Research On Testing Method Of Pile Foundation Integrity In Platform-pile System

It is not easy to control pile quality as pile foundation is a concealed work.Therefore,pile foundation detection is particularly important and it is the emphasis and difficulty in the project.Pile-cap structure is increasingly applied in actual foundation building.Under the influence of the cap,A lot of the detection signals will be interfered during the integrity testing of the pile foundation,so it is necessary to find a suitable method to reduce the influence of the cap on the integrity detection of pile foundation.The main contents of this paper are as follows:(1)After numerical simulation by software,this paper has studied the propagation characteristics of stress wave in pile-cap structure,and found by comparing with single pile detection signals that pile length and defects cannot be accurately determined directly by means of low strain method when there is a cap above the pile.(2)It is concluded through frequency domain analysis that a lot of high-frequency signals are generated when knocking on the cap surface.By adopting filter method,some clutters can be effectively filtered out and filtering frequency is subject to the value of the thickness of the cap(3)By means of shock excitation and vibration picking at different positions on cap surface,the signal curves at the measuring points were obtained for contrastive analysis to find out the position where the measuring point is under smaller reflection interference from the cap.After changing the acting area and the acting time of shock excitation,and other parameters,through comparison of the signal curve,it is found that changing the acting area of excitation has no big influence on the detection,but changing the acting time of shock excitation can effectively reduce the interference from the reflected signal at the cap.The detection effect is better when there exists a certain ratio between the thickness of cap and the propagation speed of stress wave in the pile body.(4)Effective identification of pile body integrity cannot be achieved by single point detection on cap surface which still receives reflection signals from the cap.Contrastive analysis of signals on cap surface and signals at pile side was conducted by introducing two-point test method to determine the pile body integrity according to phase position travel time.Signal comparison method could be used through numerical simulation of the signal curve in case of pile body integrity.By comparing defect signals,the defect location of pile body could be analyzed.(5)Inadequacies of each detection method could be found through above detection methods.Then,this paper proposed to exchange the location of shock excitation and vibration picking on cap surface to obtain two signal curves for signal comparison analysis.It could be found in shock excitation in the center of the cap that vibration picking signal from the center 3R/2 excited at 3R/2 from the center of the cap.Comparing vibration picking signal curve in the center of the cap can help to effectively judge the integrity of the pile body.(6)Through numerical simulation analysis of the propagation characteristics of stress wave in the double-pile-cap structure,this paper has analyzed the influence of the adjacent pile as intact piles and defective pile on detection signals.It could be found that when the detection pile and the adjacent pile are both defective piles,the detection signal has great interference influence.The comparison analysis of signals at different detection locations signals show that the detection signal is better at the excitation point 0.5R-0.6R when shock excitation point is in the center of cap surface.Changing the acting time of the excitation can reduce the interference from adjacent piles and cap reflections.By adopting two-point test method,it is easier to detect the double-pile-cap structure.(7)The intact pile,the shrinkage piles and the expanding piles of the non-pile-cap structure were detected in field experiment.The single cap pile was detected by using two-point test method,signal comparison method and two-point comparison method of the cap surface.In double-pile-cap system,defects of the adjacent pile would cause interference to the identification of the detection pile.Prolonging the acting time of the excitation appropriately can reduce the interference of adjacent piles.