By combining the basic theory of risk management with the practice of shield tunnel construction, this paper expounds the application of risk management in shield tunnel construction, probes into the key points of shield tunnel construction supervision, and provides strong technical support for the design and construction of subway tunnel.
Preface of 0
The implementation goal of Shanghai rail transit is that by 20 10, the scale of rail transit network will reach more than 400 kilometers, and the basic rail transit network in the central city will be built. In 20 10, the construction of urban sub-centers, banks of Huangpu River and Shanghai World Expo area will be strengthened. Subway engineering has the characteristics of complexity, uncertainty, high risk and great disaster loss. In recent years, the diameter and depth of subway tunnel excavation have been increasing, and the cross-section shapes are diverse, so the risks of subway engineering in the construction process are increasing day by day. After the accident of Shanghai Metro Line 4, risk management has been put on a new agenda by academic and engineering circles. Risk assessment can make decision-making more scientific, reduce the accident rate and provide a basis for determining the insurance tax rate. Einstein [1, 2], the representative of tunnel engineering risk analysis, pointed out the characteristics of tunnel risk analysis and the ideas that should be followed. Salazar of Cambridge University (1983) linked the influence of uncertainty with the project cost in his doctoral thesis "Practical Research on Uncertainty and Economic Evaluation in Tunnel Design and Construction". Reilly(2000) proposed that the construction process of tunnel engineering is a comprehensive risk management and risk sharing management process, and the International Tunnel Association [3] compiled the Guide to Risk Management of Tunnel Engineering, which provided a set of reference standards and methods for risk management of tunnel engineering. In China, Professor Ding (1992) of Tongji University has made some research on the risks and insurance modes in subway construction, such as the first phase of Guangzhou Metro and the first line of Shanghai Metro. Based on the reliability theory, Dr. Fan Yiqun of Shanghai Tunnel Design and Research Institute (2000) put forward the concept of anti-risk design of underground structures, calculated the probability of risks in underground structures such as foundation pits and tunnels, qualitatively evaluated the losses caused by risks, and proposed an improved analytic hierarchy process. Professor Huang Hongwei of Tongji University [4] studied the risk assessment project of Chongming River Crossing Channel, including pre-selection line, construction risk management, environmental protection, operational accident control and financial analysis. Risk loss includes durability loss, construction period loss, direct cost loss and environmental impact loss. The risk mechanism of shield tunnel construction is shown in figure 1.
1 risk management
The risk management of subway tunnel is based on the analysis of the risk factors of tunnel construction, and makes a reasonable evaluation of the existing risks, providing a basis for risk decision-making, and finally achieving the purpose of avoiding, reducing or transferring risks.
1. 1 risk analysis
Risk analysis includes risk identification and risk assessment. Risk identification is to find out the possible risk location and the factors that cause the risk, which is the basis of risk management. The methods of risk identification can be divided into expert investigation and tabular classification analysis, and the two methods can be combined. Expert analysis method is to obtain some empirical data in line with the actual situation through a questionnaire survey of a large number of experts involved in engineering construction. Risk assessment is a quantitative measure of the probability and consequences of danger. The main methods used in risk assessment are probability distribution method, probability tree, extrapolation method and Monte Carlo method.
The main lines of risk analysis of subway shield tunnel construction are shaft construction, shield assembly, shield tunneling, shield propulsion, segment assembly, synchronous grouting and secondary grouting shield propulsion, shield tunneling, joint plugging, hand hole plugging, waterproof and plugging, quality inspection and evaluation, steel formwork and segment production, construction monitoring, foundation reinforcement, shield machine detection and maintenance, etc. Combined with the shield selection, geological conditions and hydrological conditions of tunnel construction, the construction risks of shield tunnel are shown in table 1.
1.2 risk decision
Risk decision is how to deal with existing risks. The risk assessment of the project does not mean that the smaller the risk, the better. The smaller the risk, the greater the investment to reduce the risk. Therefore, on the basis of risk analysis, according to the overall goal of the project, we should apply the alarp principle (to make all risks of education cover a reasonable loan amount). ) Minimize the potential loss of project risks and improve the ability to control project risks. Risk management methods include risk avoidance, risk mitigation, risk transfer and risk retention.
2 Quality supervision and control
2. 1 supervision and acceptance node
Compared with the building foundation pit, the deep foundation pit of underground station has the characteristics of complex surrounding environment, dynamic, boundary requirements, all-line penetration, anti-disturbance and permanent anti-floating, so the station quality supervision and acceptance stage includes the acceptance of foundation pit excavation conditions, the completion of the construction of each end well, the acceptance of foundation pit enclosure before the shield machine is lowered, the acceptance of main structure decoration, and the acceptance of air shaft and entrance structure decoration. The acceptance of shield tunnel construction includes the acceptance of shield exit conditions, the completion of 100 ring group, the acceptance of segment repair and plugging conditions, and the acceptance of bypass structure construction and excavation conditions.
2.2 subway shield tunnel engineering quality supervision focus
(1) the overall assembly and debugging of the shield machine are qualified, and the formal acceptance report can only be issued after the on-site trial driving is qualified at a distance of 50mm~ 100m.
(2) Tunnel construction survey is mainly to determine the orientation and elevation of shield tunneling, correctly calibrate the tunnel axis, make the tunnel extend and run through along the design axis, make the three-dimensional position of tunnel lining meet the design requirements, and also make other buildings related to the project accurately built in its design position without invading the prescribed boundaries.
(3) When the shield goes in and out of the tunnel, the necessary foundation reinforcement should be carried out for the stratum in a certain range outside the hole of the working well, and sealing measures should be taken for the gaps between the holes to ensure the safety of shield construction.
(4) In the process of shield propulsion, the deviation between the shield axis and the design axis should be controlled within the allowable range.
(5) The speed of shield tunneling should be coordinated with the settlement value, excavation amount, positive earth pressure balance adjustment value and synchronous grouting controlled by the surface.
(6) If the front part of the shield collapses or encounters obstacles, the rotation angle of the shield is too large, the position of the shield is too large, and the thrust of the shield increases more than expected, the excavation should be stopped, the reasons should be analyzed, and necessary measures should be taken.
(7) During tunnel construction in soft soil stratum, the changes of soil stress, groundwater pressure and harmful gas content must be monitored to prevent them from flooding into the tunnel; When there are buildings or structures that need to be protected in the tunnel, it is necessary to monitor the ground deformation, soil deformation and structural deformation.
(8) Simultaneously collect construction parameters such as earth pressure, thrust, advancing speed, excavation amount, grouting amount and shield posture.
3 Conclusion
Combined with the basic knowledge principle of risk management and the practice of shield tunnel construction, the risk generation mechanism, risk identification, risk assessment and decision-making are studied. According to the construction process, the risk is identified by stage analysis method, and the key points of quality supervision and control are discussed. Risk management can provide a reliable scientific basis for the transformation of government quality supervision behavior and supervision mode, so as to better control the risk sources and ensure the quality of subway construction in a controllable state.
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