Interpretation of the Torsional Resistance of Shallowly Embedded Hemiball Penetrometer

Abstract

The hemiball penetrometer is a geotechnical device developed to measure the property of surficial seabed sediments (<0.5 m), which is of most significance for pipeline design and for situations in which traditional site investigation methods face challenges in accurately characterizing the soil properties. This study developed a novel method to derive the axial frictional resistance along the pipeline through a simple scaling relationship from the torsional resistance measured by the hemiball penetrometer in situ. To achieve this, the undrained and drained torsional resistances of the hemiball for idealized conditions were theoretically explored using the plasticity theory and principles of critical state soil mechanics. The link between undrained and drained torsional resistances for the idealized conditions then was established. Calibrated against those theoretical solutions, coupled finite-element analyses were conducted to explore solutions for more-realistic real-life conditions and to study the effects of hemiball embedment:diameter ratio, initial strength profile, and consolidation after hemiball or pipe installation. Based on these analyses, a simple model using two scaling factors, the effective radius of the hemiball and the equivalent length of the pipe, was proposed to derive the pipe–soil interface friction from the measured torsional resistance of the hemiball. The method presented in this paper provides a simple yet powerful approach for interpretation of hemiball testing results, and is an important step toward the eventual application of the hemiball in deep-water pipeline engineering.