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.
The
most common causes for building settlement are from underlying deposits of
compressible fill or native soils. Compressible soils which are under unchanged
building foundation loading cause settlement to start immediately and taper off
over time. Therefore, if the settlement is not noticed until much later in time,
the presence of compressible foundation soils is not likely the culprit. One
cause, which can result in building settlement at any time, would be the
shrinkage of plastic clay soils. These clay soils will shrink when they “dry
out” and are problematic where they are subjacent to the foundation and have
significant initial moisture. Shrinkage of foundation clay soils is typically
associated with added landscaping which causes water to be “sucked out” of the
soils.
Another
fairly common source of settlement are foundation soils that can collapse when
exposed to moisture. Therefore, settlement of the structure would be noticeable
after significant precipitation and is likely to occur early after and even
during construction. Soils which would exhibit this behavior are loose, drier
fine sands to silts. More common in colder climates, another typically early
post-construction source is thawing soil. More specifically, building
settlement results from thawing of frozen soils left below the foundation.
Two
other more typical causes are less time dependent but are location dependent.
These are building settlement from land subsidence in karst terrain and underground
mining. In other words, there are only certain regions where either karst
conditions and/or underground mines are present. These karst and mine
subsidence events may occur at any time. These land subsidence events are
discussed in blogs entitled “What is Karst Subsidence” and “What is
Mine Subsidence”.
There
are some causes of building settlement which are more directly identifiable.
These include from underground tunneling, structures next to temporary or
permanent yielding retaining walls, earthquake shaking of mainly loose fine
sands which can contain some silt, and high extraction underground mining which
causes immediate ground collapse.
Red
herrings of building settlement, even to the professionals, can be building
foundation heave, and from subtle landsliding. Landsliding is discussed in “Landsliding
What to Do” and building heave will be discussed in an upcoming blog. Where
the building damage is apparently from settlement but requires proper investigation
a qualified geotechnical engineer expert in forensic analysis is recommended.
If
MEA can assist you with your building settlement problems, please contact us at
314-833-3189.
Mine
subsidence is the collapse or settlement of the ground surface from failure of
an underlying mine. The most common mine subsidence events are from the
extraction of coal. However, it also occurs from underground mining of other
ores or natural resources as well. This would include mines in gold, iron, zinc,
trona, salt, gypsum, limestone, etc. The nature of the mining and depth play a
significant role in how the subsidence expresses itself on the ground surface.
Based on essentially these two factors the mine subsidence can express itself on
the ground surface as pothole sized to large sinkholes and small to very large
trough to bowl-shaped depressions.
The
mine subsidence movements can be very gradual to rapid depending on the type of
mine failure. Example of larger and smaller sinkholes are shown in Figure 1 and
2. Examples of smaller to larger sag depressions of the ground surface are
depicted in Figures 3 and 4.
For
more information on mine subsidence see: Establishing Mine Subsidence Risk. In selecting a mine
subsidence expert see: What to look for in a
Geotechnical Engineering Expert.
FIGURE
1 SINKHOLE FROM MINE SUBSIDENCE
FIGURE 2 LARGE SINKHOLE
FIGURE
3 SMALLER SAG DEPRESSION FROM MINE SUBSIDENCE
FIGURE
4 LARGER SAG DEPRESSION FROM MINE SUBSIDENCE
