3214 BROADWAY SEPA 017 - 004 Geotech Report 2025-04-14

American Property Develow..,ent ES-0599.05 January 16, 2015 Page 12 Updated January 22, 2015 Retaining walls should be backfilled with free draining material that extends along the height of the wall, and a distance of at least 18 inches behind the wall. The upper one foot of the wall backfill can consist of a less permeable (surface seal) soil, if desired. In lieu of free draining backfill, use of an approved sheet drain material can also be considered, based on the observed subsurface and groundwater conditions. The geotechnical engineer should review conditions at the time of construction and provide recommendations for sheet drain, as appropriate. A perforated drain pipe should be placed along the base of the wall, and connected to an appropriate discharge location. Where foundation walls are formed against the temporary shoring walls, the shoring wall drainage illustrated on Plate 6 can be utilized. For site retaining walls receiving backfill, the retaining wall and drainage detail illustrated on Plate 7 should be considered. Excavations and Slopes The Federal and state Occupation Safety and Health Administration (OSHA/WISHA) classifies soils in terms of minimum safe slope inclinations, In our opinion, based on the soil conditions encountered during fieldwork for this site, the weathered native soils and existing fill encountered to depths of up to four feet to seven feet would be classified by OSHA/WISHA as Type B. Temporary slopes in Type B soils should be sloped at an inclination of at least 1 H:1 V, or flatter. In our opinion, the dense to very dense native soils below the weathered native soil and fill would be classified by OSHA/WISHA as Type A and B. Temporary slopes over four feet in height in Type A and Type B soils should be sloped at an inclination no greater than 0.75H:1V or 1 H:1V, respectively. With respect to the proposed building excavations, temporary slopes inclined at 0.5H:1V are feasible within the very dense silty sand with gravel glacial till deposits. The geotechnical engineer should observe the excavations to verify the appropriate allowable temporary slope inclination. If the above slope gradients cannot be achieved, temporary shoring may be required. Permanent slopes should maintain a gradient of 2H:1V, or flatter, and should be planted with an appropriate species of vegetation to enhance stability and to minimize erosion. Seismic Considerations The 2012 International Building Code specifies several soil profiles that are used as a basis for seismic design of structures. The 2012 IBC recognizes ASCE for seismic site class definitions. If the project will be permitted under the 2012 IBC, in accordance with Table 20.3-1 of ASCE, Minimum Design Loads for Buildings and Other Structures, Site Class C, should be used for design. In our opinion, the site has a low susceptibility to liquefaction. The absence of a shallow groundwater table observed at the test sites, and the soil relative density observed throughout the test sites is the primary basis for this conclusion. Earth Solutions NW, LLC