Pillar design for St. Peter sandstone formation

Silica sands produced primarily from St. Peter Sandstone are used for hydraulic fracturing in the petroleum industry, glassmaking, chemicals, ceramics, filtration and the foundry industry in the US. The recent high demand for silica sands for hydraulic fracturing has triggered increased production and commissioning of new silica sand mines to support natural gas production from shale and tight gas deposits in the US. Most mines use surface mining methods to extract St. Peter Sandstone. The room and pillar mining method has been successfully used for St. Peter Sandstone mining in a few US locations, however, no one has proposed a rigorous pillar design method. The goal of this research is to: (1) elucidate factors contributing to ground control problems in St. Peter Sandstone mines; (2) derive a pillar design method for St. Peter Sandstone using numerical modeling; and (3) investigate the mechanics of “pinch out” failure in St. Peter Sandstone. The study found that the factors contributing to ground control problems in St. Peter Sandstone mines are: water in roofs, friability of the St. Peter Sandstone, strength variability, and reinforcement techniques. The study proposed the following pillar strength criteria for St Peter Sandstone: SP = 14.360 + 11.720C - 0.903h[0.28 + 0.53(w/h)] Where, C is the cohesion of the pillar rock in MPa, h is the pillar height in meters and w/h is the dimensionless pillar width to height ratio. The study also found that pinch out failure is influenced by the contrast in rock properties at the pillar-roof interface and shape of the mine opening --Abstract, page iii