Foundation pile and cavity detection by the 3D directional borehole radar system, ReflexTracker

Abstract

In order to evaluate our developing 3D directional borehole radar system, ReflexTracker®, we carried out experimental studies on its capability to detect foundation piles in poor subsoil in the Tokyo area, and cavities beside a housing complex in Aichi Prefecture, central Japan. To evaluate foundation pile detectability, we took omnidirectional and directional borehole radar measurements in two boreholes (vertical and tilted at 60 degrees) near known concrete piles. We utilized existing drawings and specifications that well described the piles at the site in terms of their materials, structures, locations, and depths, and conducted GPR and vertical differential magnetic surveys to confirm the exact locations of the pile heads. The measurements were successfully taken for both boreholes. The estimated 3D locations of the reflected points were in good agreement with the known pile locations, with an accuracy of 0.14 to 0.20 m from the pile for measurements with the vertical and the titled boreholes, respectively. For the cavity detectability evaluation, we collected core samples to check geological conditions and existing cavities estimated by drilling operations and N-values by SPT. The directional borehole radar measurements showed several significant reflected phases in a radargram and revealed the existence of the cavities corresponding to the drilling results obtained from 3D reflected points. As a result of the two experimental studies for foundation piles and cavity detectability of the ReflexTracker, we concluded that the system could be applied to civil engineering issues in poor subsoil ground.