Ashton Underground Mine (Ashton) is an underground longwall mine located northwest of Singleton in the Hunter Valley of NSW. The mine has extracted longwall panels in three seams with mining in a fourth seam planned and each seam progressively deeper than the last. The mining geometry in each of the seams is regular, parallel and either offset or stacked relative to the panels in the seams above. The high-quality survey monitoring dataset available from Ashton provides significant insight into the mechanics of ground behaviour in the multi-seam geometry at this site. This paper presents a summary of the observations and interpretations previously presented in 2017 to 2022 for the mining in two seams, for the mining in the third seam and is a final update after final longwall has been mined in a three seam geometry.
Observations of the characteristics of multi-seam subsidence indicate that although subsidence movements above multi-seam mining are more complex than for single seam mining, these movements are nevertheless regular and predictable. However, they are sensitive to the relative panel geometries in each seam and to the direction of mining.
Cumulative vertical subsidence after longwall mining in three seams has now reached 5.8m with incremental vertical subsidence, as a percentage of incremental mining height, increasing with each episode of subsidence. Latent (extra) subsidence from near overlying pillar edges and stacked goaf edges is recovered when mining in the seam below and contributes to all subsidence parameters. In an offset geometry, remote from pillar and goaf edges, tilt and strain levels are similar or lower than single seam levels, despite the greater vertical subsidence, due to the general softening or reduction in shear stiffness of the overburden with each episode of subsidence. At stacked and undercut goaf edges, tilts and strains are significantly elevated.
Some conventional single seam concepts such as angle of draw and subcritical/supercritical geometry require consideration of the overall mining geometry in a multi-seam environment. A site-specific methodology developed to forecast subsidence behaviour has provided reliable estimates of actual subsidence effects.
Ken Mills and Stephen Wilson, Observations of multi-seam subsidence at Ashton Underground Mine-2012 to 2022, Proceedings of the 2023 Resource Operators Conference, University of Wollongong - Mining Engineering, February 2023, 70-83.