In situ dynamic piezocone penetrometer tests in natural clayey soils — a reappraisal of strain-rate corrections

摘要

Cone penetration testing with pore pressure measurement (CPTU) is a cost- and time-efficient way of collecting in situ geotechnical parameters of near-surface marine soils for cable and pipeline tracks, offshore foundations, and geohazard identification. The measured dynamic CPTU parameters (cone penetration resistance, sleeve friction, pore pressure) are higher than the measured static CPTU parameters. This mismatch is caused by the different penetration rates used for dynamic and static CPTU tests (dynamic tests have up to 500 times higher penetration rates than the static tests; i.e., with the commonly used 2 cm/s penetration rate). This study presents comprehensive Calypso piston and gravity core as well as dynamic and static CPTU datasets acquired in the landslide-prone S?rfjorden area (Finneidfjord, northern Norway). The fjord-marine sediments at the study site are characterized as normally consolidated to slightly over consolidated clay-dominated soils with embedded layers of sandy silt to sand. The dynamic CPTU results were corrected to match the nearby static CPTU (i.e., distance less than 10 m) using strain-rate factors (SF) derived from three known correction methods. Based on a statistical test and visual comparison of dynamic and static CPTU profiles, the modified inverse sin-hyperbolic correction method is found to be best suited for the strain-rate correction of dynamic CPTU tests, and results in SF less than 1.35 for the corrected cone penetration resistance and up to 2.4 for the sleeve friction. Our data illustrate a positive correlation between penetration rate and penetration depth, which is governed by the consolidation state of the clays. The good agreement between SF-corrected dynamic CPTU data from 34 deployments (acquired within less than 36 h shiptime) with data obtained from static CPTU and laboratory experiments on sediment cores further demonstrates that the MARUM dynamic CPTU device is a powerful tool for characterizing the properties of surficial seafloor sediments in shallow-water environments.