Microstructure and geotechnical properties of St. Peter sandstone in Clayton, Iowa

The St. Peter Sandstone is significantly different from the minerals and rocks which have been studied extensively in mining. It is brittle and characterized by an unusually high friction angle. On the other hand it is friable, and it is nearly cohesionless. The scarcity of the ground control techniques for this particular mining environment has created many difficulties for mine operators. This research aims to establish a fundamental understanding of the basic mechanical and strength properties of the St. Peter Sandstone for engineering design and scientific research. The specific objectives are 1) characterizing the strength of the St. Peter Sandstone, and 2) elucidating the strength mechanics by scientific evidences. The study is essential for developing safe and reliable ground control techniques for mining under this sandstone condition. In this study, extensive conventional rock mechanics testing, as well as a detailed particle structure including optical and scanning electron microscopies studies was carried out on St. Peter Sandstone. An appropriate sample preparation technique for St. Peter Sandstone is proposed. The optimum specimen size for characterizing St. Peter Sandstone was determined. The mechanical behavior of St. Peter Sandstone was investigated under triaxial compressive condition. The results indicated that confining pressure will significantly increase the strength and change the mechanical behavior of St. Peter Sandstone from brittle to ductile. The particle structure of St. Peter Sandstone was studied in terms of porosity, particle size distribution, and density. It was demonstrated that the mechanical and strength properties of the St. Peter Sandstone are fundamentally governed by its particle structure. The systematic presence of Hertzian fractures on the St. Peter Sandstone particle structures were obtained, identified and demonstrated. The finding provides direct evidence to resolve outstanding issues regarding the depositional environment of the St. Peter Sandstone. It was indicated that Hertzian fractures were not the product of eolian action. The contact surface of St. Peter‘s sand grains was investigated. It was found that the majority of contact surfaces are smooth. Hence, the high friction angle of St. Peter Sandstone cannot be attributed to “penetrative surfaces” as hypothesized by locked sand theory --Abstract, page iii