Construction Risk Analysis of Urban Subway in Broken Rock Strata

Construction Risk Analysis of Urban Subway In Broken Rock Strata

Chun-quan Dai College of Civil Engineering, Shandong University of Science and Technology, Qingdao, China e-mail: [email protected]

Zeng-Hui Zhao State Key Laboratory of Mining Disaster Prevention and Control Co-founded by Shandong Province and the Ministry of Science and Technology, Qingdao ,China College of Mining and Safety Engineering, Shandong University of Science and Technology, Qingdao, China e-mail: [email protected]

ABSTRACT Zhongshan Park Station tunnel in Qingdao Metro possessed the characteristics of shallow buried(10.0~12.3m), large span (19.6m), complex ground traffic load with heavy traffic, and part of the tunnel in strongly weathered surrounding rock of grade V which is very bad to construction deformation control. Due to complicated station structure and potential risk factors, the construction management was difficult and easily caused engineering accidents such as collapse, ground settlement during construction, and for which risk assessment analysis should be carried out. Through established the tunnel construction mechanics model of Zhongshan Park Station in Qingdao Metro, ground settlement values during tunnel construction were calculated by adopting orthogonal simulation test, and effect weight of risk comprehensive indexes under tunnel construction factor levels was calculated by using hierarchical analysis theory. On this basis, the tunnel construction risk index value and risk grade under different working conditions were given for shallow buried tunnel with large span of Zhongshan Park Station in Qingdao Metro. Engineering practice shows that the risk assessment method has important theoretical guiding significance and practical application value.

KEYWORDS: metro tunnel; shallow buried; risk index; analytical hierarchy process; risk

assessment

INTRODUCTION Along with the development of Chinese economic society and necessity of the urbanization construction, the urban rail transit construction has been entered a rapid development period. During the Eleventh Five-Year, the cities including Beijing, Shanghai, Guangzhou, Nanjing, Hangzhou which number up to 12 had constructed a large amount of the metros and rapid railways. - 4189 -

Vol. 19 [2014], Bund. Q 4190

Compared with the general engineering, urban metro tunnel engineering has highly multidimensional uncertainty which influences each construction link and causes many malignant engineering accidents such as large strata deformation, collapse and subsidence during tunnel construction. In recent years, many tunnel engineering construction accidents have been occurred in domestic. The Shanghai Rail Transit line 9 ruptured by water and caused the ground subsidence in 2004, the Guangzhou Metro line 3 had a ground subsidence in 2005, and water gushing and collapse occurred in Guangzhou Metro line 3 in 2008, the Beijing Suzhou Road line 10 collapsed in 2007, settlement and collapse engineering accidents appeared in Shenzhou Metro line 1 in 2009, large collapse appeared in Dalian Metro during tunnel construction in 2011. Comprehensive analysis of the engineering accident, the reasons include severe hydrological and geological factors, but the more factors are feeble risk awareness and lacking risk management. Aiming at this engineering problem, many domestic and foreign scholars combined specific engineering example to carry out studies for urban shallow buried tunnel construction risk, and obtained many research results for tunnel engineering risk guidance [1-2].Based on systemic summary of urban metro tunnel construction risk and combined urban metro construction project accidents in recent years in China, Wang et al [3-4] put forward a strategy of the informational construction combined with displacement distribution principle to control the risk of urban shallow buried tunnel construction, and applied into many metro engineering in Beijing, Shanghai, Guangzhou et al. Huang et al [5-6] combined Shanghai Metro line 11 engineering project with expert investigation and the analytic hierarchy process to carry out the risk analysis, thus putting forward some rationalization proposals for construction risk control at key points. Chen [7] used system theory to analysis the probably existed risk in shield construction, and discussed the city shield construction risk aversion measures from the view of engineering application. Based on urban shallow buried tunnel construction characteristics, Zhang et al [8] analyzed environmental safety risk caused by the city tunnel excavation on adjacent buildings. From the perspective of engineering project management, Burland JB [9], Kim JH et al [10] analyzed the construction risk and risk countermeasures for urban underground engineering. Chen [11] employed analytic hierarchy process to analyze the risk of land subsidence and building damage caused by urban tunnel construction. Nevertheless the above researches have not combine risk management with underground project construction stability control organically, now risk research is still stay at a stage of which undefined risk concept, more qualitative analysis, less quantitative analysis and mismatching of risk model and practical application[12-14]. In this study, tunnel engineering risk index evaluation was studied by comprehensively employing risk analytic hierarchy process (AHP), the orthogonal experiment theory and the computer simulation. The quantitative analysis method which used to research urban tunnel construction risk research possessed the characteristics of rapid, efficient and low cost. Vol. 19 [2014], Bund. Q 4191

