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Evaluation of Bear No. 3 Mine Landslide <br />Characteristics of Bear No. 3 Mine Slide <br />The landslide whose major movement occurred on November 15, 1997 lies immediately to the <br />west of the Bear No. 3 Mine portal and across the North Fork to the southeast from the Town of <br />Somerset. The toe of the slide extends onto the flat alluvial terrace used as Bear's surface <br />facilities. On the west end of the slide the toe extends to the bank of the North Fork (Figures 2, <br />3, and 4). The slide extends approximately 400 feet in width in an east-west direction. The slide <br />head scarps are approximately 330 feet above the toe upslope to the southeast. In plan view the <br />slide length extends approximately 400 feet in asoutheast-northwest direction. Calculations <br />from available topographic maps indicate the pre-slide slope was approximately 39°. <br />Measurement of the raveling slope on the lower portion of the slide indicates apost-slide slope of <br />49 to 50°. The slope faces northwest. The lower Mesaverde bedrock beneath the slide dips 3 to <br />5° to the northeast. <br />Except for boulders which have fallen or toppled from the bedrock head scarps, no bedrock <br />appeazs to have been involved in the slide (Figure 5). Field observation indicated that only the <br />colluvium moved in the slide. Where it could be observed, slide movement occurred only at the <br />bedrock-colluvium interface (Figures 6 and 7). Observation of the head scarp areas, where <br />accessible, indicated the colluvium ranged in thickness from only 2 to 3 feet to as much as 20 <br />feet. The colluvial deposit appears to be wedge-shaped in cross section with a thin edge at the <br />top and thickens towards the base where it was deposited on the alluvial gravel terrace. <br />Thickness at the base can not be determined but is estimated as 40 to 50 feet. <br />Investigations by Rold (a geological consultant) and Robin VerSchneider (WWE), and <br />observations by Jubenville (WWE), Kathy Welt (MCC), David Berry (CDMG) and Bear Mine <br />personnel did not observe any water emerging from the slide or any wet spots on the slide. <br />Where observed, the bedrock-colluvium interface slide plane mimics surface outcrop <br />topography. That is, where the slip plane occurs at competent sandstones, it is quite steep. <br />Intervals of soft erodible shales or coals have gentler slopes. Rold and Jubenville saw no <br />831-032.411 Wright Water Engineers, Inc. Page 7 <br />