9931 19TH AVE SE 2018-01-02 MF Import

April 10, 1997 ARCO Petroleum Products Company c/o Environmental Associates, Inc. 2122 - 112th Avenue NE, Suite B-100 Bellevue, Washington 98004 Attn: Mr. Don Spencer Letter Proposed ARCO Am/Pm Mini -Market Everett, Washington NCA File No. 189996 Environmental Associates, Inc. File No. JN-6413 Dear Mr. Spencer: This letter presents the results of our evaluation of the Keystone wall in the wet cell pond of the proposed ARCO Am/Pm Mini -Market in Everett, Washington. The site is located on the northeast corner of the intersection of 19th Avenue SE (1 iighway 527) and 100th Street SE. You have provided us with comments on the plans made by the City of Everett to confirm stability of the wall, for our use in preparing this letter.. A 3-foot-high KeyStone wall is planned on the south side of the proposed wet cell pond. The manufacture's recommendation for KeyStone walls is to fill the voids in the blocks with free draining material, such as 5/8-inch crushed rock. Typical, cantilever type concrete retaining walls surround the other sides of the pond. The dead storage elevation of the pond will he 1.6 feet above the bottom. We are not aware of the maximum water height of the pond, it may be above the top of the KeyStone wall. The wet cell will be excavated into the native subsurface conditions. The exploration, performed during the preparation of our geotechnical report, encountered a very dense silty fine sand, interpreted as glacial till. This type of soil has a very low permeability, and typically has high strengths in its undisturbed slate. The wall should be stable with the planned conditions, and should not be impacted by saturation of the soils behind the block wall. Water allowed to flow through the free draining material of the wall and the