Estimation
of productivity is a key factor in the design of hydraulic mines and in
assessing their economic feasibility. A published Russian formula for hydraulic
mining monitor cutting rates overstated productivity by up to 2 tonnes per
minute (251%) when applied to Solid Energy's hydraulic mines. A new formula,
taking into account additional productivity factors, was derived by Solid Energy
and produced an acceptable correlation between predicted and actual cutting
rates inNew Zealand and
overseas hydraulic mines, with an average error of approximately 0.1 tonnes per
minute.

Another key factor in hydraulic
mine design is the transport of coal slurry via flumes. Solid Energy carried out
a series of 80 full scale tests in a surface flume installation, varying the
gradient and coal:water ratio. Both steel flumes and plastic-lined flumes were
evaluated. A modification of the Chezy Open Channel Flow formula, including a
factor for slurry density, gave a reasonable correlation between actual and
predicted flows. It is believed that this Solid Energy formula is suitable for
the design of flume installations for hydraulic mining.