8417 EVERGREEN WAY Geotech Report 2022-10-25

Geotechnical Engineering Report-Revised lierracon Proposed CFT Retail a Everett,Washington February 1, 2018 a Terracon Project No. 81185002 ■ Common Fill-"Common"fill generally consists of lesser quality, more moisture-sensitive soil that can be compacted to a firm and non-yielding condition if near the optimum moisture content. Common Fill should meet the requirements of WSDOT Section 9-03.14(3), Common Borrow. For bidding purposes, we recommend the use of this soil type be limited to construction scheduled for the summer months. 4.2.3 Compaction Requirements Structural fill materials should be placed in horizontal lifts not exceeding about 8 inches in loose thickness. We recommend that each lift then be thoroughly compacted with a mechanical compactor to a uniform density of at least 95 percent, based on the modified Proctor test(ASTM D 1557). Where light compaction equipment is used, as is typical within a few feet of retaining walls and in utility trenches, the lift thickness may need to be reduced to achieve the desired degree of compaction. Excavated soil that will be reused as structural fill should be protected from rain and other factors to aid in preventing an increase in moisture content. Moisture contents at the time of compaction should be within 2 percent of the optimum moisture content. 4.2.4 Grading and Drainage Adequate positive drainage of exposed subgrades should be provided during construction and maintained throughout the life of the development to prevent an increase in moisture content of the foundation and pavement subgrades and excavation backfill materials. Surface water drainage should be controlled to prevent undermining of fill slopes and structures during and after construction. Roof gutters and downspouts should be routed into tightline pipes that discharge into a municipal storm drain or other suitable location. Splash-blocks should also be considered below hose bibs and water spigots. 4.2.5 Storm Water Infiltration As part of the project Terracon has evaluated the feasibility of on-site infiltration of storm water run-off. At the project site we observed approximately 1 foot of uncontrolled/undocumented fill on the uphill (north) side of the site increasing to approximately 7 feet of fill on the downhill (south) side of the site which is underlain by glacially consolidated soils which have a very low rate of infiltration. Per the 2014 version of the Storm Water Manual for Western Washington site suitability criteria SSC-5 and SSC-6 would apply to the site and we recommend that it is not a candidate for on-site infiltration. Issues precluding site infiltration: Per SSC-5 the depth of the low infiltration layer(glacial till)from the base of the infiltration element should be 5 feet or greater. This is not possible for the majority of the site. The depth from the base of the infiltration element it reads may be reduced to as little as 3 feet with a mounding analysis and upsizing of the infiltration system however the site does not have adequate space and depth to accommodate such a facility. Responsive ■ Resourceful ® Reliable 7