Mapping shallow weak zones in Riyadh carbonate terrain using integrated MASW and borehole data

Reliable delineation of shallow weak zones is essential for infrastructure design in carbonate-alluvial environments where thin soils overlie fractured or karstified limestone. In this study, 15 closely spaced multichannel analysis of surface waves (MASW) profiles were acquired along urban corridors in Riyadh, Saudi Arabia, and integrated with six geotechnical boreholes (SPT and RQD) to characterize subsurface conditions to depths of approximately 20 m. Dispersion curves were inverted to produce two-dimensional shear-wave velocity (Vs) sections, which were calibrated using lithologic logs and penetration resistance data. The integrated results define a consistent three-layer structure comprising: (1) near-surface silty sand with gravel (Vs ≈ 360-760 m/s), (2) highly weathered to fractured limestone (Vs ≈ 760-1500 m/s), and (3) compact limestone bedrock with Vs locally exceeding 3000 m/s. Soil thickness above limestone varies laterally from approximately 2 to 11 m. Shallow low-Vs anomalies (30-60% velocity reduction relative to surrounding materials) are detected between 2 and 10 m depth and correspond to intervals of poor RQD and reduced SPT resistance, indicating fractured or cavity-prone zones. Vs30 estimates classify most of the area as NEHRP Site Class C, with localized tendencies toward Class D where soils are thickest and weak zones cluster. The principal contribution of this work lies in the high-resolution, corridor-scale integration of MASW and borehole data to map laterally discontinuous weak zones and generate engineering-ready spatial products (soil thickness, average Vs, and site class) suitable for foundation planning and infrastructure alignment in heterogeneous carbonate terrains.