| The cover excavation reverse construction method has significant advantages in controlling the deformation of structures and buildings around the site,controlling dust and noise pollution,etc.It is a good solution to the problems faced by the construction of underground structures such as subway stations in busy urban areas,such as land resource constraints and environmental protection requirements,and is gradually applied to the construction of underground structures.In the promotion of the method,a systematic theoretical system has not yet been formed,and the relevant theories of the open excavation method are still borrowed to a certain extent in terms of deformation control indexes and protection measures of adjacent buildings.Therefore,this paper takes Hohhot subway station project as the basis to carry out research on the key problems in the cover excavation reverse work structure,aiming to gradually improve the theoretical system of cover excavation reverse work method,to provide some theoretical references for the method to be more widely used in urban construction and to better solve the engineering construction problems.Since the vertical support system functions as both the pit support structure and an important part of the main structure,the deformation and internal force change after the external force is applied will have an impact on the pit and the main structure.These effects are mainly reflected in the deformation of the foundation pit,the settlement of the surrounding ground surface and the internal force change of the main structure,etc.At present,domestic scholars have carried out relevant research for the above problems of the cover excavation inverse structure,but there are two limitations: firstly,the object of the research is concentrated in Shanghai,Tianjin and other areas with soft soil,and secondly,the research is mainly carried out for a single problem,and the middle column and the pile foundation under the column and the support structure are not considered as a whole support system.The structure as a whole support system is not systematically studied.Therefore,in this paper,the vertical support system of cover excavation reversal structure is taken as the research object,and the three aspects of the vertical support system are studied in terms of the uplift and subsidence law,the differential uplift of the vertical support system and the horizontal displacement of the vertical support structure and the settlement law of the soil behind it,based on the cover excavation reversal subway station project in Hohhot.This paper is based on the Hohhot subway station project,which is an interchange station,and the strata in the station area from top to bottom are human miscellaneous fill,powdery clay,chalk,silt,fine sand,medium sand,coarse sand,gravelly sand and round pebble layer.The width of this section is 24.9m and the depth of foundation pit is 20.54 m.The middle column adopts steel pipe concrete column,the pile foundation under the column adopts double-layer expanded bottom pile,and the foundation support structure type is underground diaphragm wall.This paper uses theoretical analysis combined with actual measurement data to carry out the following aspects of work.1)The settlement calculation theory of double-layer belled pile and diaphragm wall under top uniformly distributed load is studied.The settlement of double-deck spread bottom pile and ground connection wall is analyzed by using elastic theory,and the Mindlin solution is applied to the case of uniform load to derive the calculation of settlement of double-deck spread bottom pile and ground connection wall under the top uniform load,which provides the theoretical basis for the study of settlement law of both.2)For the structural characteristics,the selection of the principal structure model is demonstrated and the numerical model is established.By analyzing the various types of principal structure relationship from the aspects of the rationality of the model,the applicability of the model object and the difficulty of determining the model parameters,it is determined that the modified Moore Coulomb model is used for the principal structure relationship.The advantage of the modified Moorcullen model is that it can more accurately simulate the excavation and unloading process of the foundation pit soil,and has better computational convergence,and can also more accurately simulate the compression behavior of the soil body.The numerical model of the structure is established by Midas GTS NX finite element analysis software.In order to accurately simulate the mechanical corresponding characteristics of the support structure under each working condition in the construction of the pit cover excavation inverse practice,the calculation adopts the calculation method of dynamic simulation of the construction process.3)Study of vertical support system lifting and sinking lawThrough the analysis of numerical simulation results combined with the actual measurement data,the dynamic mechanical response characteristics of the middle column,pile foundation under the column and diaphragm wall in the unloading-loading process are studied,the structural internal force changes and the law of uplift and subsidence are summarized,and the rationalized suggestions for design and construction are given accordingly.4)Research on differential settlement of vertical support structureBased on the study of the uplift and settlement law of the vertical support system,further research is carried out on the differential settlement law,causes and effects on the structure between the intermediate column and the pile foundation under the column and the underground continuous wall.A practical solution is proposed to convert the differential settlement into vertical displacement values and load them into the calculation model of the main structure for the reference of structural design.At the same time,the rationalization suggestions to reduce the differential settlement are given to reduce the safety hazards in the construction of the structure.5)Research on the horizontal displacement of the support structure and the ground settlement law behind itThrough numerical simulation and combined with the results of field measurement,we study and analyze the horizontal displacement characteristics and the ground settlement law behind the structure when the cover excavation reversal structure adopts the ground connection wall as the foundation pit support structure,and compare with the open excavation foundation pit,analyze the rationality of using the control index of the open excavation foundation pit as the basis in the design and construction of the cover excavation reversal,and provide a theoretical basis for the development of the protection plan of the structure around the foundation pit and the monitoring and measurement plan for the cover excavation reversal foundation pit.This paper provides a theoretical basis for the development of the protection scheme and monitoring and measurement scheme for the surrounding structures.In this paper,the research results based on numerical simulation analysis provide theoretical guidance for the construction of the Station.The field measured data collected during construction also agree with the numerical simulation results,which proves that the research results of this paper are reliable and can provide theoretical reference for similar projects. |
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