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Kathy Welt March 27, 2012 Page 2 Identified Seepage Locations Construction crews began draining water from the East Slide in early May 2011. A seepage area with a flow rate of 5 to 6 gpm was uncovered in early June and crews chased it back to bedrock to allow it to drain away freely. Other seepage areas were uncovered throughout construction as material within the sliding mass was excavated. The maximum flow rate observed in any of the seepage areas was about 7.5 gpm during the spring, and the flow typically emerged from thin coal seams. The primary seepage areas are shown in Figure F -3. Construction Methodology In the East Slide area just below the sandstone outcrop, a bedrock bench was exposed and an upper interceptor trench was constructed by pulling disturbed material up the slump area to the fill at the base of the cliffs. A 12" slotted HDPE pipe and gravel were placed in a 15- foot -deep trench below the wet zone and a granular drain was brought up the cliff face as the fill was raised. The HDPE pipe was sloped downward to the west as shown in Figure F -4. The granular drain was brought up to the base of the massive sandstone layer and then capped with compacted clay. Excavation north and east of the fill was started to find a suitable foundation for the drainage blanket beneath the road alignment. A competent bench consisting of siltstone /mudstone suitable for installing a drainage blanket was reached at approximately Elevation 6,180 as shown in Figure F -3, and the drainage blanket and associated chimney drain were constructed as shown in Figures F -4 and F -5. Samples of available aggregates proposed for the granular drainage material were collected and tested to aid in designing the drainage blanket. The design resulted in a blend of 35% 3/8" crushed rock and 65% 1/4" crusher fines. This blend resulted in a D 15 of approximately 5 mm, which was targeted to provide proper filtration against colluvium/alluvium intrusion and from intrusion of fine particles that could separate from the bedrock as groundwater flows through siltstone and mudstone. The laboratory performed a permeability test and obtained a permeability of2.6 x 10 -2 cm/sec for this composite. The recommendation to use this material assumed the following: • Gradations of the two materials supplied by the gravel pit remain reasonably consistent for all materials used in the construction • The gravel pit is able to blend the two materials thoroughly into a homogeneous mixture • Segregation of materials during placement is minimized Based on the permeability for this blend and an assumption that there is one 7.5 -gpm seep every 50 feet along the length of the slide area, and assuming that bedrock benches have only a gentle slope to the north, a thickness of 4.8 feet was computed for the drainage blanket. This was rounded to a nominal thickness of 5 feet for construction purposes to give a small factor of safety and to allow for variability in the bedrock surface. The recommendation for the thickness of the drainage blanket against nearly vertical rock faces was that it could be thinned to 3 feet, which was a reasonable minimum constructible dimension. In the West Slide area, a similar construction methodology was employed. The main difference was that a much shorter chimney drain was installed above the drainage blanket. This chimney drain was near vertical and placed directly against bedrock (extended about 2 feet above the top of the bedrock face to ensure that all bedrock strata are intercepted). Furthermore, a 16" HDPE pipe (slotted in the P:AMplsA06 CO \26 \06261003 Haul Road Slide RepairAWorkFiles\Report \MCC Slump Repair Letter 2012- 03- 27.docx