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Home My WebLink AboutExh.21_Wall_Stability_2Terracon 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 May 21, 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 Addendum No. 2 Cedar River Apartments 1915 SE Maple Valley Hwy Renton, Washington Terracon Project No. 81175025 Dear Mr. Loos: Terracon Consultants, Inc. (Terracon) has performed an additional Bulkhead Wall Stability evaluation that includes consideration of potential scour for the Cedar River Apartments project. The scour depth for our stability evaluation is based on the the scour analysis completed by The Watershed Company dated March 31, 2020. This letter serves as an 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 and presents updated results of geotechnical analyses for stability of the existing bulkhead wall. 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. The assumptions inherent to the stability analysis were previously presented in our Addendum letter No. 1 dated June 10, 2019. 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 EXHIBIT 21 RECEIVED 08/13/2020 MHerrera PLANNING DIVISION DocuSign Envelope ID: 99BA427F-FE67-49CC-B142-9BE0760232C1 Bulkhead Wall Stability Addendum No. 2 Cedar River Apartments ■ Renton, Washington May 21, 2020 ■ Terracon Project No. 81175025 Responsive ■Resourceful ■Reliable 1 UPDATED WALL STABILITY EVALUATIONS Inclusion of Scour Potential The existing wall conditions described in our Addendum No.1 were updated to include a potential scour depth of 8 ½ feet for sections A, B, and C (see Figure 1). In discussion with The Watershed Company, the shape of the scour at the wall is difficult to predict but an analysis assuming vertical erosion at the wall face with a slope away from the wall at or near the angle of repose is generally conservative. Therefore, Terracon assumed this geometry with an angle of repose of 32 degrees to account for potential scour. Both static and seismic scenarios were evaluated; however, only an operational-level seismic event with a return period of 100-years was considered. Consideration of larger seismic events such as the maximum credible earthquake (MCE; return period of 2475 years) in combination with an additional 8 ½ feet of scour is too conservative, in our opinion, particularly given the absence of occupied structures at the shoreline (i.e. the proposed apartment buildings are setback roughly 100 feet due to the shoreline buffer requirements). Global Stability Methodology Stability evaluations were performed using Rocscience SLIDE 6.0 software. The method of analysis for predicting potential slip surfaces used Spencer’s method (1967). Slip surfaces considered are those that initiate behind the wall and daylight through the river bed near the base of the scour depth. The outputs from the slope stability software are presented at the end of this memorandum. Geometries for the three cross sections previously presented in Addendum No. 1 were modified to include 8 ½ feet of potential scour. The scenarios previously performed were re-analyzed to include the scour potential. These scenarios are as follows: ■Static – assumes a nominal live-load of 250 psf uniformly distributed along the back of the wall and a high-water river event, which is assumed to be 5 feet above the height the stage observed at the time of drilling. ■Seismic – assumes a 100-year return period for the wall/shoreline system and an average river condition. For the horizontal acceleration, one-half of the peak ground acceleration is assumed. For all section, the soils are assumed to not liquefy for this scenario. DocuSign Envelope ID: 99BA427F-FE67-49CC-B142-9BE0760232C1 Bulkhead Wall Stability Report Cedar River Apartments ■ Renton, Washington May 21, 2020 ■ Terracon Project No. 81175025 Responsive ■Resourceful ■Reliable 2 ■Post-liquefaction – for section A-A’, the soils between 22 ½ and 37 ½ feet bgs are assumed to liquefy and mobilize a residual shear strength. Triggering of liquefaction is not expected to occur for sections B-B’ and C-C’. Global Stability Evaluation Results With a Scour Depth of 8 ½ Feet The results of the slope stability analyses for static, seismic, and post-liquefaction scenarios are presented in the table below: Cross Section Static 1 Seismic 1,2 Post-Liquefaction 1 A-A’1.82 1.3 1.0 B-B’3.15 2.0 ---3 C-C’2.24 1.8 ---3 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 at this location Typical minimum target factors of safety for static and seismic stability of existing shorelines are assumed to be 1.3 and 1.0, respectively. Although a factor of safety less than 1.0 is undesired, the result does not imply a collapse of the bulkhead wall but rather some movement should be expected. As observed in the table above, the factor of safeties are greater than 1.0 therefore only negligible wall movements are expected for a 100-year return period. In our opinion, the existing bulkhead wall is not at risk of a global stability failure as a result of the estimated scour potential. As stated previously in Addendum No. 1, larger, more rare-event earthquake will likely cause some wall movements. It is recommended