Speaking at the recent subsidence management conference organised by The Mine Subsidence Technological Society (MSTS), Ian Forster, technical specialist at Connell Wagner, presented his paper, Hydrogeological Impact of Shallow Longwall Mining at West Wallsend Colliery.

West Wallsend, near Newcastle in New South Wales, operates in the West Borehole Seam. In 2002 the mine submitted an application to the New South Wales Department of Mineral Resources for the extraction of longwalls 27 and 28.

The application was required to address the impact of the proposed underground mining on the local groundwater regime.

The major risk was if the overburden strata was fractured during mining, aquifers in the strata could be drained. In a worst case scenario water inflow could enter the underground workings.

A detailed investigation was carried out to characterise the hydrogeological regime in the area to assess the likely effects.

This study identified the existence of three aquifer systems in the overburden, including groundwater charged alluvial deposits in valley of Cockle Creek, where the depth of cover was about 125 metres.

The study found the likely impact of the longwall extraction on the aquifer would be minimal since the fractured zone above the mine was not expected to reach the ground surface, hence vertical drainage should not occur. The risk of the loss of this resource was considered to be tolerable, based on the nature of the aquifer and the assessed impact.

The only recommendations were for monitoring to provide additional data to detect any unexpected impact of mining on the groundwater regime and to verify the study. A targeted monitoring program was undertaken during extraction of the two longwalls to fulfil this recommendation.

Groundwater levels were monitored in seven exploration bores above the two longwall panels, confirming the limited extent and permeability of the aquifer in these bores and showed minimal response to mining-related ground movements.

A well at the southern end of one longwall panel was monitored, recording no abnormal trends.

The results of subsidence monitoring showed a good correlation with predictions ahead of mining. Actual subsidence over longwall 27 was 1.41m, compared with predicted 1.4. Subsidence over longwall 28 was 1.48m, compared with predicted 1.4m.

The results of the monitoring confirmed the predictions of the study, namely that the impact on the hydrogeological systems was negligible.