ENGINEERING SITUATION AND COMPLEXITY IN ZHONGSHAN PARK STATION

Engineering situation in Zhongshan Park Station

Zhongshan Park Station is located in the south of Tiantai Stadium, and the north of Hongkong Road. Rongcheng Road, Shaoguan Road and Hongkong West Road are intersection in the scope of this station, and the traffic flow is so heavy. The North Ship Playground is above the station, and no tall buildings around the station. Qingdao Zhongshan Park is near to the station north side, there are municipal pipeline below through the Hongkong West Road. The station is 176.9 meters long, and is 10 meter platform Island Station. Soil cover depth is 10~12.3 meters, the surrounding rock level is Ⅲ~Ⅴgrade. The station bottom is positioned on the micro-weathered rock stratum. The station of each rock conditions is shown in Table 1. The station is constructed in large arch-foot cover construction method with single arch upright wall section, and used the drilling and blasting excavation. The section is 19.2 meters width and 16.2 meters high. Typical temporary support structure of the station section is shown in Fig 1 and Fig 2 respectively.

Table 1: Surrounding rock condition of station tunnel

Line Mileage Length(m) Rock grade K3+604.6～K3+655.1 50.5 Ⅲ K3+655.1～K3+665.0 9.9 Ⅴ Left line K3+665.0～K3+707.8 42.8 Ⅳ K3+707.8～K3+760.9 53.1 Ⅴ K3+584.1～K3+597.3 13.2 Ⅳ K3+597.3～K3+646.7 49.4 Ⅲ K3+646.7～K3+666.0 19.8 Ⅳ Right line K3+666.0～K3+674.0 8 Ⅴ K3+674.0～K3+721.2 47.2 Ⅲ K3+721.2～K3+760.9 39.7 Ⅳ

Figure 1: Support structure-type A Vol. 19 [2014], Bund. Q 4192

The groundwater types in Zhongshan Park Station: most of Quaternary bedrock fissure water and slight of the Quaternary soil pore-water. Groundwater field depth is 4.8 ~ 13.6m. The quantity of water inflow pit is 376m3/d, and the quality is belonging to the water rich bad formation.

Figure 2: Support structure-type B

Engineering complexity analysis

From the point of the construction site, there are large deformations and local instability problems, the reasons are as the following: (1) Formation condition is poor, and the station is shallow buried, large span, within the buried depth were filling soil and the strong weathering, the tunnel span is 19.6m, 10m to 12m depth. Deformation control of construction is difficult. (2) The complexity of ground environment: the traffic flow is big above the station, and there is heavy truck load and the construction load. This condition is adverse to the stability of the supporting structure. From the point of construction progress, when enter the second stage of the construction, the upper part of the temporary support structure involving the safety of the supporting body and the permanent lining constructed. If support removing parameters were unreasonable and took improper measures, support system will damage and large area ground subsidence. Engineering construction easily becomes malignant project.

RISK ASSESSMENT MODEL BASED ON THE ORTHOGONAL EXPERIMENT OF ANALYTIC HIERARCHY PROCESS (AHP) Based on the risk classification system, the risk index assessment model can rapid estimate risk grade by analyzed the risk factor and other related preference, and then appropriate indices would be to give according to certain principles, in this way we can obtain a index or point of a subsystem or system that gather up by mathematical technique [15-16]. Vol. 19 [2014], Bund. Q 4193

Synthesized index model assessment process

Based on the basic principle and steps of Watts[17], synthesized index model assessment can be constructed observe the following steps: (1) Determination of evaluation index according to subject investigation, such as the surface subsidence value is regarded as evaluation index of urban tunnel strata deformation risk, while the risk of tunnel collapse regard arch crown settlement as the evaluation index. (2) Identification of risk factor influence. (3) Construction orthogonal experiment model according to analytic hierarchy process (AHP). (4) Carry on orthogonal experimental analysis. (5) Calculation effect value of various factors. (6) Based on the related standard and experience, the attribute factors of quantitative index is given according to the results of numerical simulation quantitative analysis and empower the weight and score. Analysis flow is shown in Fig 3.

