This paper describes field tests in which a 250mm steel sphere was allowed to free fall through water from drop heights of up to 2m and dynamically embed the soft clay underlying the water. Instrumentation housed within the sphere measured accelerations in three orthogonal axes, as well as rates of rotation about those three axes. The data were used to calculate velocities and displacements of the sphere during free fall in water and embedment in soil. Reasonable agreement was obtained between the measured velocity profiles and velocity profiles predicted using a simple approach based on strain rate dependent shearing resistance and fluid mechanics drag resistance.
Understanding the processes associated with dynamic penetration of rigid bodies from water into soft soil is challenging. There are a number of applications for dynamic penetration of rigid bodies, including installation of dynamically installed anchors, free-fall gravity core samplers and in situ characterisation tools. Previous work in this arena include centrifuge studies reported by Poorooshasb and James (1989), Richardson et al. (2006), O’Loughlin et al. (2004, 2009); field tests reported by Freeman et al. (1984), Lieng et al. (2010); and numerical studies reported by Einav et al. (2004), Nazem and Carter (2010), Raie and Tassoulas (2006). In these studies, the geometry of the rigid body tends to be rather complex, to the extent that a number of simplifying assumptions are required in order to address the problem. In this paper, the geometry is simplified to a sphere for which the soil mechanics is quite well behaved (Randolph et al., 2000). This permits a more rigor rigorous assessment of dynamic penetration effects. The data are then used to validate an embedment model based on strain rate dependent shearing resistance and fluid mechanics drag resistance.