Variability of in situ test results from a coastal silt deposit in Finland

The geotechnical characterisation of silty soils remains a challenge due to their transitional behaviour and high variability. In Finland, silts are widespread but poorly represented in existing correlations, which are largely based on clays or sands. Current practice often relies on Weight Sounding (Painokairaus, PK) and Combined Static–Dynamic Penetration Testing (Puristinheijari, PH), supported by empirical guidelines developed several decades ago. While these methods are cost-effective, their reliability in silts is uncertain and often conservative. In this study, we reported results from a benchmarking campaign at the Haistila test site in south-west Finland, where PK, PH, and piezocone penetration tests (CPTU) were performed independently by Tampere University and Ramboll Finland Oy. The objective was to quantify variability between methods and operators, and to assess implications for geotechnical design. Results showed that CPTU provided the most repeatable measurements, with cone tip resistance showing the lowest relative error and coefficient of variation. In contrast, PH results displayed greater variability, particularly in torque, and resulted in possibly conservative design parameters according to national guidelines. The findings confirmed that PK and PH are useful for stratigraphic profiling but not for parameter derivation in silts. CPTU, if calibrated with laboratory data, offers a more robust alternative and highlights the need for updated, silt-specific correlations in Finland.