that following an earthquake event in excess of a 100-yr return period, a post-earthquake reconnaissance be performed to assess any damage incurred by the wall. To support internal stability evaluations to be performed by a structural engineer, recommendations for lateral earth pressures and base sliding of the concrete blocks are providing in the following section. Building Setbacks Based on the analysis, the potential initiation of a wall failure would occur roughly 10 to 27 feet from the wall face. Given that the building setbacks for the riparian areas is a minimum of 100 feet, the proposed development does not appear to be at risk from potential failure of the bulkhead wall. DocuSign Envelope ID: 99BA427F-FE67-49CC-B142-9BE0760232C1 Bulkhead Wall Stability Report Cedar River Apartments ■ Renton, Washington May 21, 2020 ■ Terracon Project No. 81175025 Responsive ■Resourceful ■Reliable 3 GENERAL COMMENTS Our analysis and opinions are based upon our understanding of the project, the geotechnical conditions in the area, and the data obtained from our site exploration. Natural variations will occur between exploration point locations or due to the modifying effects of construction or weather. The nature and extent of such variations may not become evident until during or after construction. Terracon should be retained as the Geotechnical Engineer for further evaluations. Our Scope of Services does not include either specifically or by implication any environmental or biological (e.g., mold, fungi, bacteria) assessment of the site or identification or prevention of pollutants, hazardous materials or conditions. If the owner is concerned about the potential for such contamination or pollution, other studies should be undertaken. Our services and any correspondence or collaboration are intended for the sole benefit and exclusive use of our client for specific application to the project discussed and are accomplished in accordance with generally accepted geotechnical engineering practices with no third-party beneficiaries intended. Any third-party access to services or correspondence is solely for information purposes to support the services provided by Terracon to our client. Reliance upon the services and any work product is limited to our client and is not intended for third parties. Any use or reliance of the provided information by third parties is done solely at their own risk. No warranties, either express or implied, are intended or made. Site characteristics as provided are for design purposes and not to estimate construction costs. Any use of our report in that regard is done at the sole risk of the cost estimator as there may be variations on the site that are not apparent in the data that could significantly impact construction cost. Any parties charged with estimating construction costs should seek their own site characterization for specific purposes to obtain the specific level of detail necessary for costing. Site safety, and cost estimating including, excavation support, and dewatering requirements/design are the responsibility of others. If changes in the nature, design, or location of the project are planned, our conclusions and recommendations shall not be considered valid unless we review the changes and either verify or modify our conclusions in writing. DocuSign Envelope ID: 99BA427F-FE67-49CC-B142-9BE0760232C1 FIGURES DocuSign Envelope ID: 99BA427F-FE67-49CC-B142-9BE0760232C1 SITE EXPLORATION PLAN Cedar River Apartments ■ Renton, Washington May 21, 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 DocuSign Envelope ID: 99BA427F-FE67-49CC-B142-9BE0760232C1 1.821.82 W 1.821.82 8.5 Material Name Color Unit Weight (lbs/Ō3)Strength Type Cohesion (psf) Phi (deg) FILL 120 Mohr-Coulomb 0 32 Loose to Med. Dense Sand 125 Mohr-Coulomb 0 32 Dense Sand 125 Mohr-Coulomb 0 38 Dense Gravel 130 Mohr-Coulomb 0 42 Blocks 135 Infinite strength Concrete 145 Undrained 3600806040200 40 60 80 100 120 140 160 180 200 1 Figure No. 81175025 Project No. Section A-A' - Static Condition with ScourCedar River Apartment Bulkhead Wall Stability SLIDEINTERPRET 6.038 DocuSign Envelope ID: 99BA427F-FE67-49CC-B142-9BE0760232C1 3.163.16 W 3.163.16 8.5 Material Name Color Unit Weight (lbs/Ō3)Strength Type Cohesion (psf) Phi (deg) Dense Gravel 130 Mohr-Coulomb 0 42 FILL 120 Mohr-Coulomb 0 32 Dense Sand 125 Mohr-Coulomb 0 38 Concrete 145 Undrained 3600 Block 135 Infinite strength 271008060 402000 20 40 60 80 100 120 140 2 Figure No. 81175025 Project No. Section B-B' Static Condition with ScourCedar River Apartment Bulkhead Wall Stability SLIDEINTERPRET 6.038 DocuSign Envelope ID: 99BA427F-FE67-49CC-B142-9BE0760232C1 2.362.36 W 2.362.36 Material Name Color Unit Weight (lbs/Ō3)Strength Type Cohesion (psf) Phi (deg) Concrete 145 Undrained 3600 Fill 125 Mohr-Coulomb 0 32 Dense Sand 125 Mohr-Coulomb 0 38 Blocks 135 Infinite strength Gravel 130 Mohr-Coulomb 0 4212010080 604020-20 0 20 40 60 80 100 120 140 160 3 Figure No. 81175025 Project No. Section C-C' Static Condition with ScourCedar River Apartment Bulkhead Wall Stability SLIDEINTERPRET 6.038 DocuSign Envelope ID: 99BA427F-FE67-49CC-B142-9BE0760232C1