Study Of The Characteristic Of Vertical Dynamic Stiffness To Piles-cap-pier

With the development of high-speed and heavy-duty railways in China,the speed increase of passenger trains on existing lines and the increase of axle load on freight trains will reduce the safety margin for the bearing capacity of the pier foundations.At present,relevant methods and procedures of bridge detection in China are mainly aimed at the upper structure and the lateral vibration of bridges.The method of non-destructive testing and evaluation of the vertical bearing capacity of the pier foundations is not complete.Taking the piles-cap foundation as an example,the traditional static load test and the high strain pile testing method can’t meet the non-destructive testing demands for the rapid screening for the bearing capacity of the existing pier foundations.Therefore,it is very important to develop a set of dynamic non-destructive testing methods to evaluate the vertical bearing capacity of pier foundations.Now there are mainly two kinds of dynamic non-destructive testing methods for the evaluation of the vertical bearing capacity of bridge piles.In addition to the high strain testing method,the other is the comprehensive evaluation method of dynamic stiffness developed by mechanical impedance method.When the bearing capacity of one bridge pier is controlled by deformation,the static stiffness and the dynamic stiffness will be converted by coefficient of correlation.Existing experimental studies have shown that there is a positive correlation between dynamic stiffness and vertical bearing capacity of pile foundation when dynamic stiffness is used as the evaluation index.However,the previous research object is mostly single pile without considering the impact from the pile cap and the pier on the dynamic stiffness.For the widely used group pile-pier structure system,systematic research on the reliability of the dynamic stiffness index for assessment is inadequate.Thus,through the indoor principle experiment and numerical simulation,the influence factors and characteristics of vertical dynamic stiffness to piles-cap-pier system are analyzed systematically,the main works are as follows:(1)Based on the basic principle of transient mechanical impedance method,a principle test model of piles-cap-pier is established.By using two kinds of materials wood and gypsum,the influence of pile defect,pile diameter,effective friction area around pile and height of pier body on the vertical dynamic stiffness of piles-cap system and the relationship of system vertical dynamic stiffness between single pile and piles-cap foundation is tested and studied.(2)Resorting to static and dynamic testing data of field model pile,the rationality of the parameters of the three-dimensional finite element analysis model of pile-soil are verified,and the influence of material factors(concrete elastic modulus,Poisson ratio,elastic modulus of soil and soil changes,etc.)and geometrical factors(single pile,single pile-cap and piles-cap,pile length,etc.)on piles-cap-pier system vertical dynamic stiffness and its characteristics are studied.The conclusions drawn from the above work are as follows:(1)Compared with the intact pile,the vertical dynamic stiffness of the expanding defect pile is obviously larger,to which the shrinkage defect and the short pile(the length of the pile does not meet the design requirements)are contrary;The closer the defect part of the defect pile is to the bottom of the pile cap,the greater the influence on the vertical dynamic stiffness of the pile cap system will be;The vertical dynamic stiffness of pile is positively correlated with the pile diameter;The vertical dynamic stiffness of the piles-cap system is positively correlated with the effective friction area around pile;The vertical dynamic stiffness of the piles-cap-pier system presents a decreasing trend with the increase of the pier.(2)The vertical dynamic stiffness of the piles-cap system is positively correlated with the concrete elastic modulus and Poisson ratio;With the increase of the dynamic elastic modulus of the soil layer,the vertical dynamic stiffness value of the piles-cap system increases,and the friction pile is more sensitive to the change of the soil around the pile than the end bearing pile;The percentage of vertical dynamic stiffness decreases gradually with the decrease of the thickness of the soil around the pile.(3)The value of vertical dynamic stiffness of piles-cap system is between n times and n-1 times of single pile cap system(n is the number of piles under the cap);The dynamic stiffness of friction pile system presents an increasing trend with the increase of pile length,with the end-bearing pile system works opposite to the former,the vertical dynamic stiffness of the system tends to be stable when the pile length reaches 20m.