4730 COLBY AVE YMCA Geotech Report 2025-04-14

Subsurface Exploration, Geologic Hazard, and Snohomish County YMCA Geotechnical Engineering Report Everett, Washington Project and Site Conditions 2.0 PROJECT AND SITE DESCRIPTION The subject site consists of about 8 acres located at 4730 Colby Avenue in Everett, Washington (Parcel No. 29053100400500). The property is currently occupied by an existing office building with parking lot and grass field areas, and is bounded by Colby Avenue to the east, Rucker Avenue to the west, 47th Street SE to the north, and residential properties to the south. Site topography is generally mounded in the middle of the property and slopes gently downward toward the north,west, and east. Overall vertical relief across the site is estimated at 25 feet. We understand that the project will consist of the demolition of the existing building and parking area and will include the construction of a new two-story YMCA building. Other site features include paved parking and drive lanes, sidewalk areas, utilities, and landscaping. The proposed on-site stormwater management includes a series of infiltration and bioretention facilities. 3.0 SUBSURFACE EXPLORATION Previously completed subsurface explorations observed by AESI consisted of the excavation of four Pilot Infiltration Test (PIT) pits, nine exploration pits, and installation and development of one monitoring well between December 13 and December 20, 2017. The various types of materials and sediments encountered in the explorations, as well as the depths where characteristics of these materials changed, are indicated on the exploration logs included in the Appendix. The depths indicated on the logs where conditions changed may represent gradational variations between sediment types in the field. Our explorations were approximately located in the field relative to known subject site features shown on Figure 2, "Site and Exploration Plan." The locations of explorations completed previously (Liu & Associates, Inc., 2012) are also presented on Figure 2. Because of the nature of exploratory work below ground, extrapolation of subsurface conditions between field explorations is necessary. It should be noted that differing subsurface conditions may sometimes be present due to the random nature of deposition and the alteration of topography by past grading and/or filling. The nature and extent of any variations between the field explorations may not become fully evident until construction. If variations are observed at that time, it may be necessary to re-evaluate specific recommendations in this report and make appropriate changes. 3.1 Infiltration Test Pits and Exploration Pits Explorations IT-1 through IT-4 and EP-1 through EP-9 were excavated by Northwest Excavating, under subcontract to AESI, with a CAT 420E rubber-tired backhoe with an approximately 2-foot-wide toothed bucket. The explorations were excavated on December 13 and 14, 2017. February 26,2018 ASSOCIATED EARTH SCIENCES,INC. AWR/ms-170671E001-2-Projectsl20170671\KE\WP Page 2