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12-0a-1997 8:58at-1 FROt1 ENG. WEST ELV: rtINE 970 929505© <br />per-~e-r»~ re~uu rna•tu trvu LNwSUt+ EiN1 Sb.; 4d76 P.06 <br />Heramg Lawson AvoWtea <br />Page Five <br />Mr. Norm Every <br />Mountain Coal Cotnpany <br />Based upon the above described characteristics of the slide mass, proposed landslide <br />mitigation alternatives will be discwsed and evaluated in the context of these assumed <br />characteristics. <br />3.0 PRELIMINARY EVALUATION OF A7ITIGATION ALTERNATIVES <br />Thc evaluation ofpotentia] mitigation alternatives ate based on five principle factors: <br />+. 1. EEfettiveness <br />?. Reliability <br />3. Cost (both initial capita! eosu and future operating costs) <br />4. Life (duration of service) <br />5. Maintcnancc <br />Subsequrnt sections will address each of the proposed alternatives in detail <br />3.1 VERTICAL DEWATERiNG WELLS <br />This approach will be to place Utc 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 the ground water depth encountered, the <br />well spacing, and bulk hydraulic conductivity of the material in which the wills aze <br />completed (which would control the ult;rnate radius of influence of the wells and [hc <br />degree of water intercepted)- <br />These wells wotild be expected to be highly variable in nattue depending on the degree of <br />frreture interception. Reliability' should be higher as the drilling environment should be <br />expected to be mom stably 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 be expected to have a lottget working life and <br />compazativrly lowered maintenance requirements. <br />32 HORIZONTAL DEWATERING WELLS <br />Horimntal dewateting wells wiil consist of holes drilled into water beating strata at a <br />slightly positive inclination so a, to permit gravity drainage of the watu instead of <br />purrtping. Target horimns wilt consist of the boulder stream bad deposits at the base of <br />the slide and~or the abandoned underground workings in the area immediately behind the <br />slide. <br />P. 6 <br />