LATWAK : an impact test to obtain the lateral static stiffness of piles

Abstract

This study presents the development and application of a new method to predict the lateral static stiffness of piles. This new method is called LATWAK; it consists of a package including the LATWAK Test, the theory behind it, and the LATWAK computer program. The LATWAK Test allows to obtain time dependent data by hitting the top of a pile laterally with a sledge hammer; the hammer is instrumented with a dynamic force transducer and the pile is instrumented with a geophone. The horizontal force vs. time signal is the input and the horizontal velocity vs. time signal is the response. The pile is modeled as an elastic member inserted in a visco-elastic soil. The theory of flexural waves, applied to the assumed pile model, allows to determine the theoretical mobility of the pile head in question. Also, Discrete Fourier Transforms, DFT, applied to the time domain data given by the LATWAK test, allow to determine the measured mobility of the pile head. The System Identification Technique is employed in order to match the measured and theoretical mobility curves, thus predicting all the parameters of the assumed soil/pile model. One of the predicted parameters, the element soil spring stiffness, directly relates to the lateral static stiffness at the pile head, which can thereby be predicted. The "LATWAK" computer program was specifically developped in this study in order to automate the theoretical and the data analyses. Many features can be found in this program. The main capabilities of the program are to visually simulate the pile wave motion, obtain the theoretical mobility response, retrieve the LATWAK test data and transform them into the measured mobility response, as well as match the two curves, identify the system, and predict the lateral static stiffness. In this study, both conventional static load tests and LATWAK tests were conducted on twenty full scale piles at three different sites. LATWAK predictions of lateral static stiffness were found, on the average, ic fall within 25% of those measured conventionally by much more expensive lateral load tests.