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LSI -ADAPT <br /> continuously present at the jub sire to observe tho nature and conditions of the subsurface material <br /> encountered. The contractor should also be requirod to perform all excavations in accordance with <br /> federai,state, and local regulatious. <br /> Retaining and Subgrade Walls I <br /> Retaining and/or subgrade wa11s can be supported on shallow footings four�ded on medium dense or denser I <br /> native soils or structural fill, if properly prepared. Footings bearing on at least medium dc.nse native soits or <br /> suuctural f.11 as described above can be designed using an average allowable bearing valuc of 2,500(pounds <br /> per square foot) psf with a m�imum toc prcssure of 3,000 psf. Lateral loads on comentional retaining <br /> structures founded as described above may be resisted by friction on the base of the wzll footings and as <br /> passive pressure on the sides of footings. Wc recomme�d using an allowable coetlicient of friction oC 0.35 <br /> to calculate friction bctwecn the foundation and native soils or on swctural fill. For the condition of a <br /> horizontal ground surface beyond lhe wall,the passive pressu�e may be detertnined using an :quivalent fluid <br /> densiry of 300 per cubic foot (pc�. This assumes that the foo+ings are cast"neaf' against the cut, and that <br /> any fill soils around the wall are placed and compacted as swctural fill. A safety factor of 1.5 has been <br /> applied to these values,for sliding and overtuming. <br /> The lateral active soil pressures acting on rcinforced concrete rciaining walls depend on the nature, density <br /> and confi�uration of the soil behind the wall. Wc recommend that walls support;ng horizontal backGU Ue <br /> designed using an equivalent (luid densiry of 35 pcC for a level back-slope behind thc�vall. This pressure is <br /> based on backfill placed within 2 fect of the wall being compacted by hand-ope�ated equipmenl to a density <br /> of 90 per cent of the MDD and consisting of cican sand or sand and gravel. The rewmmended pressure does <br /> not include the effects of surcharges from surface loads, or the surcharge effecls of a slope above the wall. <br /> For the conditions of a 2H to 1 V slope above the wall, a 50 per cent increase in horizontal pressures would <br /> apply. As an altcmative to conventional poured in place concrete retaining walls, the use of segtnental <br /> coucrcte walls with gcofabric or geogrid reinforcement may be considcred (i.c., "Keystone" - type walls). <br /> Such walls have the advantagc of economy ovcr cast-in-place walls, and are :specially attractive where site <br /> grades will be raised, as the gridJfabric can be included progressivcly in thc swctural fill as site grades �re <br /> raised. If such walls �rc wnsidered, they should be conswcted in accordance with ilie manufacturer's <br /> recommendations and specifications. <br /> R�e recommend that retaining wall footings foundcd ncar fill slopes be a minimum of 2 feet in width and <br /> ha�c a minimum 2 fcet of cmbedment. Adequate drainagc tehind any retaining st�ucture is imperative. We <br /> rccommcnd that a drainagc systcm consisting of a minimum 18 inches of washcd sand and gravcl with Icss <br /> than 3 per ccnt fines be placcd along ihe back of the���all. In addition,a drainage collecror system consisting <br /> of 4-inch perforated PVC pipe should be installcd behind all walls to provide an ouqet for any accumulatcd <br /> watcr. Thc drainage material should be capped at thc ground surface with 1-foot of relativcly impermeable <br /> so�l or othcnvisc scaled. <br /> LSI-ADAPT Job No.WA02-It127 Pa9e�� <br /> August6,2002 <br />