Geotechnical Properties from Portable Free Fall Penetrometer Measurements in Coastal Environments

Abstract

Coastal environments are characterized by a variety of sediment deposits with highly diverse geotechnical properties. Particularly in energetic coastal environments, sediment type and properties may vary on small spatiotemporal scales, limited previous information may be available, and sediment coring may be difficult. This study proposes a single data analysis framework to estimate geotechnical strength parameters in the uppermost layers of the seabed surface for a wide variety of sediment types from portable free fall penetrometer (PFFP) measurements without need for previous or complementary data. The framework builds on existing approaches and modifies and integrates them toward one universal analysis procedure. Target results are to distinguish sediment type, estimate relative density and critical friction angle for coarse-grained sediments, and undrained shear strength for fine-grained sediments. Data sets and sediment samples collected from seven different sites across the US served the validation and assessment of the framework. It was found that for the PFFP used a penetration depth less than 20 cm indicated coarse-grained sediments with a certainty of 96%, whereas a penetration depth greater than 25 cm was associated with fine-grained sediments with a certainty of 100%. Friction angles yielded a favorable match with laboratory triaxial and direct shear testing for the sand within ±1° and undrained shear strength values resulted in mismatches <10% of minivane shear results for most fine-grained sediments using cone factor of 10 for two groups of sediment strength-depth profiles. Higher mismatches observed for the third type of distinct profiles (up to 48%) suggested lower cone factors, likely related to a change in sediment properties. This paper proposes a single framework that builds on and evaluates existing approaches to estimate geotechnical sediment properties in coastal environments using a portable free fall penetrometer. The framework identifies different soil types (coarse-grained versus fine-grained) and proceeds with data analysis toward strength parameters. Relative density and friction angle are estimated for coarse-grained sediments, and undrained shear strength is estimated for fine-grained sediments. Results of the framework were validated and assessed with laboratory testing based on a data set collected from over seven different sites across the US. The good agreement between field and lab results supports the feasibility for one data analysis framework for the use of portable free fall penetrometers for a rapid and preliminary seabed surface characterization, especially in areas with difficult access or under extreme conditions. This makes this method accessible to a broad group of users in the geotechnical and coastal engineering community.