2006-08-02_PERMIT FILE - C1980007 (3)

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Mountain Coal Company -West EIk Mine Seismic Refraction Survey Sylvester Gulch Road Geotechniral Investigation Summary of Operations • Summary of Operations Seismic Refraction Survey -Sylvester Gulch Road SEISMIC REFRACTION -RATIONALE The geotechnical properties of the underlying soils and geologic strata along Sylvester Gulch vary widely and rather unpredictably due to fault dispLicemem, scouring, and alluvial deposition; consequently, projecting bedrock conditions from discrete geotechnical boreholes is extremely difficult. Bedrock conditions must be mapped accurately to align the proposed Sylvester Gulch road optimally and design typical sections with appropriate load bearing capacity, retaining wall systems, and hydraulic relief systems. Sylvester Gulch lies along a probable fault lice that has been subjected to centuries of seasonal erosion and episodic out-washings. The geologic setting varies with elevation, passing through munerous horizons of thick marine sandstone, weathered sandstone, shale, clay, coal seams, and colluvial fill. The Later is an unstable, inconsistent miadure of aU the previous sedimentary horizons, from clay to sandstone boulders. In addition there aze isoLrted perennial and ephemeral springs dotting the landscape and sub- temdin along the gulch. The seismic refraction survey is to augment geotechnical drilling, providing continuous profiles of the subsurface beneath the steep terrain in the immediate vicinity of the proposed road alignme~, primarily in areas inaccessible to the geotechnical drill rig. . The seismic survey can provide data on bedrock characteristics that is valuable for slope stability modeling and geotechnical design. When combined with geotechnical borehole data (a.ka. ground truth), the seismic refraction survey can provide interpretations of bedrock depth, composition and competency in profile. The seismic survey is anon-intrusive, non-impact means for providing essemial geotechnical data in areas where drilling cannot be performed because of access limitations. The wmbination of the geotechnical boreholes along the existing all-terrain-vehicle (ATV) trail and the lateral seismic profiles can provide designers data essential for a sound road design. FIELD OPERATIONS Seismic refraction survey operations in Sylvester Gulch began Apri12, 2003 and ended Apri18, 2003. Twerrty-two survey lines were completed perpendicular to the stream bed between Stations 17+00 and 49+00 with one additional survey line parallel to the existing ATV trail centered at 47+10. The following profiles were completed to support the geotechnical analysis: 17+00, 19+00, 23+00, 25+50, 28+00, 31+00, 33+00, 34+00, 35+00, 36+00, 37+00, 38+00, 39+00, 40+00, 41+25, 42+00, 43+00, 44+00, 45+00, 46+50, 47+10(parallel), 48+00, 49+25. Incompliance with the United States Departmerrt of Agriculture Forest Service regulations, proper consideration was taken to prevent disdubance of fhe existing terrain. The site was accessed using 4- wheelers on the existing ATV trail. Work along side the ATV trail was accessed on foot, Depending on geologic and terrain conditions a Betsy shotgun or a 16 pound sledge hammer were used as a seismic source. The Betsy shotgun was loaded with bLurks and dischazged irrto the ground at a depth of 2-3 feet. 'The 2-inch diameter, 2-3 fee[ deep hole created by the discharge was subsequently filled in. • A 24~haanel portable seismograph with 14 hertz geophones was used to log seismic data. Each seismic line contained 24 geophones set in the ground with 10 foot spacing. For each set a seismic shot was taken 5 feet off the ends of the line and at the cemer of the line. A Geometrics Mode12401 seismograph stored UMS, Inc. Page 1 05/17/03