4702 W GLENHAVEN DR 2020-06-16 - Laserfiche WebLink

Oskar Property Deck Limited CAO,Everett,WA Materials Testing&Consulting,Inc. July 17.2019 19B182 No surface water features or apparent seasonal channels were observed within the subject property during MTC's site reconnaissance. No seepage was observed on the slope during our site visit in the early spring season. The ground surface was damp throughout the proposed building area, with no evidence of channeling or runoff zones encountered. No upland surface water features are mapped at the site. No pervasive groundwater table or seepage flows were observed at hand auger borings, and DCP rods were not wet upon extraction from refusal depth. MTC's scope of investigation did not include determination or monitoring of seasonal water condition variations or conclusive measurement of regional groundwater conditions past the depths explored. Summary of Geologic Literature Review: According to the 1:24,000 scale Geologic Map of the Everett 7.5-minute Quadrangle, Snohomish County, Washington, published by the Washington State Department of Natural Resources (Minard, 1985), the project site upland and top-of-slope area is mapped as underlain by Quaternary Vashon Glacial Till (Qvt). The unit is described as a diamicton, including an unsorted and unstratified mixture of boulders, gravel,sand, silt and clay. This unit is typically highly compacted from glacial consolidation below its cover deposits, and is considered to be generally stable on slopes, resistant to erosion, and impervious to water transmission. In the project area, till deposits are typically found mantlinghills, ridges and slopes at higher elevations beneath thin cover and weathered soils varying in composition and thickness locally. Also in the site area,mapped downhill from the project location at lower elevations to the north and west, is Quaternary Vashon Advance Outwash (Qva) that underlies regional till deposits. These pro-glacial deposits are described as a moderately to well sorted,well stratified sand with some pebbles and cobbles. This unit may exhibit cross-bedded sands and gravels with locally laminated, silt and clay layers. Advance Outwash tends to be moderately to strongly compacted from overriding glacial ice depending in severity on location and depth within the section, and is also typically stable at moderate slope angles that are below the angle of repose for such deposits. The NRCS Web Soil Survey (accessed online) maps near-surface soils at the project site as Alderwood- Everett gravelly sandy loam, 25% to 70% slopes, and Alderwood-Urban Land Complex, 8% to 15% slopes in upland areas. The Alderwood unit was formed on till plains from a parent material of basal glacial till. Typical profile is gravelly ashy sandy loam becoming gravelly sandy loam below about 3 feet depth. The soil is moderately well drained and has a very low to low capacity to transmit water. Restrictive conditions (densic deposits) are commonly found between 20 to 40 inches depth. Perched water may occur over the restrictive densic soils (unweathered till)within 18 to 36 inches of the surface. Soil conditions encountered in our current and prior explorations of the proposed development area consisted of lower strength cover soils and weathered glacial soils to a few feet depth over dense or hard deposits interpreted as consolidated glacial till. These findings are consistent with mapped conditions.