The risk of ground falls during pillar recovery increases in multiple-seam mining conditions.
The hazards associated with pillar recovery in multiple-seam mining include roof cutters, roof falls, rib rolls, coal outbursts, and floor heave.
The study finds that, for interburden thickness of 21 m, the multiple-seam mining influence zone in the lower seam is directly under the barrier pillar within about 30 m from the gob edge of the upper seam.
Instead of the traditional "top to bottom" coal mining sequence in multiple seams, the first mining seam (it is also often referred to as a protective seam) is so selected in the integrated technology that its mining can result in the deformation, de-stressing, enhanced permeability, and easy gas drainage of its overlying and underlying coal seams.
One of such breakthroughs is the integrated coal production and gas extraction technologies for pillarless mining of multiple seams of low permeability with the "Y" type ventilation system through retaining goaf-side roadways.
The overburden depth is responsible for the overall level of stress, but pillar design, multiple seam interactions, and/or mining activity can concentrate stresses in distinct locations.
For example, mining alternate panels far away from each other, or mining alternate panels in multiple seams is not common.
When pillar recovery is planned in multiple seams, it is critical to properly design the mining sequence and panel layout to minimize potential seam interaction.
As mining depths increase, new challenges, such as gas drainage, gas outbursts and rock bursts induced by mining extraction, have emerged, particularly in gassy, multiple-seam coal mines.
Past mining also gives rise to multiple seam interactions in some areas.
