| With the acceleration of urbanization process,deep foundation pit engineering develops rapidly.For more complex construction environment and deepening foundation pit depth,safety and stability of deep foundation pit engineering has become a hot topic.However,the deep foundation pit is faced with such problems as insufficient research theories,complex and diverse influencing factors,and non-standard construction and design,etc.Due to the lack of whole-process displacement monitoring and timely alarm to failure,accidents of the deep foundation pit engineering cannot be timely controlled,resulting in serious injuries&deaths and economic losses.Therefore,it is very important to understand the deformation characteristics of supporting structure of foundation pit,to explore its influencing factors,and to study the hierarchical warning and alarm.The main research results are in this paper as follows:(1)collecting and studying a large number of literature and materials of deep foundation pit engineering to understand the relationship between deformation characteristics of supporting structure and internal force and failure mechanism.(2)through detailed investigation,studying the occurrence process and the key nodes of deep foundation pit engineering accidents,and searching for the signs before foundation pit destruction.The range of horizontal deformation ratio at the time of normal construction or foundation pit failure is investigated.(3)combined with the construction and monitoring work of a deep foundation pit in NanChang,softwares of LiZheng and FLAC~3DD were separately used to build the model and simulate variation characteristics of the deformation and internal force of the supporting structure in the whole process from excavation to the bottom of the pit.Through comparisons of the calculation and monitoring in the whole-process tracking form,it is shown that it is beneficial to the foundation pit monitoring,and the deviation can be found in time,so as to trace the root cause of the problem.(4)through case analysis and calculation and numerical simulation,it is showed that horizontal deformation characteristics of anchor-pull supporting structure of NanChang deep foundation pit,just like so in other general soil area and have a certain difference from soft soil area.The destruction form is sepeculated that the pile-anchor structure became a cantilever structural form due to lose efficacy of all of anchor,in which giant bending moment is produced and pile broken near the position of foundation pit bottom.However,the fracture of pile in soft soil area is due to the large"bow"deformation near the foundation pit bottom and the large bending rate.(5)combining the results of numerical simulation analysis and engineering cases,the method to determine the orange and red alarm values is discussed.The red alarm value and the orange alarm value are demonstrated for the first time through theoretical analysis.(6)according to the literature survey,combined with the practical experience of foundation pit engineering,based on the current national standards and referring to the local standards of some provinces and cities,the four-level warning and alarm strategy proposed is of great significance and the implementation plan is feasible.The emergency management measures put forward are for reference.This grading scheme relieves the design pressure and is conducive to solving the contradiction existing in the current design and construction.(7)studying the horizontal displacement and internal force changes of pile body caused by axial force changes of anchor cables,overloading,over digging and groundwater level changes.It is showed from the resulting that the up and down position,the horizontal spacing and the prestress of the anchors of the pile-anchor supporting structure have great effects on the deformation control.In the foundation pit of general soil area,the role of the uper anchor is greater than that of the down anchor.So it must be carefully designed and constructed,and it is very important to strengthen monitoring of the anchor timely and effectively. |
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