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SEP-18-1997 12 44 HRRDING LRWSON 801 363 4476 P.06 <br />Page Five <br />Mr. Norm Every <br />Mountain Coal Company <br />Mara~ng Lawson Avsaciatea <br />Based upon the above described characteristics of the slide mass, proposed landslide <br />mitigation alternatives will be disewsed and evaluated in the context of these assumed <br />characteristics. <br />3.0 PRELIMINARY EVALUATION OF MITIGATION ALTERNATIVES <br />The evaluation of potential mitigation alternatives are bazed on five principle factors: <br />1. Effectiveness <br />2. Reliability <br />3. Cost (both initial capita! costs and future operating costs) <br />4. Life (duration of service) <br />S. Maintenance <br />Subsequent sections will address each of the proposed alternatives in detail. <br />3.1 VERTICAL DEWATERING WELLS <br />This approach will be to place the wells upstream of the slide to an unknown depth to <br />collect and intercept the groundwater before it can pressurize the boulder complex. The <br />spacing and depths of the wells will be based on ilte ground water depth encountered, the <br />well spacing, and bulk hydraulic conductivity of the material in which the wells are <br />compleud (which would control the ultimate radius of influence of the wells and the <br />degree of water intercepted). <br />These wells would be expected to be highly variable in nature depending on the degree of <br />fracture interception. Reliability should be higher az the drilling environment should be <br />expecud to be more stable. Initial capital cost and operating cost should be comparable to <br />the other alternatives if the number and spacing of wells is comparable. Vertical wells in <br />this stable environment would 6c expected to have a longer working life and <br />comparatively lowered maintenance requirements. <br />3.Z HORIZONTAL DEWATERINC WELLS <br />Horizontal dewateting wells will consist of holy drilled into water beating strata at a <br />slightly positive inclination so as to permit gravity drainage of the water instead of <br />pumping. Target horizons will consist of the boulder stream bed deposits at the baze of <br />the slide and/or the abandoned underground workings is the area immediately behind the <br />slide. <br />