2909 SEAWAY BLVD 2025-07-28 - Laserfiche WebLink

- :l I Lathrop Properties,LLC,c/o WH Pacific,Inc. I November 26,2009 Page 2 7. We recommend the gradation of the infiltration trench backfill meet WSDOT 9.03.12(5), Gravel Backfill for I Drywells(provided it is a crushed product), or AASHTO Grading No. 57. We estimate the porosity of both of these materials to be in the range of 38 to 50 percent, depending on the specific gradation of the supplied material and placement techniques. I I Geotextile panels should be placed with a minimum 18 Inch overlap, and we recommend the PVC membrane be ( placed as one piece or have panels welded in the field. IBased on our previous explorations completed at the site and observations during site grading, subsurface conditions below the pavement section at the proposed location of the trench consist of advance outwash(sand with less than 10 percent passing the U.S. No.200 sieve). The outwash was observed at elevations 30 feet lower than I 1 the pavement surface in other site areas, and is reportedly as much as 200 feet thick within local outcrop areas. Previous Infiltration rates used for design are appropriate for this material (2 inches per hour which includes a correction factor of 4 in accordance with the Stormwater Management Manual for Western Washington). IWe recommend that a representative from our firm be present on site during construction of the Infiltration trench to observe soil conditions,geotextile placement and backfill operations. A test pit should be excavated at the bottom of the infiltration trench to a depth of approximately 5 feet to confirm anticipated subsurface soil conditions. The test Ipit should be backfilled with gravel as described above. Q We trust this letter meets your immediate needs. Please call if you have any questions or If you need additional - •▪ c information. p^' Sincerely, ►v6,•,l '�` • •GeoEngineers,Inc. ..' '' ' c' T - 41 ` . i fir.,„ • 0 4 *:.:1;...:p.4.0.7.5-.1t. .e:_. .„6/.."ei.„,)40..... Debra C.Overbay,PE VLSI %Ifi '� Robert C.Metcalfe,PE,LEG G • Senior Engineer 1(} Associate IRCM:DCO:ta EVER\P:\08\08837001\01\Flnais\08337001011..docx Iccr Scott Lathrop(by e•mall) Copyright©2009 by GeoEngIneers,Inc.Ali rights reserved. I `' Disclaimer:Any electronic form,facsimile or hard copy of the original document(email,text.table,and/or figure),if provided,and any attachments are only a copy of the original document.The original document is stored by GeoEngineers,Inc.and will serve as the official document of record. I I GE ENGiNEE� RS He Na.8837-00101