Highwall mining is a specialised and largely automated mining method
developed in the United States for the extraction of coal left under a
shallow overburden cover after cessation of economical open cut mining
operations. As the name indicates, coal is extracted from the seam
exposed along a highwall by a remotely controlled continuous miner
operating in a single unsupported opening and brought to the surface by a
string of conveyor segments. The single entries are separated by coal
.pillars and driven normal to the highwall, over areas where the thickness
of cover and volume of recoverable coal is insufficient to allow or justify
conventional underground mining. An additional highwall mining
method is augering. The success of high wall mining operations depends significantly on
geological features, notably the stability of the working roof, determined
by the type and mechanical properties of the immediate roof strata.
Stability of the ribs, absence of floor heave, faults and dykes are further
important prerequisites for profitable highwall mining. The geometric
relationship between the spatial attitudes of existing joint systems, the
direction (azimuth) of the single entry opening and its vertical profile
determine the unimpeded depth of penetration into the coal and thus its
economic success. The mechanical implications of the lithological transition zone from a
coal seam to either primary or original roof strata, compared to an
interface between the coal seam and overlying strata, characterised by a
sharp erosional boundary between coal and a secondary immediate roof
stratum, are very significant. The compatibility between the response to
mining by the geological system (floor-seam-roof) and the capability of
the mining equipment to prevent the development of instability are
essential requirements from a mechanical viewpoint. The minimisation
of vibrations, very good horizon control, an ability to vary the profile of
the opening, and the maintenance of consistently high advance and retreat
rates, are also important aspects of profitable highwall mining.