Understanding subsidence patterns from underground mining allows us to anticipate and manage the effects on the surface.
How subsidence occurs
As the coal seam is removed the rock immediately above the mined section collapses. This fractured rock is called the goaf. Above the goaf the layers of rock crack and bend in relation to how far above the goaf they are until the bending reaches the surface where it is seen as a gradual dipping in the ground. Some cracking may occur at the surface; the nature of which is dependant upon the depth of the seam (the deeper the seam the less cracking at the surface), the moisture of the soil (soil cracks more in a drought), and how competent the surface material might be (for example an exposed sandstone will behave differently to a thick loam).
In order to better understand and manage the effects of subsidence, Bulga Underground Operations installs survey subsidence monitoring lines at various locations along each longwall panel. These markers are precision surveyed prior, during, and after mining to measure the tilts, strains and complete subsidence across and along the mined panel. The results of these surveys are compared against the predicted values and reported to government each year in the Annual Environmental Management Report. Usually the predictions are conservative and the measured values are found to be less than what the mine had planned to manage.
How subsidence is managed at Bulga underground
We have a pre-emptive approach to managing the impacts of mine subsidence. Where feasible the identified impacts are mitigated prior to mining, and when mitigation might not be practical, appropriate controls are put in place to monitor and manage the impacts during subsidence.
Depending upon both operational and geological factors, we may subside over 120 hectares during the course of a year. Because the distance between the surface and the coal seam varies substantially between the starting end of a panel and the finishing end, the appearance of subsidence on the surface also varies greatly. The area most likely to experience surface cracking is where the seam is shallowest and all the shallow areas are owned by the mine.
Because underground mining does not require any stripping of soil or rock it allows for the coincident use of the land, and a thorough understanding of the effects of subsidence means our impacts on surface infrastructure are minimised. Bulga Underground Operations owns a large portion of land that is undermined by our operations, including an olive grove, vineyards and native bushland.
However, not all subsidence occurs on land owned by Bulga Underground Operations. Where the coal seam is deepest, privately owned vineyards, houses and farming infrastructure are also undermined. For undermined private land, a Private Property Subsidence Management Plan is prepared in consultation with the respective owners and the Mine Subsidence Board prior to mining that area.
As the mining progresses along the panel, evidence of cracking is monitored on the surface. When repairs are required contractors remove the topsoil from the cracked area, then roll or clay fill the crack depending upon it's length and width. During repair works, every effort is made to ensure that existing vegetation and habitats are not disturbed. After repairing, the vegetation is replaced by sowing native pasture seed mix or tree seed.
The largest pieces of infrastructure impacted by mining at Bulga Underground Operations is Broke Road and Charlton Road. When the longwall extracts the coal underneath these roads, we have 24/7 monitoring in place. When cracking of the pavement occurs temporary repairs are made immediately. Final re-profiling of the road after subsidence has ceased is undertaken by the Mine Subsidence Board. All management of mining impacts on public roads is carried out in consultation with the Singleton Council and The Mine Subsidence Board.