In the days when mining for coal was an integral part of the region’s economy, little thought was given to the underground voids left when the fuel was removed. But as the need for land for development has increased, much thought has been devoted to the question of what to do to avoid land subsidence when buildings are constructed on land where abandoned mines exist. The issue arose earlier this year when the South Allegheny School Board decided to close two elementary schools and build a larger building on 36 acres on the border between Port Vue and Glassport. The project is estimated to cost $10.9 million. To solve the problem, the designer of the new South Allegheny Elementary School, N. John Cunzolo, will pour grout into the abandoned mines under the site to ensure against the new building’s collapse from subsidence. Construction crews at the school have been drilling into the mine from the surface above and then pouring grout into those borings. The division chief of the Office of Surface Mining’s Federal Reclamation Program agrees with Cunzolo’s assessment on how to shore up the property. Grouting, said Tim Dieringer, “is the safest way to go. In some instances people have gone and put other materials (into abandoned mines for building supports) and it hasn’t worked. “If you really want to be sure – and I think you would absolutely with a building like that – grouting is the way to go,” Dieringer said. The grouting process is a fairly simple one. Once the borings are drilled, crews pump in concrete, fly ash or a mixture of both. As the grout hits the mine floor, it builds into a conical shape up to the mine roof before filling the boring to the surface level. While other forms of support can be used, such as pilings, Dieringer does not believe they are as cost-effective for building support, especially if a mine is a couple of hundred feet deep. “By the time you drive the piling, it would be cheaper to do the grout,” he said. Cunzolo said the borings typically are on a grid that are 25 feet to 30 feet apart. On harder ground, engineers may recommend stretching the borings to as much as 50 feet. Tim Sullivan, an architect for Cunzolo’s firm, said about 225 holes going as deep as 125 feet will be drilled at the site. However, he could not say how much grout would be needed for the project. The work going on now at the school is not unusual in Western Pennsylvania, especially in areas where unauthorized or “bootleg” mining occurred, according to Cunzolo. Many times, going against common practice to leave coal pillars standing to support the land above, miners took that coal as well, leaving the ground above susceptible to subsidence, he said. “In just about every site in this region you’re going to find under mining at some point in time,” Cunzolo, whose Bellevue firm bears his name. Dieringer added that many abandoned mine maps will note coal pillars. However, mining companies different from the original ones often went back into the mine and shaved the pillars or did what is known as retreat mining, where the coal pillars were taken altogether. Nevertheless, Cunzolo was quick to note the mine under the South Allegheny site still had the pillars in place, although soils engineers recommended not to simply rely on ground to support the structure.

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