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Pacific Ridge — DRH, LLC ES-8528.01 <br /> July 18, 2022 Page 11 <br /> A capillary break consisting of a minimum of four inches of free-draining crushed rock or gravel <br /> should be placed below the slab. The free-draining material should have a fines content of 5 <br /> percent or less defined as the percent passing the number 200 sieve, based on the minus three- <br /> quarter-inch fraction. In areas where slab moisture is undesirable, installation of a vapor barrier <br /> below the slab should be considered. If used, the vapor barrier should consist of a material <br /> specifically designed to function as a vapor barrier and should be installed in accordance with the <br /> manufacturer's specifications. <br /> Drainage <br /> Groundwater seepage will likely be encountered within site excavations depending on the time <br /> of year grading operations take place. Temporary measures to control surface water runoff and <br /> groundwater during construction would likely involve interceptor trenches, interceptor swales, and <br /> sumps. ESNW should be consulted during preliminary grading to identify areas of seepage and <br /> provide recommendations to reduce the potential for seepage-related instability. <br /> Finish grades must be designed to direct surface drain water away from structures and slopes. <br /> Water must not be allowed to pond adjacent to structures or slopes. In our opinion, a foundation <br /> drain should be installed along building perimeter footings. A typical foundation drain detail is <br /> provided on Plate 4. <br /> If buildings will incorporate crawl spaces rather than slab-on-grade, in our opinion, a crawl space <br /> drain system can provide adequate drainage in lieu of perimeter footing drains if properly <br /> designed and installed. The crawl space drain must provide positive drainage to an appropriate <br /> outlet. <br /> Infiltration Feasibility <br /> In general, the relatively dense, weakly cemented, and unweathered glacial till soils (hardpan) <br /> observed at depths beginning at about one to eight feet bgs generally exhibit very poor soil <br /> infiltration characteristics, which is exhibited by the shallow perched groundwater seepage and <br /> relatively high fines content. In our opinion, the unweathered glacial till soils should be considered <br /> impermeable for design purposes, and the use of full infiltration stormwater BMPs is not <br /> recommended for this site. <br /> If determined necessary to satisfy design objectives, small-scale (limited) infiltration devices <br /> incorporating overflow provisions may be feasible within the upper weathered glacial till soils <br /> (where present) pending further geotechnical assessment. As such, if limited infiltration devices <br /> are pursued, ESNW should review the proposal and provide supplementary recommendations, <br /> as appropriate. <br /> Earth Solutions NW, LLC <br />