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Home My WebLink AboutC_BulkheadWallStabilityAddendumNo3_200724_v1.pdfTerracon Consultants, Inc.21905 64 th Ave. W, Suite 100 Mountlake Terrace, WA 98043 P (425) 771 3304 F (425) 771 3549 www.terracon.com REPORT COVER LETTER TO SIGN July 24, 2020 SRM Renton, LLC 720 6th Street South Ste. 200 Kirkland, WA 98033 Attn: Andy Loos P:(573) 555-1212 E:andy@srmdevelopment.com Re: Bulkhead Wall Stability - Response Letter Addendum No. 3 Cedar River Apartments 1915 SE Maple Valley Hwy Renton, Washington Terracon Project No. 81175025 Dear Mr. Loos: Terracon Consultants, Inc. (Terracon) has performed a revised Bulkhead Wall Stability evaluation in response to the technical review letter prepared by GeoEngineers dated July 8, 2020. This response letter addresses comments #3 and #4 from the technical review letter and assumes a revised scour depth per The Watershed Company. This response letter serves as an additional addendum to the geotechnical report for the project titled “Revised Geotechnical Engineering Report, Cedar River Apartments, Renton, Washington”, prepared by Terracon, dated October 31, 2018. Two previous addendum letters were provided addressing bulkhead wall stability and are dated June 10, 2019 and May 21, 2020. This letter presents updated slope stability analyses for the revised scour depth and to address technical review comments provided to the City of Renton. REVISED WALL STABILITY EVALUATIONS The bulkhead wall stability evaluations presented herein were performed consistent with the description provided in our previous addendum letters but include a deeper scour depth of 9.1 feet rather than 8.5 feet. This revised scour depth was communicated to Terracon via e-mail by The Watershed Company. Additionally, Terracon received technical review comments that expressed the need for consideration of potential cracking within the mass concrete in our slope stability analyses. Therefore, an alternative slope stability analysis was performed that compares different strength assumptions for modeling the mass concrete. The technical review comments received that are addressed in this letter are as follows: 3. In the stability report addendum, the concrete was modeled as a cohesive material. This assumes some tension capacity in the material and, therefore, must assume that there Bulkhead Wall Stability – Response Letter Addendum No. 3 Cedar River Apartments ■ Renton, WA July 24, 2020 ■ Terracon Project No. 81175025 Responsive ■Resourceful ■Reliable 2 are no cracks in the concrete. The concrete mass is unreinforced and, therefore, has likely cracked due to shrinking while curing. The fact that no perched groundwater was noted on top of the concrete is, in our opinion, evidence that the concrete, while massive and intact at a local scale, is likely cracked on a larger scale. We believe that a cohesive soil model can be an appropriate and conservation assumption over short distances such as when considering potential lateral earth pressures on the bulkhead wall and locally at the toe of the wall. However, over larger areas, such as were analyzed in the global stability analysis, cracks in the concrete are likely to control the strength of the mass must be considered. We suggest modeling the concrete as a high friction material in global stability analyses. A similar comment was also provided in our October 28, 2019 review. 