Developers are faced with an unusual problem when they discover a property they are considering for construction is undermined. This is particularly problematic in some areas of the country where prime land is in short supply. In certain areas, finding such locations which are underdeveloped and not undermined are becoming more difficult.

Until they are economically viable, undermined areas sit vacant for a considerable amount of time because of the potential risks commonly associated with underground coal mines. In fact, lending institutions can be hesitant to finance such developments because of the potential risk of significant future damage to the proposed structures. Therefore, banks can require assurance that this risk will be nominal.

Coal mining defined
Mine subsidence of the ground surface can unexpectedly result when the land is undermined with abandoned coal mines. Abandoned coal mines are usually no less than 30 ft to typically no more than 600 ft deep. The organization of the remnant coal workings depends upon the age of the mining.

Abandoned underground coal mines exist in all the major coal fields in the U.S. Each coal field can have more than one mineable coal seam. Therefore, in some areas there can be more than one level of coal workings. Coal extraction by underground mining consisted of leaving mine voids with columns of coal to support the overlying ground. This method is called the room-and-pillar mining. The mine void areas are usually called rooms, entries or mains and the columns of coal are called pillars. In a subsidence engineering investigation it is important to have the best available mining data in order to assess subsidence potential. Mineable coal seams are typically 3 ft thick and can reach 20 ft or more thick, but are more commonly less than 8 ft. When the coal is extracted this results in corresponding mine void heights with mine opening widths typically 15 to 30 ft. Older abandoned mine workings are much less organized and maps depicting them (if available) are much less accurate than those which are more recent. Extraction rates of underground coal mines typically range from 50 to 75%.

Mine subsidence ­defined
Ground surface subsidence resulting from the collapse of an underground mine are of two fundamental types: pit or sag. Pit subsidence expresses itself on the ground surface in a sinkhole configuration. A pit can be as small as a pothole or as big as a large crater typically 30 feet or so in diameter and on the order of up to about 20-feet deep. Some pits can be more trough shaped with similar dimensions. Land with shallow underground mines up to 80-feet deep are more susceptible to pit subsidence than deeper mines.

In general, sag subsidence can vary from fairly abrupt to a gradual depression of the ground surface which can be bowl to almost trough-like. The fairly abrupt cases are usually the smaller sags up to about 300-feet wide. These depressions typically occur from the collapse of an area of a mine room. A more gradual sag results from the yielding of multiple pillars. Even the more gradual subsidence depressions can be visually seen in relatively flat topography, but this vertical displacement can be unnoticeable to the naked eye in hilly ground or where maximum settlement is small.

Typically, sags over room-and-pillar mines have a maximum settlement of less than four feet. More massive pillar or floor failures result in sag expressions which can be quite large and are typically less than 2,000-feet wide.

Mine subsidence and damage potential
In areas where underground coal mines exist, developers are faced with the possibility of the future collapse of the mines and the associated damage of any proposed structure. Although many may focus on the potential of subsidence of the ground surface, it is ultimately the potential damage which is the most important factor. Whether or not subsidence occurred on the project site would not be a controlling issue if the damage level determined by the subsidence engineer was nominal or acceptable.

In other words, more important than the risk and severity of the subsidence is the subsidence damage potential. Subsidence damage potential can be summarized in categories such as: none, slight, moderate, severe, and very severe with detailed descriptions of each damage intensity provided.

These various levels of damage depend upon the severity of the subsidence and the building element affected. When evaluating the overall subsidence concerns, the acceptable level of damage must be identified. For example, when within the designated time span, the damage tolerance level is greater than the expected severity, then the risk becomes subservient to the severity. In other words, the risk (mine stability) will not matter even with the possibility of subsidence.

Some times the potential subsidence damage can be cost effectively identified in a Phase 1 Subsidence Engineering Study. This level of investigation does not require site specific deep drilling and testing. Phase 1 information is gathered from a variety of sources on the:

• Site
• Project
• Geologic
• Mining conditions
• Subsidence history and characteristics

Where the potential subsidence damage to the proposed structures is unacceptable to the developer, it is important to establish the potential of subsidence at the project site. The risk of the subsidence at the project site now becomes a controlling factor in determining if and what mitigation measures can be taken to reduce the subsidence potential.

Subsidence potential can be defined as having two components:

• Risk
• Severity

The risk of subsidence is determined by the likelihood of mine and overburden failure which would result in surface subsidence within expected time span of the development. The severity is assessed by the extent of the area prone to subsidence, as well as subsidence type, size, and magnitude which are a function of potential breadth and mode(s) of mine failure. The main factors which determine the subsidence type, size, and magnitude characteristics are: if the mine has collapsed and to what extent, the expected mine failure mechanism(s), and empirical subsidence correlations to the mining conditions.

