Macroscopic And Mesoscopic Study On Bearing Characteristics Of Single Pile-soil-cap In Sand

With the rapid development of engineering construction in China,the requirements for bearing capacity and anti-deformation of pile foundation are getting higher and higher,and the theory and technology of pile foundation engineering have made great progress and development.The interaction between pile and soil is the core issue in the development of pile foundation engineering theory and technology,and its essence is how to make full use of the bearing capacity of pile and foundation,so that two materials with different properties can bear the load of superstructure cooperatively,and control the structural deformation and differential settlement within the allowable range.In the existing research,there are many factors affecting the interaction between pile-soil-cap in the process of exerting bearing capacity of pile foundation with cap,and the comparative relationship between the ultimate bearing capacity of single pile foundation with cap and the sum of the ultimate bearing capacity of single pile and cap foundation is controversial.In order to explore the bearing mechanism under the synergistic action of pile-soil-cap,this paper studies the bearing characteristics and foundation deformation characteristics of single pile,single pile with cap and cap foundation under static load through macro model test,and launches corresponding meso numerical simulation inversion to explore its internal meso mechanism.The main research contents and achievements are as follows:(1)in the aspect of macroscopic model test,the static load comparative tests of single pile,single pile with cap and cap foundation are carried out in sand foundation,based on particle image velocity measurement technology,in the static load test of sand foundation,the corresponding deformation measurement system is established and applied to the static load contrast test of single pile,single pile with cap and cap foundation.By observing the bearing relationship of single pile,single pile with cap and foundation under static load and the mutual response between foundation soil and foundation,the macroscopic working mechanism of single pile with cap in the process of pile-soil-cap interaction is explored.(2)In terms of meso-scale numerical simulation,firstly,based on indoor model experiments,two-dimensional discrete element numerical models are respectively established in sandy soil,which can reflect the whole static load process of single pile,single pile with cap and cap foundation.The rationality of the discrete element numerical simulation model is verified by comparing the mechanical properties of foundation and the variation law of load-settlement curve in indoor model test.Secondly,based on the verified numerical analysis model,the dynamic changes of the micro-structural characteristics of foundation soil particles such as displacement vector,rotation field,motion trajectory,porosity,contact force chain distribution,large principal stress vector and contact fabric are compared and analyzed from a micro-perspective view.Finally,a two-dimensional particle discrete element numerical model for the whole static load process of double piles with caps under different pile distances is further established in order to find out the bearing mechanism of pile foundation with caps under the synergistic action of piles,soil and caps.The macro and micro research results show that in terms of foundation deformation mechanism,compared with single pile and cap foundation,single pile with cap has deeper and wider disturbance to foundation soil under the synergistic action of pile,soil and cap,and a larger range of soil is involved in the process of resisting load.with the increase of pile distance,the soil between two piles with cap appears the effect of upper arched soil plug.In the aspect of bearing characteristics of pile foundation,the phenomenon of arching appears in a certain range of soil on the side of single pile with cap,which strengthens the compactness of soil near pile side and pile tip,and improves the friction resistance of pile side and the bearing force of pile tip,so as to improve the overall bearing capacity.The macro and meso research results can provide reference for the design and bearing capacity estimation method of composite pile foundation with cap.