11831 AIRPORT RD WHOLE SITE FILE Geotech Report 2022-10-25

If temporary cut slopes experience excessive sloughing or raveling during construction, it may become necessary to modify the cut slopes to maintain safe working conditions and protect adjacent facilities or structures.Slopes experiencing excessive sloughing or raveling can be flattened or can be regraded to add intermediate slope benches, or additional dewatering can be provided if the poor slope performance is related to groundwater seepage. Benching In area where fill will be placed against existing slopes,the fill should be effectively keyed into the existing slope as described in Section 2-03.3(14)of the WSDOT Standard Specifications. Permanent Slopes We recommend that permanent cut and fill slopes be constructed no steeper than 2H:1V. To achieve uniform compaction, we recommend that fill slopes be overbuilt slightly and subsequently cut back to expose properly compacted fill.We recommend that the finished slope faces be compacted by track walking with the equipment running perpendicular to the slope contours so that the track grouser marks help provide an erosion-resistant slope texture. To reduce erosion, newly constructed slopes should be planted or hydroseeded shortly after completion of grading. Until the vegetation is established,some sloughing and raveling of the slopes should be expected. This may require localized repairs and reseeding.Temporary covering,such as clear heavy plastic sheeting, jute fabric, loose straw,or excelsior or straw/coconut matting,should be used to protect the slopes during periods of rainfall. Site Drainage Permanent drainage systems should intercept surface water runoff at the top and/or bottom of cut and fill slopes to prevent it from flowing in an uncontrolled manner across or off the site. The finished ground surface adjacent to new retaining walls should be sloped so that surface water runoff flows away from the structures. Infiltration Evaluation Initially, three infiltration facilities were planned as a part of the project, an infiltration chamber gallery located on the south side of 128th Street SW east of 4th Avenue West (from Station 100+69 to Station 101+99), a bioretention/rain garden on the north side of 128th Street SE west of 3rd Avenue SE (between approximate Station 124+00 and 125+00), and an infiltration chamber gallery located on the east side of SR 527 between approximate Stations 12+50 and 14+50. At each proposed location, a monitoring well was installed to evaluate preliminary groundwater conditions prior to completing PITs.After installation of the monitoring wells, it was determined that infiltration was not feasible at the proposed infiltration facility along SR 527 due to the high groundwater condition (4 feet bgs). Therefore, PITs were only completed at the remaining two proposed facilities. We understand the facilities are to be designed to meet best management practices(BMPs) and infiltrate stormwater to the extent practical,while providing an overflow structure for excess flows. Infiltration rates were evaluated by both grain size analyses and by field measurements as summarized in Appendix C, Infiltration Technical Memorandum. The results of the PITs indicate a low infiltration rate at both sites, a corrected rate of less than 1/4 inch per hour. GEOENGINEERSi April 14,2017 Page 25 File No.2095-032-00