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West Elk Mine <br />Those defined geologic hazazds to be considered in the permit azea of the West Elk Mine include <br />landslides, debris flows, potentially unstable slopes, rockfalls, seismic events, and ground <br />subsidence. A detailed discussion of each geologic hazazd follows. <br />Landslides <br />Landslides in the permit azea have been studied by Geo-Hydro Consulting Incorporated (1980), <br />Dames & Moore (1993), Intraseazch (1993 and 1994), John Rold (1994 and 1996), and Wright <br />Water Engineers (primazily R. Dunrud annually since 1996). Map 9 shows the distribution of <br />landslides in the permit area. This map is a compilation of the previous studies with <br />modifications based on recent field observation and photo studies. <br />Generally, the observed slumps on the lease block have two important chazacteristics in common: 1) <br />They occur in stratigraphic intervals of known or suspected weathering or burn, and 2) Shales <br />and/or clays in the stratigraphically affected interval usually account for more than 50 percent of the <br />total rock lithology. <br />Landslide activity in the area is quite episodic and is closely related to periods of high precipitation <br />and high groundwater saturation. Examples of this occurred during the spring of 1985, 1986, and <br />1987 when many serious landslides occurred over much of Western Colorado. At that time, a <br />period of eight yeazs of higher than average precipitation culminated in heavy snowfalls in the fall <br />before the ground froze and in heavy spring snows and rains. This combination of events triggered <br />or reactivated extensive landslide activity throughout the entire North Fork Valley. <br />Landslides in the West Elk Mine permit azea fall into three distinctive categories: (1) Deep slides <br />involving bedrock and abnormally thick colluvium; (2) Shallow colluvial slides often referred to <br />as skin slides; and (3) Debris avalanches. <br />Deep Landslides <br />The deep landslides involve bedrock and/or abnormally thick colluvium. They can be as much as <br />100 to 200 feet deep and several hundred feet wide and long. They generally result from <br />abnormally deep groundwater saturation. They usually occur in the spring or eazly summer of the <br />year and aze more prevalent following a heavy autumn snowfall before the ground freezes. Once <br />initiated, movement is usually quite slow, ranging from a few inches to several feet a day with some <br />moving a few feet an hour. The movement of active deep slides is seasonally episodic. That is, the <br />slides move mare in the spring or early summer and slow markedly or even stop in the fall and <br />winter. Lazge, deep active slides can easily be seen in the field or on aerial photographs. Old <br />dormant landslides can be difficult to locate because of erosional changes to the typical landslide <br />topography and a masking of the features with thick brush and vegetation. These dormant slides <br />can be reactivated during periods of high precipitation or by excavation and drainage changes. <br />Neither excavations nor drainage changes aze planned for the South of Divide permit revision <br />area. Fortunately, most of the current or potential slides in the subject azea would not affect <br />structures. <br />• <br />2.04-38 RevisedNovem6er 2004 PRI G <br />