Shield-driven induced ground surface and Ming Dynasty city wall settlement of Xi’an metro

Tunnelling and Underground Space Technology 97 (2020) 103220

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Shield-driven induced ground surface and Ming Dynasty city wall settlement of Xi’an metro ⁎

T

⁎

Jinxing Laia, Hui Zhoua,e, Ke Wangb,c, , Junling Qiua, , Lixin Wanga,c, Junbao Wangd, Zhihua Fenga a

School of Highway, Chang’an University, Xi’an 710064, China Institute of geotechnical engineering, School of Civil Engineering and Architecture, Xi’an University of Technology, Xi’an 710048, China c China Railway First Survey and Design Institute Group Co., Ltd., Xi’an, Shaanxi 710043, China d School of Civil Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China e CCCC Highway Consultants CO., Ltd., Beijing 100088, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Xi’an Metro Historic buildings Settlement Field monitoring Numerical simulation

The influence of metro shield construction in congested urban areas on nearby buildings is of great concern in practice. In this study, 3D-FEM model, validated through comparison with field monitoring results were used to investigate the influence of Xi’an Metro Line 4 excavation on existing city wall of the Ming Dynasty at Heping Gate in loess ground. The research results were found to be in good agreement with field measurement results. Furthermore, the results indicated that because of pre-reinforcement by sleeve valve pipe grouting and supporting by I-shaped steel at openings, the surface settlement within the foundation area on the city wall is reduced by 4 mm compared with that without the reinforcement, and the settlement of the city wall structure is reduced by approximately 2.2 mm, which prevents excessive local heave of the city wall foundation and out-oflimit of inclination. Additionally, the settlement-time curves of the city wall structure and change laws of surface settlement slot were also obtained. This study provides a valid reference value for protecting historical buildings adjacent shield construction in similar loess areas.

1. Introduction In recent decades, urban metro projects have increased substantially because of rising populations, space restrictions and growing environmental concerns. However, many engineering and environmental problems occur during construction (Ocak, 2009). Tunnels will inevitably pass through the beneath or sides of existing structures and disturb the soil mass. When the deformation of the strata exceeds a particular limit, uneven settlement of the building foundation and additional deformation of the superstructure occur (Ocak, 2013). This can lead to building cracks, structure rotation and distortion, and possible unrecoverable damages (Boone et al., 1999; Yoo and Lee, 2008; Ou et al., 2008; Xu et al., 2015; Cheng et al., 2019), particularly for buildings on shallow foundations (Li et al., 2019; Wang et al., 2020; Yan et al., 2018; Cao et al., 2018; Cheng et al., 2018b; Wang et al., 2019a; Zhang et al., 2013; Liu et al., 2020; Zhang et al., 2020). Currently, the shield method has become the prevalent choice for metro construction in congested urban areas owing to its fast speed,

safe working environment, and minimal disturbance to surrounding grounds (Weng et al., 2020; Fang et al., 2019; Fargnoli et al., 2015; Liao et al., 2009; Peila, 2014; Standing and Selemetas, 2013; Sun et al., 2019; Wongsaroj et al., 2007; Zhang et al., 2018). Various studies on the impacts of shield tunnelling on the surface settlement of residential/ commercial buildings and existing tunnels and pipelines have been conducted over the years (Clarke and Laefer, 2014; Ercelebi et al., 2011; Li et al., 2018c, 2019; Liang et al., 2016; Shi et al., 2015; Sirivachiraporn and Phienwej, 2012; Wang et al., 2019b; Xie et al., 2016). However, there are few studies on shields crossing underneath buildings with a long history and major historical significance, with structural aging, and high protection requirements as well as buildings in loess areas. Xi’an is a world-famous historical and cultural city with widespread cultural relics and historical sites. It is the location of the city wall of the Ming Dynasty, a landmark building in Xi’an, which was built in 1384. As the main city of the ‘Silk Road Economic Belt’, Xi'an is now enjoying a boom in metro construction (Wang et al., 2016; Qiu et al., 2020; Song et al., 2019). There are 16 tunnels crossing

⁎ Corresponding authors at: Institute of geotechnical engineering, School of Civil Engineering and Architecture, Xi’an University of Technology, Xi’an 710048, China (K. Wang). School of Highway, Chang’an University, Xi’an 710064, China (J. Qiu). E-mail addresses: [email protected] (J. Lai), [email protected] (K. Wang), [email protected] (J. Qiu), [email protected] (L. Wang).

https://doi.org/10.1016/j.tust.2019.103220 Received 7 June 2018; Received in revised form 21 October 2019; Accepted 25 November 2019 0886-7798/ © 2019 Elsevier Ltd. All rights reserved.

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4, the shield tunnelling process under different conditions is simulated by using a three-dimensional finite element model. The deformation law of the city wall structure and the change of surface settlement slot are obtained by combining with field measurements, and the treatment effect of the pre-reinforcement measures is analysed. This study can provide technical support for settlement prediction in similar crossing projects and safety control of ancient city walls and also accumulate relevant theoretical and engineering analogy experience.

Table 1 Main factors affecting the surface settlement (Ocak, 2013). Category

Factors

Excavation and support method

Excavation method (NATM, TBM, EPBM, etc) Excavation type (full face or sequential mining) Support (anchoring, shotcrete, steel sets, lining, etc) Shield operation factors for EPBM (Fp, penetration rate, pitching angle, tail void grouting pressure, percent tail void grout filling,etc.)

