17th U.S. Symposium on Rock Mechanics, Snowbird, Utah, 1976
Authors: J. F. T. Agapito (AAI) and J. B. Page
This paper describes part of a geotechnical study based on stress determinations and convergence measurements conducted to assess the long-term stability of an experimental room-and-pillar oil shale mine. A rock mechanics program was initiated in 1971 at the Colony Mine as part of a large study conducted for the design of a commercial operation. The Colony Development Operation is a joint venture currently consisting of four active members: Ashland Oil Inc., Atlantic Richfield Company (operator of the project), Shell Oil Company, and The Oil Shale Corporation. Although oil shale commercialization plans are presently suspended, rock mechanics instrumentation has been continued as part of Colony’s policy in maintaining readiness to proceed with a commercial shale oil plant when national energy policies became better defined. The property is located in the southern edge of the Piceance Creek Basin in northwestern Colorado, approximately 200 miles west of Denver. Experimental mining operations were conducted from 1965 to 1972, with one of the major objectives of the pilot mine being the assessment of opening and pillar sizes for the determination of extraction ratio and life of the property. Mining was conducted in a 60-ft-thick portion of the Mahogany Zone in the Parachute Creek Member of the Green River Formation, at depths of 600 and 860 ft by a one-bench system. Pillar dimensions were 58 ft by 58 ft and rooms were 55-ft wide. A well-defined system of joints and bedding planes is present in the flat-lying oil shale beds. The pillars at Colony have two joint sets approximately 90 degrees to each other. The mean strike of the major set is E-W, with 42 percent of the dips being vertical ±10 degrees and 33 percent dipping south at 24±6 degrees from the vertical. The other dips range between these two orientations. Major pillar slabbing has taken place along the south-dipping joints. Joint spacing may vary from one foot to more than 15 feet. The minor joint set dips mostly vertically and has a wider spacing than the major set. The roof shows only one joint system parallel to the major pillar joint set. Most joints are small and very tight, but a small percentage are open and can be traced across some pillars. Joint filling material when present consists mostly of calcite. Laboratory tests indicate that the uniaxial compressive strength is dependent on oil content. Above 30 gallons per ton, strength seems to remain constant at 13,000 psi, but below this grade the strength increases with a decrease in oil content. The immediate roof rock is 3,000 psi stronger than the mine horizon because of a marked difference in grade. Field instrumentation was described in detail in another paper and basically consisted of in-situ stress determinations by the overcoring technique, and rock mass displacement measurements by means of borehole and tape extensometers. Design data was greatly enhanced by unplanned pillar and roof failures.