Dynamic embedment of projectiles in clay

C.D. O’Loughlin, M. Nazem, S.H. Chow, M.F. Randolph, C. Gaudin and D.J. White

This paper provides an overview of the work of the Australian Research Council-funded Centre for Geotechnical Science and Engineering on free falling projectiles that have applications as seabed characterisation tools and as anchoring systems for floating facilities. These projectiles are released in water and dynamically embed into the seabed through the kinetic energy they gain during freefall. The high penetration velocity, which can be up to 25 m/s at impact with the seabed, induces shear strain rates in the soil that are up to eight orders of magnitude higher than in a typical laboratory test. The difficulty in quantifying the soil strength at these very high strain rates, together with hydrodynamic aspects including pressure drag and potential water entrainment along the projectile-soil interface, complicates assessment of the penetration response. A large database of centrifuge and field data has been collated by the Centre and is used in this paper to quantify embedment potential and to examine the merit of a simple analytical framework that captures the dynamic response of free-falling projectiles. Aspects of the dynamic embedment process that cannot be predicted by the analytical framework, including potential hole closure during installation and pore pressure generation are investigated in finite element analyses that model the dynamic penetration of projectiles in soil. Example results from these analyses are provided.