Integrated Geophysical and Geotechnical Investigation for Detection of Karstic Weak Zones in Limestone Terrain, Riyadh, Saudi Arabia

Carbonate terrains in central Saudi Arabia are prone to subsurface hazards due to karstification, fracturing, and differential weathering, posing significant risks for large infrastructure developments. This research presents an integrated geophysical and geotechnical investigation carried out for the proposed construction site at Riyadh, located along the Wadi Hanifah escarpment. The site is underlain by highly to moderately weathered Jurassic limestone of the Shaqra Group, characterized by vugs, fractures, and solution-filled discontinuities.

Multichannel Analysis of Surface Waves (MASW) was employed to map subsurface stiffness variations and identify potential cavities and weak zones. A total of 1400 linear meters of MASW profiles were acquired using a 24-channel system with 2.5 m geophone spacing, achieving an investigation depth of up to approximately 25 m. Shear-wave velocity (Vs) sections were generated through dispersion analysis and inversion of surface-wave data. The interpreted Vs values range from about 200 m/s to 3500 m/s, where higher velocities (>1500 m/s) represent competent limestone, while lower velocities (<1000–1500 m/s) indicate fractured, weathered, or solution-affected zones.

MASW results delineated several localized low-Vs anomalies corresponding to solution-filled vugs and cavities at depths ranging from approximately 1 m to 13.5 m. These geophysical findings were correlated with borehole data from fifteen geotechnical boreholes, including rock coring, RQD measurements, pressuremeter testing, and laboratory strength testing. Borehole logs confirm the presence of highly fractured limestone with variable RQD (0-100%) and unconfined compressive strength values between about 13 and 65 MPa. Zones identified as weak in MASW sections coincide with intervals of low RQD, poor core recovery, and solution-filled fractures observed in the boreholes.

The integrated interpretation demonstrates that MASW is an effective tool for rapid detection and lateral mapping of karst-related weak zones in limestone terrains when calibrated with geotechnical data. The results provided critical input for foundation design, ground improvement planning, and risk mitigation at the site. This study highlights the value of combining surface-wave geophysics with conventional geotechnical investigations for sustainable and safe development in karst-prone regions.