Study On Design Parameters And Deformation Control Of Deep Foundation Pit In Soft Soil Area Of Nanjing Flood Land Based On CPTU Data

With the development of urbanization along the eastern coast of China,the excavation of foundation pits in coastal soft soil areas tends to be larger and deeper.Because the large-scale deep foundation pit engineering is in the complex sensitive environment of soft soil,the design and deformation control of deep foundation pit engineering in soft soil area are very complex.Therefore,the design parameters of soft soil foundation pit are the key to the success of foundation pit engineering.The thickness distribution of soft soil in floodplain facies of the Yangtze River in Nanjing is extremely uneven.It has the characteristics of large void ratio,high compressibility,high water content,low shear strength and high sensitivity.At present,the design parameters of soft soil foundation pit mainly depend on laboratory tests,which have the shortcomings of soil disturbance,stress release and high cost.CPTU testing has the advantages of continuity,high accuracy,high reliability and repeatability,which can directly evaluate the engineering characteristics of soil and be used in geotechnical engineering design.Relying on the Jiangsu Surveying and Mapping Geographic Information Research Project?JSCHKY 201606?,the multi-functional CPTU was conducted in Nanjing Financial City foundation pit engineering.The key design parameters of the foundation pit were calculated based on the CPTU test data.The foundation pit of Nanjing floodplain area was simulated by three-dimensional finite element method combined with the measured data in the floodplain soft soil area.The main contents of this paper are as follows:?1?The present situation of deep foundation pit engineering in flood plain area and the evaluation method of design parameters of foundation pit engineering based on CPTU prediction are summarized.Relying on the project of Nanjing Financial City,the field test of CPTU is carried out in detail.Combining with the laboratory test data,the over-consolidation ratio OCR and static earth pressure coefficient K₀ of soft clay are predicted and analyzed.This paper evaluates and analyses the accuracy of indoor test method,Mayne&Kulhawy method and normalized cone tip resistance Q_t method to predict K₀,and establishes an empirical method for in situ determination of K₀ in Nanjing floodplain area.?2?Based on MIDAS finite element analysis software,considering the influence of groundwater seepage,three-dimensional modeling and numerical simulation are carried out for typical foundation pit project in Nanjing floodplain area.The deformation of the enclosure structure,the ground settlement around the pit and the heave of the pit bottom are analyzed by using the parameters determined by CPTU prediction and indoor test.The influence of internal friction angle,cohesion,coefficient of static earth pressure,thickness of soft soil layer and seepage of groundwater on deformation behavior of foundation pit during construction stage is studied.With the increase of?,c and K₀,the lateral displacement of diaphragm wall and settlement behind the wall decrease;with the increase of thickness of soft soil layer,the deformation of surface around the pit and surrounding structure increases.The wall deformation and settlement behind the wall are greatly affected by groundwater seepage.?3?Based on the detailed monitoring data of the foundation pit engineering in the floodplain area of Nanjing,the supporting structure and dewatering scheme of the foundation pit are analyzed,and the commonly used enclosure structure and dewatering scheme are given.Based on the statistical data,the influence of soft soil layer on the deformation of the retaining structure and the surface settlement outside the pit is analyzed;the average maximum settlement value and range of the surface settlement are analyzed,and the envelope curve of the surface settlement is proposed;the average value and range of the maximum lateral displacement of the retaining structure are also analyzed.According to the law of deformation,the control measures for foundation pit construction,the environmental impact and control measures for foundation pit dewatering are put forward,and the relevant protection measures for the control of underground pipelines and surrounding roads are also put forward.