Client name: University of Newcastle

Duration: June 2013 - ongoing

Location: Newcastle, NSW Australia

Coffey developed a mine void grouting strategy that used 1/3 of the grout typically required for a development of this size. This saved our client more than $1 million in remediation costs. 


Infrastructure development in Newcastle has stalled recently due to costly underground mine void remediation. Although generally considered stable, over time mine workings can become unstable if localised ground conditions change – this is particularly risky with respect to high rise property developments.

One remediation strategy is to fully grout mine workings within the zone of influence which can cost millions. For these development projects to remain viable finding ways to reduce remediation costs is essential.


Voids underneath the city aren’t always an issue. Above the workings is generally a 20m thick sandstone layer that acts as a beam to distribute loads. And parts of the city are known to have large barriers of unmined coal - along the northern side of the NeW Space site exists one such barrier.

We were engaged to undertake geotechnical mine subsidence and environmental assessments for the NeW Space Development. To assess the risk of a mine collapse on the development we developed a model in FLAC3D. This utilised finite difference methods to model the effect of the overlying strata, and then modelled the failure potential under various loading conditions.

The model was first calibrated with historical pillar strength data. We slowly reduced the strength to model the effect of a collapse of the mine workings without grouting. We then reran the model with low strength in-fill grout over a critical area of workings. This grout would provide confining pressures to the pillars rather than roof support.


Our assessment provided the Mine Subsidence Board with enough confidence to approve the mine rectification works for the NeW Space development. This was based on the grouting of one heading along the south western boundary of the site and the sandstone layer spanning the edge of workings.