4730 COLBY AVE SEPA 016 - 027 - YMCA Geotech Report 2025-04-14

October 25, 2015 YMCA - Everett L&A Job No. 15-089 Page 6 dense, fine to medium sand, with trace to some gravel, was found underlying the weathered soil within the explored depths of all test pits except 8, 9, and 11. A hog sediment of brown, medium-dense, silty fine sand with mixed-in decayed tree stumps and roots was encountered in the bottom of Test Pit 11. Groundwater Condition Groundwater was not encountered in any of the 11 test pits excavated on the project site. The advance outwash deposit underlying the site at shallow depth is of moderately high permeability and would allow water to seep through. Stormwater infiltrating into the surficial more permeable soils may perch on the more silty fill and weathered soil and form pockets of near-surface groundwater. The amount of and the depth to such pockets of perched groundwater would fluctuate seasonally, depending on precipitation, surface runoff, ground vegetation cover, site utilization, and other factors. The perched groundwater may dry up completely during the dryer summer and fall months and accumulate and rise in the wet winter and early spring months. GEOLOGIC HAZARDS AND MITIGATION Erosion and Landslide Hazards The project site is mostly gently sloped. The advance outwash deposit underlying the site at shallow depth is of high shear strength and moderately-high permeability. Therefore, the erosion and landslide hazards of the site should be minimal. To further minimize erosion and landslide hazards of the site, concentrated stormwater should not be discharged onto the ground within the site. Stormwater over impervious surfaces, such as roofs and paved roadways, driveways and parking areas, should be captured by underground drain line LIU & ASSOCIATES, INC.