| Summary: | In the framework of the largest urban-transit system ever built from scratch, the Riyadh Metro Project (6 lines totaling 176 km), a comprehensive investigation was carried out for karst detection, prevention and correction. This case study of the Metro Line 3 (41.6 km) seeks to show how the multi-technique geophysical survey (seismic refraction, electrical resistivity and ground penetration radar down to 40–50 m depth) was found to be a successful tool in detecting karst features. Preventive measures included systematic probing drilling to anticipate karst cavities below foundations (653 piers) and tunnel lining evaluation using 2D finite elements. Finally, this paper provides initial guidance of the corrective techniques used for each engineering challenge, such as cavity filling with grouting, geogrid reinforcement, etc.
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