundary on both sides, the surface settlements in sections DBC1 and DBC11 were significantly smaller than those in the other horizontal sections. When excavating large areas of soil, the final surface settlements in sections DBC3 and DBC9 were smaller than those in sections DBC5 and DBC7 because of the beam support below. For the longitudinal sections (Fig.13(d)), the difference in settlement values near the middle of the metro station was minimal, with a gradual decrease in settlement values from the middle of the station toward the north and south ends. Moreover, the surface settlement above the station is symmetrically distributed on the DBC6 section, and the shape is similar to a “basin”. The maximum settlement monitoring value after excavation of the station was 24.1 mm, which indicates that the settlement was within the allowable range of construction.

5.2 Comparison analysis of monitoring and simulation results

The monitored values for the longitudinal section at each stage were compared with the simulated values (Fig.14). In Stages 1 and 2, the simulation results for the surface settlement were almost the same as the monitoring results, and the generally changing tendencies of the monitoring points were consistent. The construction of a longitudinal pilot tunnel was mainly conducted in Stage 1, which had a limited influence on the surface settlement; therefore, the change was relatively gentle. The maximum simulation and monitoring values were 4.66 and 4.9 mm, respectively, with an error of 4.89%. Seven transverse pilot tunnels (Fig.15) were constructed in Stage 2. There were numerous pilot tunnels close to the ground; thus, the surface settlement increased significantly. The maximum difference between the simulated and monitored values at this stage was 7.1%. The negative layer of the metro station (Fig.16) was constructed in Stage 3, and the surface settlement exhibited a larger increase. At this stage, the error between the simulation and monitoring results at both ends of the station was large but acceptable, at approximately 8.45%. It is considered that both ends of the station were more disturbed during site construction. Compared with Stage 3, the surface settlement curve of Stage 4 only marginally changed, and the simulation and monitoring values were similar, with a maximum error of 6.94%.

At different stages, the monitoring results of the surface settlement variation trend at DBC6-2 were close to the simulation results (Fig.17). The maximum values of simulation and monitoring were 25.1 and 24.1 mm, respectively, and the error was only 3.98%. The stratum was briefly treated in this study to simplify the establishment and calculation of the numerical model. The irregular stratum was simplified to a homogeneous thickness. Artificial disturbances in actual construction were also not considered. The simulation results showed some discrepancies with the monitoring results; however, the maximum error was less than 8.45%. In summary, the simulation results were consistent with the monitoring results at different construction stages, confirming the reasonableness and correctness of the established numerical model. The security and adaptability of the proposed method were further confirmed.

5.3 Comparison with similar subway stations

The Pinganli station on Beijing Metro Line 19 (Fig.3(b)), the Dongbeidamalu station on Shenyang Metro Line 10 (Fig.3(c)), and the Shifu Road Station described in this paper are selected for comparison and analysis. All three stations were two-story, three-span stations with similar section dimensions, stratigraphic conditions, and burial depths, and their construction after precipitation was not affected by groundwater. The Pinganli and Dongbeidamalu stations were built using the tubular roof and STS methods, respectively. Fig.18 shows the eventual surface settlement values and construction cycles at the three stations. In terms of strata deformation, the eventual surface settlements at the Pinganli, Dongbeidamalu, and Shifu Road Stations were 62.8, 7.7, and 24.1 mm, respectively. The Dongbeidamalu and Shifu Road Stations were constructed with a pipe-roof structure supporting them, and compensatory grouting was implemented after the construction of the pipe screen; therefore, settlement resulting from jacked pipes was not considered. The construction cycle was calculated from the start to the end of the main structure of the station. The construction period was from August 2017 to September 2019 at the Pinganli Station, from January 2015 to May 2017 at the Dongbeidamalu Station, and from October 2020 to May 2022 (including the period of suspension owing to the COVID-19 pandemic) at the Shifu Road Station.

In summary, the STS method is the most effective in controlling surface settlement; however, its construction period is relatively long. This method can significantly reduce the construction period under the condition of satisfying the surface settlement control standard and is worthy of further promotion and application.

6 Conclusions

This paper proposed a small-pipe roof-beam method based on the Shifu Road Station. The supporting parameters of this method were optimized and analyzed, and the construction process was analyzed through numerical simulation. Then, the impact of practical construction on the change pattern of surface settlement was analyzed through on-site monitoring. Finally, the proposed method was compared with other construction methods. The following conclusions were drawn.

1) This method reduces the complexity of the connections between the steel tubes, and the beams are set under the pipe-roof structure to form a bidirectional support system, reducing the construction difficulty of the pipe-roof method. This method is simplified into four stages for constructing a metro station.

2) The increase in the pipe-roof spacing had no significant effect on the surface settlement and pipe-roof deformation. In practical engineering, the spacing can be selected according to the design of the interlock. When the flexural stiffness reached 2EI, the bearing capacity of the pipe roof could be fully utilized, and the flexural stiffness was no longer a key factor in controlling the deformation. The overutilization of beams can result in wasted material and low efficiency, whereas too few beams can cause greater surface settlement. With a control standard of 30 mm, the construction requirements were satisfied when the stiffness was EI and the number of beams was four. The control requirements were also satisfied when the stiffness was 2EI and the number of beams was three.

3) The key stages for controlling surface settlement are the excavations of transverse pilot tunnels and the soil between them. The proportions of settlements to the overall settlements were 27.5% and 53.3%, respectively. The deformation of the stratum could be effectively controlled by the “pipe roof-beam-central column” support system.

4) The surface settlement above the station was symmetrically distributed on the DBC6 section, and the shape was similar to a “basin”. The eventual monitoring value was 24.1 mm, which was within the allowable range for construction. In contrast to the other two pipe-roof methods, this method reduced the construction period by at least five months and is worthy of further promotion and application.

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