Tunnel geometry

Worksite conditions Depth Diameter Number of tunnels Distance between tunnels

Ground properties

2. Project profile Heping Gate is located in the southern section of the city wall of the Ming Dynasty in Xi’an, which was built between 1374 and 1378 and has undergone several renovations. It has a history of more than 600 years to the present. It is one of the most complete ancient city walls in China. The city wall has a stone foundation, which is buried at a depth of 2 m below the ground. The overall city wall is trapezoidal, narrow at the top and wide at the bottom. The bottom is 16–18 m wide and the top is 12–14 m wide. The core wall is formed by ramming within the city wall. The external wall is built with 45 cm × 22 cm × 10 cm black bricks and is 12 m high. Two to three layers of black bricks are paved on the top and built with cement mortar (Lei, 2010). Xi’an Metro Line 4 is 592 m long from Heping Gate to Dachaishi station. The tunnel crosses underneath the Heping Gate opening and the city wall from section DK14 + 769 to section DK14 + 787. The distance from the tunnel arch crown to the city wall foundation is only approximately 16 m. Before the shield crossing, two rows of 80 mm diameter sleeve valve pipes spaced at 1000 mm are embedded around the city wall opening within the range of influence of the shield. They are grouted for reinforcement to reduce the surface settlement and ensure safety of the historic buildings. They are buried at a depth of 7 m above the top of the tunnel. A secondary tracing compensation grouting was suitably performed during construction based on situation monitoring. The opening contour is provided with a steel frame made of Ishaped steel (220 × 112 × 9.5 mm) with a vertical spacing of 3000 mm. A 120 × 100 × 8 mm angle steel bar with a spacing of 1000 mm is installed in the ring direction along the opening to strengthen the longitudinal connection as well as to improve the overall stability of the city wall structure. The Heping Gate and shield tunnel locations are shown in Fig. 1. In addition, the cross-section of the tunnel is circular with an outer diameter of 6 m and a lining thickness of 0.3 m. In the process of shield construction, an earth pressure balanced machine was used, and the annular gap caused by the shield driving was backfilled using the two-component grouts at the same time, namely, the grout is composed of two components: A-liquid, which mainly consisted of cement, water, bentonite, and a retarder, and B-liquid, composed of sodium silicate and water (about 40 of Baume degree), which could help restrict ground movements and reduce volume loss.

Elasticity modulus Unit weight Cohesion Friction angle Poisson’s ratio Lateral earth pressure coefficient Groundwater Permeability

underneath the city wall, which have been constructed or planned. Therefore, achieving a win-win situation between cultural relic protection and rail transport construction is a problem that urgently requires a solution, and this falls within the scope of this research project. In addition, the Xi’an Metro was originally built in a loess area in China and there was a lack of engineering analogy and normative guidance. This resulted in the sinking of the city wall foundation and expansion of wall cracks during construction of Metro Lines 1 and 2 (Wei et al., 2019; Lei, 2010; Li et al., 2020; Luo et al., 2017b). Tunnel–soil–building interaction is a complicated process, which is very difficult to rigorously analyse such an interaction process because of the space effect of tunnel excavation, engineering characteristics of the soil, structural materials of buildings, and many other factors (Boscardin and Cording, 1989; Cheng et al., 2007; Jiang et al., 2019; Zhou et al., 2016). The main factors affecting the surface settlement are provided in Table 1 (Ocak, 2013). To estimate tunnel-induced building responses, several empirical (O’reilly and New, 1982) and analytical methods (Loganathan and Poulos, 1998; Bobet, 2001) have been employed to predict tunnel-induced ground movements. This paved the way for the establishment of various empirical relationships between tunnel-induced ground movements and associated building damages. More recently, numerical simulation and laboratory model test methods have been considered to improve the damage prediction of buildings subject to adjacent tunnelling works (Cheng et al., 2020; Wu et al., 2020; Li et al., 2020; Liu et al., 2012; Azadi et al., 2013). These methods have significantly contributed to the knowledge on surface settlement characteristics, building deformation, and other laws related to buildings on the ground. Furthermore, they have provided guidance to several engineering construction projects. In particular, numerical simulation methods can simulate actual excavation processes during tunnel construction in the study of shields crossing underneath cultural relics and historic buildings. They can also optimise and analyse tunnel construction processes, construction parameters, and reinforcement measures by considering many factors. Furthermore, they can predict surface settlements and analyse structural deformations, such as in Xi’an Metro Lines 1 and 2, which are crossing underneath the city wall and the Bell Tower (Lei, 2010; Li et al., 2009; Ren et al., 2011) and in Wuhan Metro, which is crossing underneath Roots’ formal residence (Zhang et al., 2015). However, we need to combine these methods with field measurements for comparison and validation analysis because there are differences between numerical simulation and actual situations. In this study, based on the double-line shield crossing underneath the ancient city wall at Heping Gate between the sections of Metro Line

3. Numerical simulation of Double-line shield crossing underneath the city wall 3.1. Calculation modelling In this study, a 3D–FEM model of tunnel–soil–city wall structure to predict the influence of Xi’an Metro Line 4 excavation on existing city wall of the Ming Dynasty at Heping Gate was presented by MIDAS–GTS (Geotechnical and Tunnel Analysis System) software (Midas, 2013). As shown in Fig. 2(a), the overall calculation model dimension is 84 m × 146 m × 45 m, according to the construction scheme and boundary effect. Fig. 2(b) illustrate sleeve valve pipe grouting reinforcement area at bottom of wall foundation. The tunnel advances in the X-direction, that is tunneling from south to north. The clear distance of the tunnel under the city wall is 21.5 m (at the southern end) and 22.5 m (at the northern end). The soil mass and city wall core soil are simulated by solid elements, the tunnel segment and the external wall 2

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(a) Cross section of tunnel and city wall

(b) Plan Layout of project

Fig. 1. Relative position of Xi’an metro line 4 and Heping Gate City Wall.

of the city wall is simulated by plate elements, and the beam element is used to simulate the I-shaped steel support on inner wall of the gate, as shown in Fig. 2(c). The Mohr-Coulomb elastic-plastic model is employed to define behavior of the soil, and an elastic behavior is considered for the tunnels liner and building material. As for the boundary condition, the four lateral sides are fixed by roller, the bottom is constrained in all direction, while the top surface is free. During calculation, the impact of pore water pressure of groundwater is considered.

Table 2 Physical and mechanical properties of different loess strata (Ren et al., 2011). γ (kN/m3)

Soil type Plain fill New loess Fossil soil-1 Old loess Fossil soil-2 Silty clay Sand

3.2. Material parameters The soil mass under the foundation of the city wall is classified into seven layers based on the geotechnical investigation report. The soil layers within the research area from top to bottom are as follows: plain fill, new loess, fossil soil, old loess, fossil soil, silty clay, and sandy soil.

