LONG-awaited maps showing the extent of old coalmines under the Newcastle business district have been made available by the Mine Subsidence Board.
While not the detailed three-dimensional images that some had hoped for, the online maps define the old workings into six colour-coded categories.
These depend on the amount of grouting – or partial filling with cement and fly-ash – needed to strengthen the workings before building over them.
Green areas, the largest of them at Wickham, have no workings underneath them and are given the all-clear by the subsidence board.
The Civic ‘‘wedge’’ site of the proposed new seven-storey state court building is on the border of two mid-range categories and is at least partly on top of the old and shallow Yard seam, just 30metres or so below the surface.
Subsidence board chief executive Greg Cole-Clark said the maps, known formally as a mine subsidence category plan, had been developed by the board as part of a working group involving state government, local council and private sector bodies.
Similar maps would also be prepared for ‘‘growth areas’’ at Charlestown and Glendale.
The board has noted previously that ‘‘practically all remaining vacant land in the Newcastle, Lake Macquarie and Maitland districts [is] undermined by old abandoned mine workings’’.
Mr Cole-Clark said substantial grouting had been needed for sections of the $1.7billion Hunter Expressway.
Property Council of Australia regional director Andrew Fletcher said the maps would help developers deal with the uncertainty of mine subsidence.
‘‘This information will help identify which sites carry the greatest commercial risk, but the challenge now is to develop a strategy which can reduce the risk, facilitate investment and fast-track urban renewal in the CBD,’’ Mr Fletcher said.
Part of the Honeysuckle ‘‘red’’ zone at Cottage Creek is marked as having an ‘‘in principle grouting proposal’’ but Mr Cole-Clark said this idea was still at its early stages.
As things stand, individual land-owners are responsible for the costs of grouting their sites.
To gain sufficient strength, the grout is often required to run under neighbouring properties to the one being developed.
Mr Cole-Clark said the working group was looking at ways of equitably sharing the cost of grouting.