Risk factors investigation statistics

Factor 1 Factor 2 … Factor n

Orthogonal simulation test scheme

Build risk influence weight matrix

Risk index assignment

Risk assessment

Figure 3: Analysis flow of risk index method

The AHP analysis of orthogonal simulation trial

The AHP method of orthogonal simulation trial using orthogonal experiments theory design numerical simulation test scheme, calculating the evaluation index value, and then establish the evaluation matrix according to the analytic hierarchy process in order to calculate the weighing effect on the evaluation index value of each factor. The tunnel construction risk project could be designed to a hierarchical structure without interaction on the basis of risk characteristics and risk source, as shown in Fig 4. Vol. 19 [2014], Bund. Q 4194

Text index values

A(1) A(2) … A(n)

(1) (1) (2) (2) (n) (n) A1 … An A1 … An … A1 … An

Figure 4: Hierarchy of analytic hierarchy process In order to calculation analysis convenient, the sum of simulation test value Kij that under the j level of factor A(i) is considered as the effect value of simulation test, which affected by the j level of factor A(i). Combined with engineering practice, if the test index the bigger the better, and then mask Mij=Kij, on the contrast, Mij=1/Kij. According to the analytic hierarchy process, the effect matrix on horizontal layer is A which can be expressed as:

M 11 0 … 0   …  M 21 0 0   … … … …    M n1 0 … 0   0 M … 0   12   0 M 22 … 0  (1) =  … … … …  A  

 0 M n2 … 0     … … … …   0 0 … M   1k   0 0 … M 2k   … … … …     0 0 … M nk 

Let be matrix S as:

 1 0 … 0   t1   0 1 … 0  S =  t2  (2)  … … … …   1   0 0 …   tk 

n j = = … where t j ∑M ij ( j 1,2,3 ,k) . i=1

As matrix calculation theory，we can use A right multiply matrix S, and present a normalization processing to matrix A. Thus, we regard matrix AS as standard effect matrix. Vol. 19 [2014], Bund. Q 4195

The range Ri（i=1, 2, 3, …, m）of A(i) that calculate from the orthogonal experiment result is count as influential effect on the simulation test. So the weighting effect matrix can be expressed as:

    R R R R C =  1 2 3 … m  (3)  m m m m  ∑ Ri ∑ Ri ∑ Ri ∑ Ri   i=1 i=1 i=1 i=1 

As the AHP theory, the level of each factor acting on the experimental index weights F is called matrix W=ASCT, among them the matrix W is n * 1 order matrix which indicates the effect value of each level factors act on the test in turn.

Construction risk assessment index of tunnel engineering

The index method assignment of city long span structure shallow buried tunnel adopts to rely on the orthogonal test results, adjusting with the feedback information of site construction and experience. So the specific assignment process can be expressed as:

F=kcAc (4) where F indicates comprehensive risk score of tunnel construction, Ac denotes the objective score obtained from numerical simulation test, and kc represents the adjustment factor score which can be determined on the basis of the expert experience and the situation of construction site. The bigger the F value shows that the greater the risk, we can divide the risk into five grades that consult the evaluation criteria of “Interim Provisions for risk assessment and management of Railway Tunnels”, as shown in Table 2.

Table 2: Ranking of tunnel construction risk F Value Risk grade Level code Remarks

＜20 high-low A Normal construction 20-40 low B Normal construction 40-60 moderate C Needs risk pretreatment 60-80 upstairs D Needs risk of warn ＞80 sky-high E Needs risk of warn

ANALYSIS OF THE CONSTRUCTION RISK IN ZHONGSHAN PARK STATION

Choice of the tunnel section

For analyzing the complexity problems, the sections range K3+ 760.9 to K3 + 707.8 is selected, in which the segment of the structural support is shown in Fig 2. Because of the subsection Vol. 19 [2014], Bund. Q 4196 construction method, we mainly analyze the construction risk of the upper structure. The construction process is shown in Fig 5.

