Influence of Piling on Damping Coefficients of Soil Surrounding Piles

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    The accuracy of drivability analysis for piles with large diameters is very important for pile driving safely.The prediction of soil resistance to driving (SRD) is the focus of this analysis.Numerous models have been developed to predict soil resistance.With many outstanding advantages,the technology of high-strain testing is the basis for a new advanced method for pile-driving analysis.The Case Pile Wave Analysis Program (CAPWAP) method is generally used to analyze dynamic tests data to obtain the SRD.CAPWAP is based on one-dimensional wave theory,which is widely used in pile foundation monitoring and bearing capacity evaluation in ocean engineering.Using CAPWAP to analyze the data from dynamic testing,the SRD can be determined during the entire pile installation procedure.When using CAPWAP,soil factors and pile parameters such as damping,quake force,and wave velocity should be determined.Among the parameters,the soil damping coefficient of friction Js and top of pile Jt,have the greatest influence on pile bearing capacity.Soil damping coefficients are the most important parameters of soil dynamics.Various models have proposed distinct Js and Jt,and each soil type has a specific damping value.For example,the Smith model proposed values of 0.16 and 0.48 for Js and Jt,respectively,and the Case model recommended Js values of 0.15 for cohesive soil and 0.65 for cohesionless soil;Jt was 0.5 regardless of soil type.Many models widely use the value of 2.54 mm for quakes.Although many scholars give different experience values,the Case damping coefficients are widely adopted in most cases. Engineering applications reveal that various soil parameter values strongly influence the SRD;therefore,the study of soil parameter values is meaningful.To obtain more accurate damping parameters and to improve the reliability of CAPWAP,this study examines the results of pile dynamic testing of an engineering application in the East China Sea.During pile installation,the pile is struck up to 1868 times to penetrate the design depth,with each blow containing a set of corresponding force and velocity curve.In this study,we analyze approximately 200 curves by CAPWAP to obtain a set of soil parameters for each layer.To obtain the changes in Js and Jt with depth,numerous data are analyzed.First,a study is conducted to determine Js and Jt with various penetration depths.The results show that the soil damping coefficients increasingly decay rather than remain constant during piling and change with penetration depth.Therefore,when using CAPWAP to predict the soil resistance to driving,using a constant for Js or Jt during the entire pile installation is not reasonable and will result in error.To further study the changes in soil damping coefficients during pile installation,the relationship of soil damping coefficients with blows is examined.It is determined that the change trend of blows is in contrast to that of the damping coefficient.Finally,this study analyzes the change trend of soil damping coefficients with depth.During the piling procedure,the factor value is shown to steadily decrease in a manner similar to a linear gradient.

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  • Online: January 15,2015
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