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Observations by O'Rourke and Turner (1981) that the maximum tensile strains can <br />be approximately four times larger for U. S. mining conditions (than SEH predictions) <br />should be considered when using these values. However, their quantitative data are <br />limited to one case, so it would not seem appropriate to consider their observations as <br />being necessarily accurate for the Mesaverde Formation. <br />Tilt is also sometimes used as a criterion for damage to structures. This is <br />appropriate for tall structures, such as multi- storied buildings, chimneys or transmission <br />towers, and for structures sensitive to slight changes in gradient, such as sewer lines. Tilt <br />is calculated as the change in subsidence between two points divided by the distance <br />between those points. If the ground slope is such that after tilting the center of gravity of <br />the tilted structure falls outside of the base area, it will topple, but this seldom occurs. The <br />Soviet Union allows a 0.005 tilt for tall transmission (Syd S. Peng 1992). <br />Brauner (1973) summarizes criteria used in various countries for allowable tilts and <br />strains for different structures. He concludes that the various criteria are not always <br />consistent. <br />The SEH damage classification, which is shown on Figure 15 -5, has been derived <br />from observations of damage due to mining. It therefore expresses the combined effect of <br />strain and curvature, even though only strain is used as an indicator of damage. <br />The SEH classification related damage to the length of the structure. Buildings <br />under which mining could occur in the North Fork Valley are for the most part small and <br />should not, in general, be especially sensitive to subsidence. Calculated strains indicate <br />that appreciable to severe damage could result at shallow mining depths, particularly at <br />overburden depths of less than 500 feet. By the time overburden depth reaches 1,000 <br />feet, expected damage is generally becoming slight to negligible for the type of buildings of <br />limited length common in the area. <br />Potential damage to pipelines is indicated to be in the appreciable class of damage <br />of the SEH. French experience ( Brauner, 1973) indicates tensile strains as small as 5000 <br />(u- in /in) or compressive strains of 1000 (u- in /in) to 2000 (u- in /in) can cause visible damage <br />to pipelines. <br />.7 <br />TR -65 -25- 02/10 <br />A F P-Pow- <br />f t // C /1,& <br />