17.3 18.8 20.1 20.4 20.1 20.4 20

E (kPa) 3

5.0 × 10 9.0 × 103 10.0 × 103 7.0 × 103 10.0 × 103 11.0 × 103 11.0 × 103

v

c (kPa)

φ (°)

H (m)

0.32 0.33 0.32 0.28 0.30 0.28 0.3

15 30 39 32 39 56 15

18 20 22 21 22 22 20

6.0 3.6 4.0 7.7 5.8 8.1 10.1

The parameter values for calculation are provided in Table 2 (Ren et al., 2011). The compressive modulus of the composite soil in the sleeve valve pipe reinforcement area can be calculated according to the

(a) Calculation model

(b) Reinforced area of city wall foundation

(c) I-shaped steel support Fig. 2. 3D finite element model for the case. 3

where Esp is the compressive modulus of the composite foundation, and m is the area replacement ratio. During the pile arrangement in the form of an equilateral triangle, dε = 1.05S; S is the pile spacing, Ep is the compressive modulus of the sleeve valve pipe, and Es is the compressive modulus of the soil between pipes. When the foundation soil bears the load, it will generally produce elastic–plastic deformation. The deformation modulus can reflect the elastic deformation and partial plastic deformation, which is consistent with the foundation deformation under normal circumstances. Therefore, the modulus of the foundation soil used as deformation modulus in the calculation will correspond to that in engineering practice. The deformation modulus of the soil can be determined based on equation (3):

Hardening of the grout as time passes.

Tunnel segment

Fig. 3. Major Components of an EPB Shield Tunnel.

after crossing of the left line. Fig. 3 illustrates the simulation of a stepby-step construction procedure as follows: (1) The self-weight of the ground and the static pressure of the ground water are applied to reproduce the initial state of the ground before excavation; (2) Deactivating the soil elements to simulate excavation of shield, and then face pressure is applied to the excavation face for the modeling pressure produced in the chamber of a shield machine, and simultaneously activate the shield shell that modeled by applying pressure to the surrounding soil surface in contact with the shield; (3) Activating the lining elements to simulate the tunnel lining when the excavation is conducted as far as the shield length (9 m), and than the hydraulic jack is applied to the segment; (4) Deactivating the shield pressure as much as the length of the lining that was pushed out of the shield, and simultaneously activate the fresh grout pressure. The property of the grout is changed to simulate the hardening of the grout as time passes. Moreover, the first applied jack force is removed and the new jack force is applied to the newly activated lining. The process from the steps one to four is repeated until the completion of the entire construction process.

2

2μ ⎞ E0 = ⎜⎛1 − ⎟ Esp 1 − μ⎠ ⎝

Grout pressure

(2)

Y

m = d 2/ de2

X Shield

(1)

Tunnel face (face pressure)

Esp = mEp + (1 − m) Es

Soil will be excavated

following equation (Bae et al., 2005):

Jack force

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(3)

where E0 is the deformation modulus of the composite foundation, and μ is the Poisson’s ratio of the composite foundation. The shield construction includes soil excavation, installation of segment linings, grouting of the tail void and hardening of the grout, among which the grouting of the tail void is highly important and the time-dependent hardening of the grout could help restrict ground movements and reduce volume loss (Shah et al., 2018; Wu and Shao, 2019a, 2019b). In the project (section DK14 + 769 to section DK14 + 787), the annular gap caused by the shield driving was backfilled using the two-component grouts as has been mentioned. According to the engineering design and shield construction experience in loess stratum (Lei, 2010; Ren et al., 2011), the water-cement ratio of cement slurry is 0.8:1–1:1, the mix ratios (ratio by volume ) of A-liquid to B-liquid are 2:1–3:1. Considering that the grout will harden with time, the elastic modulus of the grout is about 9.0 × 102 kPa in its fresh state and 4.0 × 105 kPa in hardened state. In addition, the face pressure, grouting pressure, and jack force J during the shield tunnelling are selected based on actual engineering, and they are 200 kPa, 250 kPa, and 3452 kPa respectively. The structural parameters of the shield in detail are listed in Table 3.

4. Analysis of calculation results 4.1. Surface settlement of tunnel axis (1) Surface settlement of tunnel axis with reinforcement As shown in Fig. 4, the city wall foundation is located within a range of 0–19 m at the X coordinate, where RL indicates the monitored value of the right line surface after the crossing of the left line. RR indicates the monitored value of the right line surface after the crossing of the right line; LR indicates the monitored value of the left line surface after the crossing of the right line, and LL is the monitored value of the left line surface after the crossing of the left line. It can be observed that the settlement above the tunnel axis obviously tends to decrease when approaching the city wall foundation owing to the sleeve valve pipe grouting reinforcement. On the other hand, the surface settlement tends

3.3. Simulation of numerical calculation process Modeling shield tunneling process, such as soil excavation, installation of segment linings, grouting, and applying external forces, such as jack forces and face pressure, are important for simulating the behavior of an actual shield tunnelling operation (Cho et al., 2016). In addition, back-filled grouting and change in time-dependent material properties (change in elastic modulus of grout) are also included in the model. In the project, the actual construction sequence is left line shield tunnelling and then right line shield construction in the same direction Table 3 Numerical simulation parameters of the structures and materials. Structure type

γ (kN/m3)

E (kPa)

v

c (kPa)

φ (°)

H (m)

Sleeve valve pipe grouting Sleeve valve pipe groutinggrouting Wall brick body foundation Tunnel segment Fresh grout Hardened grout Shield shell I-shaped steel

21 19.0 18.9 24 23 23 78 76

2.12 × 102 4.5 × 103 8.0 × 103 3.45 × 107 9.0 × 102 4.0 × 105 2.10 × 108 2.06 × 108

0.3 0.2 0.3 0.3 0.3 0.3 0.2 0.25

75 900 22 – – – – –

22 26 21 – – – – –

8 0.45 2 0.3 0.06 0.06 0.06 –

Note: H = soil layer thickness, r = unite weight, c = cohesive force, φ = internal friction angle, v = Poisson’s ratio, E = elasticity modulus. 4

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Fig. 4. Surface settlement of tunnel axis with reinforcement.

