3800 RIVERFRONT BLVD Geotech Report 2023-05-04

March 26, 2019 HWA Project No. 2015-061-21 Task 600-7 Low Permeability Soil Barrier(LPSB) Past projects within the landfill site have used low permeability soil barrier(LPSB) material as a means of stopping the migration of landfill gas to the ground surface. A minimum thickness of 3 feet of LPSB has been used as a gas barrier in the past to reduce the potential for differential strain-induced tension cracks to penetrate through the entire layer depth. Historically, LPSB import sources have been identified and tested to ensure that they meet the gradation and permeability requirements to for the site. Generally, glacial till sources, meeting the gradation specifications shown below, have met the permeability requirements for LPSB (1 x 10-5 cm/sec). Sieve Size Percent Passim 3 —inch 100 i 1 1/2—inch 75-100 U.S. No. 4 45-75 U.S. No. 200 12-45 All percentages are by weight. Although not tested to verify its classification as LPSB, the Common Borrow soils placed across the site in the summer of 2018 were derived from glacial till sources and may meet the functional requirements of LPSB. I We recommend that imported LPSB be constructed to a minimum thickness of 3 feet to reduce the potential for differential strain-induced tension cracks to penetrate through the entire layer depth. Where it is desirable to utilize in-place soils as LPSB, we recommend that test pits be conducted to confirm the presence of at least 3 feet of suitable LPSB material. Additionally, laboratory testing will need to be completed to verify that the proposed LPSB material is compacted to the appropriate level and meets the design specifications. On February 19th, 2019 an HWA geologist collected seven representative samples from the existing soil stockpile located southwest of the 41 st Street roundabout. This stockpile was placed in the summer of 2018 and consist of soils exported from the Simpson site development preload. These soils are similar in origin and constancy to the soils that were placed across the landfill site in 2018. Collected soils samples were returned to HWA's Bothell, Washington laboratory for LPSB acceptance testing. The results of these tests are provided in Appendix F. The tests indicate that under proper moisture conditioning and compaction, the soils within the stockpile southwest of the 41 st Street roundabout are acceptable for use as LPSB. All LPSB should be placed and compacted to within the acceptable zone of compaction, as provided in Appendix F. Laboratory tests suggest that the subject stockpiled soils possess natural moisture contents above the acceptable level for LPSB placement. Therefore, moisture conditioning will be required before placement and compaction. Due to the high moisture sensitivity of LPSB, placement and Geotechnical Report Revision 3 26 2019.doc 22 HWA GEOSCIENCES INC.