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P. 7 12-84-1997 8:59AP1 FR014 ENG. v~EST ELK r1INE 97Q 929505[7 801 36. aa76 P. B' per-in-177r 1C•4, rn~uiw LHWSU~ Herdiry t.aN'lOfl AllaCiateS Page Six Mr. Norm Every Mountain Coal Company The effectiveness will depend on the nature of the target intercepted by the slide. Holes which penetrate the stream bed deposits would have a high degree of effectiveness due to the high permeability of the gravel, cobble and boulder materials that are contained in the streatrt bed deposits. For wells which intercept the old workings, once again the effcetiveness would be eztrcmely high due to the virtually unlimited hydraulic wnductivity of the old workings. In other words, virtually ail of the old workings could be dewataed and depressurized by a very limited number oC well placed horizontal drains. Reliability of these systet[ts, if installed below the failure plane of the active slide, would be high. They would not be at r+sk from the expectad dlsplacernenu and stresses along the existing failure plture as in the case of the venica! wells. Initial capital costs would depend on how and where the drains were placed. if target zones could be located near the base of the slide and holes is+ralled in an upward direction initiating in the target materials. initial capital costs can be greatly minimized. In addition, the increased control coming with starting in the target ,one will further improve the reliability of these systems. Operating costs for These systems would be relatively low as it would involve little or no pumping. However both additional initial capital cows and operating costs would be incurred if the system were to incorporate header pipes and vacuum is order to accelerate the dewatering of the fine grain materials along the failure plane itself. 7hc life of the systems would be comaiderahly greats than the vertical welt alternatives again due to the placement below the active failum pleat. Maintenance of the horizontal drain systems would also be considerably lower due to the Zack of mechanical equipmem (i.e., the submersible pumps). ~. 3.3 STRUCTURAL SOLUTIOPIS USI1~G Tll'; BACK A1YCIiOR.S Use of tie back anchors has already been unployed on the site in the case of the roller compacted concrete buttress where tie back anchors have been used to improve the effectiveness and reliability of the buttress and extend its life. such systems work by adding resisting force and thereby inen:asing the factor of safety (i.e., factor of safety equals resisting force divided by driving force). !t requires adequate anchorage into stable ground, the use of steel tendons to transfer these forces to a reaction block at the surface, and the use of some form of structural element (i.e., whaler) which acts to transfer stresses from the reaction block at the end of the anchor into the slide mass itself. The effectiveness of such systems is very high_ In fact if properly de,igncd and deployed, they are capable of completely arresting al! movement.