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2.2 Repair Construction <br />The process of construction for the slide repair is included in the following sections. <br />2.2.1 Identified Seepage Locations <br />Construction crews began draining water from the East Slide in early May 2011. A seepage area with <br />a flow rate of 5 to 6 gpm was uncovered in early June and crews opened it farther to bedrock to allow <br />it to drain away freely. Other seepage areas were uncovered throughout construction as material <br />within the sliding mass was excavated. The maximum flow rate observed in any of the seepage areas <br />was about 7.5 gpm during the spring, and the flow typically emerged from thin coal seams. The <br />primary seepage areas are shown in Figure 3. <br />2.2.2 Construction Methodology <br />In the East Slide area just below the sandstone outcrop, a bedrock bench was exposed and an upper <br />interceptor trench was constructed by pulling disturbed material up the slump area to the fill at the <br />base of the cliffs. A 12" slotted HDPE pipe and gravel were placed in a 15 -foot -deep trench below <br />the wet zone and a granular drain was brought up the cliff face as the fill was raised. The HDPE pipe <br />was sloped downward to the west as shown in Figure 4. The granular drain was brought up to the <br />base of the massive sandstone layer and then capped with compacted clay. <br />Excavation north and east of the fill was started to find a suitable foundation for the drainage blanket <br />beneath the road alignment. A competent bench consisting of siltstone/mudstone suitable for <br />installing a drainage blanket was reached at approximately elevation 6,180 as shown in Figure 3, and <br />the drainage blanket and associated chimney drain were constructed as shown in Figures 4 and 5. <br />Samples of available aggregates proposed for the granular drainage material were collected and <br />tested to aid in designing the drainage blanket. The design resulted in a blend of 35% 3/8" crushed <br />rock and 65% 1/4" crusher fines. This blend resulted in a D15 of approximately 5 mm, which was <br />targeted to provide proper filtration against colluvium/alluvium intrusion and from intrusion of fine <br />particles that could separate from the bedrock as groundwater flows through siltstone and <br />mudstone. The laboratory performed a permeability test and obtained a permeability of <br />2.6 x 10-2 cm/sec for this composite. <br />The recommendation to use this material assumed the following: <br />• Gradations of the two materials supplied by the gravel pit remain reasonably consistent for all <br />materials used in the construction <br />• The gravel pit is able to blend the two materials thoroughly into a homogeneous mixture <br />P:\Mpls\06 CO\26\06261003 Haul Road Slide Repair\WorkFiles\Report\MCC_Slump_Repair-Instrumentation-Drain Rev 2012-12-12.docx 6 <br />