6624 BROADWAY 2018-01-02 MF Import

In accordance with the 2003 Ii3C, retaining wall design should include seismic design parameters. Based on the site soils and assumed wall backfill materials, we recommend a seismic surcharge pressure in addition to the equivalent fluid pressures presented above. A rectangular surcharge pressure distribution of 4H and 8H psf (where H is the height of the wall in feet) should be added for "active" and "at -rest" loading conditions, respectively. The resultant of the rectangular seismic surcharge should be applied at the mid -point of the wall. 'Tile lateral pressures presented above are based on the conditions of a uniform backfill consisting of on -site soils compacted to 90 percent of ASTM:D 1557. A higher degree of compaction is not recommended, as this will increase the pressure acting on the wall. A lower compaction may result in settlement of structures supported above the walls. Thus, the compaction level is critical and must be tested by our firm during placement. Surcharges from adjacent footings, heavy construction equipment, or sloping ground must be added to the above values. Perimeter footing drains should be provided for all retaining walls, as discussed under the section on "Drainage Considerations." It is imperative that proper drainage be provided so that hydrostatic pressures do not develop against the walls. This would involve installation of a minimum, 1-foot-wide blanket drain for the full wall height using imported, washed gravel that meets Washington State Department of Transportation (WSDOT) Standard Specification 9-03.12(4) against the walls. If a structure is below the water table, it must be designed for hydrostatic uplift and lateral pressures, as well as soil pressures. Such situations are not envisioned for the project at this time; however, if they do arise, we should be allowed to offer situation -specific recommendations. Floor Support We anticipate that floor slabs will be underlain by suitable native soils, structural fill, or possibly by existing fill. As discussed in the "Site Preparation" section of this report, existing fill soils are considered to be settlement -prone, and we therefore recommend limited removal and replacement of existing fill below floor slabs. Floor slabs should be cast atop a minimum of 4 inches of clean, washed, crushed rock or pea gravel to act as a capillary break. Areas of subgrade that are disturbed (loosened) during construction should be compacted to a non -yielding condition prior to placement of capillary break material. Slabs should also be protected from dampness by an impervious moisture barrier at least 10 mils thick. The moisture barrier should be placed between the capillary break material and the concrete slab. Drainage Considerations All foundation walls should be provided with a drain at the footing elevation. Drains should consist of rigid, perforated, polyvinyl chloride (PVC) pipe surrounded by washed pea gravel. The level of the perforationss in the pipe should be set at the bottom of the footing at all locations, and the drain collectors should be constructed with sufficient gradient to allow 0