Regional Calibration of CPT Correlations for Shear Wave Velocity in Budapest Soils

Abstract Small-strain stiffness and shear wave velocity (V s ) are key parameters in geotechnical and seismic design, yet direct in situ seismic measurements are often unavailable in routine site investigations. This study evaluates and recalibrates CPT-based V s correlations for the geological conditions of Budapest, Hungary, using a database of 1304 paired CPT and seismic measurements from 15 urban sites. Forty-two published CPT-V s correlations were examined and the 25 best-performing relationships retained for detailed statistical assessment. Most of them underpredicted the measured velocities. Site-level hypothesis testing confirmed a significant underprediction for the Robertson (2009) framework over the whole database, and for every best-performing correlation above about 400 m/s, where mean bias ratios fall to 0.72–0.91. The best existing relationships were those of Robertson (2009), Andrus et al. (2007) and Wolf (2018), but recalibrating the Robertson (2009-q c1N ) framework improved on them. New Budapest-specific correlations are proposed for all soil types represented in the database and, separately, for Quaternary and Tertiary deposits. The all-soils correlation reduced the RMSE from 64.1 to 56.7 m/s and improved the mean bias ratio K mean from 0.92 to 1.02. Leave-one-site-out cross-validation confirmed that the coefficients are not overfit, the out-of-sample RMSE increasing by only 2.2–6.3%. The deviation from global correlations is interpreted in terms of local soil microstructure: measured-to-estimated velocity ratios and normalized small-strain rigidity indices indicate ageing, overconsolidation and cementation. The proposed correlations reproduced the Eurocode 8 ground type at 13 of the 15 sites from CPT-derived V s profiles, supporting their use for seismic site characterization and microzonation in Budapest.