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ABSTRACT ABSTRACT
Observations of the land surface show that subsidence over active and abandoned underground mines is controlled to some extent by the stratigraphic and structural geology of the mines in addition to the methods of mining and the thickness of the seam mined. Exposures in highwalls of surface mines that intersect abandoned 6nderground mines in the Warrior coal basin of Alabama illustrate this, showing sandstone beds that act as beams bridging openings and interbedded shale beds crumbling. Stratigraphic changes observed in the district are extremely variable and calculations regarding subsidence used for one area will not apply to another area if there is a major change in rock types. Rocks exposed in road cuts and open pit mines demonstrate a marked difference in weathering characteristics. Some rocks, such as shale, weather very rapidly, by crumbling and becoming plastic, whereas other rocks, such as sandstone, resist weathering. The continuing rock weathering after mining in openings produced by the mining activity and in fractures caused by roof movement, apparently, is a major cause of the surface subsidence that may occur years after the mines are abandoned.
INTRODUCTION
The purpose of this report is to emphasize the importance of stratigraphic and structural geology in evaluating the surface subsidence over underground mines. Subsidence problems are in two fields. One set of problems deals with surface collapse over abandoned mines, some of which were closed more than a half century ago. The second set of problems are surface movements over active mines. The stability of the surface over abandoned mines is a major concern in Alabama because municipal developments extend over abandoned mines in both the Warrior and the Cahaba coal basins, and occasionally there are subsidence problems. The surface over the abandoned Birmingham red-iron ore mines on the southeast slope of Red Mountain and below Shades Valley is now a potential problem because this area, formerly held as a mining re- serve, is open for development (fig. 1).
Surface stability is an international problem. European problems are that mining is conducted under densely populated areas (Brauner, 1973). In the United States problems are that some cities, like Birmingham, are expanding with housing and other construction over mines that have been abandoned years ago, and they are having subsidence problems (Matheson and Pearson, 1985). A problem facing active mines is that government regulations require subsidence plans be filed before permits to mine are granted (Chen and Peng, 1985). The stability investigation of the Warrior coal basin includes observations made throughout the entire district. The writer visited surface mines in each area, examining the highwalls, which provide excellent geologic cross-sections. The subsidence and distortion of strata were mapped and photographed in the highwalls of pits that had intersected abandoned underground mines. The reconnaissance included the Birmingham red-iron ore district. Field work included traverses in old and new housing developments that overlie abandoned mines, looking for evidence of subsidence in houses, streets and walks where cracks would be visible. Approximately 12 months were spent on the field investigation.
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