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��x <br />>Hxtn <br />H X� <br />� H� <br />� M A7 <br />�p H <br />ti1 O � <br />ON <br />�/��� <br />y. Y �] <br />t" y � <br />HH <br />g `� <br />c� C y <br />��� <br />x H tn <br />H O �n <br />� <br />�s <br />i,� <br />March 20, 1990 <br />Project No. 2090 <br />Page B <br />�.ti, - -. <br />DAVID L. NELSON AND ASSOC�ATES, INC. <br />Consulting Enginccring Gcology <br />` could be developed once the downslope geometry and building designs are known. <br />an appropriate factor of safety should be applied Lo the abo�e base friction <br />� value to resist sliding. <br />, <br />� <br />a <br />'" <br />w .�� <br />'�� !d <br />�., � , . <br />The above values assume the presence of the soils encountered in our <br />explorations, and a free-draininq material behind the subgrade wall. These <br />values do not take into account hydrostatic pressures, sloping qround behind <br />the wall, or surcharges due to equipment or adjacent structural loads. These <br />values further assume the use of an effective drain system along the backside <br />of the wall. We recommend the use of a free-draining sand and fine gravel <br />backfill along the backside of the wall to reduce pressures on walls and to <br />reduce potential for moisture problems. We suggest any basement or retaininq <br />wall areas utilize the recommendations as shown on the enclosed Subdr..� & <br />Hackfill Scheme enclosed. The drainage blanket should extend to within about <br />two (2) feet of finished exterior grade and have a minimum thickness of 12 <br />inches. The remaining two feet should be backfilled with lcw permeability on- <br />site soils. A perforated drain pipe should be embedded in the drainaqe <br />blanket along the base of the wall. The pipe should be set such that a <br />positive drainage gradient is established and access to an outlet provided. <br />The use of ADS flexiblz type drain pipe is not recommended for greater than 4 <br />foot embedment depths and/or where excavation and replacement type maintenance <br />is not desired. <br />Drainaqe <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 qround cover is left intact, and surface water is controlled, is expected <br />to be ur.affected by the proposed development. Construction phase erosion <br />control could include, but not necessarily be limited to the use of hay bales, <br />geotextile barriers (silt fences), and controlled surface grading. <br />Aoofs and pavements will reduce the surface area available for natural <br />infiltration within the site. This will in turn increase surface runofE. We <br />0 <br />�� <br />