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<br /> <br />Evaluation of Bear No. 3 Mine Landslide <br />Landslides in the azea fall into three distinct categories: (1) deep landslides involving bedrock <br />and/or thick colluvium, (2) shallow colluvial slides ofren referred to as skin slides, and (3) debris <br />avalanches or debris flows. <br />Deep landslides can be as much as 100 to 200 feet deep and several hundred feet wide and long. <br />They can be either rotational or translational. They generally result from abnormally deep <br />groundwater saturation. They usually occur in the spring or early summer and are more <br />prevalent following heavy early autumn snowfall before the ground freezes. Once initiated, <br />movement is usually quite slow ranging from a few inches to a few feet per day, but some move <br />a few feet an hour. <br />Shallow colluvial slides aze quite thin, usually only 2 to ]0 feet thick. They involve slippage of <br />colluvium along steep bedrock-colluvium contacts. They are triggered by extensive, high <br />intensity rainfall, rapid snowmelt or seepage of water into the bedrock-colluvium interface. <br />Depending on the initial moisture content and in place density of the colluvium and the steepness <br />of the slope, they can slowly creep down the slope or become fast moving debris avalanches or <br />debris flows. They are usually fairly small being only a few feet to a few tens of feet wide with <br />failed material sliding a few feet to 200 to 300 feet. Many examples of these slides can be seen <br />in the area as fresh scars on the slopes or on aerial photos. Old shallow colluvial slides <br />revegetate in a few years and can be difficult to see on the ground or in photos. The erratic <br />distribution of colluvium, local variability of surface slope or the bedrock-colluvial interface <br />makes prediction ofthese slides very difficult. <br />The Beaz No. 3 slide appears to be a thin, but extensive colluvial skin slide at its upper edges and <br />a thicker translational slide of unconsolidated colluvium in its middle portion. Typical evidence <br />of a rotational slide was not observed at the toe of the slide. <br />Debris flows or debris avalanches occur on steep slopes at the base of skin slides or small <br />rotational slides where that material becomes saturated and flows rapidly downslope. Recent <br />831-032.411 Wright Water Engineers, Inc. Page 4 <br />