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Pacific Ridge — DRH, LLC ES-7852.01 March 15, 2022 Page 11 Drainage The presence of groundwater seepage should be expected in excavations. Where zones of groundwater seepage are encountered, temporary measures to control groundwater seepage may be needed. Temporary measures to control groundwater seepage and surface water runoff during construction will likely involve passive elements such as interceptor trenches and sumps, as necessary. Surface water should not be directed to the top or toe of slopes, modular block walls, or rockeries; wall and rockery drainage should not be used to temporarily control surface water during construction. Surface grades must be designed to direct water away from buildings. The grade adjacent to buildings should be sloped away from the buildings at a gradient of at least 2 percent for a horizontal distance of at least 10 feet or as setbacks allow. In our opinion, perimeter footing drains should be installed at or below the invert of the building footings. A typical footing drain detail is provided on Plate 4 of this report. If footing drains are not installed, we recommend backfilling footings with a relatively impermeable soil; native glacial till soils compacted to at least 90 percent will be relatively impermeable. If footing drains are omitted, there is a higher potential for moisture issues for slabs-on-grade or crawl space areas. If buildings will incorporate crawl spaces rather than slab-on-grade, in our opinion, a crawl space drain system can be used in lieu of perimeter footing drains. The crawl space drain must provide positive drainage to an appropriate outlet. Infiltration Evaluation The unweathered glacial till soil deposits observed at relatively shallow depths exhibit very poor soil infiltration characteristics. This is exhibited by the iron oxide staining and perched groundwater seepage observed within the test pits. Based on the observed soil conditions, we do not recommend utilizing infiltration systems for the proposed development. Stormwater Vault Design We understand a stormwater vault will be constructed to manage stormwater. Interflow groundwater seepage into the vault excavation should be anticipated, particularly if completed during the wet season. Vault foundations should be supported on dense native soil or crushed rock placed on dense native soil. Final stormwater vault designs must incorporate adequate buffer space from property boundaries such that temporary excavations to construct the vault structure can be successfully completed. Perimeter drains should be installed around the vault and conveyed to an approved discharge point. The need for a drainage blanket below the vault in the area of the existing wetland should be evaluated during construction. Earth Solutions NW, LLC