Assessment framework to understand the impact of longwall mining on hydrogeology

When coal is extracted using longwall mining, the overburden strata is disturbed causing surface and subsurface subsidence and strata fracturing. This may impact the natural state of the groundwater system. In order for a mine to remain operational it may need to dewater its underground workings from time to time. The mine dewatering and the mining induced rock deformation could lead to a decrease in pressures in subsurface aquifers as well as lowering of the groundwater table. Longwall mining may induce extensive fracturing in the caving zone. The impact of mining on the strata and groundwater resources diminishes with distance away from the mining voids both vertically and laterally. Depending upon the spatial location of a mine with respect to other strategic assets that rely on groundwater resources, the severity (i.e. effect and consequence) of the impact of mining on hydrogeology will vary. Figure below shows a methodology for assessing the impact of longwall mining on hydrogeology. The assessment process is divided into the four distinct phases as baseline data collection; preliminary impact assessment and prediction; on-going data collection; and refining and updating of the assessment for on-going and future mining.

 

Figure: Assessment framework to understand the impact of longwall mining on hydrogeology.

With this framework, for the past 15 years, our team has been undertaking research on longwall mining under or adjacent to surface water, sub-surface aquifers, water reservoirs and flooded workings at a number of mine sites in Australia and overseas.

For the past several years, the team has been involved in assessing and developing the hydrogeological response model for various mines using the data obtained from piezometer and extensometer monitoring, permeability measurements, as well as with the numerical simulations. As an example, for one of the Australian underground mines, following deformation zones with distinctive hydrogeological response characteristics have been identified and shown in below figure.

• Caving zone with broken blocks of strata (about 3 times the extraction height);
• Transition zone where the rock strata is going through gradual transition from caving zone to fracture zone (up to 7 times the extraction height);
• Fracture zone where the strata have sagged downwards and have suffered bending, fracturing, bed separation (about 33 times the extraction height);
• Constrained zone where the strata may have suffered some bed separations and fracturing without causing significant alterations to the original strata properties (about 32 to 35 times the mining height and 5 to 10 fold increase in average permeability);
• Elastic zone with minimal bed separations (about 20 to 23 times the mining height and 3 to 5 fold increase in average permeability)
• Surface zone with some bed separations and tensile fracturing up to 20 m to 30 m thick (5 to 30 fold increase in average permeability)
• The average permeability within the caving zone, including the transition zone, is predicted to increase by as much as 1000 to 2000 times. The average permeability is expected to increase by as much as 20 to 1000 times within the fracture zone. Thus any subsurface aquifers lying within the caving and fracture zones are expected to be extensively impacted by mining.

 

Figure: Hydrogeological response model for one of the Australian underground mines.