1033 51ST PL SW Geotech Report 2022-10-28

granular wall backfill be separated from general fill, native soil, and/or topsoil using a geotextile fabric that meets the specifications provided in WSS 9-33.2 -Geosynthetic Properties for drainage geotextiles. Backfill should-be placed and.compacted as recommended in the"Materials" section, except for backfill placed immediately adjacent to walls. Backfill adjacent to walls should be compacted to a lesser standard to reduce the potential for generation of excessive pressure on the walls. Backfill located within a horizontal distance of 3 feet from the retaining walls should be compacted to approximately 90 percent of the maximum dry density, as determined by ASTM D1557. Backfill placed within 3 feet of the wall should be compacted in lifts less than 6 inches thick using hand- operated'tamping'equipment(such as•ajumping jack or vibratory plate compactor). If flatwork (slabs, sidewalk, or pavement)will be placed adjacent to the wall, we recommend that the upper 2 feet of fill be compacted to 95 percent of the maximum dry density, as determined by ASTM D1557. 7.4.3 Wall Drainage The above design parameters have been provided assuming that back-of-wall drains will be installed to prevent buildup of hydrostatic pressures behind all walls. If a drainage system is not installed, our office should be contacted for revised design forces. Positive drainage should be provided behind below-grade walls and retaining walls by placing a minimum'1.5-foot-wide zone of free-draining backfill directly behind the wall. The free-draining backfill should meet the criteria for WSS 9-03.12(4)-Gravel Backfill for Drains. The free-draining backfill zone should extend from the base of the wall to within 2 feet of the finished,ground surface. The top 2 feet of fill should consist of relatively impermeable or native soil to prevent infiltration of surface water into the wall drainage zone. Perforated collector pipes should be placed at the base of the walls. The pipe should be embedded in a minimum 2-foot-wide zone of drain rock. The drain rock should meet specifications provided in the "Materials"section. The.drain rock should be wrapped in a geotextile fabric that meets the specifications for drainage geotextiles as described in the "Materials"section. The collector pipes should discharge at an appropriate location away from the base of the wall. Unless measures are taken to prevent backflow into the drainage system of the wall,the discharge pipe should not be tied directly into stormwater drain systems. 7.5 INFILTRATION 7.5.1 Design Infiltration Rate As discussed in the "Subsurface Conditions"section, the soil encountered near the base of proposed infiltration elements consists of silty sand with gravel fill material. Native glacial till, which will control the overall infiltration rate,was encountered between 2.5 and 4.0 feet BGS. The infiltration rate determined using the PIT procedure is a short-term infiltration rate. A correction factor is necessary to account for the small scale of the test. Additional correction factors are necessary to account for testing uncertainties,site variability, and long-term reduction G EODESIGN= 7 NWHome-1-01:101618