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DRAFT <br />In a longwall mining operation, the immediate roof rocks behind the face supports <br />collapse into the volume formerly occupied by the extracted coal. The face supports <br />advance following the shearer, as it cuts each slice of coal off the coal face, much of <br />the weight of the overburden arching over the longwall face will be borne by the <br />re-compressed caved material (gob). The load carried by the gob reduces the <br />abutment load and stress on the coal ahead of the face. Abutment loads acting on <br />the coal ahead of the face are smallest when the roof caves immediately behind the <br />longwall face supports. The magnitude of the weight of overburden transferred is <br />reduced when the length between the advancing face abutment and the following <br />gob abutment is shortened. <br />• Caving of the immediate roof, which is necessary to form an abutment zone on the <br />gob, is partially controlled by the lithology of the immediate roof rocks. Generally, <br />shales, mudstones and some siltstones, cave readily because the are relatively <br />weak, whereas beds of stronger sandstone and limestone frequently cave with <br />difficulty. Thin-bedded rock units, with closely spaced joints tend to cave more <br />readily than thick bedded rock units, with more widely spaced joints, particularly the <br />stronger rock types that tend to temporarily hang up and then periodically collapse, <br />occasionally violently. Coal mine bumps and outbursts from abutment loaded pillars <br />and from a longwall face, which may occur when the abutment pressure exceeds the <br />strength of the coal, are minimized both in number and magnitude where the <br />immediate and near roof rocks consist of shales and claystones, but may occur in <br />greater frequency and magnitude where the immediate and near roof rocks are <br />strong, i.e. sandstones and limestones. The thick Rollins sandstone, and numerous <br />thinner sandstone beds occur in the coal bearing lower sequence of the Mount <br />Garfield Formation (Mesaverde Group) that contains the Cameo Seam in the Coal <br />Lease Application area. The Rollins sandstone occurs over much of the western <br />Colorado coal mining districts and is locally exposed as a prominent buff-colored <br />cliff-forming outcrop in canyon walls. However, weak immediate roof rocks can <br />cause roof control and support problems in the gateroad entries and crosscuts and <br />caving ahead of the face supports between the time the shearer exposes a portion of <br />the immediate roof and the face supports can move forward to provide the necessary <br />roof support. Coal outbursts that may occur at the coal face can release weak roof <br />rocks to collapse onto the face conveyor. <br />• Ground stresses and mining induced stress concentrations increase with increasing <br />overburden above a coal seam. Room-and-pillar mining becomes significantly more <br />difficult in overburden more than 1,500 to 2,000 feet thick, because the mine roofs <br />and pillars are already more highly stressed, before coal extraction transfers <br />additional overburden stress. Miners can be forced out of an area by roof falls, pillar <br />slabbing, rib sloughing and floor bumps before planned pillar robbing can be <br />completed. The longwall method overcomes some of the room-and-pillar stability <br />problems. There are no highly stressed pillars present that are split during pillar <br />robbing on the retreat from a panel. Abutment stresses are generally lower and more <br />uniform than in coal mined by the room-and-pillar method. There is also a major <br />body of solid and confined load carrying coal immediately in front of the longwall <br />face. <br />• More frequent and larger magnitude bumps and related seismic activity may occur <br />where a large incompletely caved and consolidated gob area develops behind the <br />longwall face supports. The presence of a thick sandstone bed, such as the Rollins <br />sandstone, in the near roof can be is progressively cantilevered further out over the <br />Page 12 of 57 <br />