Internal fluidisation of granular material

Abstract

Uncontrolled seepage flow due to defects or imperfect joints in civil engineering structures (such as dams, levees, dry-dock, seepage barriers, and sheet piles interlocking) or from fractured underground pipes is a major concern. The result of such leakage is increased water pressure and hydraulic gradients at the source of leakage making the behaviour of the surrounding material complex. This is because of the interaction of particles with the source of flow, turbulent flow in the surrounding bedding and the potential of developing additional mechanisms at the source of flow. Evidence of the occurrence of internal fluidisation has been reported in a number of fields, and yet not much is known about it.In this fundamental study experimental apparatus and techniques have been developed to investigate the mechanism of internal fluidisation of granular material due to localised flow. A two-dimensional experimental model was created for the study, in which a machined box with variable orifice openings was designed and built to simulate an idealised crack for a localised leak. The box was fitted inside a modified seepage tank to fluidise a bed of granular material. Various parameters were investigated: flow rate, excess pore water pressure in the bed, pressure upstream of the orifice, particle size, particle shape, height of the bed, and orifice size on the observed mechanism. Image analysis techniques based on the Particle Image Velocimetry (PIV) have been developed in this study to monitor the behaviour of the fluidised zone.Results and observations of this study suggest that the mechanism of internal fluidisation in a bed of granular materials is associated with an uplift mechanism of the grains in the active region of the bed. This is attributed to the drag force exerted by seepage flow overcoming the downward bulk weight of the bed. From the results and observations of this study a mathematical model based on the concept of force equilibrium has been proposed to predict the pore pressure at the onset of internal fluidisation. The results show that high pressure heads can be sustained upstream of the orifice without the internally fluidised zone breaking through to the bed surface. They also show that the onset of this mechanism in a bed of granular material is highly dependent on packing properties (grain size, grain shape, height of the bed), seepage velocity and orifice size. <br/