Page 303 - Advances In Productive, Safe, and Responsible Coal Mining

Sustainable reclamation and water management practices 279

earthworms, soil microbes, etc.) contained in the soil remain alive and able to assist
the reclamation process. Soil is essential for rapid restoration of natural ecosystem
structures and processes after disturbance [31,32].
After soil removal, bulldozers construct benches for drill rigs to bore holes for
blasting reagents. Blasting activities are carefully controlled and monitored, so no
off-site damage occurs, especially damage to nearby roads, buildings and other struc-
tures. Once the overburden has been fragmented and fractured by blasting, surface
mining equipment such as draglines, front-end loaders and trucks, and bulldozers
remove the overburden to reach the coal. The blasting sequence and pattern are essen-
tial to environmental controls, as successful reclamation requires that geologic mate-
rials be managed in accord with their physical and chemical properties. The blasting
sequence and pattern will influence the ease with which adjacent geologic strata can
be separated from one another at the loading site, when such separation is required for
effective environmental control. The blasting pattern can also influence rock-
fragment size distributions as needed for reconstruction of the mined landform.

14.3.5 Backfill construction to isolate problematic materials
Once coal is removed, filling the pit or backfilling the highwall begins with material
from adjacent pits. Overburden materials with suitable chemical characteristics are
used to fill the pit. If problematic spoils have been identified, special provisions
are made to ensure that they are disposed of in a manner that achieves “hydrologic
isolation,” meaning that they are placed such that they are not subject to in situ
leaching by environmental waters. Such materials can be segregated during overbur-
den excavation and placed above the water table in a 3- to 5-m layer of problematic
material that is solidly based (i.e., compacted) on the undisturbed strata below the low-
est coal seam (“pavement”). This layer is then covered with nonproblematic material
that is also compacted and has a pitched surface to shed water. Although these pro-
cedures were developed for isolation of highly acidic strata [10,11], they can be
applied to isolate high-TDS and/or high-Se strata as well.
In some cases, acid material can be managed bymixingwith nonacidic material con-
tainingsufficientalkalinitytoneutralizetheacidity,buttheacid-neutralizationreaction
tends to generate high TDS concentrations in water. Therefore, more emphasis is being
placed on isolating acidic materials. Materials traditionally used for enveloping and
sealing acid-producing materials in the backfill, such as fly ash or kiln dust, must
now be scrutinized for use since they may cause elevated TDS concentrations.
The hydrology of mine backfills is complex and is under study [33]. Fine-grained
materials such as siltstones and shales, when compacted, tend to have lower infiltra-
tion rates than more permeable sandstones. The quality of water emerging from a
backfill is affected by all rock types found in flow paths, and constituent concentra-
tions are influenced by contact time and geochemistry. Therefore, mine water chem-
istry can be directly influenced by directing water through rock materials with
minimal chemical reactivity, such as low-TDS durable-rock sandstones [34]. Subsur-
face drains constructed of low-TDS durable-rock sandstones can be placed within and
at the base of backfills that receive groundwater influx for the purpose of draining that
water to daylight along a low-TDS drainage pathway.