10200 25TH AVE SE Geotech Report 2018-01-02 MF Import

Proposed Modernizarion and Additions to Subsurface Fxploration, Geologic Nazards, Eisenhower Middle School and Geotechnical Engineering Repnn Everert, Washington Prajecr and Site Conditions 4.1 Soil Cenditions Fil! The site appears to have previously been graded to provide the relatively level pad on which the existing school and tields are now situated. Previous grading appears to have resulted in fill across the majoriry of the site especially to the eastern portions of the pad areas. Fill ranging in depth from 1 foot to 12 feet was encountered in all of the borings advanced at the site. Fill depth generally increased from west to east across the site. The fill consisted of a loose to medium dense mixture of silt and sand with variable amounts of gravel, and organic matter. The fill was probably placed at the time the school Nas built. Because the existing fill was observed to vary in density and contained organic material, we recommend that the existing fill not be used for direct support of new foundations, tloors, or paving without prior remediation. Portions of this fill may be used as structural till if moisture-conditioned and compacted as detailed in the Structura! Fi[l section of this report. Till Below the fill, all of the exploration borings encountered dense to very dense till consisting of silry sand containing few to little amounts of graveL The till was deposited at the base of the active continental glacier during advance and retreat and was subsequently overridden and compacted by the weight of the overlying glacial ice. Till soils rypically contain a substantial fine-grained soil fraction, which makes them moisture-sensitive and susceptible to disturbance when wet site or weather conditions exist. Till soils rypically possess high strength and low compressibility ch�racteristics and are considered suitable for structural support. 4.2 Hydroloey [n general ground warer seepage and/or wet to saturated :oil was encountered above the surface ot the dense till encountered in the exploration borings advanced at the site. The observed ground water seepage/wet to sa[urated soil was iuterpreted to be in a "perched" condition where do�vmvard infiltration of surface water throu�;h the relatively permeable fill is impeded by die lower permeabiliry tili soils at depth. It should be nored that the exploration borings �vere completed during the early winter. Ground water levels are expecred to be higher and the amount of ground water is expected to be greater during and following the wetter winter and spring seasons. The quantity and duration of flow from an excavation face tha[ pierces the perching layer will depend on the soil grain size, [opography, and season. Ground water conditions should also be expected to vary in response to changes in precipitation, on- and off-site land usaQe, and other factors. Inn�mrv _'3, _'OOd ASSOCIdTED E9RTN SC/E,VCES, lNC. ,1171d-KEOJ%J3d3-Pro�rnai]IY)J%JY�KEiIVP-«"A PageS �