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DAVID L. NELSON AND ASSOCIATES, INC.
<br /> ,�, Cunsulting Engincering Geology
<br /> '�' May 31 , 1991 �
<br /> I Project No. 4791
<br /> Page 11
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<br /> values further assume the use of an effective drain system along the backside
<br /> � r, of the wall. We suggest the use of a free-draining sand and fine gravel
<br /> -' ', r backfill along the backside of the wall to reduce pressures on walls and to
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<br /> <<,z;;'.' ; E� reduce potential for moisture problems. We sugqest any basement or retaining
<br /> �'�' � wall areas utilize the recommendations as shown on the enclosed Subdrain 8
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<br /> ,�. ,. Backfill Scheme enclosed. The drainage blanket should extend to within about
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<br /> �=_' two (2) feet of finished exterior grade and have a minimum thickness of 12
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<br /> �:ki�5.i' ,`,,;, inches. The remaininq two feet should be backfilled with low permeability on-
<br /> � ' site soils. A perforated drain pipe should be embedded in the drainage
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<br /> �r blanket alonq the base of the wall. The pipe should be set such that a
<br /> ��i��F�"�-,r '' � positive drainage gradient is established and access to an outlet provided.
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<br /> f, ���j;�'� e.; � The use of ADS flexible type drain pipe is not recummended for greater than 4
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<br /> J ,� t , t •' � t foot embedment depths and/or where excavation and replacement type maintenance
<br /> f��y«. :��, ,
<br /> ' ' �' I ; is not desired.
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<br /> , - i� rB Drainage
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<br /> Erosion potential should be considered moderate to severe for stripped areas
<br /> '�% with 15} or steeper grades. Erosion potential for aree.s in which the existing
<br />� ' � � low ground cover is left intact, and surface water is controlled, is expected
<br /> f to be unaffected by the proposed development. Constr.uction phase erosion
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<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 /> � Roofs and pavements will reduce the surface area available for natural
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<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
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<br /> ;•,i,,, r , suitable system. Runoff should not be allowed to discharge onto sloping
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<br /> , ,��;'',. � ground or to saturate foundation areas. Surface drains should be maintained
<br /> separately, and not interconnected with foundation or wall drains.
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