Flow Analysis, Transportation, and Deposition of Frictional Viscoplastic Slurries and Pastes in Civil and Mining Engineering

Abstract

Backfilling and injection of granular materials into mining induced voids, separated beddings, and cracks, as either diluted slurry or concrete paste, is widely used to control coal mine subsidence. As a viable environmental solution, mine waste and rejected materials from underground coal seams are used in both backfilling and injection mine operations. During longwall mining, the grout slurry is pumped into the separated beds of the fractured rock mass through a pipeline connected to a central vertical borehole, which is drilled deep into the interburden rock strata above the coal seam. Either as dilute slurry or thick paste or cake, the fill material normally needs to travel a significant distance in a long pipeline. A blockage can occur in the pipeline when the slurry velocity falls below a certain critical threshold value, indicating a material phase change from cohesive-viscous to cohesive-frictional. In a previous study of radial flow through disks, complete analytical solutions of the required pump pressure versus fluid volume rate were presented for such slurries, categorized as frictional Bingham-Herschel-Bulkley fluids. This paper is an extension to the theory of fluid mechanics to this type of flow in uniform circular pipes. General analytical solutions were developed for complex fluids in velocity and pressure gradients and velocity and pressure as a function of pipe length, from which special and familiar equations for simpler fluids are derivable by mathematical reduction of the general equations. This study differs from the previous research in consideration of the variable shear parameters rather than fixed values, inclusion of total nonlinear behavior, and implementation of a friction function to mimic behavior of the depositing and consolidating stiff slurry, which can cause a significant pressure rise as a result of the increased shear resistance.