Longwall mining
[2] While the technology has changed considerably, the basic idea remains the same, to remove essentially all of the coal from a broad coal face and allow the roof and overlying rock to collapse into the void behind, while maintaining a safe working space along the face for the miners.
Seals are required to be monitored each shift by a certified mine supervisor for damage and leaks of harmful gases.
[9] A number of hydraulic jacks, called powered roof supports, chocks or shields, which are typically 1.75 m (5 ft 9 in) wide and placed in a long line, side by side for up to 400 m (1,300 ft) in length in order to support the roof of the coal face.
At the main gate the coal is usually reduced in size in a crusher, and loaded on to the first conveyor belt by the beam stage loader (BSL).
This collapsing can lower surface height, causing problems such as changing the course of rivers and severely damaging building foundations.
Longwall has better resource recovery (about 80% compared with about 60% for room and pillar method),[11] fewer roof support consumables are needed, higher volume coal clearance systems, minimal manual handling and safety of the miners is enhanced by the fact that they are always under the hydraulic roof supports when they are extracting coal.
[11] Longwall mining has traditionally been a manual process in which alignment of the face equipment was done with string lines.
Briefly, Inertial navigation system outputs are used in a dead reckoning calculation to estimate the shearer positions.
Optimal Kalman filters and smoothers can be applied to improve the dead reckoning estimates prior to repositioning the longwall equipment at the completion of each shear.
[12] Expectation-maximization algorithms can be used to estimate the unknown filter and smoother parameters for tracking the longwall shearer positions.
[13] Compared to manual control of the mine equipment, the automated system yields improved production rates.
At Newstan Colliery in New South Wales, Australia "the surface has dropped by as much as five metres in places" above a multi-level mine.
[16] In some cases the subsidence causes damage to natural features such as drainage to water courses[17] or man-made structures such as roads and buildings.
"[17]: 2 A 2005 geotechnical report commissioned by the Roads & Traffic Authority warned that "subsidence could happen suddenly and occur over many years".
[citation needed] Subsidence is minimised by increasing the block's adjacent chain pillar widths, decreasing extracted block widths and heights, and by giving consideration to the depth of cover as well as competency and thickness of overlying strata.
[18] On top of this, if there are present dams near to the longwall mining site, this could doubly impact the riparian ecosystems as it would have a reduced inflow rate as well as the loss to the underlying rock fracturing.
[19] As a result of bedrock cracking from mining beneath waters such as lakes or rivers, there have been groundwater losses of varying extents.
Mines within a few hundred meters of the surface are susceptible to receiving great inputs of water from these bodies.
Additional mining in concentrated areas continuously move these water flow paths, which take years to return to their original states.
Longwall mining can also result in localized water temperature change, stimulating algal bloom which can use up available oxygen required for other species health.
