2022-02-22_PERMIT FILE - C1980007 (5)

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West Elk Mine 2.04-38 Rev. 11/04- PR10, 04/06- PR10, 09/07- PR12, 10/08- PR14; 01/22- MR459 Those defined geologic hazards to be considered in the permit area of the West Elk Mine include landslides, debris flows, potentially unstable slopes, rockfalls, seismic events, and ground subsidence. A detailed discussion of each geologic hazard follows. Landslides Landslides in the permit area have been studied by Geo-Hydro Consulting Incorporated (1980), Dames & Moore (1993), Intrasearch (1993 and 1994), John Rold (1994 and 1996), and Wright Water Engineers (annually since 1996). Map 9 shows the distribution of landslides in the permit area. This map is a compilation of the previous studies with modifications based on field observation and photo studies. Generally, the observed slumps on the lease block have two important characteristics in common: 1) They occur in stratigraphic intervals of known or suspected weathering or burn, and 2) Shales and/or clays in the stratigraphically affected interval usually account for more than 50 percent of the total rock lithology. Landslide activity in the area is episodic and is typically closely related to periods of high precipitation and high groundwater saturation. Examples of this occurred during the spring of 1985, 1986, and 1987 when many serious landslides occurred over much of Western Colorado. At that time, a period of eight years of higher than average precipitation culminated in heavy snowfalls in the fall before the ground froze and in heavy spring snows and rains. This combination of events triggered or reactivated extensive landslide activity throughout the entire North Fork Valley. Landslides in the West Elk Mine permit area fall into three distinctive categories: (1) Deep slides involving bedrock and abnormally thick colluvium; (2) Shallow colluvial slides often referred to as skin slides; and (3) Debris avalanches. Deep Landslides The deep landslides involve bedrock and/or abnormally thick colluvium. They can be as much as 100 to 200 feet deep and several hundred feet wide and long. They generally result from abnormally deep groundwater saturation. They usually occur in the spring or early summer of the year and are more prevalent following a heavy autumn snowfall before the ground freezes. Once initiated, movement is usually quite slow, ranging from a few inches to several feet a day with some moving a few feet an hour. The movement of active deep slides is seasonally episodic. That is, the slides move more in the spring or early summer and slow markedly or even stop in the fall and winter. Large, deep active slides can easily be seen in the field or on aerial photographs. Old dormant landslides can be difficult to locate because of erosional changes to the typical landslide topography and a masking of the features with thick brush and vegetation. These dormant slides can be reactivated during periods of high precipitation or by excavation and drainage changes. Neither excavations nor drainage changes are planned for the South of Divide permit revision area. Fortunately, most of the current or potential slides in the subject area would not affect structures.