A nondestructive testing experimental system for hydraulic antiseepage wall based on crosshole ground penetrating radar
[Objective]A hydraulic antiseepage wall is an underground diaphragm wall built in an Earth and rockfill dam to prevent seepage.The quality of the wall directly affects the impermeable effect and structural safety.However,defects within the antiseepage wall are usually randomly distributed and relatively hidden,so it is crucial to timely and accurately capture the hidden defects within the wall.Crosshole ground penetrating radar(GPR)is an efficient geophysical method for detecting and locating inner defects in antiseepage walls without excavation.It involves inserting antennas into measurement pipes buried in the structure to conduct high-frequency electromagnetic signal(EMS)measurements.By analyzing and inverting the EMS data,an inference of the defect position,size,shape,and physical characteristics is achieved based on the differences in the electrical parameters of various media.Crosshole GPR has advantages such as high accuracy,high resolution,and sensitivity to water-containing materials,and it is not limited by penetration depth.[Methods]Herein,a teaching experiment was performed using crosshole GPR to detect concealed defects in an antiseepage wall.The experimental system comprises a visual water tank,a visual model box,and a stepped-frequency crosshole GPR system.The dimensions of the visual water tank and model box are 1.2 m×1.0 m×1.5 m and 0.6 m×0.2 m×1.0 m,respectively.The visual model box is placed inside the water tank,and the cavity between these two components is filled with water(permittivity of 78.28 and conductivity of 0.30 S/m)to simulate the surrounding backfill.The visual model box is filled with dimethyl malonate to simulate the concrete wall because the dielectric parameters of dimethyl malonate(permittivity of 15.88 and conductivity of 0.24 S/m)are similar to those of concrete wall.Two measurement pipes are placed vertically from the top to the bottom of the visual model box,enabling the antennas to move up and down in the pipes and facilitating the zero-offset profiling and multioffset gather.The stepped-frequency crosshole GPR system comprises a vector network analyzer(VNA),a pair of transmitting and receiving antennas,and a laptop.The VNA generates and collects electric signals(ESs).The transmitting antenna converts ES to EMSs,propagating through the medium between the antennas.Next,the receiving antenna receives EMSs and relays them back to the VNA.[Results]Four cases are well-designed to simulate the different defects in a hydraulic antiseepage wall,including voids full of air,water,mud,and uncompacted stone.Crosshole GPR data are collected for all the cases,and electronic parameter inversion is performed using computed tomography.The physical characteristics of the defects can be accurately revealed,indicating that the experimental system can use crosshole GPR to effectively detect concealed defects within the antiseepage wall.[Conclusions]Using this experimental system,GPR images are easily acquired and the electronic parameter inversion of the medium is achieved,enabling an inference of the defects.Meanwhile,this system inspires students'innovative thinking,enhancing their research capabilities and achieving the goal of promoting teaching through research and vice versa.
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