There are vast areas of undeveloped land which exist over underground abandoned coal mining that can be potentially used for wind farms. These land use areas can be economically feasible for this purpose even when accounting for any future land subsidence resulting from mine collapse. This feasibility depends on how much damage could occur, if any, and whether or not the damage element was repairable and not hazardous. Therefore important elements of the economic feasibility of a wind farm against mine subsidence are:
The resistance of the underlying mine structure to collapse across the project site. (i.e., more resistant leads to less mine collapse potential). See EU Issue #14 for mine subsidence risk as it relates to mine collapse.
Severity and extent of the surface subsidence across the project site.
The damage thresholds of the wind farm infrastructure to those predicted subsidence movements.
The extent and intensity of the damaged farm areas across the project site.
Moreover, based on the site specific conditions, the economics can be improved through Kaizen analysis and mitigation measures taken to reduce the expected level of damage. With cost-effective mitigation measures in place against mine subsidence risk, wind farms would be a viable land use over underground workings.
For more information contact aosouli@meacorporation.com.
Underground mines are commonly stabilized by filling the void space created by the extracted ore with material. The reason this is done is for the protection of surface structures and landfills from damage as a result of mine collapse of the mine structure some time in the future.
FIGURE 1 PHOTOGRAPH OF INJECTING GROUT INTO A MINE VOID ABOUT 190FT BELOW THE GROUND SURFACE
The material placed in these underground mines is filled in from the ground surface, requiring the drilling of injection holes that are typically 50ft to over 300ft deep to reach the mine voids (see Figure 1).
These injection holes are typically spaced from 20 to 75 ft apart. The injected material, called grout, will be made to have various fluidities depending upon its purpose. For example, stiff grouts are used to limit the spread and are thereby used to create a grout “wall” in the mine to contain fluid grout when injected into designated areas to protect the overlying infrastructure.
These projects typically consist of subjacent mines which contain a large volume of void space from mineral extraction. It is not uncommon for quantity of the grout used to stabilize the mine to be on the order of 25,000 to 50,000 cubic yards or more for commercial and industrial projects. Therefore, the grout material costs represents a significant part of the budget of project. Therefore, investigating cost-reducing grout materials can represent significant savings to the project. Also, the deeper the mine, the greater the drilling footage and costs, and therefore, in some cases, ways to reduce the drilling footage can also be cost effective.
Various ways exist to stabilize an underground mine, which involve spot injection grouting to full saturation grouting (see Issue 24). To apply the most cost effective methodology, factors that should be considered include:
• Mine depth,
• Existing mine conditions (e.g., mine gas, flooding, roof rubble, etc.),
• Surface conditions (e.g., existing structures and utilities, site topography, etc.),
• Mine failure mechanism(s) to be mitigated, and
• Acceptable damage threshold of the protected structures(s)
Smart cost effective grout design considering the above factors can save over 50% on the cost.
