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Sustainable coal waste disposal practices 265

13.2.3 Backfilling

Following the 2008 coal fly ash slurry spill at the Tennessee Valley Authority’s Kings-
ton Fossil Plant located in Harriman, TN, the United States, the US EPA initiated efforts
to classify all coal waste as hazardous. Facing the possibility of significant increases in
waste disposal costs, coal mine operators began exploring less environmentally intru-
sive waste disposal methods that could be implemented cost effectively. Backfilling or
underground disposal was to some a natural solution. Internationally, there have been
several mines employing the practice over a long period of time; however, in the United
States, the MSHA has been very opposed to underground placement of any type of
waste that could contain combustible material, especially in the wake of the Upper
Big Branch mine explosion that killed 29 miners. The US EPA also expressed concern
that underground placement would make it difficult to detect and monitor for ground
water contamination. Therefore, research on backfilling has focused on developing
materials that set up quickly, minimizing the risk of spontaneous combustion or prop-
agation of an explosion due to air passing through unconsolidated material that contains
combustible matter. Such material would have little or no permeability, which also min-
imizes the potential for ground water contamination after placement.
In addition to addressing the waste disposal issue, the authors believe that back-
filling offers other significant advantages that derive from backfill material providing
supplemental ground support in mine openings. For example, strategic placement of
backfilling material has the potential to reduce or eliminate surface subsidence caused
by mining, increase extraction ratios so that a greater percentage of reserves are recov-
ered, and eliminate the need for costly explosion proof seals in mined-out areas.
Accurately determining and properly manipulating the rheology of backfill material
is undoubtedly the key to designing a successful underground placement program. Two
studies involving the authors have shown that there are multiple possibilities for creating
backfill mixtures that can be pumped and placed underground as backfill. In one study
[23], a mixture of 67% CCRs and 33% FCPW was used to make a paste backfill that was
injected through boreholes into underground mine openings. Roughly 14,000tons
(15,300 US tons) of material was placed at an underground depth of 100m (325ft) with
a maximum flow distance of 100m (325ft). In the second study [24], coalspiralwaste
was tested (pumped) with and without admixtures in 5-cm (2-in) and 7.6-cm (3-in) pipe
loops. Results showed that paste at a specific gravity of 1.65 could be pumped at flow
3
rates approaching 30m /h with maximum pressure of 1000kPa. Rheology is not a one-
size-fits-all characteristic, and each potential fill material must be tested to obtain its
rheology before designing backfilling systems and layouts; however, these projects
demonstrated that backfill material can be produced that meets requirements for a
high-density material that gels following placement to achieve low permeability.
Engineering an economical backfilling program must examine several important
parameters. Among these are the cost and layout of pump and piping systems to achieve
maximum coverage (horizontally and vertically) in mined-out panels and the effect
of backfill material on mine floors and coal pillars. Regarding the former, in a
follow-up to the 2010 Spearing study, an economic analysis indicated that employing
paste backfilling in a typical Illinois Basin room-and-pillar coal mine would increase