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.

Nowadays global urbanization, increasingly trending towards megacities worldwide, is putting pressure on the underground environment. Consequently, a need to shed light on the metropolitan subsurface geology has arises. Different megacities around the world maintain large bibliographic repositories related to prior infrastructure construction projects, for example, metro projects which spread over entire urban areas. However, in fast growing cities like the capital of Saudi Arabia, the available technical and scientific literature regarding the city’s subsoil is scarce. This investigation shows the first full-scale geotechnical assessment of the Upper Jurassic - Lower Cretacic subcrop sequence found throughout Riyadh’s downtown region. It is based on data obtained during the investigation of Riyadh’s Metro Line 3 (41.6 km), the longest line within the largest metro project ever built from scratch and as a whole. The geotechnical testing protocol involved around 9 km of core-drilling with tests conducted inside the boreholes, collection of more than 900 samples of rock cores obtained to perform laboratory tests, and 42 km of multi-technique geophysical survey. This article provides data concerning ground properties based on the statistical evaluation of intact rock and rock mass scales. Considering the lack of pre-existing information, this paper could be useful when planning the design of future geotechnical investigations and projects spanning the city of Riyadh. Furthermore, this paper also provides empirical correlations which are usable to obtain indirect geotechnical parameters, applicable to other urban areas crossing frameworks similar to Riyadh’s subsoil.