Nowadays, most planned large infrastructure projects increasingly use multi‐technique geophysical methods integrated with geotechnical surveys to assess detection and exploration of karst‐related systems. This article focuses on the case of a large sinkhole found during the geophysical surveys that were carried out along Riyadh’s new Metro Line 3 (Saudi Arabia). This line is the longest (41 km) of the six that are currently under construction in the framework of the largest public rail infrastructure project, the Riyadh Metro Project (176 km). A multi‐technique geophysical survey combining seismic (1040 m of seismic refraction tomography and four downhole tests), electrical (1035 m of electrical resistivity tomography), and electromagnetic methods (1040 m of ground penetration radar) was conducted to shed light on the subsurface geology along the section of this case study. The combination of the geophysical methods led to early identification of a subsurface area of lower resistivity and seismic velocity than the background values of the carbonate bedrock. It also revealed smaller fractures that could lead to future sinkhole formation. A subsequent dense investigatory borehole grid (14 boreholes, five probeholes, 38 standard penetration tests, nine TV‐logging, and five pressuremeter tests) further confirmed the presence of a buried sinkhole. This paper shows the results of each individual geophysical method as well as the final geotechnical interpretation based on the combination of geophysical methods with borehole drilling. It concludes that the use of a single method for karst assessment, whether a geophysical method or borehole drilling, does not allow a sufficiently detailed geotechnical profile of the ground. This case study provides basic guidance on the most suitable and accurate techniques to detect similar karstic features across Riyadh.

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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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