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TEMPORARY DEWATERING FEASIBILITY STUDY Redmond Square

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TEMPORARY DEWATERING FEASIBILITY STUDY Redmond Square REPORT TEMPORARY DEWATERING FEASIBILITY STUDY Redmond Square Submitted to: MGRM LLC 1111 3rd Ave. Suite 2850 Seattle, WA 98101 Submitted by: Golder Associates Inc. 18300 NE Union Hill Road, Suite 200, Redmond, Washington, USA 98052 +1 425 883-0777 1661044.900 November 2019 November 2019 1661044.900 Table of Contents 1.0 INTRODUCTION ............................................................................................................................................. 1 2.0 GEOTECHNICAL REQUIREMENTS .............................................................................................................. 1 2.1 Stratigraphy .......................................................................................................................................... 1 2.2 Primary Aquifer..................................................................................................................................... 2 2.3 Aquifer Transmissivity and Hydraulic Conductivity .............................................................................. 2 2.4 Groundwater Elevations ....................................................................................................................... 2 2.4.1 Historic Record from Nearby Monitoring Wells ............................................................................... 2 2.4.2 Groundwater Levels in On-Site Monitoring Wells ........................................................................... 3 2.5 Potential Construction Dewatering Pumping Rates ............................................................................. 4 2.6 Aquifer Drawdown ................................................................................................................................ 6 3.0 PROPOSED PROJECT DESCRIPTION ........................................................................................................ 6 3.1 Proposed Excavation ........................................................................................................................... 6 3.2 Preliminary Dewatering Scheme .......................................................................................................... 7 3.3 Area of Groundwater Lowering ............................................................................................................ 7 3.4 Conveyance ......................................................................................................................................... 7 4.0 WATER QUALITY ........................................................................................................................................... 7 5.0 CLOSING ........................................................................................................................................................ 8 6.0 REFERENCES ................................................................................................................................................ 9 ii November 2019 1661044.900 TABLES Table 1: Predicted Pumping Rates, Drawdown, and Radius of Influence for Elevator Pit Dewatering FIGURES Figure 1: Vicinity Map Figure 2: Site Map and Proposed Excavation Boundaries Figure 3: Monitoring Wells in Vicinity of Redmond Square Site Figure 4: City of Redmond Monitoring Well Groundwater Elevations Figure 5: Groundwater Drawdown Profile Figure 6: Estimated Radius of Influence from Potential Elevator Pit Dewatering No table of figures entries found . No table of figures entries found . No table of figures entries found . iii November 2019 1661044.900 1.0 INTRODUCTION This report presents a feasibility study to meet the requirements for the City of Redmond’s Temporary Construction Dewatering Operating Policy (Attachment A; Redmond 2016) for the Redmond Square Site (Site), a proposed mixed-use development site by MGRM LLC in Redmond, Washington (Figure 1). The proposed project includes two buildings and will encompass approximately 3.5-acres, bounded by Redmond Way to the north, 164th Avenue NE to the west, 166th Avenue NE to the east, and Cleveland Street to the south (Tiscareno 2019). The project will be constructed in two phases. The first phase entails the construction of Building B on the eastern portion of the Site and the second phase will be the construction of Building A on the western portion of the Site (Figure 2). The current project design includes one story of below-grade parking below both buildings. The underground parking structure below Building B will encompass approximately 1.5 acres (65,000 square feet [ft2]), while the structure below Building A will be slightly smaller (approximately 1.4 acres or 60,000 ft2). Each building will have two elevators. A concrete wall will separate the two foundations. The finished floor slab elevation for the below-grade parking will be approximately 34 feet (all elevations in this report are referenced to NAVD88 datum), or approximately 10 feet below ground surface (bgs) assuming a ground elevation of 44 feet. Based on this slab elevation, the elevation of the bottom of the footings is estimated to be 32 feet. Groundwater elevations in the vicinity of the site typically range from 24 feet during the summer and early fall (dry season) to 32 feet during the winter and spring (wet season; Section 2.3). Based on these elevations and the timeline for construction (Section 3), temporary construction dewatering for the foundation will likely not be necessary. The bottom of the elevator pits will extend to an elevation of 28 feet (bottom of slab elevation) and, depending on static water levels at the time of construction, some localized dewatering may be required around the elevator pits during the winter months. Static groundwater elevations will likely be below the bottom of the elevator pits during the summer months (approximately May to November), meaning no dewatering will be needed during the summer. Groundwater elevations in the vicinity of the site have been artificially lowered from nearby construction dewatering projects for approximately the last 3 years (2017 through 2019). While there are currently no active construction dewatering projects (beginning in June 2019), groundwater elevations could remain lower than normal when project construction is planned to start (October 2020). This could reduce or eliminate the need for construction dewatering around the elevator pits during the winter season. As detailed later in this report, the potential need for temporary dewatering and the necessary dewatering rates are dependent on antecedent groundwater levels and weather conditions during construction, subsurface geologic conditions, and potential impacts of other construction dewatering projects that may be occurring in the City at the time of construction. 2.0 GEOTECHNICAL REQUIREMENTS 2.1 Stratigraphy The Site is underlain by alluvial and recessional outwash sands and gravels. These materials were encountered to a depth of at least 27 feet bgs in fourteen borings completed on or adjacent to the Site as part of the Site geotechnical investigation (Golder 2019a). Nearby City monitoring wells (MW008 for example) indicate that alluvium extends to at least 30 feet bgs in the vicinity of the Site. Information in other reports (Golder 2003a, 1 November 2019 1661044.900 2003b; Parametrix 1997) indicates these materials extend to about 35 to 70 feet bgs in the downtown Redmond area. The alluvial and recessional outwash materials are underlain by fine sand and silt materials. 2.2 Primary Aquifer The alluvial and recessional outwash sands and gravels form the primary aquifer in the vicinity of the Site. These materials extend to a depth of about 35 to 70 feet bgs. The sands and gravels are underlain by fine sand and silt materials that do not form a productive aquifer in the downtown Redmond area. 2.3 Aquifer Transmissivity and Hydraulic Conductivity The transmissivity and hydraulic conductivity of the alluvial and recessional outwash aquifer is generally very high but there is wide variability in the estimated values in the vicinity of the Site. Testing of City Wells No. 1 and No. 2 in Anderson Park suggest a transmissivity of the about 22,000 to 47,000 ft2/day (Golder 2003a, 2003b). The City’s wellhead protection report cites transmissivities of approximately 5,300 to 134,000 ft2/day near the Site (Parametrix 1997). Based on these estimates and assuming an aquifer thickness of 50 feet1, the hydraulic conductivity ranges from about 200 feet per day (ft/day) to over 2,000 ft/day. A pumping test performed at a nearby development site (Bear Creek Mixed Use), located approximately 1,500 feet west of the project site, indicated aquifer transmissivities of 39,000 to 50,000 ft2/day or hydraulic conductivities of 800 to 1,000 ft/day based on an aquifer thickness of 50 feet (Bender 2018). The City’s groundwater flow model used hydraulic conductivity values of approximately 5,000 to 7,500 ft/day for the portion of the aquiver near the Site, as part of the model calibration process (GeoEngineers 2018). The hydraulic conductivity of the alluvial and recessional outwash underlying the Site was also estimated from three samples collected from two borings during the geotechnical study of the Redmond Square. The samples were collected from depths ranging from 15 to 20 feet bgs. The hydraulic conductivities were estimated using an empirical equation developed from grain size analyses (Massmann et al. 2003), as described in the Western Washington Stormwater Manual (Ecology 2012). The estimated hydraulic conductivities of
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