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HomeMy WebLinkAboutRS_21_GE Cedar River 205 New Floodwall Letter Report_200923_v21101 South Fawcett Avenue, Suite 200 Tacoma, Washington 98402 253.383.4940 September 23, 2020 Tetra Tech, Inc. 1420 Fifth Avenue, Suite 550 Seattle, Washington 98101 Attention: Chuck Purnell, PE and Lois Loesch, PE Subject: Recommendations for Floodwall Analysis and Design Cedar River 205 Levee Renton, Washington File No. 0693-078-01 INTRODUCTION The purpose of this report is to present our recommendations for design of new or modified floodwalls for the Cedar River 205 Levee Project in Renton, Washington. More detail on the geotechnical aspects of the project, including a summary of subsurface explorations and observed soil conditions can be found in our Levee Certification report dated November 17, 2017. We prepared a similar report on November 17, 2017 providing recommended soil parameters for the analysis of existing floodwalls. Our services are being provided in accordance with our December 7, 2015 agreement with Tetra Tech, Inc. executed February 16, 2016 and the latest contract amendment dated July 30, 2019. SUBSURFACE CONDITIONS Our understanding of subsurface conditions is based on reviewed documents provided by Tetra Tech, the City of Renton (City), and from our in-house files. We also performed a subsurface investigation between January 16, 2017 and January 27, 2017 to further characterize the site soils. A total of nine cone penetration tests (CPTs) and four borings were performed at various locations along the right and left bank of the Cedar River within the project area. Details regarding the subsurface investigation are provided in our Levee Certification report for this project. The project site is located in an alluvial valley adjacent to the Cedar River. With the exception of small amounts of fill, all the soils reported in the reviewed explorations are described as alluvium. The alluvium generally consists of interbedded layers of sand, gravel, and silts or clays. The sands were typically observed to be silty sand or sand with silt and the gravels were typically observed to be silty gravels. Interbeds of sands and gravels with lower fines content were also observed within the upper layers of alluvium. The DRAFT Tetra Tech, Inc. | September 23, 2020 Page 2 File No. 0693-078-01 granular soils within the alluvium are characterized as very loose to loose with occasional zones of medium dense soil. DESIGN AND ANALYSIS RECOMMENDATIONS General Design Criteria We anticipate that cast-in-place concrete floodwalls will be designed using the guidance and criteria described in United States Army Corps of Engineers (USACE) Engineer Manual (EM) 1110-2-2502 “Retaining and Floodwalls”. Specifically, that loads will be applied as shown in Figures 4-5 and 4-11, and minimum factors of safety should be established in accordance with Table 4-2. We recommend that full soil saturation be assumed at the design water level, or the design water level plus freeboard as required for specific cases, on the riverside. On the landside or protected side, we recommend that water levels and full soil saturation be assumed at existing ground surface on landside. Seepage uplift forces on the bottom of the footing can be interpolated linearly as shown in Figures 4-5 and 4-11 of the USACE Retaining and Floodwall guidance. This method for calculating seepage and uplift pressures can be conservative. If this simplified method results in large wall footings that could come into conflict with adjacent structures or improvements, seepage pressures could be checked again using more detailed methods after a general wall geometry has been established. Lateral Soil Loads and Pressures Lateral soil pressures, both driving and resisting, can be estimated with the following design parameters. We have provided parameters for three typical soils. The Alluvium can be assumed to be present at the base or bottom of the walls and reacting against shear keys or other structural elements extending below the ground surface. The Levee Embankment fill represents soil placed to form or connect to earth berm levees. The roadway fill is intended to be used on the landside of the levee walls where adjacent to roadway sections that extend to the floodwall. LATERAL