1 project overview
A subway tunnel is located in Nanjing, Jiangsu Province. The tunnel area is located in Fuguishan, which extends westward from the middle branch of Zijin Mountain. The terrain is not undulating. The vegetation in the tunnel area is dense, the tunnel is 2200m long, the longitudinal slope in the tunnel is 12‰, the maximum buried depth of the tunnel is about 20m, and the peak acceleration of ground motion is 0. 10g. The overburden in the tunnel area is silty clay, gravelly sand and gravelly soil of Quaternary brand-new alluvium, and the underlying tertiary Pliocene basalt, Cretaceous lower siltstone and sandstone. The main rivers in Nanjing are the Yangtze River and Qinhuai River, which have water all the year round. The groundwater level in the tunnel area can be divided into weathered bedrock fissure water and structural bedrock fissure water according to its occurrence conditions.
2 the significance of tunnel construction risk assessment
According to the general situation of this tunnel project, there are problems such as water gushing, landslide and water inrush in the construction process, which have caused great harm to the construction safety. Through risk assessment, we can identify potential risk factors that may occur in the construction stage, determine the risk level, and put forward risk treatment measures for each risk factor, so as to reduce all kinds of risks to an acceptable level and ensure construction safety.
3 Risk assessment
Based on the design drawings and geological data, this paper comprehensively uses the methods of risk analytic hierarchy process, matrix method, fuzzy comprehensive evaluation method and brainstorming method, and analyzes and estimates the probability of basic risk events, loss value and the probability of each consequence according to the experience of previous and similar projects.
According to the Interim Provisions on Risk Assessment and Management of Railway Tunnels, Measures for the Administration of Safety Production in Railway Construction and other relevant requirements, combined with the actual situation of this subway tunnel project, the following steps are adopted for risk assessment: first, collect information, identify the initial risks in the construction stage, and form a risk list; Secondly, the initial risk is evaluated according to the risk list, and the probability and loss value of each risk factor to the construction safety risk are determined respectively; Third, take corresponding measures according to the evaluation results. In the process of risk assessment, we should pay attention to collecting relevant experience of similar projects and apply them to actual projects according to local conditions.
There are many influencing factors, uncertainties and risks in the tunnel construction stage. In order to ensure the smooth progress of the project, risk assessment must be carried out. Through the statistics of risk factors in each process of the tunnel project, it is preliminarily identified and evaluated that the main safety risk factors are mud gushing, water inrush, landslide and large deformation. In the process of risk assessment, quantitative and semi-quantitative methods should be adopted, combined with existing statistical data and current codes and standards, and based on field investigation and design data analysis, the possible probability and possible consequences of risk events caused by various risk factors should be determined. After the risk assessment of this tunnel, the following conclusions are drawn: the total loss value of each basic risk of this tunnel project is estimated to be 6.5438+million yuan, and the risk assessment loss value of this project is 6 million yuan considering the probability of each basic risk. Through analysis, calculation and sequencing, it can be found that the mud gushing and water inrush in this tunnel is a grade A risk, which is the focus of risk control. Landslide and deformation belong to Grade B risk, which is the secondary focus of risk control.
What needs to be emphasized here is that risk assessment should not be limited to one project, but should learn from the experience of other similar projects. And should establish the corresponding organization, responsibility to people, form a complete risk assessment and risk control team, * * * The same goal is to avoid the occurrence of risks or reduce risks to an acceptable range, so as to reduce the probability of accidents, improve resource utilization, reduce the overall cost of the project and improve the overall economic benefits.
4 Control measures taken
The purpose of risk assessment is to find the risk factors leading to safety accidents, so as to provide the necessary basis for taking corresponding control measures. According to the above risk assessment, the losses caused by risk factors in tunnel construction are considerable, and necessary control measures must be taken to reduce the probability of tunnel safety accidents before the accidents occur, and measures should be taken to solve them as soon as possible after tunnel safety construction, so as to reduce construction costs and maximize economic benefits.
Risk control measures in tunnel construction mainly include risk avoidance, loss control, risk dispersion and risk transfer. The selection of control measures should be determined by comprehensive comparison according to the engineering practice. According to the characteristics of high risk probability and serious economic loss in this tunnel project, loss control measures have been taken in this project.
4. 1 Risk control measures for mud gushing and water inrush in tunnel
According to the actual situation of the tunnel, the following control measures are taken: before and during the construction, according to the construction drawings and geological data, strengthen the advanced geological prediction of key parts of the tunnel, and comprehensively use advanced technologies such as horizontal drilling and geological radar to make timely and accurate geological prediction so as to take necessary measures in time; The tunnel is reinforced in advance, that is, the method of strengthening first and then excavating is adopted. Although this method is adopted, the deformation measurement and monitoring of surrounding rock should be strengthened in the construction process to minimize the risk. If the dangerous area of mud gushing and water inrush is determined according to the geological forecast, the method of pre-grouting in advance can be adopted, and the tunnel construction can be carried out after the plugging is qualified. This method can effectively reduce the probability of mud gushing and water inrush risk.
Using the above three risk control measures, the probability of risk control is expected to reach more than 92%, and the cost of risk control scheme is expected to reach about 1 10,000 yuan.
4.2 Landslide and deformation risk control measures
At present, blasting is still widely used in tunnel excavation, which is easy to cause tunnel collapse and deformation due to improper operation and other factors. According to the actual situation of this tunnel, the following control measures are taken: when blasting, the holes are arranged according to the geological conditions around the tunnel and the thickness of the covering soil, and the charge is carried out in strict accordance with the calculated charge. Clean up the dangerous rocks in time after blasting and take corresponding reinforcement measures. In addition, the necessary detection of tunnel collapse and deformation is also an effective means to reduce the risk probability of collapse and deformation.
Because the probability that the tunnel collapse deformation risk is caused by human factors accounts for a large proportion, after taking necessary measures, the risk control probability can be guaranteed to be above 96%, and the estimated cost is about 300,000.
5 concluding remarks
Tunnel construction is characterized by many unstable factors, high risks and great losses in case of accidents. Therefore, we should conduct a comprehensive risk assessment in the construction process and indicate the preventive measures taken for this project in the construction organization design, so as to realize the quantitative assessment of tunnel construction risks and form a systematic and scientific management goal, so as to reduce losses and improve overall economic benefits. As technicians, we should constantly sum up experience in the construction process, attach importance to the construction experience of similar projects, actively learn new technologies and new processes in the project, and make greater contributions to the safe construction of tunnel projects.
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