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Geotechnical Engineering Report <br /> Proposed Flight Line Steam Line Project lierracon <br /> Boeing Everett Facility • Everett, Washington <br /> January 17, 2012 • Terracon Project No.: 81115085 <br /> ' Based on the subsurface explorations, significant groundwater seepage is not anticipated within <br /> planned excavation depths. However, perched groundwater seepage may be encountered, <br /> particularly near existing utility trenches. <br /> We recommend that all project excavations be adequately dewatered so that work can be <br /> performed in the "dry." Given the perched nature of the site groundwater conditions, we <br /> 1 anticipate that a combination of sumps and pumps within utilidor excavations may be suitable <br /> for this purpose. In our opinion, the contractor should be responsible for designing and <br /> installing appropriate dewatering systems, if needed, to complete the work. The dewatering <br /> plan should include provisions for disposal of the collected water. We recommend that the <br /> contractor be required to submit the proposed dewatering plan to the engineer for review prior to <br /> start of construction. <br /> The contractor pothole data suggests that existing utility trenches, particularly the conduit <br /> trenches with their sand bedding material, may contain a significant amount of perched <br /> groundwater. The dewatering plan could include provisions to dewater these existing utility <br /> trenches prior to excavation of the utilidor trench. This could be accomplished using dewatering <br /> excavations located adjacent to existing trenches that cross the planned utilidor alignment. The <br /> dewatering excavations should be located about 10 feet from both sides of the planned utilidor <br /> excavation, should extend down to the bottom of the existing trench backfill material, and should <br /> freely communicate with the trench backfill. We anticipate that sumps and pumps would likely <br /> be suitable for the dewatering excavations. <br /> We recommend that the earthwork portion of this project be completed during the dry season <br /> (typically June through October), if possible. We anticipate that perched groundwater flows <br /> would be reduced during this period, resulting in decreased dewatering volumes. <br /> 4.2.6 Subgrade Preparation <br /> ' After site preparation activities and excavation to design grade is completed, the exposed <br /> utilidor foundation subgrade soils are anticipated to consist of dense to very dense glacial till <br /> deposits generally consisting of silty sand with gravel. We anticipate that the native glacial till <br /> deposits will provide adequate support for the planned utilidor. However, the glacial till soils are <br /> moisture sensitive and the utilidor excavation may extend below zones of perched groundwater. <br /> Excessive disturbance of the subgrade soils may impact support of the utilidor and could result <br /> in undesirable post construction settlement. We therefore recommend that the exposed <br /> subgrade be evaluated by a Terracon representative before the placement of engineered fill or <br /> foundations. Disturbed areas or other unsuitable soils identified by the Terracon representative <br /> should be removed and replaced with compacted engineered fill meeting the requirements of <br /> Section 9-03.17, Foundation Material Class A, as presented in the WSDOT Standard <br /> Specifications for Road, Bridge, and Municipal Construction. <br /> I <br /> Responsive • Resourceful ■ Reliable 7 <br />