SOIL PRESSURE PARAMETERS FOR NATIVE ALLUVIUM (LOOSE TO MED. DENSE SILTY SAND) Soil Parameter Alluvium Saturated Alluvium Soil Unit Weight Total Weight = 110 pcf Total Weight = 115 pcf Buoyant Weight = 53 pcf Friction Angle 30 degrees 30 degrees Cohesion 0 psf 0 psf Active Earth Pressure Ka = 0.33 Equivalent Fluid Pressure: Ka*Unit Weight = 36.7 pcf Ka = 0.33 Equivalent Fluid Pressure: (Ka*Buoyant Unit Weight) = 17.5 pcf Passive Earth Pressure Kp = 3.00 Equivalent Fluid Pressure: Ka*Unit Weight = 330 pcf Kp = 3.00 Equivalent Fluid Pressure: (Ka*Buoyant Unit Weight) = 158 pcf Notes: pcf = pound per cubic foot DRAFT Tetra Tech, Inc. | September 23, 2020 Page 3 File No. 0693-078-01 LEVEE EMBANKMENT FILL (COMPACTED SILTY SAND WITH GRAVEL) Soil Parameter Levee Fill Submerged Levee Fill Soil Unit Weight Total Weight = 120 pcf Total Weight = 125 pcf Buoyant Weight = 63 pcf Friction Angle 32 degrees 32 degrees Cohesion 0 psf 0 psf Active Earth Pressure Ka = 0.31 Equivalent Fluid Pressure: Ka*Unit Weight = 36.9 pcf Ka = 0.31 Equivalent Fluid Pressure: (Ka*Buoyant Unit Weight) = 19.2 pcf Passive Earth Pressure Kp = 3.25 Equivalent Fluid Pressure: Ka*Unit Weight = 390 pcf Kp = 3.25 Equivalent Fluid Pressure: (Ka*Buoyant Unit Weight) = 204 pcf Notes: pcf = pound per cubic foot ROADWAY FILL (AGGREGATE FOR GRAVEL BASE PLACED AND COMPACTED AS PART OF THIS PROJECT) Soil Parameter Roadway Fill Submerged Roadway Fill Soil Unit Weight Total Weight = 130 pcf Total Weight = 135 pcf Buoyant Weight = 53 pcf Friction Angle 38 degrees 38 degrees Cohesion 0 psf 0 psf Active Earth Pressure Ka = 0.24 Equivalent Fluid Pressure: Ka*Unit Weight = 30.9 pcf Ka = 0.24 Equivalent Fluid Pressure: (Ka*Buoyant Unit Weight) = 17.3 pcf Passive Earth Pressure Kp = 4.20 Equivalent Fluid Pressure: Ka*Unit Weight = 546 pcf Kp = 4.20 Equivalent Fluid Pressure: (Ka*Buoyant Unit Weight) = 305 pcf Notes: pcf = pound per cubic foot BEARING RESISTANCE ON NATIVE ALLUVIUM Footing Width Unfactored Bearing Resistance (KSF) 4.0 5.4 5.0 6.1 6.0 6.7 7.0 7.3 8.0 7.8 9.0 8.3 10.0 8.8 11.0 9.2 12.0 9.6 13.0 10.0 14.0 10.3 DRAFT Tetra Tech, Inc. | September 23, 2020 Page 4 File No. 0693-078-01 Global and Seismic Stability Analysis Global and Seismic stability of the I-Walls have been evaluated separately. The results of this analysis are provided in our Levee Certification report. Our analysis indicates that minimum recommended global stability factors of safety have been achieved for static, non-seismic cases. Our analysis also predicts that some movement of the riverbanks is expected during the design seismic event. In these cases, the predicted slip surface extends below the floodwalls. This indicates that the floodwalls are completely within the sliding soil mass and are therefore expected to displace during the design seismic event regardless of the internal strength of the walls. Our seismic evaluation of the earth embankment levees focuses on the risk of such displacement resulting in flooding and potential need for repair after a major seismic event. We recommend that a similar approach be taken when evaluating the floodwalls for seismic resilience. LIMITATIONS We have prepared this report for the exclusive use of Tetra Tech, Inc. Tetra Tech may distribute copies of this report to the City of Renton, the City’s authorized agents, and regulatory agencies including Federal Emergency Management Agency (FEMA) and FEMA’s designated reviewers, as may be required for the project. Qualified engineering and construction practices can help mitigate flooding risks, but they cannot completely eliminate those risks. Favorable performance of structures in the recent past provides useful information for anticipating likely near-term future performance, but it cannot predict or imply a certainty of similar long-term performance. Levee systems require periodic inspection to confirm that all critical components continue functioning as intended. Confirmation that design flood flows and/or elevations have not significantly changed also requires the periodic review of design criteria and other potential contributing factors including, but not limited to, changes in surrounding development, weather patterns, system operational policies, or sedimentation. Within the limitations of scope, schedule and budget, our services have