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

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