Laboratory investigation of polydisperse rock releases into deep receiving waters through vertical, enclosed fall-pipes

Laboratory experiments were conducted to investigate the transient release of graded particle mixtures into deep receiving waters through submerged vertical pipes with varying diameters and lengths. The experiments were scaled through geometric and dynamic similarity considerations to model the dumping of waste rock fragment masses into deep coastal waterbodies via enclosed vertical fall-pipes. The model rock fragment mixture was generated from colour-coded particle size classes, with their in-pipe vertical transport and release from the pipe exit measured from time-synchronised camera recordings. Quantitative image analysis methods were developed to determine the bulk and fractional particle velocities and concentrations at different pipe elevations and immediately below the pipe exit. For larger mass inputs, induced in-pipe fluid motions and particle mixing were strongly three-dimensional, characterised by upward and downward velocity fluctuations associated with oscillation and drawdown of the enclosed water surface. Equations of motion were developed to describe this transient free-surface behaviour within the pipe. Increased pipe confinement of the mass input was shown to hinder sedimentation from the fall-pipe, whilst promoting in-pipe segregation of the particle size classes released from the pipe exit. Finally, a series of sequential mass release experiments was conducted to investigate the quantity and fate of residual fines remaining in the fall-pipes after each mass input. These fines were ejected from the pipe exit as a puff or puffs before being re-entrained into the subsequent coarser particle stream. The implications of these scaled experimental measurements for optimising waste rock fragment mass disposal through vertical fall-pipes are discussed.