Indentation of a free-falling lance penetrometer into a poroelastic seabed

Abstract

A solution is developed for the buildup, steady, and postarrest dissipative pore fluid pressure fields that develop around a conical penetrometer that self-embeds from free-fall into the seabed. Arrest from free-fall considers deceleration under undrained condi- tions in a purely cohesive soil, with constant shear strength with depth. The resulting decelerating velocity field is controlled by soil strength, bearing capacity factors, and inertial components. At low impact velocities the embedment process is controlled by soil strength, and at high velocities by inertia. With the deceleration defined, the solution for a point normal dislocation migrating in a poroelastic medium is extended to incorporate the influence of a tapered tip. Dynamic steady pressures, PD , develop relative to the penetrating tip geometry with their distribution conditioned by the nondimensional penetration rate, UD , incorporating impacting penetration rate, consolidation coefficient, and penetrometer radius, and the nondimensional strength, ND , additionally incorporating undrained shear strength of the sediment. Pore pressures may develop to a steady peak magnitude at the penetrometer tip, and drop as PD51/x D with distance x D behind the tip and along the shaft. Induced pore pressures are singular in the zone of tip taper for the assumed zero radius of the penetrometer, negating the direct evaluation of permeability magnitudes from pressures recorded on the cone face. However, peak induced pressure magnitudes may be correlated with sediment permeabilities, postarrest dissipation rates may be correlated with consoli- dation coefficients, and depths of penetration may be correlated with shear strengths. The magnitudes of fluid pressures evaluated on the shaft may be correlated with sharp penetrometer data ~reported by Urgeles et al. in 2000! to independently evaluate magnitudes of strength and transport parameters.