What is a Highwall?

In coal mining terms, a highwall is the excavated face/slope which advances down-dip with each successive strip that is mined in an open cut strip mine.

How are Highwalls Excavated?

Following the excavation of a boxcut to establish the initial open cut pit for a new coal mine, highwalls are typically prepared by means of drilling and blasting of the overburden material (i.e., non-coal coal measure rocks and soil which overlie the target economic coal seam(s) to be mined).

Excavation Sequence

A typical excavation sequence will include:

• Pre-stripping and stockpiling of topsoil for future mine rehabilitation.

• Free-dig removal of soil and weathered rock overburden by means of mechanical excavation (e.g., excavators, rope shovels, etc), without requiring drilling & blasting.

• Drilling & blasting the fresh overburden coal measure rocks to achieve the required geotechnical and mine design.
• Blocks mined laterally along the strike of the coal seam
• Strips are mined in succession down-dip (i.e., from shallowest to deepest depth of cover/lowest to highest stripping ratio) until the stripping ratio becomes uneconomic for continued open cut mining methods.

Machinery & Equipment

In large strip mines, draglines are typically employed to move the blasted overburden material to expose the target/economic coal seam(s). Capital costs aside, draglines offer some of the highest production rates in terms of bank cubic metres (BCM) moved at the lowest cost.

Truck-and-excavator fleets, with the assistance of dozer-push operations, are also extensively used where draglines are impracticable or the deposit is more structurally-complex.

Highwall Geotechnical Design

Highwall design is governed by numerous factors, particularly relating to geotechnical and mine planning requirements. As a general rule of thumb, highwalls in Australian coal mines are often designed as follows:

• Strips are typically orientated to avoid interaction with major geological structures (such as faults), and limit kinematic availability of planar, toppling, and wedge-type failures.
• Batters (i.e., the slope face) are typically pre-split at 70° angles.
• Benches may be incorporated into the highwall to reduce the overall slope angle, as well as provide an area to catch failed/rilled/eroded material.

Highwall Access to Underground Coal Reserves

Highwalls can also provide a means of access to additional underground coal reserves when the stripping ratio becomes uneconomic for continued open cut mining. In this regard, portals and drifts can be driven into final highwalls (and endwalls) to develop longwall and/or bord-and-pillar operations.

Highwall Mining Techniques

Coal reserves that lie beyond final highwalls, which would otherwise be sterilised, can be recovered by means of highwall auger mining (HWAM) and/or continuous highwall mining (HWM or CHM), if practicable. These methods are typically employed when thermal and metallurgical (i.e., steelmaking) coal prices increase during the window of opportunity between the final highwall being excavated and in-pit dumping and mine rehabilitation commencing. 

An example of Coal Augering Services' highwall auger mining (HWAM) operations in action at an open cut metallurgical coal mine in Queensland's Bowen Basin is shown below.