Posts Tagged ‘mine subsidence’

Solar Farms Over Mined-Out Areas

FIGURE 1

There are vast areas of undeveloped land which exist over underground abandoned coal mining that can be potentially used for solar farm development (see Figure 1). These land use areas can be economically feasible for this purpose even when accounting for any future land subsidence resulting from mine collapse. Therefore, prudent due diligence requires an expert analysis on how much damage could occur.

It is MEA’s experience from previous project investigations that even solar development was viable with the identified subsidence risk and predicted damage.

The economics of subsidence damage depends on the predicted number of subsidence events which would result over the lifespan of the farm and the amount of associated damage to the farm infrastructure and whether the damaged element is repairable. Therefore important elements of the economic feasibility of the farm against mine subsidence are:
  • The resistance of the mine structure to collapse across the project site (i.e., more resistant leads to less collapse over time).
  • Severity and extent of the surface subsidence across the project site.
  • The damage thresholds of the various farm infrastructure to those subsidence movements. For example, a significant part of this subsidence damage analysis is assessing the subsidence interaction of the tracker whose piers would be exposed to a range of horizontal and vertical movements.

    FIGURE 2 A SKETCH ILLUSTRATING INDUCED TRACKER BEHAVIOR FROM A SINGLE SAG EVENT OVET AN ABANDONED MINE

    This is illustrated in Figure 2 when sag subsidence is expected.
  • 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. To understand more about mine subsidence risk see Engineering Update #14 – Establishing Mine Subsidence Risk.

Causes for Building Settlement

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.

FIGURE 1: SETTLEMENT DAMAGE FROM MINE SUBSIDENCE

FIGURE 2: SETTLEMENT DAMAGE FROM KARST SUBSIDENCE

FIGURE 3: FOUNDATION SETTLEMENT OF A BUILDING IN MEXICO CITY
(Photo Credits: Tim Leffel) https://www.perceptivetravel.com/issues/0409/mexico_city.html

What is Mine Subsidence?

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

What to Look for in a Mine Subsidence Expert

The most appropriate mine subsidence expert for your case depends on the nature of the problem to be investigated. Mine subsidence investigations can require various expertise depending upon the focus of the problem. Some of the questions that may need answers are:

• Is the damage from mine subsidence?

• Will the underground mine result in subsidence in the future?

• If subsidence were to occur, what is the range of movement you would expect?

• If subsidence does occur, how severe could the resulting damage be?

• Was the mine designed properly so that mine subsidence would not result in the future?

• How can we mitigate the subsidence risk to a tolerable level?

• Can you stabilize the mine and how much would that cost?

As you can see from the above, the subject of mine subsidence can actually involve a number of expertise depending upon focus of the investigation. Also, there is the context of the expert mine subsidence investigation: Is it being done for existing or new construction, mine design, review of a mining permit application, or is it for tort litigation? Therefore, in addition to having the technical know-how, and expert with oratorical skills may also be necessary.

You may also be interested in:
Issue #14: Establishing Mine Subsidence Risk
Traits to Dig For in an Engineering Expert
Issue #24: Anatomy of Grouting Mine Voids
Issue #25: Transmission Pipeline Subsidence from Mining
Issue #27: Borehole Radar Used to Identify Deep Coal Pillars