Research On Working Characteristics And Settlement Calculation Of Composite Foundation Improved By Concrete-cored Deep Cement Mixing Pile Under Embankment

The concrete-cored deep cement mixing pile(CDCM pile)combines the advantages of high strength of precast pile and large specific surface area of deep cement mixing pile,which effectively improves the vertical and horizontal bearing performance of the pile.With the rapid development of domestic infrastructure construction and the accelerated construction of urban expressway network,CDCM pile composite foundation technology has been rapidly promoted and applied in the foundation treatment of highways,railroads and other projects.However,the research on the load and deformation characteristics of CDCM pile composite foundation under embankment is not sufficient,and the simplified calculation method of settlement is not yet perfect.In this paper,firstly,indoor experimental research on the mechanical properties of cemented soil was carried out,and the relevant parameters in numerical analysis were calibrated;then,numerical analysis was used to study the load-deformation characteristics of CDCM monopiles under horizontal load and the working properties of CDCM pile composite foundation under embankment;finally,based on the orthogonal analysis method and the results of a large number of finite element calculations,the prediction formula of the average pile-soil stress ratio and the simplified calculation method of settlement of composite foundation are proposed.The main contents and conclusions are as follows:(1)Unconfined compressive strength test,unconsolidated undrained shear test and scanning electron microscope test were carried out for cemented soils with different cement content and curing time.The results showed that the cement hydrate between soil particles increased significantly with the increase of the cement content and curing time,and the strength of cemented soil increased because of its better structrue,and the ductility characteristics of the cemented soil was more obvious under the higher confining pressure.The internal friction angle and cohesion were 1.4～2.7 times and 18.8～89.4 times of the in-situ soil respectively.(2)The numerical analysis model of CDCM monopile under horizontal load was established to analysis its loading and deformation characteristics and be compared with deep cement mixing pile and PHC pipe pile of the same diameter.The results showed that the cemented soil shell effectively limited the development of bending moment and horizontal displacement of concrete core pile,reduced the scope and degree of tensile damage of concrete core,accelerated the radial stress decay around the pile,and improved the horizontal bearing capacity of CDCM pile;the increase of core rate had the most significant improvement on the horizontal bearing capacity of CDCM pile;when the length of concrete core was less than the critical value,its bending resistance couldn’t be fully developed,and the horizontal bearing capacity of CDCM pile decreased.(3)The numerical analysis model of CDCM pile composite foundation under embankment was established by "typical section method",and the working characteristics were studied and compared with deep cement mixing pile composite foundation and concrete pile composite foundation.The results showed that CDCM piles made full use of the characteristics of high strength and high stiffness of concrete cores,and the concrete cores shared most of the vertical load,which effectively improved the stress state of the cemented soil shell,and reduced the compression of the reinforcement area and lower penetration deformation;the concrete cores under the center of the embankment were mainly subject to compression,and the concrete cores under the shoulder and foot of the slope were mainly subject to bending;the cement soil shells mainly played the role of transferring shear force,and there was a sudden increase of stress at the bottom of the concrete cores.In engineering practice,foundation settlement can be controlled by increasing the pile replacement rate,core rate and core length ratio.(4)For the average pile-soil stress ratio of CDCM pile composite foundation under embankment,sensitivity analysis of its main influencing factors was carried out by orthogonal analysis;on this basis of orthogonal analysis,the stress ratio coefficient K was constructed to normalize the average pile-soil stress ratio,and the average pile-soil stress ratio prediction formula was proposed;based on the stress correction method and pressure diffusion method,a simplified calculation method for settlement of CDCM pile composite foundation under embankment was proposed,and its reasonableness was verified by three engineering cases.