than those in the area where the city wall foundation is located. This is basically due to weight exerted by the city wall. After the crossing of the left line, the surface settlement value of the left line is approximately 10 mm in other sections and significantly increases to 12.5 mm in the city wall foundation area. The trend of the settlement curve is concave, and the maximum settlement value is increased by approximately 2.5 mm. The excavation of the left line has a small impact on the surface settlement above the right line, and the surface heave and settlement above the right line is only ± 0.3 mm. After the crossing of the right line, the surface settlement above the right line is obviously larger than that of the left line, and the difference between the settlement values of the left and right lines ranges from 2 to 4 mm. The curve of the surface settlement above the right line tunnel axis is concave in the area where the city wall foundation is located, and the settlement value is increased by approximately 3 mm. The effect of the right line excavation on the surface settlement value of the left line remains the same at the surface of the tunnel axis and is approximately 1 mm. After the double-line crossing, the maximum value of the surface settlement of the right line is 15 mm, the surface settlement of the left line is 12 mm, and the average difference between the settlement values is 3 mm. It can be observed that it is difficult to balance the soil pressure with the shield frontal pressure and grouting pressure in this area because of the effect of the load on the city wall; thus, the settlement value increases in the area. After the shield passes through the area, the soil layers have a tendency to uplift within a particular area at the rear end

to gradually increase when moving away from the city wall foundation after passing through the city wall. After the crossing of the twin tunnels, the maximum surface settlement value is 13.0 mm, which appears at the southernmost point of the model. The effect of the right line excavation on the surface settlement value of the left line is highest within the city wall foundation area and decreases with distance from the city wall foundation area. The curve of the surface settlement above the right line tunnel axis is convex upward at that area. The mean value of the settlement is approximately 11 mm, which is approximately 2 mm less than that in other sections. The difference between the surface settlement value of the left line and right is not large. The settlement curves (LR and RR) above both tunnels intersect at the city wall foundation area after crossing of the right line, indicating that there is an overall settlement in the city wall foundation area after the doubleline crossing. This also shows that because of the sleeve valve pipe grouting reinforcement, the deformation characteristics of the soil layers in the city wall foundation area tend to approach those of elastic materials. (2) Settlement of tunnel axis without reinforcement

Foundation of city wall

Fig. 5 illustrates the surface settlement of the tunnel axis without reinforcement, where the settlement value above the tunnel axis obviously tends to increase when approaching the city wall foundation after the construction of the double-line shield is completed. The settlement values in the front and rear sections of the city wall are less

Fig. 5. Surface settlement of tunnel axis without reinforcement. 5

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foundation of the city wall. L is the distance between the two monitoring points, whereas Si and Sj represent the settlement values after the double-line crossing. Therefore, the local inclination (K) can be obtained by the ratio of the settlement difference of the two monitoring |si − Sj | points to their distance; that is, K = L . Fig. 9 illustrates the local inclinations of the city wall foundation under both conditions after the double-line crossing. With the reinforcement, the maximum local inclination of the city wall foundation area is 0.00032, which mainly appeared at both sides of the east and west gates. On the other hand, the inclination near the two partitions at the middle of the gate without the reinforcement is 0.0013. Furthermore, the foundation inclination is much larger at the center than at the south and north ends of the city wall. With the reinforcement, the foundation under the city wall opening forms a hole because of the effect of the sleeve valve pipe grouting, and its deformation characteristics are similar to those of elastic materials. The excavation of the left and right lines causes an overall torsion settlement in the stratum reinforcement area. Without the reinforcement, the surface settlement is basically similar to that of the original stratum when tunnelling at the south end of the city wall because the foundation in the city wall opening area retains the status of the original soil layers. When tunnelling at the centre of the city wall, the curve of the settlement slot is rapidly reduced in the foundation area of the gate partitions and represents a narrow and deep double-peak settlement slot owing to the boulder foundation under the gate partitions (three partitions) and the barrier action of the gate partitions. The mutual effect caused by the excavation of the left and right lines is weakened; when tunnelling at the north end of the city wall, the curve of the surface settlement is again similar to that of the excavation of the original stratum because the barrier action of the gate partitions is weak.

Fig. 6. Plan layout of research sections.

of the city wall because of the effect of forward leaning of the city wall. The mutual offset of upheaval and settlement values during the construction leads to an obvious decrease of the settlement values of the soil layers at the rear end of the city wall. 4.2. Analysis of surface settlement in all sections 4.2.1. Surface settlement curve in transverse direction To obtain the change law of the transverse slots on the surface where the double-line tunnel crosses underneath the Heping Gate, three research areas included the city wall foundation (Including three sections), south of city wall (Including three sections) and north of city wall (Including three sections) are taken for comparison and analysis, and the plan layout of research sections is shown in Fig. 6. Fig. 7 illustrates the final settlement law for all sections. It can be observed from the figure that the curves for settlement in the north and south sections of the city wall are double-peak curves. The peaks occur at the surfaces above the tunnel axis of the left and right lines, and the peak of the settlement value for the left line is less than that of the right line. The average difference between the settlement peaks is approximately 3–4 mm. As the studied sections gradually approach the city wall, the settlement peak of the left line increases while the settlement peak of the right line decreases. However, the settlement peaks of the left and right lines tend to increase without the reinforcement. At the foundation of the city wall, the maximum surface settlement value is approximately 12 mm after the double-line crossing, which is 3 mm less than that without the reinforcement. The surface settlement value has no double peaks. The curve is a wide and shallow settlement slot curve. The difference of the values of the surface settlement above the right and left lines is less than or equal to 0.5 mm. Combining with Fig. 4, the excavation on the right line has an obvious impact on the surface settlement above the left line tunnel and the value is approximately 4 mm. However, the curve of the surface settlement still shows a narrow and deep double-peak settlement slot without the reinforcement. The difference between the surface settlement values of the left and right lines is approximately 2 mm after excavation of both lines.

