Structural Stability Study For Large-Scale Shield Shaft

Deep excavation engineering is one of the hottest and most difficult problems in rock engineering at present. It is largely dependent on the determination of the support parameter to guarantee the stability of the foundation structure. If the values of support parameters are too large, although the stability of the structure can be guaranteed, it will waste much money; otherwise, the foundation structure will be at risk. It is always a task for people to investigate how to effectively control the deformation of the foundation, and at the same time make the engineering both secure and economical.Relative domestic and overseas research achievements about excavation engineering are summarized systemically, and the related soil pressure theory and retaining structure design theory of deep excavation support system are concluded. This article deals with the excavation of Jiang Bei shield shaft of Weisan Road over-river tunnel at Nanjing City. Using beam on elastic foundation method and continuous medium finite element method to simulate the shaft excavation respectively, the distribution and magnitude of the structure stress and foundation deformation are obtained, and then the reinforcement ratio for each support composition is calculated on account of the most unfavorable stress state. After that, use four-factor at three-level orthogonal array experimental design method to proceed multi-factor system analysis, which proposes the minimum material cost design scheme on the basis of the structure safety. Main works and conclusions are as follows:(1) The simulation research of the joint of the retaining wall. Considering the existence form of the joint of retaining wall in the foundation support system, according to the mechanical property, the simulation method and the according theory is studied. The result shows that:the numerical results which consider the simulation of the joint of the retaining wall can well reflect the real situation of the structure stress and the deformation.(2) The stress analysis of the shield shaft support structure. Use beam on elastic foundation method and continuous medium finite element method to simulate the stress state and the strain state of the shield shaft support structure respectively. The results demonstrate that:the maximum value of the horizontal displacement of the retaining wall occurs just above the bottom of the foundation, with the maximum positive moment at the same place; the ring beams are poured together to the retaining wall, and the second ring beam at the most important place has the maximum moment; the retaining wall, ring beams and lining wall are compounded together to form the permanent support structure, which decreases the stress o: the lining wall; and because of the rationality of the support structure, the excavatior influence of the around soil can be controlled in the reasonable scope.(3) Parameter optimization design research of the shield shaft support structure. Adopt orthogonal array experimental design method to proceed multi-factor system analysis studying the effect rule of deformation and stress to the whole support structure caused by the change of the thickness of ring beams, retaining wall, lining wall and bracings, and the material cost of the engineering is quantitatively analyzed. The result shows that:①The increase of the thickness of the retaining wall can improve the stress condition of the ring beams, bracings and lining wall, decrease the reinforcement ratio of each support structure and the horizontal displacement of the retaining wall.②The increase of the ring beam; key-place can improve the stress state of other ring beams and the retaining wall, making the structure more safe.③The increase of the thickness of lining wall has little influence to the horizontal displacement of the structure, but can improve the stress condition of the retaining wall and ring beams during the process of the breaking wall by shield.④The increase of the bracing section can decrease the horizontal displacement of the shield shaft and improve the stress condition of the retaining wall sharply.⑤At some degree, the thickness increase o: the retaining wall and the decrease of the lining wall can reduce the support cost obviously.Summarizing the research result of this article, we can see that: using orthogona experimental design method to decide the reasonable support parameters of the shield shaf can minimize the calculation. Comparing to the traditional analogies method of experience in the foundation design, the optimal answer given by this article is more reasonable, which meets the requirement of the structure safety and minimum material cost, breaking through the traditional equivocal shortcomings of structure design specifications. As a result, thi: research method is suitable to popularize.