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June 13,2023 <br />HWA Project No. 2022- 1 88-2 I <br />3.2 GrNsnq,L Gror-ocv <br />Geologic information for the project area was obtained from the Geologic Map of the Ellensburg <br />North and Southern Half of the Reecer Canyon 7.5-minute Quadrangles, Kittitas County, <br />Washington (A.J. Sadowski, et a\,2020). According to this publication, the project site is <br />underlain by Holocene to Pliocene aged alluvial deposits composed of sand and gravel from <br />channel deposits on the active flood plain of the Yakima River. To the west, the site is bordered <br />by modified land (Fill) likely constructed after removal of Holocene to Pliocene non-glacial <br />deposits consisting of organic and organic rich sediment, peat, muck, silts, and clay that were <br />deposited in wetland areas that formed in closed depressions on the alluvial plain. The relevant <br />portion of this map is shown on the Geologic Map in Figure 3. <br />3.3 SussuRFA,cECoNDITIoNs <br />3.3.1 SoilDescriptions <br />Subsurface conditions encountered in the field consisted of I to 1.5 feet of topsoil (agricultural <br />soil) composed of silty sand to slightly organic sandy silt overlying native alluvial deposits <br />ranging from sandy lean clay and fat clay to silty sand and poorly to well-graded gravel with silt <br />and sand. The following are descriptions of soil units encountered during our explorations. <br />Topsoil (Aericultural Soil): Topsoil consisting of loose silty sand to medium stiff sandy <br />organic silts containing organic matter and root fragments were encountered at the surface at <br />all of the boring locations. The topsoil appears to consist of material similar to the <br />underlying fine-grained alluvial deposits that have been modified for agricultural use. This <br />layer ranged from 1.5 to about 2 feeL in thickness. <br />Fine-Grained Alluvium-Beneath the topsoil at all locations, fine-grained alluvial deposits <br />consisting of medium stiff, sandy silts, to medium stiff to soft clays of moderate to high <br />plasticity were encountered at all locations. These deposits range from 1.5 to 8.5 feet thick <br />in our borings and are thickest in the northeast corner of the site in the vicinity of borings <br />BH-3, BH-4, and BH-5. This soil is likely the result of deposition in shallow depressions or <br />wetland areas developed on, or marginal to, the floodplain of the Yakima River. These soils <br />exhibit relatively low strength and moderate to high compressibility. As such, the fine- <br />grained alluvial soils are not suitable for supporting structures or thick fills. At the locations <br />of BH-4 and BH-5, the water table was encountered in the lower 0.5 to I foot of this layer at <br />the time of drilling. <br />Coarse-Grained Alluvium- The deepest soil units encountered consisted of medium dense <br />to very dense, gravelly, poorly graded sand with silt or gravelly, silty sands to poorly or <br />well-graded gravels with silt. Though never fully penetrated by any of our borings, these <br />deposits were encountered at depths ranging from 3.5 to 10 feet below the existing ground <br />surface and extend to the termination depths of our borings with layer thicknesses ranging <br />62022-188-21 KCTS - Final Geotechnical Report HWA GEoSCIsNcss INc