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tl4~1 ~b~ 44 ~b r.Yl.] <br />btr-ltl-lDDf 1G•y~ MMttU1rvU LHWbVrv <br />Page Two <br />Mr. Norm Every <br />Mountain Coal Company <br />Harding Lawson 0.saociatea <br />facility scar Somerset. Colorado. In addition. it will evaluate and discuss a number of <br />landslide mitigation altematives which have been proposed for use on the site. These <br />alternatives include vertical dewatering well alurttatives, horizontal dewatering well <br />alternatives and tendon anchor systems for structtual stabilization. These alternatives ate <br />evaluated on the basis of potential risks and system reliability. A benefit cost ratio type of <br />approach is used to comport alternatives. Where possible, risks are assessed objectively <br />and quantitatively. Where this is not possible, risks are subjectively assigned based on <br />experience. <br />2.0 CHARACTER OF THE SLIDE <br />Recent investigations on site completed during the spring and summer of 1997 have <br />disclosed additional information about the chazacter of the existing landslide. Test <br />borings utilized for the installation of piezometers and slope inclinometer casing have <br />consistently encountered a layer of rounded gravel, cobble, and boulders ranging from 6 <br />to 30 feet in thickness with some clay fines in the matrix (but generally quite clean and <br />permeable). This layer of coazse alluvial material separates the weathered underlying <br />bedrock from the overlying colluvium and slide debris. The specific origin of this layer is <br />not known absolutely, however two possible explanations would seem to exist: <br />1. These materials consist of stream bed deposits associated with a network of old <br />paleochannels that passed through the area prior to the existence of the existing <br />slide. There appears to be some crude correlation between the locations where <br />these deposits were encountered and existing surface channel locations near the <br />back plain of the slide. <br />2. These materials may be associated with stream bed deposits that were placed <br />by the North Fork of the Gunnison River when it was located at a higher elevation <br />in the geologic past (i.c., these materials would be associated with a former retrace <br />location that has since been abandoned by additional incision along the current <br />alignment of the North Fork of the Gunnison River). <br />Given tht spatial extent of these deposits and the degree of consistency with which they <br />were rncuunterrd in all test borings recently completed through the slide m:LSS. it is felt <br />that the second scenario is mots probable. IF they are in fact old point bar and terrace <br />deposits associated with the North Fork of the Gunnison, [hen they will likely form an <br />extensive "drainage blanket" which could be expected to lx present beneath most or all of <br />the existing landslide mass. As such, these materials could represent both a major <br />potential asset and a major potential liability. if this layer could be tapped and drained, <br />then it could function as a large drain blanket that could assist in the dewatering and <br />depressurization of the slide's failtue plane. However, if this layer is not intercepted and <br />