3620 BROADWAY VERIZON 2019-01-11

� 1 Geotechnical Engineering Evaluation NGA File No.983917 Stadium Flowers Addition March 10,2017 Everett,Washington Page 10 Retaining Walls For retaining walls on this site, the lateral pressure acting on subsurface retaining walls is dependent on the nature and density of the sod behind the wail, the amount of lateral wall movement which can occur as backfill is placed,wall drainage conditions,the inclination of the backfill, and other possible surcharge loads. For walls that are free to yield at the top at least 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 recommend that walls supporting horizontal backfill and not subjected to hydrostatic forces be designed using a triangular earth pressure distribution equivalent to that exerted by a fluid with a density of 40 pcf for yielding (active condition) walls, and 60 pef for non-yielding (at-rest condition)walls. These recommended lateral earth pressures are for a drained granular backfill and are based on the assumption of a horizontal ground surface behind the wall for a distance of at least 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 distance equal to the subsurface height of the wall. This would include the effects of surcharges such as traffic loads, floor slab, foundation loads, slopes, or other surface loads. Also, hydrostatic and buoyant forces should be included if the walls could not be drained. We could consult with the structural engineer regarding additional loads on retaining walls during final design,if needed.. The lateral pressures on walls may be resisted by friction between the foundation and subgrade soil and by passive resistance acting on the below-grade portion of the foundation. Recommendations for frictional and passive resistance to lateral loads are presented in the Foundation Support subsection of this report. All wall backfill should be well compacted as outlined in the Structural Fill subsection of this report. Care should be taken to prevent the building up of excess lateral soil pressures due to over-compaction of the wall backfill. This can be accomplished by placing wall backfill in thin loose lifts and compacting it with small, hand-operated compactors within a distance behind the wall equal to at least one-half the height of the wall. The thickness of the loose lifts should be reduced to accommodate the lower compactive energy of the hand-operated equipment The recommended level of compaction should still be maintained. NELSON GEOTECHNICAL ASSOCIATES,INC