Study On Optimization Decision Of Supporting Schemes For Deep Foundation Pit Of Metro

With the development of urban rail transit in China, the number of subway foundation pit gets more and more, and foundation pit of subway is becoming deeper, larger, closer, more and more adjacent and difficult. Deep foundation pit is a comprehensive system engineering. The choice of its supporting system plays an important role in the whole foundation pit engineering and has a direct influence on its process, construction cost and quality. The optimization for supporting system of deep foundation pit is a multivariate, multi-level, multi-objective and complex system decision problem, including quantitative index and qualitative index, which is involved in many aspects. So based on characteristics of concrete foundation pit engineering, to establish the scientific, comprehensive optimization evaluation index system of supporting schemes for foundation pit, to determine a rational optimization decision method of foundation pit supporting schemes and to get the optimal supporting scheme of the foundation pit from the multitudinous and flexible supporting schemes and the best supporting scheme seems especially important, which have theoretical study and practical application value. Taking the deep foundation pit engineering of metro for metro line 2 of urban rail transit in Wuhan for research background, the main research work and results of the paper are listed as follows:1.Base on consult and collect the relative literature data, the present situation and the existent problems of the optimization method for foundation pit supporting schemes are analyzed and summarized in detail, thus, the direction and content of the paper are advanced.2.Research on comprehensive optimization evaluation index system of supporting schemes for foundation pit. The main factors and evaluating indices were decided based on the deep researches on factor to determine supporting schemes for foundation pit, and a comprehensive evaluation index system for the optimization of foundation pit supporting scheme is established from mainly considering technology, economy, environment and construction period.3.Study on optimization decision model of supporting schemes for deep foundation pit. According to the characteristics of the decision factor of supporting schemes for foundation pit, such as fuzziness, uncertainty and their gray correlation, a grey fuzzy variable decision-making model is established based on gray theory and fuzzy mathematics theory. The model not only considers the influence of single factor on optimization decision of supporting schemes for foundation pit, but also the influence of grey correlation among influencing factors.It is characterized by accurate mathematic deduction, clear concept, easy calculation, clear mathematical and physical significance. In order to more rationally solve the critical problems which affect the accurate degree of the result of the model, according to AHP method(or improved AHP), which belongs to subjective weighting method, and information entropy method, which belongs to objective weighting method, a combination weighting approach for the weight of evaluation indexes is put forward based on the maximum closeness degree, this method increases rationality of weight determination; and qualitative indexes is quantified by set-valued statistics methods, which reduces the quantization distortion and evaluation random error, so the quantization of qualitative indexes is more conform to reality and reasonable.In addition, grey fuzzy variable decision-making model is established in this paper is different form previous evaluation methods, it can be changed multiple models to carry out optimization decision by transformation parameters (optimization criterion a and distance parameter p), so this method can heighten the rationality and reliability of decision-making.4. Preliminary study on optimization decision system of supporting schemes for foundation pit. Optimization decision system for foundation pit supporting scheme is established by using advanced computer programming language C#, which can be effectively applied to practical engineering. It has advantages of convenient and visual, friendly interface, easy learning and operation and so on. In addition, Combined with practical work the application methods and steps of the system is showed in detail.5.Analysis of Engineering Example. The paper takes the deep foundation pit engineering of metro for metro line 2 of urban rail transit in Wuhan for research background. Firstly, engineering geology and hydrogeology characteristics along the first-phase project of Wuhan No.2 subway line is briefly summarized. Then, the application scope and the advantages and disadvantages of the common forms of retaining structure for foundation pit in Wuhan are systematic summarized. Finally, the best deep foundation pit supporting scheme of Jiyuqiao and Guanggu square railway station of No.2 subway in Wuhan city is obtained by using the established optimization decision system.Finally,the paper puts forward the prospect of further combining with the content and conclusion of the paper.