been executed in accordance with generally accepted practices in the field of geotechnical engineering in this area at the time this report was prepared. The conclusions, recommendations, and opinions presented in this report are based on our professional knowledge, judgment and experience. No warranty or other conditions, express or implied, should be understood. DRAFT Tetra Tech, Inc. | September 23, 2020 Page 5 File No. 0693-078-01 Please refer to Appendix A titled “Report Limitations and Guidelines for Use” for additional information pertaining to use of this report. Sincerely, GeoEngineers, Inc. Lyle J. Stone, PE Associate Geotechnical Engineer LJS:ch Attachment: Appendix A. Report Limitations and Guidelines for Use Disclaimer: Any electronic form, facsimile or hard copy of the original document (email, text, table, and/or figure), if provided, and any attachments are only a copy of the original document. The original document is stored by GeoEngineers, Inc. and will serve as the official document of record. DRAFT APPENDIX A Report Limitations and Guidelines for Use DRAFT Tetra Tech, Inc. | September 23, 2020 Page A-1 File No. 0693-078-01 APPENDIX A REPORT LIMITATIONS AND GUIDELINES FOR USE 1 This appendix provides information to help you manage your risks with respect to the use of this report. Report Use and Reliance This report has been prepared for Tetra Tech, Inc. GeoEngineers structures its services to meet the specific needs of its clients. No party other than Tetra Tech, Inc. may rely on the product of our services unless we agree to such reliance in advance and in writing. Within the limitations of the agreed scope of services for the Project, and its schedule and budget, our services have been executed in accordance with our Agreement with Tetra Tech, Inc. dated July 30, 2019 and generally accepted geotechnical practices in this area at the time this report was prepared. We do not authorize, and will not be responsible for, the use of this report for any purposes or Projects other than those identified in this report. If changes to the Project or property occur after the date of this report, GeoEngineers cannot be responsible for any consequences of such changes in relation to this report unless we have been given the opportunity to review our interpretations and recommendations in the context of such changes. Based on that review, we can provide written modifications or confirmation, as appropriate. Information Provided by Others GeoEngineers has relied upon certain data or information provided or compiled by others in the performance of our services. Although we use sources that we reasonably believe to be trustworthy, GeoEngineers cannot warrant or guarantee the accuracy or completeness of information provided or compiled by others. Conditions Can Change This report is based on conditions that existed at the time the study was performed. The findings and conclusions of this report may be affected by the passage of time, by events such as construction on or adjacent to the site, new information or technology that becomes available subsequent to the report date, or by natural events such as floods, earthquakes, slope instability or groundwater fluctuations. If more than a few months have passed since issuance of our report or work product, or if any of the described events may have occurred, please contact GeoEngineers before applying this report for its intended purpose so that we may evaluate whether changed conditions affect the continued reliability or applicability of our conclusions and recommendations. Professional Judgment It is important to recognize that the geoscience practices (geotechnical engineering, geology and environmental science) rely on professional judgment and opinion to a greater extent than other engineering and natural science disciplines, where more precise and/or readily observable data may exist. To help clients better understand how this difference pertains to its services, GeoEngineers includes these explanatory “limitations” provisions in its reports. Please confer with GeoEngineers if you need to know how these “Report Limitations and Guidelines for Use” apply to your Project or site. 1 Developed based on material provided by ASFE, Professional Firms Practicing in the Geosciences; www.asfe.org. DRAFT