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~ iii iiiiiiiiiiiii iii <br />~ gg <br />' MEMORANDUM <br />`FCF~V ~r,~/v,~_ <br />' To: Henry Barbe ~ FQ ~ 4,T HICf JN <br />Mountain Coal Company, L C Cj,2 ~ c3~ ~c 7 <br />~1rysono7M,n 3?Q~Q < ~ National y ~ <br />Erhics ~ <br />From: Wright Water Engineers, Inc. Pais ward <br />C. Richard Dunrud, P.E. ~Gty~~y A ~ <br />~~~. <br />Date: September 27, 2000 <br />' Re: Subsidence Field Observations-August 28-30, 2000 <br />' SUMMARY OF MAJOR FINDINGS <br />I. Subsidence cracks, as discussed in the report at Location 7, were observed in a sandstone <br />' outcrop at Apache Rocks. However, in other areas, more cracks were observed in soil <br />and colluvium above longwall panels 12 and 13 in the Apache Rocks Permit Area prior <br />to mining in 1998, than were observed after mining had been completed. <br />' 2. Subsidence cracks and compression features observed in sandstone outcrops above <br />longwall panel 13 last year are either closed or nearly closed this year. Cracks observed <br />' in soil and colluvium, more than one foot thick, aze now completely healed. <br />3. Subsidence cracks in the sandstone outcrop above longwall panel 13 have closed <br />' noticeably now that mining is complete. The azea was subjected to temporary tensile <br />strain on September 16, 1999, because the longwall face was located below the area at <br />the time. On August 30, 2000, the author observed that tensile strain had decreased <br />' significantly after longwall mining had been completed. This phenomenon of crack <br />closure, after mining and attendant reduction of the temporary tensile strain, has been <br />observed by the author in other mining areas, and also documented in the literature. <br />4. The subsidence cracks observed at Apache Rocks (Location 7) occur within the limits <br />projected in Exhibit 60 for the Apache Rocks and Box Canyon Mining Permit Area (see <br />' Table 2 of Exhibit 60). The projected limits and field calculation estimates are as <br />follows: <br />' a. The projected angle of draw (from Table 2 of Exhibit 60) ranges from ] 0° to 20°. The <br />field measurements range from 11° and 12°. The crack area is within this area of <br />mining influence of the combined mining areas of longwall panels 12 and 13. <br />' b. Projected horizontal tensile strain (Table 2, Exhibit 60) ranges from 0.8 percent to 1.8 <br />percent (0.008 to 0.018 ft/ft). By making a conservative estimate of the crack widths <br />' Wright Water Engineers, Inc., 2490 W. 26°' Avenue, Sle. 100A, Denver, CO 80211 <br />Tel. 303/480-1700; Faz. 303/480-1020, e-mail:krHright@wrightwater.com <br />