ABSTRACT: Ashton Underground Mine (Ashton) is an underground longwall mine located northwest of Singleton in the Hunter Valley of NSW. The mine has so far 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. A subsidence line crossing all panels in each seam has been regularly surveyed in three dimensions since the commencement of mining. The high quality data set available from this line provides insight into the mechanics of ground behaviour in a multi-seam environment. This paper presents an update of the observations and interpretation presented in Mills and Wilson (2017) for mining in two seams with the inclusion of results from mining in a third seam.
Observations of the characteristics of multi-seam subsidence continue to indicate that although subsidence movements above multi-seam mining are more complex than single seam mining, these movements are nevertheless regular and predictable. 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, transient tilts and strains are significantly elevated.
Cumulative vertical subsidence after longwall mining in three seams has now reached 5.8m with incremental vertical subsidence increasing as a percentage of incremental mining height with each episode of subsidence. Latent subsidence from near stacked goaf edges is recovered when mining in the seam below. A site-specific methodology developed to forecast subsidence behaviour is allowing measured subsidence effects to be estimated reliably.