In situ fluidisation by a single internal vertical jet

Abstract

Investigations have been conducted of in situ (unbounded) fluidisation produced by a vertical jet acting internally within saturated sands. This produces a sharply defined in situ fluidised zone, which changes with increasing jet depth from an open, axisymmetric, approximately ellipsoidal form to an asymmetric, spouted profile, and thence to a submerged fluidised cavity. Measurement and dimensional analysis of fluidised zone geometries indicate in situ fluidisation to be controlled by two mechanisms: (i) scour below the jet tip, which follows the linear velocity decrease of a submerged jet; and (ii) the ability of the flow to maintain fluidisation, which controls the zone diameter. The depth of fluidised cavity formation is shown to be a function of the ratio of the flow rate to that required for minimum (turbulent) fluidisation. The observations are justified in terms of jet diffusion, sediment transport, fluidisation and slope stability theory.