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r~~-~.. <br />Mr. Brent Anderson <br />Page Three <br />October 1, 1990 <br />IMPROVING DRAINAGE AT THE TOE OF THE SLIDE <br />Providing effective drainage at the toe of the slide would help to prevent <br />any buildup of significant pore pressures within the slide. At present, <br />the open drainage ditch at the toe of the slide serves this purpose <br />adequately. However, the drainage ditch also carries runoff from the road <br />and upgradient areas across the toe of the slide and could result in <br />erosion of the toe. Maintaining a stable toe is important to assuring <br />overall stability of the currently stable slide mass. <br />Two measures for improving toe conditions are recommended. First, a Swale <br />should be constructed across the road above the slide. This Swale would <br />intercept all up gradient cut side drainage and divert it to the existing <br />drainage ditch on the fill side of the road. This would minimize erosion <br />of the toe of the slope. Second, a 2' x 2' rock drain wrapped in filter <br />fabric should be constructed across the toe of the slide. The drain <br />should be constructed by excavating a trench at the toe of the slide with <br />a backhoe, installing filter fabric and drain materials in the trench, and <br />recovering the trench. This would insure adequate drainage at the toe of <br />the slide regardless of natural sedimentation or any additional minor <br />movements of slide material. The attached figure shows a cross section of <br />the proposed drain. <br />INSTALLING A TOE BUTTRESS <br />In a previous report dated 10/23/85, ACZ recommended installation of a toe <br />buttress in this area. Construction of the toe buttress was deferred <br />pending development and implementation of stability monitoring measures. <br />At this time it appears that a large toe buttress is unnecessary as the <br />slide has stabilized itself above the road. The current configuration <br />would provide the best alternative for long term use of the road. <br />Therefore, at this time a small buttress of dimensions approximately 2' <br />high x 3' wide is recommended. The purpose of this buttress is more to <br />provide cover and protection for the drain at the toe of the slide than to <br />add weight to the toe. <br />ANNUAL INSPECTIONS <br />The slide area should be inspected annually for the next five years by a <br />competent professional. Annual inspections would provide a record of the <br />conditions of the area as well as give timely warning of any changes in <br />stability. <br />Very truly yours, <br />ACZ INC. <br />~,, ~~ ~~~ <br />Conrad H. Parrish <br />Colorado PE 11932 <br />CHP:rce <br />attachment <br />„K, :~: <br />