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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 " 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 />• Segregation of materials during placement is minimized <br />Based on the permeability for this blend and an assumption that there is one 7.5 -gpm seep every 50 <br />feet along the length of the slide area, and assuming that bedrock benches have only a gentle slope to <br />the north, a thickness of 4.8 feet was computed for the drainage blanket. This was rounded to a <br />nominal thickness of 5 feet for construction purposes to give a small factor of safety and to allow for <br />variability in the bedrock surface. The recommendation for the thickness of the drainage blanket <br />against nearly vertical rock faces was that it could be thinned to 3 feet, which was a reasonable <br />minimum constructible dimension. <br />A toe drain consisting of an aggregate filter drain around a slotted high- density polyethylene drain <br />pipe will be constructed adjacent to the northern limit of the toe drain to collect seepage water and <br />convey this clear water past the sediment ditch, ultimately discharging to the North Fork of the <br />Gunnison River. The toe drain is shown on Figure 5. <br />In the West Slide area, a similar construction methodology was employed. The main difference was <br />that a much shorter chimney drain was installed. The soils to the north of the bedrock bench were <br />found to be dry when excavated, so these soils were backfilled to approximately pre -slide slopes and <br />compacted. Since the soils to the north of (lower than) the bedrock bench were dry, drain <br />construction in this area was limited to the chimney drain only; a drainage blanket was not placed on <br />the bedrock bench. The chimney drain was near vertical and placed directly against bedrock <br />(extended about 2 feet above the top of the bedrock face to ensure that all bedrock strata are <br />P Mpls 06 CO 26 06261003 Haul Road Slide Repair WorkFiles Report MCC Slump_ Repair - Instrumentation- DramRev_2012 -05 - docx <br />8 <br />