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• many years and there is little probability that any man-made <br />mitigation is feasible because of the considerable thickness <br />of the alluvial deposits. <br />In summary, the impacts of potential subsidence on <br />ground water in the Mesaverde Formation are negligible. <br />Potential fracturing of the formation and the local increase <br />of permeability above the extracted coal seam would only <br />affect potential perched water bearing zones. As none of <br />them has been used in the past and is not likely to be used <br />in the future, the impact is negligible. The subsidence <br />impact on Stevens Gulch alluvium is of more importance. <br />The impacts on local colluvial aquifers appear to be <br />negligible with the exception of the area within Section 8. <br />6.9 BUILDING AND STRUCTURES <br />C[iteria For Evaluating Impacts to Buildings and <br />Structures <br />Horizontal strain is a measure of the change in length <br />of a piece of ground or structure. Damage to surface <br />structures is frequently a function of horizontal strains. <br />Most relevant codes specify maximum allowable horizontal <br />strains in order to prevent damage to various structures. <br />The complete strain profile at ground surface above an <br />extraction area can be developed using methods detailed in <br />SEH. However, for the current study, it is considered <br />sufficient to estimate maximum strains, since the maximum <br />damage to any structure which could develop can be estimated <br />from these values. <br />Strain is proportional to subsidence (S) and inversely <br />proportional to depth (h), so *_he maximum strain (E) over an <br />excavation is proportional to 5/h. Using the graph presented <br />in SEH, which is reproduced as Plate 15, enables an estimate <br />of maximum strains. Since this graph has been developed for <br />a simple profile, that is, panels with no center gates or <br />barrier pillars, its application to the Orchard Valley case <br />is only an appro:timation, but does provide a basis for a <br />quantitative initial estimate of maximum horizontal strains. <br />Plate 15 indicates that for 2,500 foot wide extraction <br />areas, where the w!1~ ratio will be in excess of 1.0 at all <br />overburden depths of less than 2,500 felt, maximum <br />compressive and tensile strains .gill oe on the order of 0.5 <br />f" 1 <br />U <br />- 47 - <br />