A palaeochannel is a remnant of an inactive river or stream that has been filled or buried by younger sediment.

A geotechnical review of mining in those areas will be completed before mining takes place which may include more infill drilling and increased material strength testing in those areas, according to an investigation by the New South Wales Mine Safety Inspectorate.

Regular open cut mining operations were being undertaken on the morning of 6 June, 2015 when a section of the highwall adjacent to a public road failed. There was nobody in the immediate area at the time of the slump.

The failure was about 160m wide and approximately 55m high. The highwall edge before the failure was 40m from a public road and 12m after the failure.

The site emergency response team established road blocks on the public road as a precautionary measure immediately after the incident and local authorities were contacted.

Mine geotechnical inspections and assessments were conducted and strategies for remedial work developed.

Remedial work to stabilise the failed area and adjacent areas began, following risk management processes, and these have been completed.

A palaeochannel infill sequence of materials had been exposed during recent mining operations. It included materials that had not been identified in geological modelling.

This palaeochannel was locally deeper than previously exposed and comprised of differing lower level sediments. The increased thickness of the channel consisted of relatively loose silty sands and weaker clays which were at the base of the palaeochannel infill at the top of the coal seam.

The upper 25m of materials were stiffer and stronger and it is believed these have caused overstressing of the clays and infill sands. Undrained pore pressure is believed to have triggered localised liquefaction and lubricated the fissures within the clays.