Detection and Treatment of Sinkholes and Subsurface Voids along Guilin-Yangshuo Highway, Guangxi, China

The 66 km long Guilin - Yangshuo highway is located in northern Guangxi, China. The entire highway was built through an active karst area characterized by tower karst, karst valleys, and watershed divides. The highway construction has induced many sinkholes since the construction started in 2004. The primary goals of this research are to detect locations of subsurface voids and potential sinkholes and to treat these voids to prevent future collapses. A 200m section representing typical karst setting of the highway was chosen for a preliminary survey. Ground Penetrating Radar (GPR), multi-electrode electric resistivity, and shallow seismic reflection were used for the preliminary survey. Comparison of the three geophysical methods indicates that GPR can detect subsurface voids in a more effective and timely manner. Therefore, GPR was applied to a 12 km section highway to located subsurface voids in a highly active karst area. 337 subsurface voids, 1–6 m in diameter, 1–5.5 m deep, and density up to 48 per 100 m, were detected using GPR. Further assessment of the study area indicates that water table fluctuations around bedrock and soil interface cause the formation of subsurface voids. Excavation for the foundation of the highway induced more surface water into the karst aquifer and water pumping from the bridge foundation significantly lowered the water table of the karst aquifer. Highway construction changed the natural drainage pattern and caused more hydrodynamic changes of the karst aquifer, which in turn formed more subsurface voids or enlarged existing voids. Finally excavation, graded-filter, and geofabric lining techniques were suggested to treat clustered subsurface voids and grouting was suggested to treat isolated voids. This treatment plan has been accepted by the highway construction company.