Author(s): A. G. Tsakiris; A. N. Thanos Papanicolaou; B. M. Billing
Linked Author(s): Thanos N. Papanicolaou
Keywords: Fluidization; Porewater pressure fluctuations; X-Ray CT; Kaolin; Porousnetwork; Estuary
Abstract: Aggregative fluidization occurs when soil grains detach from each other and a zero effective stress condition is initiated. Such conditions leading to aggregative fluidization are typically observed in estuarine environments, resulting from extreme porewater pressure fluctuations or the sedimentation of cohesive materials. The effects of fluidization on cohesive soil properties, near bed turbulence and sediment entrainment have been thoroughly investigated in the literature. Nevertheless, little quantitative information exists to date regarding the threshold conditions, which characterize the commencement of aggregative fluidization in a cohesive mud. The objective of the present experimental investigation is to quantify the onset of fluidization in a pure kaolin mud layer. A fluidization column attached to a pressurized vessel (plenum) was carefully designed to ensure replicate fluidization conditions of an advective regime. Under the advective regime, fluidization was accompanied by the formation of a vertical vent (elutriation pipe) across the kaolin mud layer and a volcano-type structure on the surface of the mud layer. A first set of preliminary experiments revealed that in order to characterize the onset of fluidization, two criteria are necessary, namely the differential plenum pressure and a characteristic time scale, ∆T, of the fluidization phenomenon. A final set of threshold experiments determined the critical plenum inflow rate at which the onset of fluidization occurs. The threshold experiments further suggested that ∆T is indicative of the porous network development within the kaolin layer due to the fluidization action, because the plenum pressure was released during ∆T, prior to the rupture of the fluidization vent. The development of the porous network within the kaolin layer was shown to induce a decrease in the kaolin mud concentration via gamma source system measurements. The use of X-Ray CT will enable the visualization of the porous network, whose key characteristics will be estimated via image analysis.
Year: 2009