Research On Support Scheme Optimization And Risk Control Of Deformation Of Metro Foundation Pit Nearby Buildings

The main risk is uneven settlement and demolition of adjacent buildings when carrying on the construction of metro foundation pit nearby buildings.So,in order to effectively control the deformation of foundation pit and make sure the safety of adjacent buildings,the article makes a research on the risk of deformation of metro foundation pit nearby buildings through many kinds of theoretical analysis,numerical simulation and field monitoring.The main issues are as follows:Using the method of theoretical analysis and numerical simulation study,the paper made a study on the influence of construction of metro foundation pit on adjacent existing buildings.On this basis,from the perspective of bearing capacity of the building,a method of determining the controlling deformation of foundation pit which dominated by the bearing capacity of the building was proposed.The essence of the method is that the building is divided into two categories: large stiffness and small stiffness building based on the type of the building and the ratio of length and height of the building.According to the relationship between general building damage rating standard and the differential settlement,the maximum differential settlement control value can be worked out when the appearance damage of the building happened.Also because the maximum differential settlement control value is the function of maximum horizontal displacement of foundation pit.So the maximum horizontal displacement or the control standard of foundation pit can be obtained by solving equation.In order to optimize the foundation pit support program,an optimization method based on reliability analysis was put forward.That is,on the basis of considering the effects of stochastic factors,fuzzy factors,and uncertain factors,the function of deformation failure of retaining structure was refactored by response surface method(RSM).After that,the failure probability of support scheme was calculated using D-S theory.If the failure probability is satistied with the risk acceptance criteria,then the scheme is considered to be feasible.Otherwise,the scheme needs to be optimized through calculating the reliability sensitivity of the solution parameters.In order to overcome the bottleneck of a costly calculation as using D-S theory to compute the failure probability,a method of constructing a secondary zone was brought up to reduce the number of actual participation focal element in reliability calculation.As a result,the efficiency of reliability calculation was effectively improved.A risk assessment and early warning system of metro deep foundation pit,including three levels: monitoring index,monitoring section and overall pit,was constructed.As the characteristic of risk monitoring is generally divided into sections in metro foundation pit,the risk assessment and forewarning is divided into three levels: monitoring index,monitoring section and overall pit.Of which,the indicators risk assessment and forewarning can be achieved by comparing the monitoring value with the value of risk assessment criteria.Taking advantage of the characteristic that the projection pursuit method can change high-dimensional data into low-dimensional space,thus realize investigation on the characteristics of high-dimensional data within low-dimensional space.Multi-dimensional indicators of risk monitoring data were directly fused into one-dimensional risk value of cross-section,thereby effective avoiding the influence of contrived factors in traditional evaluation methods.On the basis of section risk assessment,the monitoring section could be viewed as an organic component of the overall foundation pit,through spreading the risk information of multiple cross sections to the overall pit,a comprehensive evaluation of the whole excavation was worked out based on the information diffusion theory.A method combing prospect theory and evidence theory was put forward to optimize the risk control scheme for deep excavation timely and reasonably when the construction of deep excavation is in a state of crisis.In this method,An index system composed of a target layer,factors layer and index layer was established by applying the concept of analytic hierarchy process.the owners’ desired goals of the project were chosen as the reference points,and the prospect decision-making matrix was constructed.With the prospect value of each scheme as evidence,the belief and plausibility interval of each scheme was received by evidence combination.According to that,the best scheme can be selected.The main findings of the study were applied to a deep excavation project in Changsha Metro Lines 3 and validated by the field observation.The controlling deformation of foundation pit which dominated by the bearing capacity of the building was received through theoretical calculation and numerical simulation.On this basis,the paper took account of the effect of 9 uncertain factors including the density of strong weathered politic siltstone layers,the diameter of bored pile,the thickness of the first reinforced concrete brace,and then made a reliability analysis and optimization of the foundation pit support program.The construction was organized according to the optimized programme,meanwhile,the dynamic evaluation and stratified pre-warning of the construction risk was carried out.When the risk assessment value exceeded the warning criteria,namely,the foundation pit was in a state of crisis,the decision of risk control scheme was carried out using the proposed method.The case study showed that the content of this paper ran through the whole construction process of metro foundation pit.A theoretical basis was provided for controlling deformation risk in construction of metro foundation pit nearby building.The practical value of this study was proved.