2201 124TH ST SE 2022-05-11 - Laserfiche WebLink

Stormwater Infiltration Letter NGA Project No. 1035918 Aikins Residence Development Stormwater Infiltration May 16,2018 Everett,Washington Page 2 For our use in preparing this letter, we have been provided with a site plan titled"Aikins New Residence," dated February 20,2018,prepared by Fred Baxter&Associates. 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 was 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 roughly rectangular-shaped parcels covering approximately 1.15-acres.The property is currently occupied by a single-family residence within the western portion of the southern parcel and detached shed sturcures within the northern parcel. The ground surface within the site is relatively level to gently sloping from north to south. Vegetation surrounding the site structure generally consists of grass- covered yard areas, landscaping plants, and few young to mature trees bordering the property. The subject site is bounded in all directions by existing residential properties. We did not observe any surface water within the site during our site visit on April 27,2018. Subsurface Conditions Geology: The geologic units for this area are shown in the Geologic Map of the Everett 7.5-minute Quadrangle, Snohomish County, Washington,by James P. Minard(1985). The site is mapped as till (Qvt). The glacial till is described as a compact diamict of mud, sand, pebbles, cobbles, and boulders. Our explorations generally encountered silty fine to medium sand with gravel soils consistent with the description of glacial till soils at depth. Explorations: The subsurface conditions within the site were explored on April 27, 2018 by excavating three test pit explorations. The approximate locations of our explorations are shown on the Site Plan in Figure 2. A geologist from Nelson Geotechnical Associates, Inc. (NGA) was present during the explorations, collected samples of the soils encountered, and maintained a log of the explorations. The soils NELSON GEOTECHNICAL ASSOCIATES, INC.