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June 13,2023 <br />HWA Project No. 2022-188-21 <br />2.3 ExrstrivcGnorncnivrc.llDA,IA, <br />Site conditions have been explored previously by Jacobs Engineerin g in 2019 and 2020a and <br />2020b. The initial investigation appears to consist of over 30 test pits (Jacobs, 2019). The <br />available Groundwater Memorandum discusses the excavation of 15 test pits and installation of <br />piezometers to measure seasonal groundwater fluctuations. The Memorandum concluded that <br />seasonal variation in groundwater levels was significant and on the order of 5 to 8 feet over the <br />course of the local irrigation season from May to October when groundwater levels rose to <br />within 3 feet or less below the existing ground surface. The report further concluded that <br />shallow groundwater is a consideration for the design and construction of the new transfer station <br />(Jacobs, 2020a). Review of the test pit logs indicate that surface soils within the interior of the <br />site consist of coarse-grained alluvium overlain by a layer of fine-grained silts and clays of <br />relatively high plasticity and varying organic content that generally thickens from 0 to 10 feet <br />from the southwest to northeast across the site. <br />The second investigation consisted of drilling 6 borings around the site perimeter to assess <br />subsurface conditions and collect data in support a utility extension along the US 97 collector <br />road into the site (Jacobs, 2020b). The borings were each conducted to depths of 16.5 feet and <br />samples were obtained using SPT sampling methods as well as from grab samples obtained from <br />the cuttings. Selected soil samples were analyzedto veriff soil classifications assess soil <br />properties. The borings indicate that coarse-grained alluvial sands and gravels are overlain by <br />about 5 feet of medium stiff to stiff, silt at the northwest (B-3), south central (B-5) and southeast <br />(8-6) corners of the site. <br />3.0 GENERAL SITE CONDITIONS <br />3.1 Srrn DpscRrPTIoN <br />The project site is located along the east side of US 97 and consists of a portion of SW /+ andthe <br />SE % of Section 28, Township 18 North, Range 18 East. We understand that Kittitas County <br />owns 50 acres at this location, of which approximately 38.84 acres encompassing the transfer <br />station development (as shown on Figures I and2). The remaining 10.87 acres (not shown on <br />Figures I and2),located to the north and abutting the BNSF right-of Way are set aside for future <br />development of a new lower county vehicle maintenance facility. <br />The transfer station site topography is relatively flat, and slopes gradually from elevation (EL) <br />1,556 feet near the northern boundary to EL I,544 feet near the southern boundary. The ground <br />surface includes several earthen ditches which are roughly 3 to 5 feet deep. The lowest point on <br />site is EL 1,537 feet at the ditch invert in the southeastern corner (See Figure 2). Previously, the <br />site was used for agricultural purposes with notable irrigation infrastructure which seasonally <br />influences groundwater levels. Recently, Kittitas County constructed a new paved two-lane <br />access road into the site along the southern lot boundary. <br />52022-188-21 KCTS - Final Geotechnical Report HWA GEOSCIENCES INC.