The importance of establishing accurate subsidence risk
The most critical and driving factor for land development is the assessment of the risk of mine subsidence related damage above underground coal mines. However, where potential damage is of concern then establishing the risk of mine subsidence becomes a controlling factor. This does not mean that any undermined site will be a serious subsidence risk. Based on an in-house survey, only about 50 percent of the undermined sites investigated for mine subsidence were found to have significant risk. Half of the serious risk sites required only partial mine grouting to reduce them to low risk. The risk of land development above coal mining is established by:

• The integrity of the mine structure against failure;
• The nature and magnitude of the subsidence based on past events;
• The tolerance of the proposed structure(s) to the estimated subsidence movements.

The evaluation of the integrity of the mine structure must consider the various ways the underground old works can collapse resulting in surface subsidence. Although there may be other concerns, a mine stability investigation should consider that:

• The rock roof above the mine can fail;
• The coal pillars can crush over time;
• The coal pillars can sink into the roof or floor over time.

The greater the experience and knowledge of the investigative engineer the more accurate and certain he is of his assessment of the mine’s integrity.

In other words, it is easy and safe for the investigative engineer who is less confident, and who has less knowledge, training and experience to conclude that significant risk exists and that the mine should be stabilized.

From the numerous investigations and engineering analyses of old mine works performed, the risk of mine subsidence can range from very low to high. This is because the geologic and mining conditions vary from site to site. From a stability standpoint, very low to low risk project sites are determined to have adequate coal mine support. This determination takes into consideration the surrounding geologic conditions or when the mine is found to be already collapsed. Obviously higher risks are present where analyses indicate the rock geology is too weak to support the mine over time.

For large project sites the integrity of the mine structure can vary across the site. Project sites investigated where there was a significant variation in the mine stability, have involved the presence of greater coal support over a portion of the project, or the roof or floor geology had changed from weak to resistant.

Therefore on some undermined sites it was found that it was only necessary to stabilize the portion of the mine workings of unacceptable risk instead of the common practice of grouting the entire mine workings under a proposed structure. Significant savings to the owner on stabilization costs is therefore realized.

Mineable coal seams
The evaluation of subsidence potential should also consider the possibility of future mining beneath the project site. If the mineral rights are not owned by the property owner and mineable seam(s) exists beneath the site, future subsidence risk may exist even if the project site may presently have no underground coal mining. From our experience, this can occasionally occur. For example a gas processing plant was placed over mineable coal reserves about 10 years ago. Today there are plans to mine these reserves with a high extraction technique which will cause immediate subsidence of the ground as the coal is mined.

Mitigation measures
Where possible a cost effective option can be to design the proposed structure(s) for the potential subsidence movements. The assessment of less expensive surface mitigation compared to mine stabilization involves the consideration and integration of a number of areas of engineering including mining, geotechnical, structural, and architectural. This becomes evident when realizing the important factors that must be assessed:

• Mine stability and the nature of existing mine conditions;
• Ground stabilization alternatives;
• Specific nature of the subsidence ground movement and the foundation loading;
• Structural response of the foundation;
• Structural and architectural response of the structure(s).

The greater the integration and understanding of these factors along with ingenuity, the more efficient and the less conservative the design rendered.

For example, where the potential subsidence was determined to be limited, mine grouting would be necessary and a subsidence-resistant design of the foundation and superstructure could be cost-effective. Also, where the project site is fairly large, and there is some flexibility in the location of surface structures, a surface risk map can be prepared. Using this map, structures can be more strategically placed. This can significantly reduce the cost of development by reducing the subsidence mitigation needed.

In approaching the potential damage from subsidence at the site one can:

• Ignore the possibility of mine subsidence;
• Implement subsidence resistant measures at the ground surface;
• Employ mine stabilization measures with or without making the structure itself more subsidence ­resistant.

Obviously, the more pro-active the approach one takes against subsidence the lower the risk but the greater the cost. Ignoring the possibility of subsidence can subject the proposed structures to a range of potential damage from minor to very severe intensities depending upon the project site.

Taking cost-effective, surface-resistant measures can significantly reduce the level of damage under certain circumstances especially if the exposure to subsidence movements is mild. If, however, the structure is exposed to a significant subsidence event, major damage can result despite taking some surface mitigation measures. Although more costly, mine stabilization measures will significantly reduce to virtually eliminate the subsidence potential and the risk of subsidence depending upon what methodology is used. Certain mine stabilization measures in combination with surface abatement can provide even more protection against subsidence.

In closing, developers are confronted with an uncommon risk assessment when contemplating building above an underground mine. This situation can be problematic and can alone result in whether the project is viable or not. In other words, the advice obtained from a mining consultant can make a considerable difference in development potential. SLDT