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SEP-18-1997 12 a5 HGIRDING LawSON Page SLr Mr. Norm Every Mountain Coal Company 801 363 4x76 P • 0'7 HarCing Lawson IWociatas The effectiveness will depend on the nature of the target intercepted by the slide. Holes which prnetrate the stream bed deposits would have a high degree of effectiveness due to the high permeability of the gavel, cobble and boulder materials that are contained in the stream bed deposits. For wells which intercept the old workings, once again the effectiveness would be extremely high due to the virtually unlimited hydratilic conductivity of the old workings. Ia oth~Y words, virtually all of the old workings could be dewatered and depressurized by a very limited number of well placed horizontal drains. Reliability of these systems, iF installed below the failure plane of the active slide, would be high. They would not he at risk from the expected displacements and stresses along the existing failure plane as in the cae of the vertical 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 installed 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 tazgct ,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 costs and operating costs would be incurred if the system were to incorporate header pipes and vacuum in order to accelemtc the dewatering of the fine grain materials along the failure plane itself. The life of the systems would be considerably greater than the vertical. well alternatives again due to the placement below the active failtue plane. Maintenance of the horizontal drain systems would also be considerably lower due to the lack of mechanical equipment (i.e., the submersible pumps). 33 STRUCTURAL SOLUTIONS USING'L1E BACK ANCHORS Use of tie back anchors has already been employed on the site in the case of the roller compacted concrete buttress where tie back anchors have been used to improve the effectivrness and reliability of the buttress and extrnd iu life. Such systems work by adding resisting force and thereby increasing the factor of safety (i.e., factor of safety equals rcsis[ing firer divided by driving force). it requires adequate anchorage into stable ground, the use of steel tendons to transfer these forces to a reaction block at the surt3ce, and the use ot'somc 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 dasigncd and deployed, they are capable of completely amsting all movement.