1215 MADRONA AVE 2016-08-24 - Laserfiche WebLink

' ' ' ' ' , ' ' ' ' , ' � ' �I, ' ' � ' Gustafson Residence Retaining Wall NGA File No 9241 15 1215 Madrona Avenue June 12, 2015 Everett, Washington Page 4 maintained logs of the explorations. A Standard Penetration Test (SPT) was perforined on each of the sainples during drilling to document soil density at depth. The SPT consists of driving a 2-inch outer- diameter, split-spoon sampler 18 inches using a 140-pound hammer falling 30 inches. The number of blows required to drive the sampler the final 12 inches is referred to as the "N" value and is presented on the boring logs. The N value is used to evaluate the strength and density of tl�e deposit. The soils were visually classified in general accordance with the Unitied Soil Classification System presented in Figure 4. The logs of our borings are attached to this letter and presented as Figures 5 and 6. We present a brief summary of the subsurface conditions in the following paragraph. For a detailed description of the subsurface conditions, the boring logs should be reviewed. In bot11 of our borings, we encountered approximately 2.5 feet of very loose dark brown to brown silty fine to inedium sand with gravel and varying amounts of organics, interpreted as undocumented fill soils. Underlying tlie fill in the borings, we encountered approxiinately 5.0 to 7.5 feet of very loose to loose brown fine to coarse sand with silt and trace gravel interpreted to be native advance outwash soils. Below the advance outwash, we encountered very stiff brown to gray sandy silt with and medium dense silty fine to medium sand with trace gravel that we interpreted as native transitional bed deposits. Borings B-1 and B-2 were terminated within the native transitional bed deposits at a depth of 21.5 feet below the existing ground surface, respectively. Hydrologic Conditions Groundwater seepage was encountered in Borings B-1 and B-2 at a depth of 10.0 feet and 6.0 feet below the existing ground surface, respectively. We also observed natural groundwater seepage einitting from the steep slope along the eastern property line within the steep slope area below the retaining walls. We interpret all of the groundwater found on this site to be a chronic perched groundwater condition. Perched water occurs when surface water infiltrates through less dense, more penneable soils, such as topsoil and the weathered harizon, and accumulates on top of a less permeable soil, such as the dense/hard glacial soils. Perched water typically does not represent a regional groundwater "table" within the upper soil horizons; however, due to the relatively large recharge area that excludes well beyond your property, it is our opinion tliat groundwater within tl�is ravine on this site is a cl�ronic condition that will persist throughout the year. Perched water tends to vary spatially and is dependent upon the amount of rainfall. We would expect the ainount of groundwater to slightly decrease during drier times of the year and increase during wetter periods. NELSON GEOTECHNICAL ASSOCIATES, INC.