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II <br />' ~ AGAPITO ASSOCIATES, INC. <br />CONSULTING ENG/NEERS <br />' GR,\dD JnN'(:TION OFFICE <br />t April 8, 2002 134-20 <br />RECEIVED <br />' Ed Baker AFR 2 ~ 2003 <br />Plant Manager <br />' White River Nahcolite Minerals, LLC <br />3200 County Road #31 Division of Minerals and Geology <br />PO Drawer 72 <br />' Rifle, CO 81650 <br />RE: Evaluation of Geotechnical Impacts-Pane12 Solution Mining <br />Dear Ed, <br />' Per your request, we have reviewed your plan for Panel 2 solution mining, as shown in <br />Figure 1, and have evaluated the potential for adverse subsidence-related impacts to (1) the <br />integrity of three overlying USDW aquifers (Perched, A Groove, and B Groove aquifers) and <br />' (2) the future mineability of the Mahogany Zone high-grade oil shale resource (Figure 2). This <br />letter presents our estimate of ground response and assessment of geotechnical risk for Panel 2 <br />mining. Relevant background information i:; discussed as it pertains to our conclusions. <br />' Conclusions <br />t Based on evidence from Panel 1 mining and plans for utilizing Cavity #lA to support <br />future Panel 2 mining, we expect volumetric extraction ratios in Panel 2 to exceed the Panel 1 <br />design extraction ratio of 40%. We anticipate continued cavern growth in Cavity #lA while the <br />' intersecting Panel 2 cavities are mined. Surrounding the intersection area of the Panel 1 and 2 <br />cavities, high to total nahcolite recovery is possible, including recovery from nahcolite-bearing <br />beds above the Boies Bed (L-SB, L-SC, and L-SE). Full cavity height in this vicinity is likely to <br />' extend from the bottom of the Boies Bed to the dissolution surface. Approximately one-third of <br />this volume is expected to fill with deposited insolubles. Cavity widths are expected to grow <br />substantially larger than achieved to date in Panel 1. <br />1 Cavity width will depend largely upon dissolution efficiency along the sides of the cavity. <br />This efficiency is expected to diminish as width increases and is, ultimately, expected to limit the <br />' size of the cavities. The practical limit for cavity width is Jikely to be narrower than typical <br />longwall panels, suggesting that caving effects over the solution cavities will be less than typical <br />for longwall mines. The comparison with longwall caving is important considering that longwall <br />' mining provides the most complete knowledge base from which practical estimates of caving- <br />and subsidence-related impacts, including hydrologic changes, over the solution cavities can be <br />made. Case histories fr om longwall mining and subsurface subsidence characterization studies <br />1 <br />' - CEOENGINEERING MINING ENGINEERING CIVII. ENGINEERING <br />7/5 Hori.on Drive, Suite 3{0, Grand Juncfimt C081506 Ph: (970) 2{2-{220 FA.t': 2{5-923{ F,-mail: A.4lna~ ir~r <br />2 fast 22nd Slreef. Suite 307, Lmubard, /L 60/{8 Phr (630J 792-1520 FAX: 792-1580 E-mail:Frnnknmv~rifo(ula0/ rom <br />