Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, TN 380, 1991 Apr; :1-2
Objective: Provide a full-scale test facility where the effectiveness of all types of subsidence abatement techniques can be evaluated under controlled conditions. Background: Subsidence is ground movement resulting from the collapse of overlying strata into a mine void because of failure of the mine roof, mine floor, or support pillars. The failure may propagate to the surface and be expressed as a depression, cracks, or a sinkhole. The surface expression of such features may be delayed until long after mining has been completed. Subsidence has the potential to affect more land area than any other type of abandoned mine land problem. Subsidence also causes direct damage to real property, with significant financial loss and more importantly, presents a danger to public health, safety, and general welfare. Subsidence-related problems are considered to be one of the most important abandoned mine land issues. Backfilling of mine voids is the most common stabilization method used to abate subsidence and protect surface structures. Backfilling or placement of material is performed by hydraulic flushing or pneumatic stowing, using either in-mine or remote methods. The majority of the past subsidence abatement projects has been performed using remote methods from single or multiple boreholes. Other subsidence abatement techniques are available. For example, various point support methods are used to protect small areas of the land surface and surface structures, without completely filling the mine void. These may be more appropriate under different conditions. Little documented information is available concerning the capability of the various backfilling methods for completely filling a mine void or providing long-term structural strength. Therefore, the Bureau of Mines developed a laboratory for controlled evaluation of backfilling technologies. Approach: The SAIL design is quite flexible, and can be most simply described as a surface structure that simulates a borebole drilled into a mine void. The configuration illustrated in the figure was used to simulate remote backfilling in a typical coal mine. However, with slight modifications the laboratory may be configured to a number of different conditions, i.e., the backfilling of open stopes, tunneling and backfilling in a flooded mine. General specifications of the SAIL are listed in the table. Capability: The laboratory structure consists of a 3 story, 300ft by 300ft steel tower with two elevated working platforms. The upper platform is 36 ft above the simulated mine floor. Access to the working platforms is by a catwalk on the elevated conveyor bridge. The conveyor bridge provides the supporting structure for a 24-in conveyor belt, a 6-in conveying pipeline and utility conduits. The combination of the conveyor belt and the conveying pipeline permits the delivery of material to the top of the borehole in bulk-form, as a slurry or by pneumatic transport. The tower is a framework for the borehole and its supporting equipment. However, the design loading of the tower is such that, if needed, additional equipment required for a specific series of tests may be mounted on either of the work platforms. The borehole is sized to provide an 8-in-diameter opening. Borehole simulations of up to 10-in in diameter can be accommodated without modification to the structure. The simulated borehole can be configured to extend from either the upper or lower platform to the mine roof. A 1-1/2 ton hoist is located above the borehole for use in lowering casing or backfilling tools into the borehole. The simulated mine is also designed for maximum flexibility. It has been constructed from concrete block walls with steel bridge planks serving as the mine roof. The concrete blocks used for the walls are interlocking "Jumbo Blocks" usually used for retaining walls. Each block measures 2-ft high by 2-ft wide by 6-ft long, and weighs 1-1/2 ton. This size allows the blocks to be set in place on a level fill without a permanent foundation. Thus, the configuration of the mine is not restricted by a foundation and can be rearranged as needed for a specific test. An intersection can be established beneath the tower, since it is designed with a wide enough base to allow 20 ft wide openings in each of four directions. The roof of the simulated mine is removable, permitting direct observation of the fill placement, in-situ testing of the fill and excavation of the fill material. When needed, the roof can be secured to a series of studs which are embedded into the upper row of blocks. Research Accomplishments: To date, a prototype "Air-Jet" style injector and four collapsible elbow/nozzle combinations for use with conventional stowing equipment have been evaluated at the laboratory. The tests were conducted to demonstrate the capability of pneumatic stowing as a subsidence abatement method and to measure the performance of the various techniques. In comparison to the plain pipe elbow currently used, all of the devices tested significantly improved the state-of-the-art by providing a greater trajectory and void filling capability. Based on this evaluation, it has been technically proven that pneumatic stowing is a viable means for backfilling abandoned mine openings, especially in areas where water is unavailable. Pneumatic stowing techniques may be used to place fill in over 200 linear ft of limited-height mine openings from a single borehole by directing the discharge down the center of the mine void. The fill is placed in a medium-dense condition and can achieve roof contact for more than 100 ft of the overall length.
Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, TN 380