1530 51ST PL SW 2016-01-01 MF Import

Gcolechnical 1?nginecring L-valuao�n [.etter NGA F�Ic No. 886013 Dtosman Bazement December 17, 2013 Evcrett, Washington Page S drainage conditions, and thc inclination of ihc backfill. For walls that arc frcc to yield at the top at Ieast one thousandth of the height of the wall (active condition), soil pressures will be less than if movement is limited by such factors as wall stiffness or bracing (at-rest condition). We rccummend that the existing basement watls be analyzcd using a tnangular earth pressurc distribution equivalcnt to that excrted by a fluid with a dcnsity of 40 pcf for yielding (active condition) walls, and 60 pcf for non-yielding (at-resl condition) walls. These recommeoded lateral earth pressures are for a drained granular b^r.kfill and erc based on the assumption of a horizontal ground surface behind the wall for a distance of at Ieast the subsurface height of the wall, and do not account for surcharge loads. Additional lateral earth pressures should be considered for surcharge loads acting adjacent to subsurface walls and within a distancc equal to the subsurface heirJ�t of the wall. This would include the effects of surcharges such as tra!'fic loads, floor slab loads, slopes, or other surface loads. We could consult with the structural engine� ::garding additional loads on retaining walls during final dcsign, if needed. The latcral pressures on walls may bc resisted by friction betwccn thc foundation and subgrade soil, and by passivc resistance acting on the below-grade portion of tbe foundation. Recommendations for frictional and passive resistance to lalcral loads are presented in thc Foundallons subsection of this Icttcr. All wall backfil] should be well compacted as outlined in thc Shvctural Fill subsection of this report. Can should bc takcn lo prevent the buildup of cxcess latcral soil pressures due to over�compaction of thc wall back6lL This can be accomplished by placing wall backfill in 8-inch loose lifts and compacting thc backfill with small, hand-operated compactors within a distance hehind the wall equal to at least one-half the height nf thc wall. 7�he thickness of thc loosc lifts should bc rcduced ro xcommodate thc lowcr compactive encrgy of thc hand-operated equipmem. Thc rocommended level of compaction should still bc maintaincd. Pcnnanent dninagc systems should be installcd for retaining walls. Rccommendations for thcse systems are found in the Subsurface Drainage subsection of this report. Wc recommend that we be retained to evaluatc the proposed wall drain backfill material and observe installation of the drainage systems. Wc NELSON GEDTECHNICAL ASSOCIATES, INC.