Model Test Study On The''Cross-Shaped"Plate Underpinning Method For The Underground Layer Of Existing Buildings

The underground addition of existing buildings is a new way to develop urban space,which is a major trend in urban development.The method is applicable to the central area of the city where the traffic is crowded.It can not only maintain the original cultural features of the city,but also increase the underground space,to a certain extent alleviate the shortage of urban land resources and urban traffic congestion.It involves a wide range of engineering techniques and requires very high technical difficulties.Among them,basic underpinning and earth excavation are the core technologies of underground layering,and they play a direct role in the success or failure of underground layering projects.At present,researchers at home and abroad have conducted some research on this field,but most of the simulations and cases are researches on pile foundation underpinning,and the research and application of plate foundation underpinning are relatively rare.The slab foundation underpinning method is a new type of basic underpinning technology for the development of underground space of existing buildings.It has far-reaching social benefits and broad application prospects.Therefore,model tests and related theoretical research are urgently needed.In view of the above problems,this paper explores the technical scheme of underground foundation plate type underpinning,and explores the feasibility of this type of exchange method through design model test.Based on the settlement data obtained by the model test and the finite element software,the deformation and the settlement of the column foot during the underpinning process are studied.The main research contents and results are as follows:(1)Firstly,it summarizes the research and existing problems of underground building in existing buildings at home and abroad,and introduces the common types of basic underpinning and discusses the basic underpinning method and its characteristics.This paper puts forward the framework of this paper to study the framework of the existing framework structure.(2)This paper analyzes the technical characteristics of underground layer-adding construction,selects a reasonable replacement method,and designs a set of “cross-shaped”plate foundation underpinning construction plan for underground layering,and studies the design and construction of the raft foundation.Consider the economic,applicable and other factors to select the type of the raft foundation,and then determine the size,the thickness of the replacement top plate and the replacement base plate.In this model test,the steel pipe fastener scaffolding model is used to simulate the existing frame structure,the foundation soilis made of natural foundation soil,and the earthwork excavation adopts “double-sided”diagonal excavation.(3)In order to study the settlement law of the column foundation underpinning method more comprehensively,the model tests were carried out on the equal cross-frame structure and the non-equal cross-frame structure.The study of the underpinning process is divided into three conditions: top plate replacement,one direction diagonal earth excavation,long column internal column,another direction diagonal earth excavation,long pit internal column,and undercut roof soil excavation.The dial gauge is used to measure the change of the settlement of each working condition at the bottom position of the pole and calculate the settlement difference.It can be seen from the test results that the settlement of the cross-frame structure is the final settlement of the column foot exposed by the excavation and the final settlement of the column post exposed by the excavation;and the final settlement of the frame structure is related to the span size,and the general span is more The conference resulted in a larger final settlement.(4)Using ABAQUS finite element software,the construction process of the plate foundation underpinning method was modeled in six working conditions.The settlement law of each part was obtained by analyzing the numerical calculation results.In the construction of the underpinning condition,the column foot of the middle part is larger than the column foot of the surrounding part;In the later stage of earth excavation,the column foot of the middle part is smaller than the column foot of the surrounding part.The maximum value of uneven settlement occurs between the completion of the second working condition and the beginning of the working condition,so the construction time is minimized during the construction.By comparing the experimental data,the experimental study of the plate foundation underpinning method verifies the correctness of the numerical analysis.In addition,the strain plate of the replacement plate shows that the earthwork excavation process has a greater influence on the strain of the top plate,and the maximum strain value occurs at the edge of the roof plate,and the strain value is closer to the earthwork excavation.