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Home My WebLink About20 - Geotechnical Report12/19/2014 1:16 PM J:\Data\REN\113-104\09 Permitting\REN Highline Background Geo Review Memo.docx 12/19/14 12/19/14 12/19/14 TECHNICAL Memorandum Client: City of Renton Project: Highlands 435 Pressure Zone Reservoirs and Emergency Generator Project File: REN 113.104.01.109 Project Manager: Kyle Pettibone, P.E. Composed by: Steve Nelson, L.HG., L.E.G. Reviewed by: Kyle Pettibone, P.E. Subject: Review of Existing Geologic, Hydrogeologic, and Geotechnical information Date: December 19, 2014 _____________________________________________________________________________ Background The City of Renton (City) intends to construct a new reservoir on the site of its existing Highlands Reservoirs on the uplands east of central Renton. The existing reservoirs were constructed in 1941 and 1960, respectively. Preliminary designs indicate that the new reservoir would store 6.3 million gallons and with dimensions of approximately 170 feet by 250 feet and 25 feet high. A 2009 geotechnical report was completed by ZZA-Terracon to assess the site conditions and recommend foundation design and construction approaches for a new reservoir founded on the native subgrade material. This memorandum reviews the findings and recommendations of the geotechnical report and provides a summary of infiltration potential for possible on-site stormwater management. City Conditional Use permitting requires sufficient geotechnical assessment of the site to support the design and construction of the project and to evaluate the potential effect of construction and operation on geologic hazards. This memorandum includes RH2 Engineering, Inc.’s (RH2) review of the existing geotechnical report (ZZA-Terracon, 2009), and a review of available geologic mapping and boring logs. Technical Memo re Review of Existing Geologic, Hydrogeologic, and Geotechnical Information December 19, 2014 Page 2 12/19/2014 1:17 PM J:\Data\REN\113-104\09 Permitting\REN Highline Background Geo Review Memo.docx Summary of Available Data The geotechnical report (ZZA-Terracon, 2009) summarized the findings of an on-site exploration program and review of available geologic mapping. Six test borings were drilled to depths of 32 to 40 feet below ground surface (bgs). Three borings were completed as monitoring wells at depths of 32, 38, and 39 feet. All of the borings encountered dense to very dense sand with silt and gravel from a few feet bgs to 40 feet bgs. No groundwater was observed in the soil boring or in the monitoring wells at the time of drilling in October 2008. The available geologic maps identified the native earth material at the site as dense glacial till. The geotechnical report evaluated potential geologic hazards at the site and concluded that the risk for liquefaction, erosion, and unstable slopes was very low. The report provided geotechnical design parameters and seismic risk conditions appropriate for the type of native earth materials, and the site location. The report assigned a Seismic Site Class C for the site and provided earth pressures allowable bearing values references, and seismic spectral acceleration values per the International Building Code (IBC) of 2006. Review of current geologic database included Washington Department of Natural Resources (WADNR) mapping and Washington State Department of Ecology (Ecology) well log database. Per the geotechnical report, the WADNR geologic mapping indicates that the site is underlain by glacial till. Boring logs within ¼ mile of the site consistently encountered dense sand, silty sand, and sandy silt with gravel, characteristic of glacial till. The depth to groundwater in nearby borings was greater than 50 feet bgs. The WADNR assigns a low risk for liquefaction, seismic shaking, erosion, and steep slope instability for the site, in concurrence with the conclusions of ZZA-Terracon (2009). Review of the City Sensitive Areas mapping, indicates that the site is within Aquifer Protection Zone 2, and that a few engineered slopes at the site (soil berms surrounding the existing reservoirs) are at slopes between 15 and 25 percent. No erosion or landslide hazards are identified at the site. Summary of Site Conditions Geologic conditions at the site are uniform and consistent with local conditions, consisting of dense glacial till to a depth of at least 50 feet. This soil type characteristically has a low permeability. The infiltration capacity is low, expected to be less than 0.75 inches per hour; the site does not have a suitable soil type or area for on-site infiltration of stormwater or reservoir overflow. Groundwater exists at a depth of more than 50 feet bgs. The thick, low permeability soil provides substantial protection against the migration of accidentally released contaminants into underlying aquifers. The soil bearing capacity of the native glacial till appears sufficiently high to support the proposed reservoir without adverse settlement. Geologic hazards at the site are mapped as low. Technical Memo re Review of Existing Geologic, Hydrogeologic, and Geotechnical Information December 19, 2014 Page 3 12/19/2014 1:17 PM J:\Data\REN\113-104\09 Permitting\REN Highline Background Geo Review Memo.docx Conclusions The existing geotechnical report and more recent mapping and soil boring information developed since the report was completed in 2009 contain sufficient geologic, hydrogeologic, and geotechnical information to support permitting, design, and construction of the proposed reservoir without the need for additional on-site investigation of soil or geologic properties. The geologic hazards at the site are sufficiently documented in existing and available geotechnical report and geologic information. The spectral seismic spectral acceleration values will need to be updated to reflect the 2012 IBC. Reference ZZA-Terracon, 2009, Geotechnical Report, City of Renton – Water Distribution System Storage Planning Study, Renton, Washington. Prepared for HDR Engineering, Inc.