Applicability of the Vesic-Speck Floor Bearing Capacity Model to Size Pillars in the Illinois Basic Coal Mines

摘要

Owing to a unique geologic depositional feature called, the cyclothem, all major mineable coal seams in the Illinois Basin are associated with weak underclay floor materials. Consequently, pillar design in the Basin's coal mines is almost always based on floor bearing capacity rather than the traditional coal strength. Towards this end, Speck's adaptation of the Vesic's non-homogeneous bearing capacity model has become the standard tool over the past three decades. Despite this popularity, no rigorous attempts were made in the past to understand the applicability of the Vesic-Speck model across the Basin. This is particularly surprising given that the Speck's floor strength equation was based on a handful of tests conducted at just two nearby coal mines and his verification of the Vesic-Speck model was based on a few cases from the same two mines. In this paper, a rigorous analysis of the applicability of the Vesic-Speck model is made by applying it at several mines scattered across the Basin. For this purpose, first, the largest database of in-situ plate tests from several Basin's mines was created to test the validity of the Speck's floor strength equation. Then, with the help of a large number of actual cases of stable and unstable floors collected from several mines spread across the Basin, the predictability of the Vesic-Speck model has been assessed. This analysis revealed that the Vesic-Speck approach had limitations in explaining what was really happening in the mines. Besides, under certain conditions, such as those noticed at several mines from the western shelf of the Basin, the Vesic-Speck approach produced physically meaningless negative safety factors.