1215 W MUKILTEO BLVD 2017-07-28 - Laserfiche WebLink

April 29,2015 HWA Project No.2014-162-21 relatively near the ground surface. Whidbey formation generally consists of medium-bedded sand, silt and clay that were deposited in lake environments prior to the last glaciation. These soils have been glacially overridden and generally possess high shear strengths and low permeability. Subsurface Soil Conditions Our interpretations of subsurface conditions are based on results of our field explorations, review of available geologic and geotechnical data, and our general experience in similar geologic settings. In general, soil conditions at the subject site consist of surficial topsoil over fill soils that are underlain by Whidbey formation soils. At the toe of the rockery wall, we encountered surficial topsoil over advance outwash soils. Our interpretation of geologic conditions perpendicular to the length of the wall alignment is shown in Figure 3, Rockery Wall Profile A-A'. Each major soil unit is described below, with materials interpreted as being youngest in origin and nearest to the surface described first. • Topsoil—A thin layer of topsoil was encountered in all of the boring locations advanced along the wall alignment. The topsoil consisted of a combination of silty fine sand with abundant organics throughout. • Fill—A thin layer of fill was encountered in all of the boring locations. This fill consisted of loose, silty, fine gravelly sand with some thin areas of organic deposits. It is assumed that this fill was placed during the initial construction of the rockery walls. • Whidbey Formation—Whidbey formation soils were encountered in hand borings HH-1 and HH-2, located at the top of the upper and lower rockery walls. The Whidbey formation consists of stiff to very stiff, slightly gravelly, sandy silts and clays. The top of the formation was encountered at 4.6 feet bgs in boring HH-1 and 2.5 feet in boring HH-2. DCP test results obtained in the Whidbey Formation soils suggest that they posses high shear strength and significant cohesion. • Advance Outwash — Advance outwash soils, consisting of very dense, poorly graded sand with gravel, were encountered in hand boring HH-3. Advance outwash deposits consist of clean to slightly silty, fine to medium sand, with varying percentages of gravel. This advance outwash deposit was likely deposited during the Salmon Springs glaciation. Ground Water Conditions Ground water observed at the project site was limited to perched ground water near the base of the fill deposit encountered in hand boring HH-1. It should be noted, however, that these borings were advanced in October and do not reflect the maximum seasonal ground water levels. We would expect that seasonal perched ground water will accumulate over the Whidbey formation 2014-162 Letter Report 3 HWA GEOSCIENCES INC.