4.3. Analysis of settlement of city wall structure To determine the settlement change law of the city wall structure due to shield tunnelling, the contour map for the settlement of the city wall structure is selected as follows: when the left line is excavated at the lower part of the city wall (stage 1), the left line runs through (stage 2), the right line is tunnelled below the city wall (stage 3), and the right line runs through (stage 4). As shown in Fig. 10, in stage 1, the displacement mainly occurs near the south end of the west gate of the city wall; with the reinforcement, the maximum settlement of the city wall is 3.75 mm, with a wide range. Without the reinforcement, the maximum settlement of 5.85 mm mainly occurs at the north end of the sidewall foot of the city gate. In stage 2, with the reinforcement, the maximum settlement is 7.9 mm with a wide range, and it mainly occurs in the area surrounding the west opening of the city wall. Without the reinforcement, the maximum settlement of the city wall is 11.8 mm with a narrow range and it mainly occurs at the sidewall foot of the city gate. In stage 3, the maximum settlement is 9.88 mm with the reinforcement, and it occurs in a specific area of the north end of the two city gates in the west. Without the reinforcement, the maximum settlement of the city wall is 11.8 mm, with almost the same range as that with the reinforcement. In stage 4, the large settlements mainly occur at the three partitions of the city gate and its upper part, and the second city gate from east to west has the maximum settlement of 12.2 mm. Without the reinforcement, the maximum settlement of the city wall is 14.4 mm with a similar range as that with the reinforcement, and the second city gate from east to west has the maximum settlement. Based on the above analysis, the settlement of the city wall structure decreases by approximately 2.2 mm with the sleeve valve/pipe grouting and the support of steel arches, indicating that there is an overall settlement instead of a differential settlement in the city wall.

4.2.2. Local inclination of the city wall foundation area The code for the design of building foundations pointed out that the deformation analysis of masonry load-bearing structures should consider the local tilt rate (GB 50007-2011, 2011). In this study, the deformation of the wall is investigated using the transverse local inclination after the subway crossing. As shown in Fig. 8, points i and j are adjacent to the two points that monitor the settlement of part of the 6

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(a) With reinforcement: south section of the city wall

(b) Without reinforcement: south section of the city wall

(c) With reinforcement: the city wall foundation section (d) Without reinforcement: the city wall foundation section

(e) With reinforcement: north section of the city wall

(f) Without reinforcement: north section of the city wall

Fig. 7. The final settlement law for all sections.

4.4. Analysis of overall surface settlement

city gate foundation at the east side of the city wall owing to the frontal pressure of the shield. With the reinforcement, the eminence of the front surface is only 1.0 mm. In stage 2, with the reinforcement, the maximum settlement of 10.9 mm occurs at the south end of the surface above the left line tunnel axis. The settlement ranging from 0.1 to 7 mm occurs within the city

Fig. 11 illustrates the contour map of the displacement of the uppermost stratum for the four stages in the two conditions. In stage 1, the overall settlement is the same under the two conditions. However, without the reinforcement, a local eminence of 7.5 mm occurs at the 7

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similar buildings (structures), national standards, numerical simulation calculation data, and measured data for the settlement of the city wall of the Ming Dynasty (Lei, 2010). From the above mentioned numerical simulation, when the twin metro passes through the city wall, the maximum surface settlement within the city wall is approximately 12 mm, the maximum settlement of the city wall structure is 12.2 mm, and the maximum local inclination is 0.00032 after the sleeve valve pipe grouting was implemented for the city wall foundation and the Ishaped steel support was installed into the gate opening. These values meet the settlement and deformation control standard. However, if no reinforcement measures are considered, the settlement of the surface and city wall structure approach or exceed the alarming value, threatening the safety of the ancient city wall.

Fig. 8. Schematic diagram of local inclination calculation.

wall foundation area, and the settlement at the west side is larger than that at the east side. Without the reinforcement, the maximum settlement of 13.7 mm occurs within the city wall foundation area above the left line tunnel axis, and there is a small eminence of 7.5 mm on the city gate foundation at the east side of the city wall. In stage 3, with the reinforcement, the maximum settlement of 14.8 mm occurs on the surface above the right line tunnel axis, and the settlement within the city wall foundation area is between 0.2 mm and 9.5 mm, with a wide range and a smooth change of overall settlement. Without the reinforcement, the maximum settlement of 15.4 mm occurs on the surface above the right line tunnel axis with a narrow range and large change, and the local eminence of the city wall foundation decreases to 1.47 mm. In stage 4, with the reinforcement, the maximum settlement of 14.9 mm occurs on the surface above the south end of the right line tunnel axis. Without the reinforcement, the maximum settlement of 17 mm occurs on the surface above the south end of the right line tunnel axis. The maximum settlement within the city wall foundation area is 12 mm with the reinforcement, while it is 15 mm without the reinforcement. The forementioned phenomena indicate that the sleeve valve pipe grouting reinforcement makes the city wall foundation within the crossing area a stable integral structure, which results in a gentle and small deformation of the city wall foundation during tunnelling and prevents the foundation structure from large local eminence during shield tunnelling.

5. Field monitoring

4.5. Deformation control standard and safety assessment

Fig. 14 illustrates the surface settlement curves above the tunnel axis of various monitoring sections. It can be observed that the surface settlement of the right line is obviously larger than that of the left line after the double-line tunnelling. The average final surface settlement of the right line is 13.5 mm, and a maximum settlement of 14.8 mm occurs at section DK14 + 805 (north section of the city wall). The average final surface settlement of the left line is 8.5 mm, and a maximum

In order to verify the accuracy of numerical simulation, the city wall structure deformation and ground settlement were monitored in the reasonable period of time. 5.1. Monitoring plan Figs. 12 and 13 illustrate the positions of monitoring sections and monitoring points for city wall structure and the ground surface of Heping Gate. 12 monitoring points are arranged in sequence at the south and north sides of Heping Gate from west to east (serial numbers at the north side are BCQ1–BCQ12 from west to east while serial numbers at the south side are NCQ1–NCQ12 from west to east), so as to monitor the deformation characteristics of city wall structure during the shield tunnelling. To further determine the disturbance of double-line shield for surrounding soil mass, several monitoring points are installed in section DK14 + 705-DK14 + 815 to monitor the change law of surface settlement during shield tunnelling. In this study, nine sections are selected from the south sections (DK14 + 745, DK14 + 755, DK14 + 765), north sections (DK14 + 796, DK14 + 805, DK14 + 815) and foundation sections (DK14 + 769, DK14 + 777, DK14 + 786) of the city wall to verify the change of surface settlement slot in transverse direction. 5.2. Analysis of final settlement of surface above tunnel axis

