Research On Design Method Of Deep Foundation Pit Based On Insitu Test Parameters

With the development of urbanization and the implementation of the strategy of underground space development,the number and scale of deep foundation pit projects in China are at an unprecedented level in the world.At present,the relevant specifications and engineering design of foundation pit at home and abroad mainly use the geotechnical parameters obtained from indoor tests as the basis for the design and calculation of deep foundation pit engineering,which often leads to problems such as excessive deformation of foundation pit and instability of foundation pit.Whether in-situ test parameters can be used in foundation pit engineering design is a technical problem faced by geotechnical engineering.Based on the project of Jiangsu Science and technology plan"Research on Key Technologies of deep foundation pit support of metro station in Changzhou area rich in confined water strata"(2018ZD173),combined with the project of Changzhou Rail Transit Line 1 and line 2,this paper takes typical shallow soil layer in Changzhou area as the research object,and adopts the method of combination of in-situ test technology(CPTU,DMT),indoor test,on-site monitoring and theoretical analysis,In this paper,the application method of deep foundation pit engineering design based on in-situ test parameters is studied systematically.The main research contents and achievements are as follows:1.This paper systematically reviews the research status of in-situ test and its application in engineering at home and abroad,summarizes the research results of the application of CPTU and DMT in foundation pit engineering,points out the existing problems and problems to be solved in these studies,and puts forward the research ideas and contents of this paper.2.In order to accurately determine the design parameters of typical soil foundation pit along Changzhou rail transit line,based on 41 metro stations of Changzhou Metro Line 1 and line 2,this paper makes a comprehensive investigation and analysis of the topographic and geomorphic zoning characteristics,Quaternary geological overview,engineering geological and hydrogeological conditions in Changzhou area,and makes a large number of geotechnical parameters statistics for 41 stations’deep foundation pit projects,Through statistical analysis,the recommended value range of Changzhou deep foundation pit engineering design parameters is given.3.Based on the comparative analysis of the geological history of Changzhou and its surrounding areas(Suzhou,Shanghai,Wuxi)in the alluvial plain of the middle and lower reaches of the Yangtze River,the stress history of typical soil layers in Changzhou is analyzed in depth.It is found that the shallow typical soil layers in Changzhou have a large over-consolidation ratio due to the stress history.The over-consolidation characteristics should be considered in the design of foundation pit.This paper systematically summarizes the research results of determining the over-consolidation ratio of soil based on in-situ test at home and abroad.Combined with indoor test and in-situ test,a method of determining the over-consolidation ratio of soil based on in-situ test is proposed.4.This paper analyzes the main influencing factors of the coefficient of static earth pressure,which is an important parameter in the design of foundation pit engineering,summarizes a series of research results of determining the coefficient of static earth pressure based on in-situ test at home and abroad,and compares the indoor test results with the predicted values of in-situ test at site.The research shows that the method of determining the coefficient of static earth pressure by flat dilatometer test is better than other methods More reliable.5.Based on the typical station Qingfeng Park Station,combined with the in-situ groundwater table penetrometer developed independently,the pumping test of the typical confined aquifer in the construction stage is carried out,the calculation method of the foundation pit ground settlement is summarized,and the in-situ test and determination method of the foundation pit ground settlement is evaluated.Through the inversion analysis method,the sand layer(5)in Changzhou area is obtained(E_s=3.83×q_c).The empirical formula of the layer compression modulus and the correction of the compression coefficient in the calculation method of cohesionless soil foundation proposed by Debeer are presented.The comparison between the predicted settlement and the measured settlement shows that the prediction of the foundation settlement based on CPT is closer to the actual situation than the prediction of the settlement based on the indoor test.