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, . . BASEMENT/RETAINING WALL BACKFILL AND WATER CONTROL - DRAINAGE Below grade basements should include a perimeter backfill drainage system on the eXlerior side of the walL The exception may be where basements lie within free draining sands where water will not perch in the backfill. Dramagc S"Slems should consist of perforated or slolled PVC drainage pipes located at the bottom of the backfill trench. lo\\er Ihan Ihe interior floor grade. The drain pipe should be surrounded by properly graded filter rock. The drain pipe should be connected to a suitable means of disposal, such as a sump pump basket or a gravity outfall A SIOnll SC\"-:''I'" gravity outfan would be preferred over exterior daylighting, as the latter nn\! freeze during winter f'(~:' :l~-.;.I-ii1l::1dmg. .2:,:!ci"ior retaining walls. weep holes ~t the base of the w;lll (:;1;\ t,{':-':;~ibs!jl'ned !O~. 2 drain plpC BACKFILLING Prior to b~lckfilIing_ damp/water proofing should be applied on perimeter basement walls- The backfill materials placed ,lgainst basement walls ,,,ill exert lateral loadings. To reduce this loading by allowing for drainage, we recoIlll1lend using free draining sands for backfill. The zone of sand backfill should extend outw<lrd from the vl-'all at least 2' _ ~ll1d then upw<.lrd and outward from the \vall at a 30') or greater angle from vertical. The sands should contaiu no grC:llcr than 12%1 by weight passing the #200 sieveo which ,,,"auld include (SP) and (SP-SM) soils_ The sand backfill should be placed in lifts and compacted with portable compaction equipment This compaction should be to the specified levels if slabs or pavements are placed. above. Where slab/pavements are nol above, we recommend capping the sand backfill with a layer of clayey soil to minimize smface water infiltration. Positive surface drainage away from the building should also be maintained Backfilling with silty or clayey soil is possible but not preferred. TIlese soils can build-up water whicb iucreases lateral pressures and,results in wet wall conditions and possible water infiltration into the basement. If you elect to place silly or clayey soils as backfitL we recommeud you ptace a prefabricated drainage composite against tlte walt which is hydraulically connected to a draiuage pipe at tlte base of tlte backfill trench. High plasticity clays should be avoided as backfill due to their swelliug potentiaL LATERAL PRESSURES Lateral earth pressures on betow grade walls vary, depeuding on backfill soil classlficatlOn, backfill compaction and slope of the backfill snrface. Static or dynamic surcharge loads near the wall wtll also iucrease lateral wall pressure. For design. we recommend the following ultimate lateral earth pressure values (given in equivalent fluid pressnre \'<llues) for a dr<lined soil compacted to 95% oftbe standard Proctor density and a level ground sruface. Equivalent Fluid Deusity Soil Type Active (pef) At-Rest (pel) Sands (SP or SP-SM) 30 , 45 SillY Sands (SM) 40 60 Finc Gr<lilled Soils (SC CL or ML) 70 90 Basement walls are normaHy restrained at the top which restricts movement. In {his case, the design lateral pressures should be Ihe "at-rest"" pressure situation. Retaining walls which are free to rotate or deflect should be designed using thc 'Ictive case. Lateral earth pressures will be siguificantly Ingher than that shown if tlle backfill soils ~lre not dr<lined and become saturaled_