5402 S 1ST AVE 2022-05-23 - Laserfiche WebLink

Stormwater Infiltration Letter NGA Project No. 1040718 Zillah Street Residential Development Infiltration June 20,2018 Everett,Washington Page 2 For our use in preparing this letter, we have been provided with a topographic survey titled "Elm Homes, LLC,"dated April 3,2018,prepared by A.S.P.I. SCOPE The purpose of this study is to explore and characterize the subsurface conditions within the site and to provide opinions and recommendations for stormwater infiltration. Specifically, our scope of services included the following: 1. Review available soil and geologic maps of the area. 2. Provide long-term design infiltration rates per the 2014 SWMMWW based on on-site Small Pilot Infiltration Testing (PIT) with two trackhoe-excavated test pits. Trackhoe to be provided by NGA. 3. Perform laboratory grain-size sieve analysis on soil samples, if necessary. 4. Provide our opinion on the feasibility of infiltration for the onsite soils. 5. Document the results of our findings, conclusions, and recommendations in a written geotechnical letter. SITE CONDITIONS Surface Conditions The site consists of two separate rectangular-shaped parcels covering approximately 0.32 acres. The properties are generally situated on gently to moderately sloping ground that descends from the upper western property line along an alley way down to the lower eastern property line along 1st Street at gradients in the range of approximately 6 to 20 degrees(11 to 36 percent) as shown in Cross Section A- A' in Figure 3. The properties are currently vacant. Vegetation within the site generally consists of grass, underbrush and scattered young to mature trees. The subject site is bounded to the north by Zillah Street, to the east by 1st Street, to the south an an existing residence property, and to the west by an alleyway. We did not observe any surface water within the site during our site visit on May 24,2018. Subsurface Conditions Geology: The geologic units for this area are shown on the Geologic Map of the Everett 7.5-Minute Quadrangle, Snohomish County Washington, by James P. Minard (U.S.G.S., 1985). The site is mapped as glacial till(Qvt). Till deposits are described as a compact diamict of sand, silt,and subrounded to well- rounded gravel. Our explorations generally encountered gray silty fine to medium sand with gravel, consistent with the description of Qvt at depth. Explorations: The subsurface conditions within the site were explored on May 24, 2018 by excavating four test pits to depths ranging from 4.5 to 6.0 feet below the existing ground surface using a mini- trackhoe. The approximate locations of our explorations are shown on the Site Plan in Figure 2. A NELSON GEOTECHNICAL ASSOCIATES, INC.