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<br />CHAPTERFOUR Enuironmenta~ i~onseguences ' <br />Information from Agapito's two reports was used to assess potential impacts on the mineability , <br />of oil shale and possible impacts from surface subsidence. <br />4.1.2 Direct and Indirect Impacts ' <br />Proposed Action 1 <br />Piceance Site <br />Initial modeling conducted by Agapito of surface subsidence resulting from mining of nahcolite ' <br />assumed the following: <br />• A maximum solution mining cavity diameter of 200 feet would be developed ' <br />• Maximum height of the solution mining cavity would be 600 feet (cavity roof would be <br />approximately 150 feet below the Dissolution Surface) <br />• Initial solution well spacing would be 600 feet , <br />• Final solution well spacing would be 300 feet <br />• Solution well field development as described in the Mine Plan ' <br />For the single-cavity analysis, the following conclusions were drawn by Agapito. Yield zones <br />were predicted both in the cavity wall and portions of the roof for the single-cavity analysis. The <br />thickness of the yield was estimated at approximately 25 to 32 feet. The impact of the yield zone <br />on cavity stability differs for the roof and walls. The yielded rock would exhibit enhanced <br />permeability and promote the dissolution process in the walls, while the roof wotdd be acted ' <br />upon by gravity and may collapse. However, insoluble residuals in the cavity should stabilize <br />the walls and help support overlying strata. , <br />For the multi-cavity analysis, the following conclusions were drawn by Agapito. With 200-foot- <br />diameter cavities, 300-foot spacings, and no adjacent well operating at the same time, the intact <br />core of the pillaz was predicted to be approximately 40 feet. The intact 40-foot-diameter core ' <br />represents the central part of the 100-foot pillar remaining between solution cavities that is not <br />subject to yield from heat and pressure effects of the solution mining process. Given that the <br /> <br />actual diameter of the solution-mined cavity would change along its height, the 40 feet of intact ' <br />material in the pillar is considered a minimum to maintain long-term stability and hydraulic <br />isolation between solution cavities. , <br />Several other multi-cavity scenarios were also considered (Agapito 1998b). One .>cenario <br />considered oversized cavities of 260 feet in diameter, and a second scenario modeled the effects <br />of a zone of structural weakness (i.e., fault zone) resulting in weakened pillazs remaining , <br />between the solution cavities. <br />From Agapito's work, it was determined that direct impacts from the project include surface , <br />subsidence potentially ranging from 1 to 3 feet. However, the most likely scenari~~ is surface <br />subsidence of less than 1 foot. Secondly, the small amount of predicted surface subsidence <br />would not cause visual surface cracks, and the subsidence would not be catastroptuc but would , <br />occur over geologic time. <br />4-2 Geology ' <br />