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•.12-t7d-1997 8:56414 FRl7t4 ENG. WEST ELK raINE 9?C~ 9295C~Se F.3 <br />~H-lkf-1`!H'/ 11:4.5 HHKU1t~W LHWSUN CYJl ~t:~r.alb r.U~ <br />Nartling Lawson Associates <br />Page Two <br />Mr. Norm Every <br />Mountain Coal Company <br />facility near Somerset, Colorado. In addition, it will evaluate and discuss a utunber of <br />landslide mitigation alternatives which have been proposed for use on the site. These <br />alternatives include vertical dewatering well altcmativcs. horizontal dewatering well <br />alternatives and tendon anchor systems for s~ucttaal stabilization. These altatsatives ors <br />evat»ated on the basis of potential risks and system reliability. A benefit cost ratio type of <br />approach is rued to compare 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 /> <br />Recent iaves[igationa on site rnmpleted during the spring and summer of 1997 have <br />disclosed additional information about the character of the existing landslide. Tess <br />borings utilized for the installation of piezometers and slope inclinometer casing have <br />consistently cacourtteted a layer of rounded gravel, cobble, and boulders ranging 5rom ti <br />to 30 feet in thickness with some clay fines in the matrix (but generally quiu clean and <br />permeable). This layer of coarse alluvial material separates the weathered underlying <br />bedrock from the overlying colluvium and slide debris. The specific origin of this layer is <br />tmt 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 />paleochatrrtels that passed through the area prior to the existence of the existing <br />slide. There appears to be some etude com]ation between thr locations whttc <br />these deposits wets 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 whrn it was located at a higher elevation <br />in the geologic past (i.c., these materials would be associated with a former terracz <br />location that has since been abandoned by additional incision along the current <br />~ aligttmcnt of the North Fork of the Gunnison River). <br />Given the spatial extern of these deposits and the degree of consisuney with which they <br />were encountered in al[ test borings recently completed through the slide mass. it is felt <br />that the second scenario is more probable. If thry are in fact old point bar and terrace <br />deposits associated wish the Nurth Fotk of the Gunnison, then they will likely form an <br />extensive "drainage blankei' which could be expected to he present beneath most or all of <br />the existingt landslide mass. As such, these materials could represent both a major <br />poundal asset and a major potential liability. If this layer cotild be tapped and drnined, <br />then it could function as a large drain blattkct that could assist in the dewatering and <br />depressuriza[ion of the slide's failtue plant. However, if this layer is not intercepted and <br />