2008-02-22_APPLICATION CORRESPONDENCE - C2008086 (43)

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• Compression arches in intact rock can typically support relatively high compressive stresses, compared to tensile stresses, because rock is much stronger in compression than in tension. Major abutment zones in a longwall mining operation will develop on (1) the unmined coal ahead of a longwall face, (2) the unmined coal behind the starter room, (3) the caved zone (gob) behind the supports and possibly on (4) rigid gateroad pillars on either side of the longwall panel. If the planned gateroad pillars do not have sufficient strength to support the arch load abutment, they will yield, transferring that arch load abutment onto unmined coal on one side of the panel and onto the gob left behind the previously mined adjacent panel on the other side. See Figure 2. Plan View of Three Adjacent Longwall Panels. • In a longwall mining operation, the immediate roof rocks behind the face supports collapse into the volume formerly occupied by the extracted coal. The face supports advance following the shearer, as it cuts each slice of coal off the coal face, much of the weight of the overburden arching over the longwall face will be borne by the re-compressed caved material (gob). The load carried by the gob reduces the abutment load and stress on the coal ahead of the face. Abutment loads acting on the coal ahead of the face are smallest when the roof caves immediately behind the longwall face supports. The magnitude of the weight of overburden transferred is reduced when the length between the advancing face abutment and the following gob abutment is shortened. • Caving of the immediate roof, which is necessary to form an abutment zone on the gob, is partially controlled by the lithology of the immediate roof rocks. Generally, shales, mudstones and some siltstones, cave readily because the are relatively weak, whereas beds of stronger sandstone and limestone frequently cave with difficulty. Thin-bedded rock units, with closely spaced joints tend to cave more readily than thick bedded rock units, with more widely spaced joints, particularly the stronger rock types that tend to temporarily hang up and then periodically collapse, occasionally violently. Coal mine bumps and outbursts from abutment loaded pillars and from a longwall face, which may occur when the abutment pressure exceeds the strength of the coal, are minimized both in number and magnitude where the immediate and near roof rocks consist of shales and claystones, but may occur in greater frequency and magnitude where the immediate and near roof rocks are strong, i.e. sandstones and limestones. The thick Rollins sandstone, and numerous thinner sandstone beds occur in the coal bearing lower sequence of the Mount Garfield Formation (Mesaverde Group) that contains the Cameo Seam in the Coal Lease Application area. The Rollins sandstone occurs over much of the western Colorado coal mining districts and is locally exposed as a prominent buff-colored cliff-forming outcrop in canyon walls. However, weak immediate roof rocks can cause roof control and support problems in the gateroad entries and crosscuts and caving ahead of the face supports between the time the shearer exposes a portion of the immediate roof and the face supports can move forward to provide the necessary roof support. Coal outbursts that may occur at the coal face can release weak roof rocks to collapse onto the face conveyor. C-9 DBMS 301