The roof collapse, which occurred at a portal of the mine, involved a section of roof 14 metres long and 5m thick.
“The investigation is being carried out determine the cause of the blocked underground roadway,” a BHP Billiton spokesperson told ILN.
“Importantly, no one was injured or trapped as a result of the roof fall.”
According to a report carried out by Cardno Forbes Rigby, in situ horizontal stresses at Dendrobium are relatively high, in line with other mines in the Southern Coalfields of NSW.
As mining occurs, the subsidence of the strata above the goaf areas can lead to some fracturing of the strata, which can in turn increase vertical permeability, according to the report.
Coking coal production at Illawarra Coal was slashed by 20% year-on-year for the June quarter because of longwall moves at both its Dendrobium and Appin collieries.