1618 HOLBROOK AVE 2018-01-02 MF Import

Retaining Wall Letter Lyashuk Residence, Everett, WA January 29, 2007 NGA File No. 755806 Page 6 Passive resistance may be calculated as a triangular equivalent Fluid pressure distribution. An equivalent fluid density of 200 pounds per cubic foot (pcf) should be used for passive resistance design for a level ground surface adjacent to the footing. This level surface should extend a distance equal to at least three times the footing depth. These recommended values incorporate safety factors of 1.5 and 2.0 applied to the estimated ultimate values for frictional and passive resistance, respectively. To achieve this value of passive resistance, the foundations should be poured "neat" against the native medium dense/stiff soils or compacted fill should be used as backfill against the front of the footing. We recommend that the upper one -foot of soil be neglected when calculating the passive resi t;mce. We anticipate compaction of most of the southern wall backfill will be difficult given the proximity of the wall the planned ultra block wall. All backfill behind the southern wall should consist of freely draining rock or pea gravel. Care should be taken to prevent the buildup 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. Permanent drainage systems should be installed for retaining walls. The drains should consist of a minimum four -inch -diameter, rigid, slotted or perforated, PVC pipe placed along the base of the wall and surrounded by free -draining material, such as washed rock, wrapped in a filter fabric. We recommend that an 18-inch-wide zone of clean (less than three -percent fines), granular material be placed along the back of the walls above the drain. Pea gravel is an acceptable drain material or it drainage composite may be used instead. The free -draining material should extend up the wall to one foot below the finished surface. The top foot of backfill should consist of impermeable soil placed over plastic sheeting or building paper to minimize surface water or fines migration into the footing drain. We recommend that we be retained to evaluate the proposed wall drain backfill material and drainage systems. NELSON GEOTECHNICAL ASSOCIATES, INC.