4. In the Stability report addendum, some portions of the analysis figures appear to be missing. Some legends appear incomplete and don’t include all soil units used in the analysis, specifically the green unit shown in Figure 3. Although we agree that there are likely cracks within the mass concrete, we did not observe cracking nor a high-degree of fracturing that would suggest the overall behavior of the mass concrete would be similar to that of a granular material. Therefore, we are assuming any potential cracking is sparse and random. Along such potential cracks, the roughness of the surface is expected to be very high due to the propagation of cracks around aggregates and in non-linear planes. This result causes interlocking of the mass concrete blocks such that the behavior is more like large riprap such as that of a jetty or rockeries. These features can be constructed considerably steep and tall due to the interlock between particles. Furthermore, we did not observe areas of spalling or sloughing of the wall that would otherwise suggest deterioration of the mass concrete despite the wall’s age, ongoing seasonal river stage changes, and very occasional seismic activity (e.g. 2001 Nisqually earthquake). In our opinion, a purely frictional model for the mass concrete strength is overly conservative and unrealistic for the existing condition of the wall. However, a strength model that assumes both frictional and cohesive components to account for both cracking and apparent cohesion from interlocking and surface roughness along those cracks could be considered representative. For comparison purposes, we performed parallel, revised analyses (i.e. with scour depth of 9.1 ft) assuming the following for the mass concrete strength: 1.An unconfined compressive strength of 7,200 psf (i.e. modeled as an undrained shear strength of 3,600 psf; see previous addendum letters) 2.An internal frictional angle of 45 degrees and apparent cohesion of 1,800 psf (i.e. half the undrained shear strength assumed previously) A summary of the results for the two concrete strength models for static and seismic conditions are presented in the table below: Bulkhead Wall Stability – Response Letter Addendum No. 3 Cedar River Apartments ■ Renton, WA July 24, 2020 ■ Terracon Project No. 81175025 Responsive ■Resourceful ■Reliable 3 Cross Section Concrete Strength Model Static 1 Seismic 1,2 Post-Liquefaction 1, 3 A-A’1 1.66 1.3 1.0 2 1.09 0.8 0.8 B-B’1 2.98 2.0 --- 2 2.19 1.5 --- C-C’1 2.22 1.9 --- 2 1.46 1.1 --- 1.Factors of safety presented are global minimums and do not consider shallow, surficial failures that may occur in front of the wall. 2.Horizontal acceleration value of 0.17 was assumed (i.e. PGA of 0.17 for 100-year return period). 3.Liquefaction is not expected to trigger near Sections B-B’ or C-C’ Based on our analysis, the bulkhead wall in the current condition does not appear to be at risk of a global stability failure from scour assuming either strength assumption for the mass concrete though the factor of safety is low for concrete strength model 2 at section A-A’. For a seismic event, including post-liquefaction, the factor of safety is acceptable for concrete strength model 1 while model 2 suggests some movement of the bulkhead wall will occur at Section A-A’. As mentioned in the previous addendum letters, the maximum scour which occurs at Section C-C’ was applied to Section A-A’ and Section B-B’. The maximum scour depth is expected to be less at these two locations though an estimated value was not made at these two locations. Other assumptions inherent to the stability analysis were presented in previous addendum letters. We anticipate that any undesired wall movements from a seismic event can be addressed through maintenance. We recommend that a post-event reconnaissance be performed following any seismic events similar to that of the operational event assumed in the analyses presented herein. Maintenance via deconstruction and regrading of any failed areas should sufficiently address additional stability concerns associated with a damaged section of the wall. We appreciate the opportunity to be of service to you on this project. If you have any questions concerning this report or if we may be of further service, please contact us. Sincerely, Terracon Consultants, Inc. Zachary L. Koehn, P.E.Richard D. Luark, P.E. Project Engineer Principal Responsive ■Resourceful ■Reliable 4 FIGURES SITE EXPLORATION PLAN Cedar River Apartments ■ Renton, Washington July 24, 2020 ■ Terracon Project No. 81175025 Note to Preparer: This is a