2320 CALIFORNIA ST Geotech Report 2025-07-25

Administration Building Office Expansion Everett,Washington if necessary. The backfill should be compacted in accordance with the criteria discussed above. Figure 4 illustrates the recommended trench compaction criteria under pavement and non-structural areas. 1 Sedimentation and Erosion Control In our opinion,the erosion potential of the on-site soils is low to moderate. Construction activities including stripping and grading will expose soils to the erosion effects of wind and water. The amount and potential impacts of erosion are partly related to the time of year that construction actually occurs. Wet weather construction will increase the amount and extent of erosion and potential sedimentation. Erosion and sedimentation control measures may be implemented by using a combination of interceptor swales,straw bale barriers,silt fences and straw mulch for temporary erosion protection of exposed soils. All disturbed areas should be finish graded and seeded as soon as practicable to reduce the risk of erosion. Erosion and sedimentation control measures should be installed and maintained in accordance with the requirements of the City of Everett. Stormwater Detention Pond General We understand the stormwater pond that will be constructed as part of the Pole Storage Addition project will likely be expanded. We understand that the pond will be 7 feet deep with a 4-foot deep permanent wet pool. Preliminary geotechnical considerations for expansion to the permanent stormwater pond are discussed below. ' Soil Conditions Test pit TP-5 was completed within the footprint of the stormwater pond to a depth of 10 feet below grade. The soils below the grass layer consisted of very dense glacial till to the depth explored. Glacial till generally has relatively low permeability rates and act as a barrier to water flow; however layers and/or lenses of more permeable sandy soils can exist with the till deposits. We recommend that all loose fill and organic materials near the ground surface be removed to expose medium dense to very dense native glacial till. The pond subgrade should be evaluated by our firm when the excavation for the pond expansion is completed. If significant sand lenses are observed in the till subgrade, then it may be necessary to remove up to 2 feet of the sand and replace with compacted glacial till to provide a low permeability layer. Permanent Slopes We recommend that permanent cut and fill slopes within the detention pond be no steeper than 3H:1V. To achieve uniform compaction on interior fill slopes, we recommend that fill slopes be overbuilt slightly and subsequently cut back to expose well compacted fill. This should be discussed with the contractor and incorporated into the project plans. Erosion Control To reduce potential erosion and to help establish permanent vegetation on the inboard and outboard slopes (if any), we recommend that erosion protection of the slopes include hydroseeding in conjunction with installation of an erosion control blanket. We recommend that the erosion control blanket be staked to GEOENGINEERSS October30,2009 Page 11 Ale No.0482-031-02