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REVIEWED 230007 - Geotech report addendum_RODRIGUEZ-As ect p A Geosyntec Company October 4, 2023 Raphael Rodriguez 18418 Olympic View Drive Edmonds, WA Re: Response to City of Edmonds Review Comments Project No. 230007 Dear Raphael: ,.,...,.,. REVIEWED BY CITY OF EDMONDS BUILDING DEPARTMENT? RECEIVED Oct 04 2023 G1 OE EDMONDS DEVE-MENTSERVICES DEPARTMENT BLD2023-1063 Aspect Consulting, LLC (Aspect) produced Geotechnical and Basis of Design reports for the slope stabilization (Project) at 18418 Olympic View Way in Edmonds, Washington (Site; Figure 1), also known as Snohomish County (County) parcel number 00565600200300 (Aspect, 2023a; 2023b). The reports were submitted to the City of Edmonds (City) as parts of a building permit application. The City responded with requests for comment that referred to specific sections of the Edmonds Community Development Code (ECDC). This addendum to our reports presents each of the City's comment requests followed by our response. Responses 1 through 7 address comments from the City's Planning Division received in a letter dated September, 19, 2023; response 8 addresses a comment from the City's Engineering Division received in a letter dated September 20, 2023; and responses 9 through 11 address comments from the City's Building Division received in a letter dated September 25, 2023. Response to City Planning Division Comments Comment 1 — Mitigation Sequencing (ECDC 23.40.090.D.5) Responsel Although the construction of the soldier pile wall in the critical area will temporarily increase erosion hazard at the site, that temporary increase will be reduced with diligent attention to temporary erosion and sedimentation control (TESC) measures. In the long term, construction of the wall will mitigate, not exacerbate, erosion and slope instability hazard for the steep bluffs at the site. Comment 2 — Statement of Accuracy and Limitations (ECDC 23.40.090. D.7) Response 2 The accuracy and limitations of our geotechnical report, building plans, and other products are addressed in Appendix B of the Geotechnical Report. Appendix B is also attached to this addendum. Raphael Rodriguez October 4, 2023 Project No. 230007 Comment 3 — Description of sewage disposal system(s) (ECDC 23.80.050.F.2.c) Response 3 The sewage conveyance at the Site consists of a side -sewer connection that runs approximately west to east between the house and the sewer main. The sewer main runs north -south on the east side of the house, as shown on Sheet C-4 of the plan set. The side sewer and main sewer lines will not be impacted by the construction of the wall. Comment 4 — Recommendations for drainage improvements (ECDC 23.80.050. F.3. b) Response 4 Some of the stormwater drainage infrastructure appears to have been damaged by the flood and landslide that occurred in December 2022, as described in the Geotechnical Report. Some other stormwater infrastructure may become disturbed or altered by construction of the wall. After construction of the wall, we recommend repairing and restoring all damaged or disturbed stormwater infrastructure and conveying stormwater to the bottom of the slope. All stormwater collected on the site from roof drains, yard drains, footing drains, catch basins, or other collection infrastructure should be conveyed in solid -wall PVC pipe (not flexible, corrugated polyethylene pipe) to an anchor catch basin near the top of the steep slope close to the south edge of the parcel. PVC drain lines should include cleanouts to allow periodic maintenance and inspection. General recommended guidelines for the components of the stormwater improvements follow. Anchor Catch Basin The anchor catch basin should be buried at least 2 feet below existing ground surface and should be positioned several feet back from the crest of the steep slope. From the anchor catch basin, fuse - welded HDPE pipe should extend to an outfall at the bottom of the slope. The HDPE should be secured to the anchor catch basin with a flange inside the outlet to the catch basin. Alternatively, a cast -in -place concrete anchor block, placed just downstream of the catch basin, could be used in combination with a fuse -welded flange. In either case, the attachment should be designed and installed such that it will transmit axial tensile loads on the HDPE pipe to the anchor catch basin. The catch basin or cast -in -place concrete anchor block should be sized to accommodate the axial tensile loads imparted on it by the HDPE pipe. The designer should confirm that the passive soil resistance and friction resistance on the anchor are greater than the anticipated static weight and dynamic forces on the outlet pipe. Outfall Pipe Starting at the catch basin, the outfall pipe should consist of a fuse -welded, thick-walled HDPE pipe. The HDPE outfall pipe should be mechanically secured to the catch basin or cast -in -place concrete anchor block as described above. The outfall pipe should extend in a trench from the catch