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SubsurJacr Erplaration, Geologic Natard, and <br />Betiederr Lnu East Preliminar�� Gco�echnical Engineering Repert <br />E��cretr, P'ash,inFro�r Prelimmary Desipn Recommmdations <br />ion Te�t No. <br />IF-] <br />IF-2 <br />TabIC 5 <br />Summary o( Infillration Testing Results <br />Depth of Test <br />4"' <br />Run No. <br />1 <br />2 <br />3 <br />1 <br />2 <br />Infiltration i2ate <br />38 <br />37 <br />34 <br />52 <br />47 <br />47 <br />°i IF-] Icst lonunn approxirtulely S feet eas� of west lo� line. <br />"' Ezcavalion for the IF-? tcsi was initially corduned approximstely 5 fre� east of �Ae west lol line, but dean sud I�ye• not <br />encnunmmd within 6 Irei of Um gmund sur(ace; «v IF ? was mrduned �pproxima�ely IG fea catt of the west loi :;a wherc <br />enplmaiion �¢nching encoumertd �hr de�n sarxl layer �i � d<pdi n( 5 feet. <br />AI[hougl� infiltration ratcs of 36 and 49 inches per hour were measured for infiltration tests IF- <br />I and IF-2, resrectively, actual infiltration rates achievable on a long-term basis in a full-sized <br />infiltration gallcry will �ikely be substantially less. This is due to such factors as ground water <br />mounding, the effect of lateral infiltration during smzll-scale testing, and the accumulation of <br />fine•grained sediments. AdditionallV, exploration piL EP-4 and EI'-5, excavated adjacent to <br />infiltration tests 11=-1 and IF-2 encountered increased silt and moisture content with depth. <br />This suggests that the permeabiliq' c�f the sediments maq also decrease with depth. <br />Consequently, if storm N•ater infiltration is conducted at the site, we recommend the more <br />cnnsen�atiee 5.27 inch per hour infiltration rate derived from the USDA textural classification <br />mcthod. <br />Infiltration testing conducted far this project indicates that the permeability of d�e near-surface <br />sediments in the areas tested is suitable for storm water infiltration. However, infiltration of <br />storm ���ater may reduce the stability of the steep slope in the eastern portion of the site. If on- <br />site infiltration of storm �eater is desired, we recommend that the infilvation sites should be <br />limitcd to the N�estern margin of the lots, adjacent to Bel��edere .Avenue. No infiltration should <br />occur between the homes and slope crest. <br />Alternati��ely, it may be feasible to discharge storm water into the creek at the base of the steep <br />slope east of ti�c site. If this option is selected, the storm w�ater should be conveyed downslope <br />using a continuous Drisco tightline pipe. The outfall should be provided with an energy <br />dissipaeor to prevent scouring at the point of discharge. Conceptuallp, the energy dissipator <br />cnuLi c�nsist uf a crushed rock mat sized according to the ��elocity of the discharge anticipated. <br />I lnaer no circunutances shnuld storm ���aier he dischar�ed on or direedy above the steep slope. <br />������.�� t,-��ia � ASSOCI.�TED E�RTN SC/£NCSS, lNC. <br />; r�ie er_,,,�ri,� r•,.�.,��,.a�r,i�,nf_��r �c� Pag,e20 <br />