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' -19- •~ PnTENT1AL SUBSIDENCE EFFECTS Pillars tend to deteriorate as time passes after mining. Pillar failures have led to serious subsidence incidents long after mining is finished. Scranton, PA and Rock Springs, WY have experienced such phenomena It was, therefore, necessary to evaluate the 'tivorst case" subsidence potential for pillar failure at some remote time in the future. ~. The maximum vertical subsidence ($m~ predicted in the very unlikely event •- the pillars in Znd East and 3rd East Sectioru within the angle of draw from the Ute i_ , Water Co's. surface facilities fail were cnlcvlated using the statistical evaluation method ~ presented by Wardell and Eynon (1968) and expanded by Abel and Lee (1980). This method is shown in Figure 7. The calculated maximum vertical subsidence values in i the affected area are presented in Table 7. (. The predicted maximum horizontal tensile and compressive strairu and tilt were calcutated using the Subsidence Engineers Handbook (NCB, 1975) and the previously predicted maximum vertirnl subsidence (Sma~. These predicted values are also presented in Table 7. I If the Ute Water facilities were fully over the 3rd East Section, the recommended 14-ft wide pillars would, if they were To foil, subject the surface structures to unacceptable strairu (Appendix E). Fbwever, most of the structures ore over the entry barrier and chain pillars which will prevent the maximum compressive strains predicted from acting on the Ute Water facilities. Unfortunately, the tensile strain and tilt ` • maximums predicted occur over or near ribsides such as the barrier pillars adjacent to the 3rd East Section of the Roadside Mine. Only the 24-in. water mein can probably tolerate The subsidence, strain and tilt magnitudes predicted. • The only effective method to reduce damaging subsidence effects accompanying the long-term deterioration and eventual failure of pillars is to backfill around the _~ Z