basin to the top of the steep slope. Backfill for the trench between the catch basin and the top of the slope should consist of relatively impermeable material such as bentonite-amended soil or controlled density fill (CDF). Where the pipe daylights on the steep slope, an elbow fitting should be installed that approximates the declination of the steep slope. From the elbow down to the toe of the slope, the pipe should be placed on the surface of the steep slope, not in a trench. Aspect Consulting, LLC Page 2 Raphael Rodriguez October 4, 2023 Project No. 230007 All joints between lengths of HDPE pipe and fittings (including flanges, elbows, tees, etc.) should be heat -fused. Solvent welds and mechanical connections are not recommended. At the outfall, the diffuser tee should be "stapled" and secured to the ground using small diameter steel pipe sections driven into the slope with a sledgehammer. Diffuser Tee We recommend the outfall discharges to a perforated diffuser tee. The diffuser should consist of perforated pipe, oriented horizontally such that the discharge is evenly distributed onto the ground surface. The ground surface should be covered with quarry spalls to dissipate the energy of the outfall and reduce erosion. Post -Construction Maintenance Drainage pipes should be regularly inspected (at least once per year) and repaired immediately if deficiencies or damage are observed. Inspection of the inlets for clogs and backed -up water can be done visually, and proper flow and leaks can be checked with a garden hose at a cleanout, catch basin, or at the base of the gutter near the ground surface. Screened inlets and cleanouts should be maintained and used to keep the drain system free of debris and backups. Comment 5 — Estimated Bluff Retreat Rate or Landslide Area Expansion Risk (ECDC 23.80.050.F.2.e) Response 5 The proposed soldier pile wall will greatly reduce the bluff retreat rate and the risk of landslide area expansion. Bluff retreat and landslide area expansion will both be essentially stopped for the lifetime of the wall, which is expected to be several decades. Comment 6 — Consideration of Run -Out Hazard (ECDC 23.80.050.F.2.f) Response 6 The landslide that occurred in December 2022 flowed down the steep slope and to the railroad tracks at the bottom of the slope. The landslide was triggered by a flood and debris flow that emitted from damaged stormwater infrastructure uphill from the property. Because of the high water content in the debris flow, material from the top of the bluff was mobilized and entrained and then flowed all the way to the bottom of the slope. The damaged stormwater infrastructure has now reportedly been mitigated. Therefore, recurrence of a landslide with similar water content and run -out is extremely unlikely. In addition, the construction of the soldier pile wall will greatly reduce the rate of ravel and topple failures at the top of the bluff and therefore greatly reduce deposition of bluff material onto the steep slope below the bluff. The reduction of debris accumulation on the slope will further reducing potential run -out hazard. Comment 7 — Seasonal Restriction (ECDC 23.80.070.A.6) Response 7 Construction of the soldier pile wall will occur after October 1. To reduce hazards associated with construction in the wet season, temporary erosion and sedimentation control (TESC) measures and site best management practices (BMPs) will be applied and monitored with extra diligence. TESC measures are described on the plan set on Sheets G-1 and C-1. As conditions demand, additional Aspect Consulting, LLC Page 3 Raphael Rodriguez October 4, 2023 Project No. 230007 applicable TESC measures and BMPs from ECDC 18.30 and the Washington Department of Ecology Stormwater Management Manual for Western Washington (Ecology, 2019) will be applied at the work site. During construction, TESC and BMPs will be frequently monitored by Certified Erosion and Sedimentation Control Leads (CESCLs) and modified or corrected when appropriate. Response to City Engineering Division Comments Comment 8 — Please show on -Site storm system on the plans. Response 8 On -site storm system is shown on the plans, Rev 2, dated 9/29/2023, on Sheet C-1. The storm drain line location is based on verbal report of inspection by The Drain Doctors, 9/26/2023, and has not been field verified. Some of the stormwater drainage infrastructure appears to have been damaged by the flood and landslide that occurred in December 2022, as described in the Geotechnical Report. Some other stormwater infrastructure may become disturbed or altered by construction of the wall. After construction of the wall, we recommend repairing and restoring all damaged or disturbed stormwater infrastructure and conveying stormwater to the bottom of the slope. A summary of our recommendations for stormwater system restoration is above. Response to City Building Division Comments Comment 9 — Minimum buffer and setback recommendations (ECDC 23.80.050(C)6, 23.80.070(A)1, and 