Miners at Illawarra Coal's Dendrobium mine were lucky to escape injury recently when a slab of the roof collapsed above an access road within the mine.BHP Billiton is investigating the collapse, which involved a section of roof 14m long and 5m thick.It fell on an access road used by miners entering and leaving the mine, which is near Mt Kembla.The fall is understood to have happened on Friday about 5km from the entrance to the mine and about 700m underground. Mining operations have now resumed.A miner and the union said no-one was hurt in the collapse and miners were not trapped. It is not the only access to Dendrobium."Nobody was injured or trapped, with two alternate roadways available at the time of the incident for egress of our people," said the firm's general manager for sustainable development and external affairs, John Brannon.The Construction, Forestry, Mining and Energy Union has requested a report into the roof fall."No-one was in the area at the time," said CFMEU industry check inspector Garry Horne.Mr Horne said he was not sure how large the collapsed roof area was but added the incident "wasn't that serious".He said the roof is inspected at least every 24 hours, and that the roof inspections are carried out "sometimes on foot, sometimes in a vehicle, depending where it is".Mr Horne said he was generally satisfied with safety at the mine - but when the report was completed it might show areas where improvements could be made."That will come to me and the mines inspector," he said."They will investigate what the root causes are."Out of that they'll put things in place hopefully to stop it happening again," he said.Mr Brannon said operations and safety inspections are carried out "strictly in accordance with the provisions of the Coal Mines Regulation Act".The Dendrobium mine is understood to employ about 270 people, including miners and contractors.The mine produces about 1.7 million tonnes a year of high-quality coking and energy coal from the Wongawilli seam.