Abstract: There are various uncertainties in the foundation pit engineering, so the foundation pit engineering has a high risk. Through risk identification and measurement, the risk evaluation in foundation pit engineering construction is realized, and verified by engineering examples. Reasonable conclusions are drawn, which has certain practical significance for risk management of foundation pit engineering.
Key words: foundation pit engineering; risk identification; risk measurement
1 Overview
With the development of China's economy, more and more construction projects have been built, and the number of foundation pit projects has increased dramatically. Because of the long excavation period, difficult construction, cumbersome construction procedures, complex geological conditions and surrounding environment, and many unknown factors in the excavation process, the risk of foundation pit construction is much higher than that of general projects. Therefore, in order to ensure the normal construction of foundation pit, it is necessary to carry out risk assessment of foundation pit construction. Foundation pit engineering refers to an underground space excavated downward by open-cut method and its supporting system. Its construction process is as follows: foundation groove acceptance pile foundation construction pile foundation acceptance foundation cushion foundation waterproofing protection layer foundation laying out foundation raft reinforcement construction foundation ground beam, post-pouring belt formwork construction wall, column insertion reinforcement inspection acceptance foundation raft plate and ground beam concrete construction 1]. The construction risks of foundation pit engineering are mainly manifested in the following aspects: (1) the selection of physical and mechanical parameters of soil in support structure; (2) the limitation of soil sampling; (3) the inadequate consideration of spatial effect in foundation pit excavation; (4) the design calculation of support structure is not in conformity with the actual stress.
2. Identification of Construction Risk in Foundation Pit Engineering
2.1 Cause Analysis of Foundation Pit Engineering Risk
The common causes of engineering accidents are as follows: (1) the factors of foundation pit engineering itself: the regional nature, concealment and complexity of foundation pit engineering; (2) geological exploration and design factors: the premise and foundation for safe construction of foundation pit engineering; (3) construction factors: rush construction period; overexcavation; overload; construction of steel girder and diagonal support connection point. Insecurity; Water-stop curtain leakage; (4) Other factors: lack of scientific technical plan, punishment and case education, etc.
2.2 Risk Identification Process of Foundation Pit Construction
(1) Establish a special risk assessment team to clarify the division of labor, the object and scope of assessment, and collect data; (2) conduct overall measurement and analysis in the whole foundation pit project; (3) conduct special risk measurement and analysis for projects whose overall risk assessment level is above grade III; (4) conduct risk measurement and analysis for special construction to identify the types of accidents in the construction process; (5) conduct specific risk measurement and analysis for projects whose overall risk assessment level is above grade III; (4) conduct risk measurement and analysis for special construction to identify the types of accidents in ) Determine risk control measures.
3. Measurement of Construction Risk of Foundation Pit Engineering
3.1 Risk Measurement Procedure for Foundation Pit Construction
(1) Pre-preparation; (2) carrying out overall risk assessment; (3) determining the scope of special risk assessment; (4) choosing assessment methods; (5) carrying out special risk assessment; (6) putting forward assessment countermeasures and suggestions; (7) drawing assessment conclusions; (8) preparing assessment reports.
3.2 Method of Risk Measurement for Foundation Pit Construction
There are many methods to measure the risk of foundation pit construction, such as BP neural network method, expert scoring method, AHP analytic hierarchy process, etc. In this paper, expert scoring method and AHP analytic hierarchy process are used to measure the risk. Both methods are simple, practical and require less data information.
3.3 Establishment of Evaluation Index System for Foundation Pit Construction
The evaluation index system of foundation pit construction is the basis and standard for comprehensive evaluation of foundation pit construction safety. It should be set up in accordance with the principles of systematic, comprehensive, concise, scientific, stable and comparable, flexible and operable. According to the actual situation of foundation pit construction, this paper establishes a set of evaluation index system with complete structure by using the theory of system engineering. The main factors are engineering geological conditions, climate conditions, surrounding site conditions of foundation pit and excavation depth of foundation pit. There are many sub-factors in these four aspects. Through the analysis of the above index system assignment and expert scoring, the risk of foundation pit engineering is obtained. Grade I criteria are as follows: 18 points or more are the first-level extremely high risk, 13-17 points are the second-level high risk, 9-12 points are the third-level moderate risk, and less than 8 points are the low-level risk.
Case Analysis of Construction Risk Assessment of Foundation Pit Project
The foundation pit of No. 3 cap of XX Bridge is located in the upper reaches of Yangtze River and Qingjiang River, the first tributary of the bridge. It has typical characteristics of mountain rivers, winding rivers and narrow and deep riverbed. 4.1XX Bridge 3 # Cap Foundation Pit Risk Analysis XX Bridge 3 # Cap Foundation Pit Risk mainly has the following two aspects: (1) Foundation Pit Landslide, Collapse. The main reasons are as follows: abundant groundwater, not timely drainage in rainy season or not using deep foundation pit drainage; not using foundation pit support measures or improper foundation pit support; not avoiding dangerous areas in excavation, pile-up materials around the foundation pit make the wall bear too much load. (2) improper construction organization. Mainly include: the periphery of the foundation pit does not set up guardrails in accordance with the requirements of safety production; there is no safety ramp in the foundation pit; the constructors do not wear safety hats as required; the constructors do not walk and stay around the foundation pit at will as required; the site layout is chaotic, and the tools are piled up in disorder. 4.2 The evaluation index system of the construction accident possibility of No. 3 Foundation Pit of Qingjiang Bridge has been proved by experts. The evaluation index system X1-X6 of the construction accident possibility of No. 3 cap foundation pit of XX Bridge is: 2, 1, 5, 2, 1, 1, 1 in turn, and the total evaluation score is 12. According to the classification of accident risk grade in Table 4-2, the risk grade of foundation pit of No. 3 cap of XX Bridge is grade 2, which belongs to high risk. Therefore, in the construction process, it is necessary to strengthen the prevention and prevent the occurrence of safety accidents.
5 Conclusion
Safety risk of foundation pit construction is an important aspect of construction safety risk. This paper has some practical significance for safety risk assessment, but there are still many shortcomings: (1) The evaluation index system is not comprehensive enough, and the establishment of a comprehensive and scientific index system is the content of follow-up research. (2) This paper does not validate the validity of the evaluation index system and the reliability of the evaluation results. In the face of the accidents that have happened, we can analyze and investigate them, but when some unknown risks occur, what we should do is to guard against them. (3) The method of risk identification of foundation pit engineering introduced in this paper is relatively simple. In the process of risk measurement of foundation pit engineering construction, it is necessary to rationally use a variety of methods to evaluate. The results obtained by two or more methods should be considered comprehensively to ensure that the risks brought to people in the process of foundation pit engineering construction are minimized.
Reference:
[1] Liu Qi. Research on early warning system of deep foundation pit construction based on risk management [D]. Southeast University, 2016.
[2] Luo Feng. Research on risk management of deep foundation pit engineering [D]. Chengdu University of Technology, 2018.