Investigate and demonstrate the possible use of low-density (45-lb/ft³) foam concrete for sealing abandoned mine openings. Although foam concrete has been used in the construction and geotechnical industries tor some decades, it has not previously been used in abandoned mine shaft reclamation.
Approach
A structurally engineered permanent mine seal for an abandoned coal mine shaft, using low-density foam concrete and other appropriate structural materials, was designed by the Bureau and installed at a field site, the No. 22 Shall in Logan County, West Virginia. The field work was conducted by a private contractor under the supervision of the West Virginia AML Section (WV AMI).
The No. 22 Shaft is situated immediately adjacent to a frequently traveled county road, A few feet away from the shaft is a branch of Pine Creek. On the other side of the road is a railroad line that serves an active mining operation located several miles away. The shaft measures 9.5 by 9 feet in cross section and is about 300 feet deep. It was used at one time as a dewatering shaft and still contains two large-diameter dewatering pipes with steel drive shafts inside them. Before the beginning of the reclamation, the shaft included a concrete cap a few inches thick, with a 4- by 5-foot access opening covered by a steel grate. The interior of the shaft was quite humid, and considerable flow of water at the bottom of the shaft could clearly be heard from the surface. Methane tests were conducted that showed the shaft to be clear of this gas.
How It Works
The shaft seal is designed to very conservative specifications with the goal of achieving at least a 100-year service life without noticeable deterioration of the seal structure. The load-bearing capacity of the seal is also conservatively designed for a heavy-duty seal, which requires a 5-psi load-bearing capacity over the cross-sectional area of the shaft. Preparatory work included removal of the preexisting temporary cap and concrete shaft liner walls down to ground level, removal of steel piping in the shaft to a depth of about 14 feel below ground level, pouring of the new reinforced concrete footers adjacent to the old shaft liner walls, and installation of the steel platform inside the shaft.
The completed seal consists of a reinforced structural concrete cap resting on reinforced footings adjacent to the shaft; it incorporates a suspended foam concrete plug that provides lateral stability inside the shaft. The foam concrete plug was poured on top of a steel platform suspended by steel tie rods from two steel I-beams resting on the reinforced footings. The two steel I-beams were later encased inside the reinforced concrete surface cap. This entire structure is protected from potentially acid-bearing moisture by a combination of geomembrane liners and fiber-reinforced sodium silicate coatings. The entire seal structure, including concrete cap, concrete footings, and foam concrete plug, forms a structural unit designed to support an external superimposed load of at least 72,000 pounds.
As a part of this design concept, the foam concrete plug serves to stabilize the original concrete shaft liner and the adjacent soil, thus preventing the possibility of slope failure that might otherwise undermine a concrete shaft cap. Use of foam concrete rather than normal density concrete for the suspended plug greatly increases the practicality of this design concept, since the suspended weight is only about 30 percent of the weight of an equal volume of normal density concrete. Cost savings were realized in the entire supporting structure because of the reduced design weight of the suspended plug.
By agreement with WV AML, the seal design was modified in two ways to accommodate weak soil conditions at the site. One modification was to increase the total area of the seal cap from 21 by 14 feet as originally designed to 21 by 17 feet, to provide more footing area and thus reduce the expected soil loading. The other modification was to allow for differential movement between the reinforced concrete cap and the suspended foam concrete plug, thus accommodating possible soil settlement. This decoupling of the cap and the suspended plug was accomplished by introducing a compressible layer between the two. This layer, 4 inches thick as built, will allow for settlement of the cap over time while preventing the cap from pushing down on the foam concrete plug.
The foam concrete is expected to bond to the existing shaft walls after the foam concrete cures. If, for some reason, the bond between the foam concrete plug and the existing concrete shaft liner fails, the steel I-beams and suspension tie rods are designed to suspend the entire weight of the foam concrete plug.
