Simulation Analysis And Optimization Research On Geotechnical Robustness Of Foundation Pit Engineering

With the rapid development of urbanization in China,the characteristics like“big,deep,tight,near”of foundation pit engineering are becoming more and more prominent,so the requirements for safety,stability and economy of excavation and support process are becoming more and more stringent.At present,there are few studies on the influence of the supporting system performance from uncertainties factors in the design process of foundation pit engineering.Especially,the treatment of uncertainties in geotechnical engineering parameters is relatively vague,which mostly relies on safety factor method,reliability method and fuzzy method.These methods can not guarantee the performance of support system on the basis of increasing engineering cost.This paper applied robustness theory to deal with the uncertainty of geotechnical parameters in foundation pit engineering,in order to minimize the sensitivity of support system to geotechnical parameters disturbance.The research was carried out from the aspects of geotechnical robustness theory,foundation pit simulation and double objective optimization design.At present,there are a few of studies on the geotechnical robustness by scholars at home and abroad.However,they paid more attention to the application and calculation of robustness method in geotechnical engineering,and paid less attention to systematic research of geotechnical robustness theory.In this paper,the definition of geotechnical robustness was defined,and decomposed by“two subjects and two properties”.Then the concepts of noise factor,design parameters and system response were elaborated.In order to expound the application value of robustness theory more comprehensively,the theories of robustness,security,stability and reliability were comparatively analyzed in this paper.The different angles,emphases and research degree describing system performance of the above theories were analyzed,then the progressive development relationship from security,stability,reliability to robustness theory was revealed by taking"system disturbance"as the main line.The correlation theory of multi-objective optimization design was introduced,and the concept of"Knee Point"was put forward.Based on the definition of knee point,a new method of Sacrifice Gain Ratio Method?SGR Method?to determine knee point was proposed,which can deal well with a series of problems such as similarity point,head and tail point and local optimum.In order to comprehensively introduce the advancement of SGR method in searching knee point,this method was compared with the existing method of reflex angle method,normal boundary intersection method and origin pedal method.A robust optimization design model for supporting structure of foundation pit engineering was constructed.The surrounding rock weight,internal friction angle and cohesion of geotechnical parameters were selected as noise factors,and the supporting forms of bored piles,inner supports and columns were selected as design parameters.Finally,the horizontal displacement of retaining structure,settlement displacement of soil outside foundation pit and uplift displacement at the bottom of foundation pit were selected as system response to reflect the supporting effect.The supporting form and deformation mechanism of foundation pit engineering were introduced and the open-cut foundation pit of Qingxiushan Station of Nanning Metro was simulated by FLAC^3D software.Based on the analysis of deformation law of each part of the foundation pit in the process of excavation and support,three representative locations were selected as system response index to reflect the effect of supporting system.A nested robust optimization design process with internal and external cycles was designed,and the cost of 96 feasible designs in the design domain was calculated simplistically.Then the robustness-cost bi-objective optimization design was carried out by non-dominated algorithm.Finally,the only optimal solution was searched on Pareto Front by SGR method,and the results were verified by reflex angle method,normal boundary intersection method and origin pedal method.