What is Karst Subsidence?

Karst subsidence is land subsidence that is caused by cavities or voids in the underlying bedrock which collapse or from soil filling them in from above resulting in surface subsidence. Under normal circumstances, the voids or cavities were created by the flow of groundwater in fractures in soluble bedrock over a great deal of time. The most significant land subsidence effects occur over voids which have been solutioned in limestone bedrock but also result in other soluble rocks such as dolomite, gypsum, and halite. The most typical land subsidence results from groundwater draining downward into these solution voids carrying soil particles with it. This results in the ground settlement in the form of a sinkhole to a more gradual depression on the ground surface. Therefore, when downward drainage of groundwater is caused into open bedrock voids, the potential for subsidence results. Some more common triggers are: unlined surfaced drainage trenches, pumping of water wells, quarry pit dewatering and retention/detention ponds.

Figures 1 and 2 are examples of this.

FIGURE 1: SINKHOLE CAUSED BY DOWNWARD DRAINAGE FROM DEWATERING OF NEARBY QUARRY PIT

FIGURE 2: IRREGULAR DEPRESSION WHICH FORMED FROM DOWNWARD SEEPAGE OF WATER STORED IN A RETENTION POND

For more information see: Risk Investigation of Karst on Sinkhole/Subsidence Prone Land.

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

What to Look for in a Karst Subsidence Expert

Karst subsidence are typically in the form of sinkhole to bowl-shaped depressions. They can occur unexpectedly and fairly abruptly and can cause significant damage. Because of their erratic geologic nature, karst it is often difficult to quantify the subsidence risk and associated damage potential. Therefore, the expert which is hired should be well versed in all the subsidence engineering aspects of interest.

More common questions the karst subsidence expert will be asked to answer are:

• What is the chance that there will be subsidence in the future?

• If there is a subsidence(s), how severe will it be?

• If there is subsidence, how much damage can we expect?

• What are my options to reduce the risk of subsidence in the future?

• A sinkhole has appeared, what do I do?

• Is there any way to virtually eliminate the risk of subsidence in the future?

Subsidence investigations in karst terrain are most commonly related to new construction or encountering unanticipated subsidence or other karst features which disrupt construction progress, insurance claims, or subsidence damage. Given the context of the investigation and the amount of risk which may be involved should determine the level of expertise that the subsidence engineer should have. If tort litigation is involved the karst subsidence expert should also be competent in this area. Also, with greater knowledge and experience in karst subsidence problems, the more cost effective the solution.

You may also be interested in:
Issue #34: Risk Investigation of Karst on Sinkhole-Subsidence Prone Lands
Traits to Dig For in an Engineering Expert