The shield construction of the Xi’an Metro adopts the settlement and deformation control standard to control the maximum settlement of the surface and city wall structure within +5 to −15 mm and the local inclination to be less than 0.001 in combination with the surface deformation control materials of the as-built metro passing through

(a) With reinforcement: local inclination of wall foundation

(b) Without reinforcement: local inclination of wall foundation

Fig. 9. Local inclination of wall foundation. 8

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(a) With reinforcement: stage 1

(b) Without reinforcement: stage 1

(c) With reinforcement: stage 2

(d) Without reinforcement: stage 2

(e) With reinforcement: stage 3

(f) Without reinforcement: stage 3

(g) With reinforcement: stage 4

(h) Without reinforcement: stage 4

Fig. 10. City wall structure settlement contour map.

line is 0.5 mm. After the crossing of the right line, the average surface settlement of the left line is 7.4 mm, increasing by 1.2 mm, whereas that of the right line is 11.5 mm, increasing by 11 mm. It indicates that double-line tunnels affect each other during shield tunnelling, and the

settlement of 11.2 mm occurs at section DK14 + 805 (north section of the city wall). After the crossing of the left line, the average surface settlement of the left line is 6.2 mm at the city wall foundation, while that of the right 9

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(b) Without reinforcement: stage 1

(a) With reinforcement: stage 1

(d) Without reinforcement: stage 2

(c)With reinforcement: stage 2

(f) Without reinforcement: stage 3

(e) With reinforcement: stage 3

(g) With reinforcement: stage 4

(h) Without reinforcement: stage 4

Fig. 11. Ground settlement contour map.

From section DK14 + 705 to section DK14 + 825, the clearance between the two tunnels becomes larger, and the effect of shield tunnelling of the left and right lines on the settlement of the monitoring points decreases gradually. The settlement within the city wall foundation

shield tunnelling of the left line causes obvious disturbance to the soil mass at the right side. The right line tunnel’s deformation is bigger than left one which is a good agreement Ocak (2014); Peck (1969); Addenbrooke (1996); Cooper et al. (2002); Chapman et al. (2004). 10

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D3~D7 DK14+705

D3~D7 DK14+715

D5 D6 D7 DK14+725 D3 D4

DK14+735 D3~D7

D1~D10 DK14+745

D6 D7 D8 D9 D10 DK14+755 D2 D3 D4 D5

4 4 5 5

DK14+769

lines reaches 7 mm. The final settlement for the surface above the right line tunnel axis is approximately 13.8 mm, which is less than the alarming value of 15 mm. After the crossing of the left line tunnel, the surface settlement mainly occurs within the area above the left line tunnel, the maximum settlement occurs at the monitoring points above the left line tunnel axis, and the settlement is between 5 mm and 6 mm. The shield tunnelling of the left line tunnel has a small effect on the right tunnel. After the crossing of the left line, the maximum settlement for the soil layer above the right line tunnel ranges from 1 to 2 mm only. Similarly, the shield tunnelling of the right line tunnel has a small effect on the settlement of the soil layer above the left line tunnel. After the crossing of the right line, the settlement of the soil layer above the left line tunnel merely increases by 1 to 2 mm.

D1

9 9 4 4 5 D1~D10 DK14+765

city wall 5 4.5 3.5 3.5 3.5 3.5 4.5 5

DK14+777

N

D1~D10 DK14+815 5 5 4 4 8 8 4 4 5 D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 DK14+805 Unit:m D1~D10 DK14+796 DK14+787 DK14+786 D3~ D12

Fig. 12. Layout of monitoring points for settlement of city wall structure.

5.3.2. Surface settlement for the city wall foundation sections The surface settlement curve for the city foundation sections is shown in Fig. 16. After the crossing of the left line tunnel, the surface settlement mainly occurs above the left line tunnel. The maximum settlement occurs at the monitoring points above the left line tunnel axis, and section DK14 + 787 has a larger surface settlement above the left line tunnel axis, which is 10 mm. The settlement for the other sections is 7 mm above the left line tunnel axis. The shield tunnelling of the left line tunnel has a small effect on the right tunnel. After the crossing of the left line, the maximum settlement of the soil layer above the right line tunnel is in the range of approximately 2–3 mm, and it occurs at the point near the left line tunnel (monitoring point D8). The excavation of the right line tunnel has a small effect on monitoring points D3, D4, and D5 above the left line tunnel, and the settlement is less than 2 mm; however, it has a large effect on monitoring points D6 and D7. The settlement is approximately 3.5 mm, and its effect on the surface above the left line decreases with increase in the distance from the right line tunnel. After the crossing of the two lines, the settlement curve shows two peaks at section DK14 + 787. The maximum surface settlement of the city wall above the left line is 9.8 mm while that above the right line is 12.5 mm. The maximum local inclination curve value is 0.0009 as shown in Fig. 17. Those values are less than the warning values of +5 to −15 mm and 0.001, respectively.

Fig. 13. Plan layout of ground settlement monitoring points.

area is much smaller than that in other areas, and the final settlement of the double-line tunnels complies with the control standard for surface settlement.

