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32.05 APPLICATION FOR PERMIT POR SORFACE OR IIRDERGROUND MINING ACTIVITIES -- MINIMUM REQUIREMENTS FOR OPERATION AND RECLAMATION PLANS. 2.05.6 Mitigation of Surface Coal Mining Operation Impacts. (Cont'd.) (6) (b) (i) (B) (Cont'd.) Federal lease C-37210 stipulation "g" limits extraction beneath Steven's Gulch and East Roatcap Creek to neces- sary development access entryways below 600 ft. of overburden thickness. Seven entries, including three developed on 120' X 150' centers, and four developed on 120' X 100' centers will result in nominal pillar sizes of 100' X 130' and 100' X 80', respectively. Design cri- teria developed by Wilson (1972) for entries developed on 120' X 80' cen- ters, estimates a safety factor in ex- cess of 2, which should preclude any long-term pillar failure. 'lhe larger 120' X 150' centered entries and 120' X 100' centered entries would increase the safety factor over the previously approved pillar size of 120' X 80' (re- fer to Pillar Stability Calculations in Volume 2). the remainder of these buf- fer zones, projected to seam level at 20.5 degrees, will not be undermined. See Map 2-9. In the above mentioned drainages from 600 to 800 ft, overburden depths, de- velopment mining with no pillar recov- ery will be undertaken with entries de- veloped on 80' X 80' centers, resulting in a nominal pillar size of 60' X 60' . Pillar stability, by employing Wilson (1972), is ensured by a safety factor greater than 1-1 /2 at 800 ft. overbur- den (refer to Pillar Stability Calcula- tions in Volume 2). this zone of de- velopment mining between 600 and 800 ft. of overburden will also serve to dissipate abutment stresses from full extraction to the virtually solid buf- fer area barrier blocks stipulated by the BLM. In essence, these subsidence Added 12/26/84 Revised 12/26/84 Revised 10/12/86 Revised 10/01/87 142 _, ~.