large table with outside borders. Just click inside the table above this text box, then paste your GIS Toolbox image. When paragraph markers are turned on you may notice a line of hidden text above and outside the table – please leave that alone. Limit editing to inside the table. The line at the bottom about the general location is a separate table line. You can edit it as desired, but try to keep to a single line of text to avoid reformatting the page. MAP 2 LANDSCAPE DIAGRAM IS FOR GENERAL LOCATION ONLY, AND IS NOT INTENDED FOR CONSTRUCTION PURPOSES MAP PROVIDED BY MICROSOFT BING MAPS 1.661.66 W 1.661.66 Material Name Color Unit Weight (lbs/Ō3)Strength Type Cohesion (psf) Phi (deg) Cohesion Type Water Surface FILL 120 Mohr-Coulomb 0 32 Water Surface Loose to Med. Dense Sand 125 Mohr-Coulomb 0 32 Water Surface Dense Sand 125 Mohr-Coulomb 0 38 Water Surface Dense Gravel 130 Mohr-Coulomb 0 42 Water Surface Blocks 135 Infinite strength None Concrete 145 Undrained 3600 Constant None 9.1806040 200-2040 60 80 100 120 140 160 180 200 1-1 Figure No. 81175025 Project No. Section A-A' - Static - Concrete Strength Model 1Cedar River Apartment Bulkhead Wall Stability SLIDEINTERPRET 6.038 1.31.3 W 1.31.3 Material Name Color Unit Weight (lbs/Ō3)Strength Type Cohesion (psf) Phi (deg) Cohesion Type Water Surface FILL 120 Mohr-Coulomb 0 32 Water Surface Loose to Med. Dense Sand 125 Mohr-Coulomb 0 32 Water Surface Dense Sand 125 Mohr-Coulomb 0 38 Water Surface Dense Gravel 130 Mohr-Coulomb 0 42 Water Surface Blocks 135 Infinite strength None Concrete 145 Undrained 3600 Constant None 9.1 0.1780 6040200-2040 60 80 100 120 140 160 180 200 1-2 Figure No. 81175025 Project No. Section A-A' - Seismic - Concrete Strength Model 1Cedar River Apartment Bulkhead Wall Stability SLIDEINTERPRET 6.038 2.982.98 W 2.982.98 Material Name Color Unit Weight (lbs/Ō3)Strength Type Cohesion (psf) Phi (deg) Cohesion Type Water Surface Dense Gravel 130 Mohr-Coulomb 0 42 Water Surface FILL 120 Mohr-Coulomb 0 32 Water Surface Dense Sand 125 Mohr-Coulomb 0 38 Water Surface Concrete 145 Undrained 3600 Constant None Block 135 Infinite strength None 9.1806040 200-20 0 20 40 60 80 100 120 2-1 Figure No. 81175025 Project No. Section B-B' - Static - Concrete Strength Model 1Cedar River Apartment Bulkhead Wall Stability SLIDEINTERPRET 6.038 2.02.0 W 2.02.0 Material Name Color Unit Weight (lbs/Ō3)Strength Type Cohesion (psf) Phi (deg) Cohesion Type Water Surface Dense Gravel 130 Mohr-Coulomb 0 42 Water Surface FILL 120 Mohr-Coulomb 0 32 Water Surface Dense Sand 125 Mohr-Coulomb 0 38 Water Surface Concrete 145 Undrained 3600 Constant None Block 135 Infinite strength None 9.1 0.17 1251007550250-40 -20 0 20 40 60 80 100 120 140 160 180 2-2 Figure No. 81175025 Project No. Section B-B' - Seismic - Concrete Strength Model 1Cedar River Apartment Bulkhead Wall Stability SLIDEINTERPRET 6.038 2.222.22 W 2.222.22 Material Name Color Unit Weight (lbs/Ō3)Strength Type Cohesion (psf) Phi (deg) Cohesion Type Water Surface Concrete 145 Undrained 3600 Constant None Fill 125 Mohr-Coulomb 0 32 Water Surface Dense Sand 125 Mohr-Coulomb 0 38 Water Surface Blocks 135 Infinite strength None Gravel 130 Mohr-Coulomb 0 42 Water Surface 9.1806040 200-20 0 20 40 60 80 100 120 140 3-1 Figure No. 81175025 Project No. Section C-C' - Static - Concrete Strength Model 1Cedar River Apartment Bulkhead Wall Stability SLIDEINTERPRET 6.038 1.91.9 W 1.91.9 Material Name Color Unit Weight (lbs/Ō3)Strength Type Cohesion (psf) Phi (deg) Cohesion Type Water Surface Concrete 145 Undrained 3600 Constant None Fill 125 Mohr-Coulomb 0 32 Water Surface Dense Sand 125 Mohr-Coulomb 0 38 Water Surface Blocks 135 Infinite strength None Gravel 130 Mohr-Coulomb 0 42 Water Surface 9.1 0.1780 6040200-20-20 0 20 40 60 80 100 120 140 3-2 Figure No. 81175025 Project No. Section C-C' - Seismic - Concrete Strength Model 1Cedar River Apartment Bulkhead Wall Stability SLIDEINTERPRET 6.038 1.091.09 W 1.091.09 Material Name Color Unit Weight (lbs/Ō3)Strength