(a) Grouting of the upper small duct

(b) Excavation and supporting of the left side

(d) Excavation and supporting of the middle

(e) Demolition and reinforcement of the support

Figure 5: Construction process Vol. 19 [2014], Bund. Q 4197

Design of the orthogonal simulation test

Identification of risk factors Comprehensively analyzing geological engineering conditions and construction environment, combining with the construction characteristics of the Zhongshan Park Metro Station, the construction risk index can be determined through expert questionnaire. In fact, the main influencing factors of the index were the surrounding rock level, the tunnel section shape, the buried depth, the tunnel span, the construction plan, the external environment and so on. However, for the established tunnel, as the cross section shape has been designed, and the external environment was with high uncertainty, three influence factors of tunnel buried depth, rock class and the construction scheme were analyzed in the selection. (1) Buried depth of the tunnel. Generally, when the buried depth is shallow, the maximum surface subsidence and sedimentation rate decreases with the increase of buried depth, and the width of the settling tank also decreased accordingly. (2) Rock level. Rock level not only produced stress adjustment and deformation caused by tunnel excavation formation, but also directly affect the choice of the excavation scheme in the design of construction. (3) Construction schemes. At present, the main methods of building railway tunnels are mining method, shield method, immersed method and cutting and covering method. Then the mining method can be divided into different excavation methods, for example: the bench cut method, CD method, CRD method, which were shown in Table 3.

Table 3: Comparison of construction schemes Constructi Construction Deformation Construction Temporary on schemes situation period quantities cost Bench bad shorter none lower CD better longer bigger higher CRD good long big high full-face excavation worse short small low

Choice of the affecting factors level The buried depth of Qingdao Zhongshan Park tunnel is 10.0 ~ 12.3 m. The level of surrounding rock is Ⅲ-Ⅴ level. The construction schemes can be bench method, CD method, CRD method respectively. The detailed information is shown in Table 4.

Table 4: Level for risk factors of tunnel construction level depth Rock grade schemes 1 8.0 Ⅲ Bench method 2 12.0 Ⅳ CD method 3 16.0 Ⅴ CRD method Three dimensional numerical model Vol. 19 [2014], Bund. Q 4198

According to the construction situation, a model of 120 * 80 * 60 was established as shown in Fig 6. Numerical simulation calculation was carried out by FLAC3D. The maximum vertical displacement was monitored.

Figure 6: The tunnel model

Figure 7: The tunnel model

Selection of mechanical parameters of surrounding rock Because the formation parameters have spatial and temporal variability [18-19], mechanical parameters obtained by engineering geological exploration of surrounding rock, cannot accurately reflect the characteristics of rock deformation, so parameter analysis should be optimized. According to the comparative data monitored in the field, the selection of surrounding rock mechanics parameters was obtained by inversion method based on numerical results. The detailed arrangement of monitoring section is shown in Fig 8. Vol. 19 [2014], Bund. Q 4199

Ground settlement monitoring points B C D E F G

Ground settlement curve Arch deformation monitoring point Horizontal convergence

monitoring point

Figure 8: Layout of watching points in tunnel construction According to the results from inversion analysis and comparison to the exploration data of geological engineering, mechanics parameters of the tunnel surrounding rock are shown in Table 5.

Table 5: Mechanical parameters of surrounding rock density deformation friction angle Cohesion Rock level Poisson's ratio /kN/m3 modulus/MPa /° /MPa

Ⅲ级 22.5 8560 0.3 19 2.5

Ⅳ级 21.1 7980 0.3 17 1.5

Ⅴ级 19.2 5226 0.27 12 0.4

Model support parameters were showed in Table 6.

Table 6: Model supporting parameters Support Parameters advanced small pipe entity unit：bulk=1.2e10,shear=0.65e10,fric=32,coh=5e6,tens=5e6 Cable unit：xcarea=4e-3, emod=100e9, ytens=1e19, gr_k=7e6 gr_coh=2e2, anchorage gr_per=0.314, gr_fric=25 support entity unit：bulk 2e10, shear 1e10, fric 40, coh 5e8, tens 5e8 Shot concrete iso=(6.5e9 0.25) thick=0.3 cs_nk=3e10 cs_sk=3e10 cs_scoh=1e6 + grille arch

Design scheme and the calculation results of orthogonal

simulation

Based on the above analysis, the optimization of the scheme was orthogonal experiment of 3 factors and 3 levels. And using orthogonal Table of L9 (33), the maximum deformation in tunnel construction were obtained, and values of the K, R, M were calculated by the theory of orthogonal test, as shown in Table 7. Vol. 19 [2014], Bund. Q 4200

