Electrically induced pore pressures in high salt content clay slurry

The development of stagnant pore water pressures due to non-uniform and non-steady distribution of voltage gradients were investigated in high water and salt content clay slurry using a floor-scale apparatus. The experiment was intended to simulate electrically enhanced dewatering of sea harbor sediments by horizontal drainage towards vertically installed electrodes. The scale of the test apparatus allowed for accurate measurements of the spatial and temporal variations of water pressure head, temperature, electrical potential and current throughout the test duration of 5 weeks. The pore water pressure dissipation measured at one-third the depth of the clay bed from mud-line appeared to alternate in rate and magnitude between the equally separated adjacent probe-stations, before it ceased completely. The voltage gradients between these stations fluctuated in the similar alternating manner indicative of significant variations in the local electro-osmotic water mass transport rates among the regions. It is postulated that the non-uniform voltage gradients give rise to competing pressure gradients; and the electrical potential applied orthogonal to the settling direction polarize clay particles hence disrupting the dewatering process by pore water pressure dissipation and compaction in high salt content clay sediments.

► The electrically induced settlement of mud-line and the pore water pressure dissipation patterns of a high water and salt content clay slurry were examined. ► The floor scale experiments were designed to simulate potential dewatering applications in sea harbor sediments using vertically placed electrodes and horizontal drainage. Hence the electric field was applied in the orthogonal direction to particle settlement. ► The long term experimental results showed that the settlement was hindered due to spatially nonlinear and temporally oscillatory voltage gradients which give way to non-uniform and stagnant pore water pressures within the clay slurry. ► The possible reasons for nonlinear and oscillatory voltage gradients were attributed to streaming potential and migrating fronts of ions in the high salinity environment. ► The resistance to pore water pressure dissipation by compaction of the slurry was attributed to potential shrinkage of particle DDL and the resulting polarization of the particles in the direction of the electric field. These effects would synergistically create competing pressure gradients and forces that keep the clay particles from settling as long as the electrical potential is maintained at the same level. ► Hence, in high water and salt content environments electrically enhanced settlement may not be a viable option unless the electrical field design is tailored and well controlled for the particular configuration.