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i • <br />Piceance Site access road, the main well field access road, and some of the main mining <br />panel access roads, will undergo periodic maintenance inspections. Repairs will be <br />made when necessary to preserve drainage structures and other road components. The <br />main Piceance Site access road will be paved with asphalt concrete to maintain its <br />stability. <br />P~elines The pipeline is described in Exhibit D, Section D.2.5. The trench for the <br />two pipes will have a total depth of 54 to 60 inches and a width of 72 to 80 inches. The <br />trench will be backfilled promptly after the pipes are installed. Slope stability for the <br />pipeline is not a concern due to the shallowness of the trench and the short time period <br />during which the trench will be open. <br />Soil Stockpiles Soils will be temporarily stockpiled at each drill pad and at the <br />Piceance Site initial processing facility. The topsoil stockpiles for each of the drill pads <br />will be in place for several years, and the stockpile at the processing facility will be in <br />place for the life of the project. All stockpiles will have a vegetative cover established as <br />soon as possible when it is determined that the stockpile will be inactive for a period <br />greater than 12 months. Side slopes for stockpiles will be approximately 3:1. Instability <br />of topsoil stockpiles is not anticipated. <br />U.2.2 Solution Mining Cavity Stability <br />As described in Exhibit D, solution mining cavities will not actually be 200-foot <br />diameter, 650-foot-thick, cylindrical voids. Approximately 75 percent of the original <br />cavity volume (oil shale and other insolubles) will remain in place after solution mining. <br />Furthermore, cavities will have an areal extent of only 40 percent of each mining panel <br />area because they will be spaced on 300-foot centers. Therefore, the total volume to be <br />mined from each panel will be a mere 12 percent (25 percent of 40 percent) within the <br />mining interval (located approximately 1,550 to 2,200 feet below ground surface). <br />Substantial modeling has been performed to estimate the effects of solution mining <br />cavities on overlying strata. The details and results of this modeling are presented in <br />the enclosed, confidential Agapito Associates, Inc. (Agapito) report entitled "An <br />Evaluation of Stability and Potential Environmental Impact of the Commercial Solution <br />Mine Plan, Yankee Gulch Lease, Rio Blanco County, CO" (Agapito Stability Report) <br />(Agapito 1998). Additional studies were performed by Agapito and were provided in <br />its confidential January 7, 1999, correspondence to Steigers Corporation (Agapito 1999), <br />which is also enclosed. <br />It should be noted that the worst-case scenario presented in the Agapito Stability Report <br />was based on very conservative assumptions. During the evaluation of the Yankee <br />Gulch Project for the BLM Environmental Impact Statement (EIS), it was determined <br />that this worst-case analysis of subsidence was overly conservative and was not <br />representative of a realistic worst-case scenario. Consequently, a more realistic worst- <br />U-3 <br />