5.3. Analysis of surface settlement for each section near the city wall 5.3.1. Surface settlement for south sections of the city wall The surface settlement curve for the south sections of the city wall is shown in Fig. 15, where L represents the monitored value of surface settlement in transverse direction after the crossing of the left line, R represents the monitored value of surface settlement in transverse direction after the crossing of the right line, namely, the final settlement after completion of the twin tunnels. And DS represents the differential settlement of ground surface caused by the excavation of left and right tunnel. It can be observed that when the deformation is stable after the crossing of double-line tunnels, the settlement above the right line is larger than that above the left line, and the maximum settlement difference between the surfaces above the tunnel axis of the left and right 11

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Fig. 14. Measured maximum surface settlements of tunnel axis.

settlement value occurs in the range of monitoring points of D2 to D6, maximum settlement value occurs in upper monitoring points of left line tunnel axis, which is approximately 8 mm. The crossing of left line has a more obvious effect on the settlement of monitoring point D7 in

5.3.3. Surface settlement for north sections of the city wall The surface settlement curve for the north sections of the city wall is shown in Fig. 18. After the crossing of the left line, the settlement value of surface mainly occurs in the upper area of left line tunnel, and larger

(a) Section DK14+745

(b) Section DK14+755

(c) Section DK14+765

(d) Final settlement of three cross sections

Fig. 15. Ground surface settlement curve for south sections of the city wall. 12

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(a) Section DK14+769

(b) Section DK14+777

(c) Section DK14+787

(d) Final settlement of three cross sections

Fig. 16. Ground surface settlement curve for the city foundation sections.

monitoring points D1, D2, and D3 above the left line tunnel, which is within 1 mm, but has a slightly large effect on points D4 and D5 at approximately 4 mm. Its impact on the upper surface of the left line tunnel becomes smaller with the increase of the distance from the right line tunnel. The increase of the upper surface settlement value of the right line tunnel is more obvious at the crossing of the right line, particularly in the settlement peak value. For example, the increase in the settlement value of monitoring point D8 is approximately 13 mm in the process of shield crossing of the right line. 5.4. Analysis of settlement of city wall structure To clearly express the rule on the development of the settlement of the city wall structure with time in the crossing of the left line, the south section installed with settlement monitoring points NCQ1, NCQ3, NCQ4, NCQ5, and NCQ6 and the north section installed with settlement monitoring points BCQ1, BCQ3, BCQ4, BCQ5, and BCQ6 were selected for detailed monitoring. The city wall structure settlement trough and time curves for the crossing of the left line are shown in Fig. 19. The settlement value of each monitoring point increased with time and reached a stable state on July 25, 2015. The differential deformation of each monitoring point is small, and NCQ4 has the largest settlement, which is 3.9 mm. The differential settlement value of the other points is merely 1.5 mm and the maximum settlement rate is only 1.7 mm/d. Both settlement value and its rate are less than the alarming values (the empirical warning value of sedimentation rate is ± 3 mm/d (Lei,

Fig. 17. Local inclination of wall foundation.

upper ground surface of right line tunnel. Settlement value of point D7 is at 2 to 4 mm, while settlement value of monitoring points of D8 to D10 is smaller in the excavation of left line, for about 1 mm. After crossing of twin tunnels, the final surface settlement curve averagely is manifested as that surface settlement above right line is larger than that above left line, maximum settlement curve value of every cross section appears in the monitoring points of D4 and D8. Maximum differential value is about 5 mm for the surface settlement above tunnel axles of left and right lines. The excavation of the right line tunnel has a small effect on

13

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(b) Section DK14+805

(a) Section DK14+796

(d) Final settlement of three cross sections

(c) Section DK14+815

Fig. 18. Surface settlement curve for north sections of the city wall.

Fig. 19. Settlement-time curves of the city wall structure during the left line crossing.

Fig. 20. Settlement-time curves of the city wall structure during the right line crossing.

2010)). In the same way as the left line, the south section installed with monitoring points NCQ7 to NCQ11 and the north section installed with monitoring points BCQ7 to BCQ11 were selected for detailed monitoring in the crossing of the right line. The city wall structure settlement trough and time curves for the crossing of the right line are shown in Fig. 20. The settlement value of each monitoring point increased rapidly from September 21 to September 25. The settlement rate

reached 2.7 mm/d, and differential settlement appeared in each monitoring point. Since September 27, 2015, the deformation tended to be stable. Afterwards, the differential settlement of each monitoring point becomes obvious, in which, NCQ8, NCQ9, BCQ8, and BCQ9 have the largest settlements of approximately 11–12 mm. Most settlement values in the southern monitoring points are larger than those of the northern monitoring points. The minimum settlement value occurred in BCQ11, which is merely 4.2 mm. In addition, the settlement values of BCQ10 14

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a) Settlement distribution of wall during the left line crossing

b) Settlement distribution of wall during the right line crossing

c) Final settlement of city wall Fig. 21. The settlement curve of the city wall when the left and right lines pass through.

numerical simulation results show that the surface settlement on the right line tunnel is larger than that on the left line. The measured right peak of the settlement curve for each monitoring section is in the range of 11–13.8 mm; the left peak ranges from 7 to 9.5 mm. Based on the numerical simulation result, the right and left peaks range from 12.5 to 14 mm and from 8.8 to 9.8 mm, respectively. It indicates that double-line tunnels affect each other during shield tunnelling, and the shield tunnelling of the left line causes obvious disturbance to the soil mass at the right side. The settlement regularity is similar. The measured settlement curve in the city wall foundation area does not show the wide and shallow settlement slot indicated in the numerical simulation result. However, the smooth change in the surface settlement on both tunnels coincides better with the predicted deformation law; therefore, numerical simulation is more accurate in predicting the surface settlement. (3) As for the settlement change characteristics of the city wall structure, the results of the numerical calculation and field measurement agree in terms of the stability law after the deformation. That is, the settlement in the south is larger than that in the north, and that in the east is larger than that in the west. Based on the comparison of the settlement values, each settlement monitoring point has a small difference except for NCQ4, and the results agree well. The maximum settlement occurs at NCQ4 after the crossing of the left line, the measured settlement is 3.9 mm, and the calculated settlement is 7.9 mm. In addition, the numerical simulation result in Fig. 9 shows that the maximum displacement in the city wall foundation area is 7.9 mm in the west gate area; therefore, the predicted location is accurate although the error in the settlement value is larger. After

and BCQ7 are relatively small, at approximately 9.3 mm. Fig. 21 illustrates the entire settlement distribution curves of the city wall structure. Here, LS indicates the final settlement value on the south side of the wall structure after the left line crossing; LN indicates the final settlement value on the north side of the wall structure after the left line crossing. RS indicates the final settlement value on the south side of the wall structure after the right line crossing, and RN indicates the final settlement value on the north side of the wall structure after the right line crossing. It can be observed that after the deformation becomes stable, the settlement value in the south is larger than that in the north, and that in the east is larger than that in the west. At the crossing of the left and right lines, the settlement value is smaller than the upheaval and settlement alarming values (+5 mm to −15 mm) of the city wall. 5.5. Discussion (1) Both numerical simulation and field monitoring results of the ground settlement above the tunnel axis indicated that because of the sleeve valve pipe grouting reinforcement, the surface settlement in the city wall foundation area is less than that of the other sections. The excavation on the right line in this section has a smaller effect than that on the left line. The predicted maximum surface settlement in the city wall foundation area is 10.2 mm for the left line and 12 mm for the right line. Moreover, the measured surface settlement is 9.8 mm for the left line and 12.5 mm for the right line. Both results are basically consistent. (2) For the surface cross-sectional settlement, the left tunnel is first excavated and the right tunnel is second, both measured and 15

