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Section - 2.05.6 <br />Given the projections of probable and worst-case subsidence and the considerations noted in the <br />previous discussions for potentially affected structures and renewable resource lands (or lack thereof), <br />the potential fox material damage or reduction in the value or reasonably foreseeable use of stmcmres <br />or lands due to mining-related subsidence is remote. With limited or no potential fox material damage <br />or reduction in value or use, the need and requirements For specific subsidence controls, monitoring, <br />and surveys are similarly limited. <br />Subsidence Control Plan - As described in the previous section, specific subsidence control <br />measures, including maintenance of a subsidence barrier adjacent to old State Highway 133 and <br />limitations on coal extraction in both the portion of the Elk Creek Mine near Hubbard Creek and the <br />south portion of the Sanborn Creek Mine, have been implemented where necessary to assure <br />protection of important surface structures. The only other area where specific subsidence control ~3y't'~G <br />measures were considered was the portion of the Bear Creek drainage where the Elk Creek Mine Sb' Jy` `~ <br />entries will pass under an area of limited cover. A separate subsidence evaluation for this area ~ „ep~ <br />completed by Dr. John Abel and included in Exhibit 2.05-E8, Subsidence Control and Monitoring, ' ~ <br />indicated that since mine development in this area will involve development of main entries with <br />intervening barrier pillazs, adequate support will be maintained to prevent subsidence and protect the <br />surface drainage, so no supplemental subsidence control measures axe necessary. <br />Subsidence Monitoring Ptogram -Subsidence and mine inflow monitoring were conducted to <br />evaluate the accuracy of subsidence predictions indicating that the historic Somerset mining <br />operations would result in no adverse effects to overlying portions of the Hubbard Creek drainage. <br />Monitoring results confirmed the accurary of the original subsidence predictions. The Somerset `/ <br />Mine, closed in 1985, has gone through its subsidence phase (subsidence typically occurs within three ~j <br />years following final coal extraction) with no indications of discernable surface damage or drainage of ~~ <br />surface water sources. The DMG approved suspension of subsidence and mine inflow monitoring <br />Eor the Hubbard Creek area in conjunction with approval of Technical Revision TR-03 in December <br />1986. The six Hubbazd Creek subsidence monuments were left in place for future monitoring, if <br />required. <br />Areas overlying both the Sanborn Creek Mine and the Elk Creek Mine will undergo either areal <br />subsidence (those portions of Sanborn Creek Mine where continuous miner room and pffiaz methods <br />were used) or trough subsidence (those portions of Sanborn Creek Mine and the entire Elk Creek `~,`~ ~ <br />Mine where the longwall mining method was used). The permit area for the Sanborn Creek Mine is ~e`J1" ~ <br />visually monitored in June and October for indications of rock fall and/or mass movement activity, yPJr 5~ <br />Particular attention is focused on the historic landslides in Coal Gulch, in the unnamed drainage Ly ~' <br />between Sanborn Creek and Coal Gulch, and on Hawksnest Creek. Similar visual monitoring will be <br />conducted on azeas overlying the Elk Creek Mine plan area where regularly traveled roads are located <br />below historic landslides in the Beaz creek drainage. If any significant indications of surfiual mass <br />movement or significant rock fall are detected by the visual monitoring, the DMG will be timely <br />notified and OMLCC will initiate joint discussions with the DMG of the need for and nature of any <br />appropriate supplemental monitoring measures. A written report documenting the visual subsidence <br />monitoring inspections will be prepared and submitted to the DMG. <br />While subsidence monitoring focuses on the identified pre-existing mass-movement features mapped <br />on the Regional Geology and Hydrology Map, (Map 2.04-M2), it should be noted that activation or <br />movement of these features, whether through subsidence or natural causes, does not necessarily <br />represent a significant risk to the environment or public health and safety. Most of these features are ~ <br />located high in the upper reaches of the drainages and steep canyons and well away from both <br />structures and human activity. <br />Subsidence Survey -Prior to conducting retreat or longwall mining the baseline condition of <br />identified rock cliffs and/or mass movement features located above regularly traveled roads will be <br />documented. Given the limited potential for significant surface subsidence effects and the ragged <br />terrain of the area, with consequent constraints on physical access, visual surveys Eor indications of <br />PROS <br />2.05-104 <br />Revised <br />2002 <br />