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~ 2.05 APPLICATION FOR PERMIT FCR SURF7iCE OR UDIDERGROUN[) MINING ACTIVITIES -- MINIt4[II~i REQUIRFMENFS FOR OPERATION AND RECLAMATION PLANS. 2.05.6 Mitigation of Surface Ooal Mining Operation Impacts. (Cont'd.) (6) (f) (ii) (Cont'd.) In the 600 to 800 foot overburden depth buffer zones defined in the above mentioned drainages, permanent protective pillars are designed to provide safety factors in excess of 1.5 using the Wilson pillar design method. Panel designs for specific development geometries and mining heights vary within these buffer zones and are optimized to provide maximum economic recovery of the coal resource, however, the minininn safety factor criteria for all panels developed within this buffer zone remains constant and 1.5 (refer to Pillar Stability Calculations in Volume 2). Secondary extraction or retreat mining is limited to the recovery of floor coal within previously developed entries. All development entries are roof-bolted in accordance with the approved roof control plan in effect. This zone of development mining between 60o and 80o feet of overburden also serves to dissipate abutment stresses from full extraction to the virtually solid buffer area barrier blocks stipulated by the BLM. In essence, these subsidence control measures of limited and partial extraction beneath Stevens Gulch and East Roatcap Creek will prevent subsidence in the areas defined as Renewable Resource Lands, during subsequent permit terms. • Structures within the permit area are depicted on Map No. 9-7. Structures located near projected undermining within Lease COC- 37210 are discussed in special stipulation "1" which states a 300 foot buffer zone will be maintained around occupied structures. Strain projections derived by Geo-Hydro Consulting (Subsidence Appendix, Volume 3) indicate material damage or diminution will not occur. Structures located within the Lease COC-37210 area generally overlie the "D" coal seam at depths less than 850 feet. F]~loying a draw angle of 20.5 degrees, the area of subsidence influence is approximately 320 feet. The coincidental concurrence between federal lease stipulation "1" and site conditions at Orchard Valley will minimize structural damage due to reduced strains and predicted tilt. Structures located near projected undermining within Lease COC- 53356 are protected by virtue of the overburden thickness occurring at these locations. All structures identified in this lease area occur in areas of over 2000 feet of overburden cover. Employing the subsidence projections developed by Geo-Hydro Consulting, only slight to negligible damage is to be expected at these structures. COVCC believes any subsidence caused damage can be promptly rectified on a case-by-case basis, using economically and technologically feasible means. PR 93-03 Revised 04/17/93 157