Type Cohesion (psf) Phi (deg) Cohesion Type Water Surface FILL 120 Mohr-Coulomb 0 32 Water Surface Loose to Med. Dense Sand 125 Mohr-Coulomb 0 32 Water Surface Dense Sand 125 Mohr-Coulomb 0 38 Water Surface Dense Gravel 130 Mohr-Coulomb 0 42 Water Surface Blocks 135 Infinite strength None Concrete 145 Mohr-Coulomb 1800 45 Water Surface 9.1806040 200-2040 60 80 100 120 140 160 180 200 4-1 Figure No. 81175025 Project No. Section A-A' - Static - Concrete Strength Model 2Cedar River Apartment Bulkhead Wall Stability SLIDEINTERPRET 6.038 0.80.8 W 0.80.8 Material Name Color Unit Weight (lbs/Ō3)Strength Type Cohesion (psf) Phi (deg) Cohesion Type Water Surface FILL 120 Mohr-Coulomb 0 32 Water Surface Loose to Med. Dense Sand 125 Mohr-Coulomb 0 32 Water Surface Dense Sand 125 Mohr-Coulomb 0 38 Water Surface Dense Gravel 130 Mohr-Coulomb 0 42 Water Surface Blocks 135 Infinite strength None Concrete 145 Mohr-Coulomb 1800 45 Water Surface 9.1 0.1780 6040200-2040 60 80 100 120 140 160 180 200 4-2 Figure No. 81175025 Project No. Section A-A' - Seismic - Concrete Strength Model 2Cedar River Apartment Bulkhead Wall Stability SLIDEINTERPRET 6.038 0.80.8 W 0.80.8 9.1 Material Name Color Unit Weight (lbs/Ō3)Strength Type Cohesion (psf) Phi (deg) VerƟcal Stress RaƟo Minimum Shear Strength (psf) Water Surface FILL 120 Mohr-Coulomb 0 32 Water Surface Loose to Med. Dense Sand 125 Mohr-Coulomb 0 32 Water Surface Dense Sand 125 Mohr-Coulomb 0 38 Water Surface Dense Gravel 130 Mohr-Coulomb 0 42 Water Surface Blocks 135 Infinite strength None Concrete 145 Mohr-Coulomb 1800 45 Water Surface Liquefied Sand 120 Strength=F(overburden)0.08 200 Water Surface120100806040200 40 60 80 100 120 140 160 180 200 220 4-3 Figure No. 81175025 Project No. Section A-A' - Liquified - Concrete Strength Model 2Cedar River Apartment Bulkhead Wall Stability SLIDEINTERPRET 6.038 2.192.19 W 2.192.19 Material Name Color Unit Weight (lbs/Ō3)Strength Type Cohesion (psf) Phi (deg) Cohesion Type Water Surface Dense Gravel 130 Mohr-Coulomb 0 42 Water Surface FILL 120 Mohr-Coulomb 0 32 Water Surface Dense Sand 125 Mohr-Coulomb 0 38 Water Surface Concrete 145 Mohr-Coulomb 1800 45 Water Surface Block 135 Infinite strength None 9.1806040 2000 20 40 60 80 100 120 140 5-1 Figure No. 81175025 Project No. Section B-B' - Static - Concrete Strength Model 2Cedar River Apartment Bulkhead Wall Stability SLIDEINTERPRET 6.038 1.51.5 W 1.51.5 Material Name Color Unit Weight (lbs/Ō3)Strength Type Cohesion (psf) Phi (deg) Cohesion Type Water Surface Dense Gravel 130 Mohr-Coulomb 0 42 Water Surface FILL 120 Mohr-Coulomb 0 32 Water Surface Dense Sand 125 Mohr-Coulomb 0 38 Water Surface Concrete 145 Mohr-Coulomb 1800 45 Water Surface Block 135 Infinite strength None 9.1 0.17 8060402000 20 40 60 80 100 120 140 5-2 Figure No. 81175025 Project No. Section B-B' - Seismic - Concrete Strength Model 2Cedar River Apartment Bulkhead Wall Stability SLIDEINTERPRET 6.038 1.461.46 W 1.461.46 Material Name Color Unit Weight (lbs/Ō3)Strength Type Cohesion (psf) Phi (deg) Cohesion Type Water Surface Concrete 145 Mohr-Coulomb 1800 45 Water Surface Fill 125 Mohr-Coulomb 0 32 Water Surface Dense Sand 125 Mohr-Coulomb 0 38 Water Surface Blocks 135 Infinite strength None Gravel 130 Mohr-Coulomb 0 42 Water Surface 9.1806040 200-20-20 0 20 40 60 80 100 120 140 6-1 Figure No. 81175025 Project No. Section C-C' - Static - Concrete Strength Model 2Cedar River Apartment Bulkhead Wall Stability SLIDEINTERPRET 6.038 1.11.1 W 1.11.1 Material Name Color Unit Weight (lbs/Ō3)Strength Type Cohesion (psf) Phi (deg) Cohesion Type Water Surface Concrete 145 Mohr-Coulomb 1800 45 Water Surface Fill 125 Mohr-Coulomb 0 32 Water Surface Dense Sand 125 Mohr-Coulomb 0 38 Water Surface Blocks 135 Infinite strength None Gravel 130 Mohr-Coulomb 0 42 Water Surface 9.1 0.1780 6040200-20-20 0 20 40 60 80 100 120 140 6-2 Figure No. 81175025 Project No. Section C-C' - Seismic - Concrete Strength Model 2Cedar River Apartment Bulkhead Wall Stability SLIDEINTERPRET 6.038