Table 7: Orthogonal simulation test result of AHP Number Depth/m Rock grade Schemes Maximum displacement/mm

1 6.0 Ⅲ Bench 17.62 2 6.0 Ⅳ CD 14.31 3 6.0 Ⅴ Glasses 12.68 4 12.0 Ⅳ Glasses 9.42 5 12.0 Ⅴ Bench 21.34 6 12.0 Ⅲ CD 7.63 7 18.0 Ⅴ CD 15.69 8 18.0 Ⅲ Glasses 3.41 9 18.0 Ⅳ Bench 12.84 K1 7.41 14.68 5.92 K2 15.62 13.47 8.69 K3 18.36 10.93 22.18 R 8.68 6.45 16.14 M1 0.117 0.056 0.138 M2 0.049 0.062 0.097 M3 0.053 0.079 0.041 Analysis of the effect on construction risk The matrix of horizontal layers affecting numerical simulation experiment is

0.117 0 0    0.049 0 0  0.053 0 0     0 0.056 0  A =  0 0.062 0     0 0.079 0     0 0 0.138  0 0 0.097    0 0 0.041 Calculation results of the matrix S is：   1 0 0  t1  9.82 0 0      S = 0 1 0 = 0 9.82 0  t2     1   0 0 9.82  0 0   t3  Influence matrix of various factors test is:      R R R  = 1 2 3 = C  m m m  [0.24 0.32 0.42]  R R R  ∑ i ∑ i ∑ i   i=1 i=1 i=1  Vol. 19 [2014], Bund. Q 4201

According to the above test results, the weighted value of the tunnel buried depth, rock mass and the construction plan were respectively 0.24, 0.32, 0.42. And calculation results show that the tunnel settlement risk is the most sensitive to construction schemes, then followed is the rock mass.

Risk index assessment of supporting in excavation upper

cross-section

According to statistics of questionnaires to experts and analytic hierarchy orthogonal simulation test and Eq.4, Risk indexes of tunnel construction were confirmed ad shown in Table 8. Table 8: Division table of tunnel construction risk index Factor Weigh Value Grade 0-3 20-24 Burial depth 3-6 14-19 0.24 6-12 8-14 >12 1-7 Ⅱ 1-6 Ⅲ 7-13 rock grade 0.32 Ⅳ 14-22 Ⅴ 23-32 Spectacle method 1-8 construct CD method 9-18 0.42 project Bench method 19-29 Full-section method 30-42

According to the regulation of value in Table 8, the risk rankings of level Ⅲ surrounding rock were calculated. (see Table 9~Table 11).

Table 9: Risk ranking of level Ⅲ surrounding rock

Burial depth/m Project 6-8m 8-10m 10-12m ＞12m Spectacle method 28-45 22-40 16-35 9-28 CD method 36-55 30-50 24-45 17-38 Bench method 46-66 40-61 34-56 27-49 Full-section method 57-79 51-74 45-69 38-62

Table 10: Risk ranking of level Ⅳ surrounding rock

Burial depth/m Project 6-8m 8-10m 10-12m ＞12m Spectacle method 35-52 29-47 23-42 16-35 Vol. 19 [2014], Bund. Q 4202

CD method 43-62 37-57 31-52 24-45 Bench method 53-73 47-68 41-63 34-56 Full-section method 64-86 58-81 52-76 45-69

Table 11: Risk ranking of level Ⅴ surrounding rock

Burial depth/m Project 6-8m 8-10m 10-12m ＞12m Spectacle method 45-62 39-57 33-52 26-45 CD method 53-72 47-67 41-62 34-55 Bench method 63-83 57-78 51-73 44-66 Full-section method 74-96 68-91 62-86 55-79

According to Table 9~Table 11, the risk ranking of construct project and different burial depth in level Ⅲ surrounding rock is between A and D .The risk ranking of construct project and different burial depth in level Ⅳ surrounding rock is between A and E. The risk ranking of construct project and different burial depth in level Ⅴ surrounding rock is between B and E. Therefore, considered the burial depth of Zhongshan Park Station of Qingdao Metro which constructed with spectacle method in K3+707.8～K3+760.9 is 12.0m and the level of surrounding rock is V，the value of F is 53, that is, the risk ranking is C.