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Underground Space Engineering (No. YT201905) and the Fundamental Research Funds for the Central Universities, CHD (No. 300102219723, No. 300102219716, No. 300102219711, No. 300102219117) and the National Key R&D Program of China (No. 2018YFC0808706).

the crossing of the right line, the maximum settlement values from the numerical calculation and field measurement occur at monitoring point CQ8, which are approximately 12.2 mm and 11.8 mm, respectively with a settlement error of only 0.4 mm. The overall settlement error is small and the predicted location is accurate. In conclusion, although there is a small difference between the results of the numerical calculation and field measurement, the area with a large measured settlement and the settlement characteristics of the city wall structure agree well with those predicted by numerical simulation, and the maximum settlement value is almost equal. Hence, the above mentioned comparative analysis with field measurements verifies the reliability of the numerical simulation and the acceptability of its result to some extent.

References Addenbrooke, T.I., 1996. Numerical analysis of tunnelling in stiff clay. PhD Theses. Imperial College, London, pp. 373. Azadi, M., Pourakbar, S., Kashfi, A., 2013. Assessment of optimum settlement of structure adjacent urban tunnel by using neural network methods. Tunn. Undergr. Space Technol. 37, 1–9. Bae, G.J., Shin, H.S., Sicilia, C., Choi, Y.G., Lim, J.J., 2005. Homogenization framework for three-dimensional elastoplastic finite element analysis of a grouted pipe-roofing reinforcement method for tunnelling. Int. J. Numer. Anal. Meth. Geomech. 29 (29), 1–24. Bobet, A., 2001. Analytical solutions for shallow tunnels in saturated ground. J. Eng. Mech. 127 (12), 1258–1266. Boone, S.J., Westland, J., Nusink, R., 1999. Comparative evaluation of building responses to an adjacent braced excavation. Can. Geotech. J. 36 (2), 210–223. Boscardin, M.D., Cording, E.J., 1989. Building response to excavation-induced settlement. J. Geotech. Eng. 115 (1), 1–21. Cao, L.Q., Fang, Q., Zhang, D.L., Chen, T.L., 2018. Subway station construction using combined shield and shallow tunnelling method: case study of Gaojiayuan station in Beijing”. Tunn. Undergr. Space Technol. 2018. https://doi.org/10.1016/j.tust.2018. 09.010. Chapman, D.N., Rogers, C.D.F., Hunt, D.V.L., 2004. Predicting the settlements above twin tunnels constructed in soft ground. Tunn. Undergr. Space Technol. 19 (4/5), 378–380. Cheng, C.Y., Dasari, G.R., Chow, Y.K., Leung, C.F., 2007. Finite element analysis of tunnel–soil–pile interaction using displacement controlled model. Tunn. Undergr. Space Technol. 22 (4), 450–466. Cheng, W.C., Li, G., Liu, N., et al., 2020. Recent massive incidents for subway construction in soft alluvial deposits of Taiwan: A review. Tunn. Undergr. Space Technol. 96. https://doi.org/10.1016/j.tust.2019.103178. Cheng, W.C., Ni, J., Arulrajah, A., Huang, H., 2018b. A simple approach for characterising tunnel bore conditions based upon pipe jacking data. Tunn. 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6. Conclusions (1) When shield structure crossing underneath the ancient wall with the reinforcement, the settlement above the tunnel axis obviously tends to decrease when approaching the city wall foundation, and the impact of right line excavation on surface settlement of left line gradually increases; but without the reinforcement., the law is in the opposite. In addition, after the completion of double lines, surface settlement value of right line tunnel axis is larger than that of left line tunnel. The reinforcement measures in city wall section make the settlement value in foundation area decreases by about 4 mm than that of original condition, and efficiently prevent excessive local inclination of city wall foundation, and the reinforcement effect is relatively obvious; the surface lateral settlement curve is a wide and shallow single peak settlement slot with the reinforcement, and it is a narrow and deep double-peak settlement slot without the reinforcement due to the barrier action between foundation boulder under the city wall and gate partitions; the curves in south and north areas are double-peak. (2) The calculation results indicate that deformation settlement caused by metro construction shall have great effects on ancient city wall in case that any reinforcement measurements have not been taken. Through conducting sleeve valve pipe grouting and setting up steel frame and other pre-support measures under city wall opening, the uneven settlement produced by city wall is restricted, and final maximum settlement value is approximately 12 mm, which is 2.2 mm less than that without the reinforcement. The maximum settlement rate is 2.7 mm/d. These values meet the requirements of settlement control standard and preservation of cultural relics. (3) Through designing deformation monitoring scheme, comparison and analysis are conducted for actual monitoring value and calculation result at the same time, the larger area of actual measured settlement for city wall structure is in good agreement with the predicted occurring area of numerical simulation as well as settlement characteristics, and the surface settlement law is also similar. The prediction for numerical simulation provides effective guidance for real-time monitoring in construction process; the actual measurement indicates that both settlement value and settlement rate are less than alarming value in crossing process of left and right lines. At present, tunnel shield construction has passed through the city wall smoothly. The research conclusions can provide a reference for similar crossing projects in the future. Declaration of Competing Interest The authors declare that they have no conflicts of interest. Acknowledgments The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China (No. 51978066) and the Open Fund for Shaanxi Key Laboratory of Geotechnical and 16

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