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Mountain Coal Company-West EIk Moe Seismic Refraction Survey <br />Sylvester Gulch Road Geotechnical Irnestigation Summary of Operations <br />• Longitudinal Line Alone ATV Trail. Centered at 47+10 <br />Seismic refraction results reveal a second layei of higher velocity material which occurs nearly parallel to <br />the surface at an average depth of 17 feet (Figure 32). The velocity of the first layer is typical of <br />unconsolidated sediments. Velocities for the second layer indicate highly weathered bedrock. <br />>~ <br /> ~ N °u s a Tr ~ <br />g ~ . <br />T32o c. m. m.w -os m m m p1° yix <br />~ --..__.. ._.. ._ _.____ __.__---.. _ .. <br /> V1 = 1100 R/s <br /> <br />reto ... _ <br />, _ . _ .. _......- , _ <br />-- - --- - <br /> <br /> YG = .J~GJ IYO ~ aUlY1d8I~ ~ <br />~M <br />7290 <br />0 10 20 3D 40 $C1 G6 10 60 80 700 110 120 730 140 7.W 160 770 180 1f10 20D 210 220 230 240 260 260 <br /> D~IIL'! <br />Figure 32. Seismic refraction profile along the ATV trail centered at 47+10. <br />SUMMARY AND CONCLUSIONS <br />The data quality for this seismic survey could be classified as excellent in most cases. Data processing <br />included corrections for the steep terrain. Interpretations generally agreed well with geotechnical <br />borehole data. The excellent data gnality and processing resuhs were verified with seismic line 47+10, <br />where twain corrections were not significant, yet the velocity data agreed very well with the Endings <br />obtained over the rest of the survey. <br />Depth of exploration typically extended to about 75 feet from the surface. Seismic data occasionally <br />• displayed some scatter of emergent rays; however, it is most probable the data represents the refractive <br />character of bedrock transitioning between sandstone and shale sub-crops as the seismic rnys traveled <br />along and up the steep flanking slopes. This interpretation is consistect with local sttatigraphic sequences <br />and is also reflected in the geotechnical borehole data. For this analysis a single average bedrock velocity <br />was used to process the data and profile the bedrock contact. AcNal depth to bedrock may vary by +/- <br />15% in areas of sub~ropping occurrence, shale bedrock being slightly deeper and sandstone slightly <br />shallower; however, for the purpose of this investigation the processed results are sufficie~. Subsequem <br />analysis and fiuther processing may be performed to reEne depth data if deemed necessary. <br />Geotechnical borehole data (Environmental Strategies Corporation, March 2003) indicates bedrock is <br />comprised of weathered shale and weathered sandstone layers. This is consistent with V2 velocities <br />obtained during the seismic refiactioa campaign and with the regional geologic strata. The seismic <br />refraction derived profiles appear to adequately define the sub-terrain overburden thickness, bedrock <br />depth for inclusion in the geotechmcal slope stability modeling within the immediate vicinity of Sylvester <br />Gulch. <br />UMS RESPONSIBILITES AND PROJECT SUBMITTALS <br />UMS provided equipme~ and services to complete the subject project. UMS adhered to all West Elk <br />Mine and U.S. Forest Service policies and procedures during the performance of these services. <br />UMS performed interpretation of seismic, geologic, and geomorphologic data and prepared <br />corresponding cross sections on AutoCad compatible digital drawing Eles for geatechnical design <br />purposes, 'This is the written report summarizing operations, i~erpretations, and recommendations for the <br />geotechnical design. <br />• <br />UMS, Inc. Page 21 05/17/03 <br />