Study On Dynamic Stiffness Characteristics Of Existing Bridge Pier Foundation

As an important part of bridge structure,the property of pier foundation is directly related to the bearing capacity and service life of the whole bridge structure.The existing verification contents and evaluation criteria of railway and highway Bridges mainly focus on the superstructure of Bridges.While the evaluation indexes of the bridge substructure,especially the pier subfoundation,are seriously lacking.Due to the bridge foundation embedded in soil,the conventional detection method cannot detect the operating state and bearing capacity at the present stage.The traditional static load test method of bridge pile foundation and the high strain dynamic test method can not meet the requirement of evaluating the basic state and bearing capacity of the bridge in service on a large scale.With the large-scale development of China’s highway and railway,traffic density,vehicle load increasing,at the same time,the operating time of existing bridge piers are increasing.At present,a set of methods which can quickly and overall test the working state of existing bridge foundation and evaluate its bearing capacity is urgently needed to evaluate the existing bridge piers and foundation scientifically and effectively.The dynamic stiffness method establishes the static and dynamic contrast coefficients between the static stiffness of pile foundation and the static stiffness of static pressure test,which shows that the dynamic stiffness is positively correlated with the bearing capacity of pile foundation.At present,the research of domestic and foreign scholars is mainly limited to the dynamic stiffness of single pile,while the research on the overall dynamic stiffness of pier-bearing cap-pile group system in service is relatively less.The dynamic stiffness of the existing bridge pile foundation is studied by combining field test with finite element calculation.The main contents are as follows:⑴ The finite element simulation method of dynamic stiffness of existing bridge pile foundation is studied.The effectiveness and rationality of the finite element simulation method are analyzed by combining the field test data of single pile,group pile-bearing cap and group pile-bearing cap.⑵ Based on the basic principle of transient mechanical impedance method,the sensitivity analysis of dynamic stiffness of existing bridge piers was carried out by establishing a three-dimensional solid finite element model.⑶ Based on the field test data and the finite element analysis,the dynamic stiffness is used to evaluate the working state of pile group foundation.The conclusions of this paper are as follows:⑴ Through the sensitivity analysis of the single pile dynamic stiffness parameters,the dynamic stiffness is independent with the excitation load;The greater the elastic modulus is,the greater the dynamic stiffness is.The dynamic stiffness has a linear relation with the elastic modulus of pile concrete.The greater the defect,the smaller the dynamic stiffness is.The larger the pile diameter,the greater the dynamic stiffness of the pile foundation.For friction pile foundation,the longer the pile is,the greater the dynamic stiffness will be.For end-bearing pile foundation,the smaller the pile length is,the greater the dynamic stiffness is.(2)By comparing the dynamic stiffness of single pile,single pile-cap,pile groupcap three structural systems,it is found that the dynamic stiffness of single pile-cap system is slightly larger than that of single pile.