Monitoring Soil Void Formation along Highway Subgrade Using Time Domain Reflectometry (TDR): A Pilot Study at Guilin-Yangshuo Highway, Guangxi, China

Soil voids and sinkhole collapses are main geological hazards during highway construction in the karst regions of southwestern China. Controlled by karst development and groundwater fluctuation, soil void usually forms abruptly and then expands progressively until a sinkhole develops. The goal of this research is to develop a cost-effective and efficient method to monitor the soil void formation and propagation, which would potentially prevent and greatly reduce damages caused by future sinkhole collapses. Recent monitoring methods include Ground Penetrating Radar (GPR), inclinometers, Interferometric Synthetic Aperture Radar (InSAR), real-time water pressure monitoring in karst opening, the Brillouin Optical Time Domain Reflectometry (BOTDR), and Time Domain Reflectometry (TDR). The unique features of the BOTDR and TDR techniques such as distributed measurement, long distance range, high resolution, and remote monitoring make them more effective to monitor soil voids and sinkholes along highways in active karst area. Because of the high cost of the Optical Fiber Strain Analyzer, the equipment for BOTDR, TDR was used as a cost-effective method to monitor soil voids and sinkholes. In a pilot study, a 110 m section of subgrade along Guilin-Yangshuo highway was selected to establish a long-term monitoring site, where soil voids formed frequently and more than 43 voids were delineated by GPR. Five lines of coaxial cables were installed on the surface of the subgrade. These lines were parallel to the main axis of the highway with a 3 m separation interval in between. A monitoring station was established on December 21, 2006, and soil deformation measurements were recorded monthly. In addition, water pressure in karst opening was monitored by piezometer and precipitation was recorded by rain gauge at the station. Results of the monitoring measurements show no major subgrade deformation in the past 10 months. One cable fault caused by construction truck was detected by TDR. One minor cable fault caused by subsurface voids or uneven subsidence during construction was detected. Further testing or GPR scan are needed to interpret the minor cable fault.