Risk index assessment of dismantling upper temporary

supporting

According to the primary design, the reinforcement measure is pre-stressed anchorage whose length is 4.0m and prestress is 10000N.The arrangement of anchorage is illustrated in Fig 9.

Figure 9: Reinforcement with pre-stressed anchorage The parameters of anchorage are listed in Table 12. Table 12: Parameters of pre-stressed anchorage Supporting parameters Cable element：xcarea=4e-3, emod=100e9, ytens=1e19, gr_k=7e6 gr_coh=2e2, Anchorage gr_per=0.314, gr_fric=25, preptension=10000 Vol. 19 [2014], Bund. Q 4203

Through the analysis of the numerical model, the pre-stressed anchorage were constructed after dismantling the temporary supporting by 10m.The axial force results of anchorage were presented in Fig 10.

Figure 10: The force of anchor after dismantling temporary supporting Through the result of numerical simulation, the value of maximal subsidence of ground is 31.5mm which exceed the the warning value (24mm). However the force of anchor is 9.8e3N, that is to say, there is no effect in controling the ground settlement.The reasons is as follows: the strength of strata is soft, and the bond stress not the pre-stress decide the force between anchors and surround rock. In order to study the risk of dismantling the temporary supporting, the weight of influence through the orthogonal test in chapter 4.2 was analyzed, and the results were: the influence weight of dismantling lenght was 0.42. the effect weight of dismantling project was 0.58. According to the questionnaires to experts, the risk index are shown in Table 13. Table 13: Division Table of tunnel construction risk index Factor Weight value Grade 6-8 1-8 8-10 9-17 Dismantling length 0.42 10-12 18-29 ＞12 30-42 Pre-stressed anchorage 25-32 Dismantling project 0.58 Anchor cable 11-25 Dismantle directly 33-58

According to the rule of assignment in Table 13, the construction risk under different dismantling length and different dismantling project was listed in 14. Table 14: Risk ranking of level Ⅲ surrounding rock Dismantling length/m Project 6-8m 8-10m 10-12m ＞12m Anchor cable 26-38 36-48 45-56 52-68 Pre-stressed anchorage 42-59 48-69 55-72 66-84 Dismantle directly 62-78 68-80 71-85 81-93

According to the result of Table 14, the risk ranking is between D and E if using the method of dismantling directly. The risk ranking is between B and D if using the reinforce method of anchor Vol. 19 [2014], Bund. Q 4204 cable. The risk ranking is between C and E if using the reinforce method of pres-stressed anchor. So it is better constructed with anchor cable. The parameters of anchor cable are as follows: L=8m, full length anchoring, and row distance is 2m.The position of anchor is shown in Fig 11.The parameters of anchor cable is listed in Table 15.

Figure 11: Reinforce with anchor cable Table 15: Parameters of anchor cable Supporting Parameters Cable unit: xcarea=6.1e-2, emod=100e9, ytens=1e19, gr_k=7e6 Anchor cable gr_coh=2e2, gr_per=0.879, gr_fric=25, pretension=50000

According to the numerical simulation result, after removing the temporary supporting by 10m ，the ground settlement and the force of anchor is presented in Fig 12.

Figure 12: Z-displacement of ground after dismantling supporting by 10m and reinforce with anchor cable The maximum ground settlement is 22.6mm and the force of anchor in anchorage zones is 8.6e5N after removing temporary supporting by 10m.The value of ground settlement is within the safe range and the effect of controlling the ground settlement by anchor cable is great.

CONCLUSIONS (1) Effect weight matrix of risk factors was established through orthogonal simulation analytic hierarchy process, and by which risk indexes were assigned to analyze tunnel construction risk quantitatively. (2) Surrounding rock mechanical parameters were optimized by comprehensively employing the engineering geological exploration data, the site monitoring data and the inversion calculation Vol. 19 [2014], Bund. Q 4205 of numerical simulation, then on this basis orthogonal simulation test which has a higher precision was carried out. (3) The numerical analysis model was established by three major risk factors which were optimized through risk identification to analyze the effect weight of each risk factor and to establish effect weight matrix of each risk factor. (4) The tunnel construction risk and risk level range under different support removing schemes were analyzed combined with the situation of shallow buried tunnel with large span of Zhongshan Park Station in Qingdao Metro.

ACKNOWLEDGEMENTS This Project is supported by the National Natural Science Foundation of China (Grant No.51174128), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No.20123718110007).

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