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1 , <br /> GEOENGiNEERQ <br /> 2924 Colby Avenue <br /> Everett,Washington <br /> 425.252.4565 <br /> November 25,2009 <br /> ' Lathrop Properties,LLC <br /> c/o WH Pacific,Inc. <br /> ' 12100 NE 195th Street,Suite 300 <br /> Bothell,Washington 98011 <br /> Attention: Michael A.Gomez,PE <br /> ' Subject: Seaway Lot 2A <br /> Proposed Infiltration Trench <br /> ' Geotechnical Consultation <br /> Everett,Washington <br /> File No.8837-001-01 <br /> This letter provides our recommendations for backfill materials to be utilized for the proposed infiltration trench at <br /> the Seaway Lot 2A property, and a summary of subsurface soil conditions previously encountered in the proposed <br /> location of the trench. We provided our review comments and recommendations for the infiltration trench in <br /> ' previous mails dated September 21,October 14,and October 19,2009. As requested,this letter summarizes our <br /> recommendations provided in previous emalls as follows: <br /> 1. A nonwoven drainage geotextile, Mirafi 140N or equal, should be placed across the bottom of the infiltration <br /> trench. <br /> 2. A higher strength nonwoven geotextile equivalent to Mirafi 160N should be used for the up-gradient sides(north <br /> ' and east sides)of the trench. <br /> 3. The west (down-gradient) and south (building) sides of trench should be lined with an impermeable PVC <br /> membrane. The PVC membrane should consist of one continuous panel, which can be welded in the field if <br /> ' needed. The PVC membrane should have a minimum average roll value (MARV) thickness of 20 mil, and a <br /> geotextile cushion should be placed between the membrane and rock backfill for protection. The geotextile <br /> cushion should be similar to Mirafi 160N. <br /> ' 4. Mirafi 140N should also be utilized as the separation/filter layer between the top of the trench and sand <br /> backfill. No equipment should operate directly on the geotextile.Backfill should be end-dumped and spread into <br /> a full lift with tracked equipment prior to compaction. Loose lift thickness should not exceed 18 inches. <br /> 5. Stormwater carrying fine sediment should be intercepted before entering the infiltration trench, which may <br /> require additional filter fabric at the pipe connection,and/or a sediment trap upstream of the infiltration facility. <br /> ' 6. All penetrations through the recommended PVC membrane should be bootfitted and sealed per manufacturers <br /> recommendations. <br /> 1 <br />