Risk management and supervision of subway tunnels?

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Using the method of combining the basic theory of risk management with the construction practice of shield tunnel, the application of risk management in shield tunnel construction is expounded, and the key points of construction supervision of shield tunnel are carried out. Discuss and provide strong technical support for the design and construction of subway tunnels.

0 Foreword

The implementation goal of Shanghai's rail transit is to expand the rail transit network scale to more than 400 kilometers by 2010, build a basic rail transit network in the central city, and strengthen the urban sub-center, Huangpu The construction of collection and distribution rail transit on both sides of the Yangtze River and the 2010 Shanghai World Expo area. Subway engineering is characterized by complexity, uncertainty, high risk and large disaster losses. In recent years, the excavation diameter and depth of subway tunnels have continued to increase, and the cross-sectional shapes are diverse. Therefore, the construction period of subway engineering Risks are increasing day by day. After the accident on Shanghai Rail Transit Line 4, risk management was put on the new agenda by the academic and engineering circles. Risk assessment can make decision-making more scientific and reduce the incidence of accidents. It can also provide a basis for determining insurance tax rates. Einstein [1, 2], a representative figure of risk analysis in tunnel engineering, pointed out the characteristics of risk analysis on tunnels and the concepts that should be followed. Salazar (1983) of the University of Cambridge linked the impact of uncertainty to project cost in his doctoral thesis "Research on Uncertainty in Tunnel Design and Construction and the Practicality of Economic Assessment". Reilly (2000) proposed that the construction process of tunnel projects is a comprehensive risk management and risk sharing management process. The International Tunneling Association [3] wrote Guidelines for Tunneling Risk Management to provide a set of reference standards and methods for tunnel project risk management. In China, Professor Ding Shizhao (1992) of Tongji University conducted certain research on the risks and insurance models in subway construction such as the first phase of the Guangzhou Subway and the Shanghai Subway Line 1 project. Dr. Fan Yiqun (2000) of Shanghai Tunnel Design and Research Institute proposed the concept of anti-risk design of underground structures based on reliability theory, calculated the probability of occurrence of risks in underground structures such as foundation pits and tunnels, and qualitatively evaluated the losses caused by risks, and An improved analytic hierarchy process is proposed. Professor Huang Hongwei [4] from Tongji University conducted research on the risk assessment project of the Chongming Cross-River Channel. The research content included early route selection, construction risk management, environmental protection, operational accident control, and financial analysis. Risk losses include durability losses, construction period losses, direct cost losses, environmental impact losses, etc. The risk mechanism of shield tunnel construction is shown in Figure 1.

1 Risk Management

The risk management of subway tunnels is to conduct a reasonable assessment of existing risks based on the analysis of tunnel construction risk factors, provide a basis for risk decision-making, and finally achieve the goal of avoiding , the purpose of reducing or transferring risks.

1.1 Risk analysis

Risk analysis includes two parts: risk identification and risk assessment. Risk identification is to first find out the location of possible risks and the factors that cause them, which is the basis of risk management. Risk identification methods can be divided into expert survey method and table classification analysis method, and the two methods can be used in combination. The expert analysis method is to conduct questionnaire surveys on a large number of experts involved in project construction, so as to obtain some empirical data that is consistent with the actual situation on site. Risk assessment is the quantitative measurement of the probability of a hazard occurring and its consequences. The main methods used in risk assessment include: probability distribution method, probability tree, extrapolation method, Monte Carlo method, etc.

The main lines of risk analysis for subway shield tunnel engineering construction are shaft construction, shield assembly, shield exit, shield advancement, segment assembly, simultaneous grouting and secondary grouting for shield advancement. , shield entry, caulking, hand hole sealing, waterproofing and leakage plugging, quality inspection and assessment, steel mold and segment production, as well as construction monitoring, foundation reinforcement, shield machine inspection and maintenance, etc., combined with tunnel engineering construction Detailed risk identification can be carried out based on shield selection, geological conditions, hydrological conditions, surrounding buildings and structures, underground pipelines, overlapping tunnels, etc. The risks of shield tunnel construction are as shown in Table 1.

1.2 Risk decision-making

Risk decision-making is how to deal with existing risks.

Risk assessment for a project does not mean that the smaller the risk, the better. The smaller the risk, the greater the investment in reducing the risk. Therefore, based on the risk analysis, the ALARP (toreduceallriskscoveredtoalevelalalowasreasonablypracticable.) principle should be used according to the overall goal of the project. Reduce the potential losses of project risks as much as possible 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 Nodes for Supervision and Acceptance

Compared with foundation pits for housing construction, the characteristics of deep foundation pits for underground stations include complex surrounding environment, Dynamics, boundary requirements, full-line penetration, anti-maze flow, and permanent anti-floating requirements. Therefore, the quality supervision and acceptance stage of the station includes acceptance of foundation pit excavation conditions, completion acceptance of end well construction on each side, and acceptance before the shield machine is lowered. , foundation pit enclosure is all completed and accepted, the main structure is inspected before decoration, the air shaft, and the entrance and exit structure are inspected and accepted before decoration. The acceptance of the shield for shield tunnel construction includes the acceptance of the shield tunnel exit conditions, the acceptance of the first 100 ring assembly completion, the acceptance of the segment repair and caulking conditions, and the acceptance of the excavation conditions for the construction of the side channel structure.

2.2 Key points of quality supervision of subway shield tunnel projects

(1) The complete assembly and debugging of the shield machine is qualified, and the formal acceptance report can only be issued after passing the on-site trial excavation for a distance of 50mm~100m. .

(2) Tunnel construction measurement is mainly to determine the orientation and elevation of shield excavation, correctly calibrate the tunnel axis, so that the tunnel extends and penetrates along the design axis and the three-dimensional position of the tunnel lining meets the design requirements. It should also ensure that Other buildings related to the project are constructed exactly at their designed locations and do not intrude into the prescribed boundaries.

(3) When the shield tunnel enters and exits the tunnel, necessary foundation reinforcement should be carried out within a certain range outside the working shaft door, and sealing measures should be taken to seal the gap around the hole to ensure the construction safety of the shield tunnel.

(4) During the process of shield advancement, the deviation value between the shield axis and the design axis should be controlled so that it is within the allowable range.

(5) The shield excavation speed should be coordinated with the surface-controlled uplift value, the amount of soil entering and exiting, the front earth pressure balance adjustment value, and synchronous grouting.

(6) During shield excavation, if the front of the shield collapses or encounters an obstacle, the rotation angle of the shield is too large, the position of the shield deviates too much, the thrust of the shield is increased than expected, etc. If the problem occurs, excavation should be stopped, the cause should be analyzed and necessary measures should be taken.

(7) When a shield boring machine performs tunnel construction in soft soil, it must monitor changes in soil stress, groundwater pressure, and harmful gas content to prevent them from influxing into the tunnel; when the tunnel hole has When it is important to protect buildings or structures, monitoring of surface deformation, soil deformation, structure deformation, etc. must be carried out.

(8) Construction parameters such as soil pressure, thrust, propulsion speed, soil excavation volume, grouting volume, and shield posture on the shield excavation surface should be collected simultaneously.

3 Conclusion

The article combines the basic knowledge principles of risk management with the practice of shield tunnel construction, and conducts research on the risk generation mechanism, risk identification, risk assessment and decision-making. Study, use the phased analysis method according to the construction process to identify risks, and explore the key points of quality supervision and control. Risk management can provide reliable scientific basis for the transformation of government quality supervision behaviors and supervision models, thereby better controlling risk sources and ensuring that the quality of subway construction is under control.

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