Research On Supporting Behavior Of Anchor Pile In Deep Foundation Pit Based On In-situ Monitoring Test And Dynamic Assignment Method

With the rapid development of economy,science,technology and culture,the urban land area is growing rapidly.There is a new trend in foundation pit engineering,such as intensive,deep,dangerous foundation pit,complex environment.Pile anchor combined support system is suitable for the first-class foundation pit engineering,which will gradually replace the soil nailing wall,composite soil nailing wall and other secondary support structures.At present,the design method of anchor pile support mainly adopts idealization,simplification,planarization and soil simplification,which is not suitable for complex deep foundation pit engineering.At present,the gap between the monitoring results and the design prediction effect is increasingly obvious.It has become an era task to study the supporting characteristics of anchor piles in deep foundation pit,provide theoretical basis for new design methods,and help the rapid construction of large-scale new foundation pit engineering.Therefore,this paper summarizes and analyzes the existing support design and monitoring methods,carries out in-situ monitoring test,summarizes the dynamic response of pile anchor support structure of complex deep foundation pit on the basis of monitoring data,as well as the differences and reasons between elastic fulcrum method,simple numerical simulation method and actual monitoring value,and puts forward a new numerical simulation strategy.The main contents and conclusions are as follows(1)In this paper,the current mainstream design method of pile anchor support system is briefly introduced,and its characteristics are summarized and analyzed.It is found that the current commonly used design technology adopts the idealized assumption of pile-soil interaction,and adopts uniform and constant soil parameters,which is not suitable for multicomponent and deep foundation pit.It is pointed out that the current monitoring technology has few monitoring indexes and can not provide sufficient research data for the development of support design technology.(2)Relying on the Xi’an Hi Tech International Conference Center project,the in-situ monitoring tests of soil pressure and pile axial force were carried out,and the monitoring data of pile top deformation and surface settlement were collected.The stress and deformation state of pile body during excavation were studied.The results show that with the increase of excavation depth,the variation range of soil pressure,reinforcement axial force,horizontal and vertical deformation of pile top increases.(3)This paper analyzes the difference between the actual support design scheme(elastic fulcrum method)and the actual monitoring results: the pile earth pressure is the ultimate earth pressure,which is quite different from the monitoring results;when calculating the deformation,the anchor cable has a significant "leverage effect",and when the excavation is deeper,the pile top will deform outward,and the deeper the excavation is,the greater the difference between the actual monitoring data and the pile earth pressure.(4)The general numerical simulation is carried out by FLAC3 D,and the development process of pile deformation is analyzed.The results show that the excavation of foundation pit is a process in which the earth pressure behind the pile is gradually transferred from the action and the soil in the pit to the pile body,and its morphological change reflects the dynamic balance process of the pile body,the soil behind the pile and the anchor cable.Analysis of the simulation results shows that the simulation results of shallow soil pressure are similar to the monitoring data,but with the deepening of foundation pit excavation,the distortion gradually becomes smaller-the soil pressure at the bottom of the pile is smaller,the total deformation of the pile is larger,and the deformation increment caused by the last excavation is smaller.(5)The reason of distortion is summarized,that is,a simple constitutive model is selected and a constant deformation parameter is used when the stress level changes greatly.According to the test data provided by general engineering,the dynamic assignment method is proposed.It is found that this method can effectively reduce the simulation distortion of deep excavation.It is considered that the rapid loading caused by deep excavation leads to large deformation.