Theoretical Study On Low Strain Integrity Testing Of Platform-pile System

To study the three-dimensional characteristics of wave propagation in platform-pile system, the three-dimensional computation model for transient vibration of platform-pile-soil system is established. Based on initial and boundary conditions, the definite solution of this model is obtained. The MATLAB program is compiled via using staggered grid finite difference method. The dynamic responses of the integrate pile’s platform-pile-soil system under vertical impact force is got, and the reliability and feasibility of the numerical simulation are verified by comparing calculation results with measured dates of platform-pile-soil system’s low strain integrity testing. The stress wave reflection at artificial boundary is eliminated by introducing o absorbing boundary. The second-order Higdon absorbing boundary is compared with the Clayton-Engquist absorbing boundary, the result show the second-order Higdon absorbing boundary is better than the Clayton-Engquist absorbing boundary absorption.The best sensor location at platform top and pile side are studied by numerical simulation. The effects of platform size, neighboring pile, pile length, surrounding soil, pile subsoil, contact area and contact time on active signal at platform top are also analyzed. Then, the three-dimensional characteristics of stress wave propagation at platform surface for platform-pile system of three piles and four piles are shown. The best sensor location at platform top of three piles and four piles are studied. Comparison of the responses and two velocity measurement are used to judge pile length and the location of pile defects. At last, the parallel seismic method using in detecting the pile length for platform-pile system is studied. A correction factor is introduced to reduce the pile length error, at the same time, the effect of pile-to-borehole distance and surrounding soil is analyzed.The results show the location distance pile heart 0.5R-0.6R(R is pile radius) where through pile heart impact point and parallel to the short side distance is the best sensor location at platform top for platform-pile system of two piles. It has a certain role to reduce three-dimensional interference via increasing shear wave velocity of surrounding soil and appropriately increasing contact time. In addition, pile length, pile subsoil and contact area have less influences on low strain integrity testing of platform-pile system. Comparison of the responses and two velocity measurement are a good method to detect the pile integrity and pile length for platform-pile system. The location distance impact location 0.6R-0.7R at Line 3(Fig.6.3) is the best sensor location at platform top for platform-pile system of three piles. The location distance pile heart 0.6R-0.7R at Line 3(Fig.7.3) is the best sensor location at platform top for platform-pile system of four piles. The parallel seismic method is a good method to detect pile length for platform-pile system, Correction factor greatly reduces the pile length errors. The original pile length increases with pile-to-borehole distance and shear wave velocity of surrounding soil increases, and the error also increases. However, When pile-to-borehole distance is 0.4 m, the error of modified length of pile is minimum.