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DAVID L. NELSON AND ASSOCIATES, INC. <br />Consulting Engineering Geology <br />July 13, 1990 <br />Project No. 9490 <br />Page 5 <br />Permanent fill embankments are expected to stand at various slopes depending <br />on the material and compaction effort used. For structural till, t <br />hat is f ill <br />comprised of clean imported or on site soils with a relative compaction in <br />excess of 90% (ASTM D-1557), embankment slopes should be no steeper than <br />2N:1v, and properly protected from erosion. We do not recommend placing <br />additional fill on the slopes unless the entire embankment is reconstructed as <br />structural fill. <br />Dewatering a Drainage <br />Depending on excavation configuration and ground water conditions at the time <br />of construction, temporary dewatering may be necessary. Typt=ally, cut off <br />drains are used to intercept groundwater before it emerges into the <br />excavation. The cut off drain consists of a trench filled with Pea gravel <br />that intercepts the ground water flow. A perforated pipe is placed near the <br />bottom of the pea gravel and is routed to a suitable discharge point. The <br />actual need and design of the cut off drain should be based on conditions <br />observed in the field at the time of construction. We suggest that a <br />geotechnical consultant be on -site during drain development and construction. <br />Wall and footing drains should be constructed as outlined in the retaining <br />wall section of this report. <br />Erosion potential should be considered moderate to severe for stripped areas <br />with 15% or steeper grades. Erosion potential for areas in which the existing <br />low ground cover is left intact, and surface water is controlled, is expected <br />to be unaffected by the proposed development. Construction phase erosion <br />control could include, but not necessarily be limited to, the use of hay <br />bales, geotextile barriers (silt fences), and controlled surface grading. <br />Roofs and pavements will reduce the surface area available for natural <br />infiltration within the site. This will in turn increase surface runoff. We <br />recommend that roof drains, catch basins or other devices used to collect <br />surface runoff be directed by tight line into the storm drain or other. <br />suitable system. Au:._:: should not be allowed to discharge onto sloping <br />