Borehole radar survey to explore limestone cavities for the construction of a highway bridge

Abstract

During excavation work for the construction of a highway bridge in a limestone area in Korea, several cavities were found, and construction work was stopped temporarily. Cavities under the bridge piers might seriously threaten the safety of the planned bridge, because they could lead to excessive subsidence and differential settlement of the pier foundations. In order to establish a method for reinforcement of the pier foundations, borehole radar reflection and tomography surveys were carried out, to locate cavities under the planned pier locations and to determine their sizes where they exist. Since travel time data from the crosshole radar survey showed anisotropy, we applied an anisotropic tomography inversion algorithm assuming heterogeneous elliptic anisotropy, in order to reconstruct three kinds of tomograms: tomograms of maximum and minimum velocities, and of the direction of the symmetry axis. The distribution of maximum velocity matched core logging results better than that of the minimum velocity. The degree of anisotropy, defined by the normalized difference between maximum and minimum velocities, was helpful in deciding whether an anomalous zone in a tomogram was a cavity or not. By careful examination of borehole radar reflection and tomography images, the spatial distributions of cavities were delineated, and most of them were interpreted as being filled with clay and/or water. All the interpretation results implied that two faults imaged clearly by a DC resistivity survey were among the most important factors controlling the groundwater movement in the survey area, and therefore were closely related to the development of cavities. The method of reinforcement of the pier foundations was based on the interpretation results, and the results were confirmed when construction work was resumed.