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

Abstract

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 Sørfjorden 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.Key words: dynamic cone penetration testing with pore pressure measurement (CPTU), static CPTU, clayey soils, in situ comparison, Finneidfjord, weak layer, strain-rate effect, soil-specific rate coefficient.Résumé : L'essai de pénétration du cône avec la mesure des pressions interstitielle (CPTU) est une façon efficace d'un point de vue temps et coûts pour récolter des paramètres géotechniques in situ de sols marins près de la surface pour des applications comme le placement de câbles et pipelines, les fondations en mer et l'identification des risques géotechniques. Les paramètres dynamiques CPTU mesurés (résistance à la pénétration du cône, friction de l'enveloppe, pression interstitielle) sont supérieurs aux paramètres statiques CPTU mesurés. Cette différence est due aux taux de pénétration différents qui sont utilisés pour les essais dynamiques et statiques de CPTU (les essais dynamiques ont des taux de pénétration jusqu'à 500 fois plus élevés que les essais statiques, par exemple un taux de...

Validation of impact penetrometer data by cone penetration testing and shallow seismic data within the regional geology of the Southern North Sea

“…Consequently, penetrometer measurements solely qualify to derive undrained parameters such as undrained shear strength (as found by Danziger and Lunne 2012;Steiner et al 2014).…”

“…7) to correct for this effect. The relationship has been applied to impacting penetrometers by Dayal (1974) as well as Dayal and Allen (1975) for a velocity range of 0.13-5.50 m/s, and subsequently widely adopted for dynamically penetrating impactors such as small penetrometers (Stoll et al 2007;Stark et al , 2012, model penetrometers (O'Loughlin et al 2004), and lance-shaped impact penetrometers (Steiner et al 2014):…”

“…A soil viscosity coefficient of 1.5 denotes a 1.5-fold increase in dynamic cone resistance q t,dyn if velocity increases by one order of magnitude with reference to v ref . The strain rate correction after Dayal (1974) and Dayal and Allen (1975) is meaningful only where v dyn ≥v ref , because of the logarithmic term in Eq. 7.…”

“…Typically, a reference velocity of 0.02 m/s is chosen to compare impact penetrometer data with static velocity CPT measurements. The soil viscosity coefficient has been applied for dynamic penetrations with penetrometers in clayey sediments, where it ranges between 0.04 and 1.50 (Dayal and Allen 1975;Dayal 1980;Randolph 2004;O'Loughlin et al 2004O'Loughlin et al , 2013Einav and Randolph 2005;Aubeny and Shi 2006;Low et al 2008;Zhou and Randolph 2009;Young et al 2011;Nazem et al 2012;Steiner et al 2012Steiner et al , 2014Airey 2013, 2014). Applications of the strain rate correction on silty sediments yield a soil viscosity coefficient between 0.13 and 0.36 (Perlow and Richards 1977;Biscontin and Pestana 2001).…”

“…Applications of the strain rate correction on silty sediments yield a soil viscosity coefficient between 0.13 and 0.36 (Perlow and Richards 1977;Biscontin and Pestana 2001). Several authors employing lightweight dynamic penetrometers penetrating sands at high rates have used values of 0.7-1.5 for the soil viscosity coefficient λ, although Dayal (1974), Dayal and Allen (1975), and Lunne et al (1997) concluded from laboratory observations that the strain rate correction is applicable only to cohesive sediments.…”