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An offshore geotechnical site investigation campaign was completed for a large wind farm development project along the US Atlantic Outer Continental Shelf (OCS) offshore New Jersey, a frontier location with few published data on soil characterization. Field exploration and a comprehensive onshore geotechnical laboratory testing program have been performed to understand the site-specific soil behavior. This paper describes the geotechnical properties of the fine-grained cohesive sediments encountered at the study site interpreted based on a consistent framework leveraging the sitewide soil data. Discussions of sample quality, soil stress history, soil compressibility and permeability, peak and critical state shear strength, strength anisotropy, and shearing rate effect for the Atlantic OCS fine-grained cohesive soils are presented from oedometer consolidation, permeability, direct simple shear, ring shear, and K0-consolidated triaxial compression and extension tests along with other conventional index and property tests. Furthermore, the Stress History and Normalized Soil Engineering Properties (SHANSEP) parameters, namely S and m, for the cohesive soils, are developed based on the specific monotonic constant volume direct simple shear (CVDSS) tests. The undrained shear strength Su profiles within the specific geotechnical cohesive soil unit developed from the SHANSEP and SP-SPW methods (Quiros, 2000) is compared to the site-specific PCPT data and laboratory undrained shear strength measurements. Comparisons of the discussed engineering properties of the Atlantic OCS fine-grained soils with other published databases for soils of the Gulf of Mexico (GOM), Offshore Trinidad, and Offshore Mozambique also are included. This paper is in a collaborative series that demonstrates the value of an integrated geoscience approach considering regulatory requirements and project design essentials. It provides a comprehensive overview of the engineering characteristics of the Atlantic OCS fine-grained soils and can assist engineers with the assignation of rate-dependant undrained shear strength parameters developed specifically for wind farm foundation design with applicability in a regional setting.

The assessment of monopile installation by means vibratory driving is discussed for a platform located in the Dutch Sector of the North Sea. The vibratory driving analysis of the 4.7 m diameter monopile was performed using the “Hypervib” method in order to evaluate the performance of a 960 kg.m excentric moment vibrohammer. The results of the installation study indicated that the monopile could be installed up to a penetration depth of 31.5 m considering the full nominal power and frequency (23 Hz) of the specified vibrohammer. The frequency of the vibrator is shown to influence the projected penetration of the monopile. In particular, the “Hypervib” method indicated that refusal could be encountered if the effective frequency of the vibrator dropped below 20 Hz. As a consequence, monitoring measures were recommended to mitigate/characterize the risks of early refusal.

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This study presents site-specific CPT-based correlation relationships for preconsolidation pressure, undrained shear strength, relative density and effective friction angle, developed for geotechnical characterization of foundation zone sediments at the Atlantic Shores offshore wind farm development. Results of laboratory geotechnical tests on samples from 49 soil borings performed at the Lease Area were correlated with corresponding measured cone resistance values from companion seabed or downhole CPTs. Ranges of overconsolidation ratios estimated using CPT-derived preconsolidation pressures from site-specific correlation relationships are in good agreement with overconsolidation ratios derived from laboratory measured preconsolidation pressures for various cohesive soil units. Interpreted ranges of cone resistance factors (i.e., Nkt) required in CPT-based undrained shear strength evaluation for cohesive soil units are significantly wider compared to the typical range of 15 to 20 representative for Gulf of Mexico clay sediments, as a result of large variability in soil plasticity as well as intermixed nature of the soils. Representative ranges of relative density and effective friction angle obtained from CPT data using site-specific correlation relationships, developed for the cohesionless soils at the Lease Area, are consistent with ranges of laboratory measured values.