23.80.070(A)2) Response 9 The soldier pile wall will be constructed within the buffer or setback for the erosion and landslide hazard critical areas. During construction, diligent attention to TESC and BMPs, as described in the previous section of this addendum, will reduce erosion and landslide hazards. After construction, the wall will greatly reduce erosion and landslide hazard for the site, both within and outside buffers and setbacks. Comment 10 — Details about slope (ECDC 23.80.050(F)1) Response 10 A text box on Sheet C-1 of Rev 2 of Project Plans shows the height of slope and slope gradient. There are no springs, seeps, or other surface expressions of ground water on or within 200 feet of the project area or that have the potential to be affected by the construction. The location and description of surface water runoff features (stormwater infrastructure) are shown on Sheet C-1 of Rev 2 of Project plans. Aspect Consulting, LLC Page 4 Raphael Rodriguez October 4, 2023 Comment 11 — Consideration of Run -Out Hazard (ECDC 23.80.050. F.2.f) Response 11 See Response 6 above. Project No. 230007 Comment 12 — Slope stability analysis with static and seismic factors of safety (ECDC 23.80.050(F)2.d, 23.80.050(F)2.e, 23.80.060(A), 23.80.070(A)4.a) Response 12 Slope Stability Analysis Based upon the topography shown in LiDAR (DNR, 2023) and detailed surveying on the site (as shown on the plan set) as well as observations from our Site reconnaissance, we conducted two- dimensional limit equilibrium stability analyses using the Slide2 computer software program (Rocscience, 2022). We assessed slope stability under static and seismic loading conditions for a representative critical section in the location shown on Figure 1. Output from the computer analysis is shown in Appendix A. Soil Unit Engineering Properties As previously discussed in our Basis of Design Report (Aspect, 2023b), we designated the soil/material units and assigned the engineering parameters shown in Table 1 for our slope stability analyses. The strength values assumed for the geologic units are roughly correlated with the site exploration blow -count data and are based upon our experience with similar geologic units. Table 1. Engineering Soil Parameters Geologic Unit Unit Weight (pcf) Strength Parameters Friction Angle (deg) Cohesion (psf) Fill 115 28 0 Transitional Beds 125 32 200 Olympia Gravels 130 36 500 Notes: pcf = pounds per cubic foot; psf = pounds per square foot; deg = degrees These soil engineering parameters were back -calculated with a sensitivity analysis by modeling the pre -failure slope using the subsurface data collected at the Site, and two-dimensional limit equilibrium methods within Slide2. With these parameters and the pre -landslide slope conditions, the calculated factor of safety (FS) was approximately 1.0, which indicates a marginally stable slope conducive to failing. Surcharge Loading from Structures To represent structural loads exerted on the soils from the existing residence, we modeled a surcharge load equal to 250 pounds per square foot (psf) over the footprint. Aspect Consulting, LLC Page 5 Raphael Rodriguez October 4, 2023 Project No. 230007 Groundwater No groundwater was encountered in the subsurface explorations on the Site and there are no surface expressions of groundwater (springs or seeps) on the steep slope. Therefore, groundwater level was not included in the slope stability analysis. Seismic Slope Stability The Edmonds Community Development Code (ECDC) Sections 23.80.050(F)2.d and 23.80.050(F)2.e state that a hazard analysis should include an estimate of slope stability that considers catastrophic events such as seismic activity. We evaluated seismic slope stability using a pseudo -static analysis. This entails applying a constant horizontal acceleration in the downward (de -stabilizing) direction to the two-dimensional slope model. The constant horizontal acceleration used in this analysis, or pseudostatic coefficient, (kn) value, is taken as one half of the design earthquake with peak ground acceleration (PGA; i.e., kh= 0.278g) for an earthquake ground motion with a 2 percent probability of exceedance in 50 years, in accordance with the American Society of Civil Engineers (ASCE) 7-16, Minimum Design Loads an Associated Criteria for Buildings and Other Structures (ASCE, 2017). Slope Stability Analysis Results The Slide2 program performs slope stability computations based on the modeled slope conditions and calculates a factor of safety against slope failure, which is defined as the ratio of resisting forces to driving forces. A factor of safety of 1.0 indicates a "just -stable" condition, and a factor of safety less than 1.0 would indicate unstable conditions. The ECDC Section 23.80.070(A)4.a requires a minimum factor of safety of 1.5 for static conditions and 1.2 for seismic conditions. The calculated factors of safety shown in Table 2 below meet the minimum values required. Table 2. Summary of Slope Stability Analysis Results Existing Conditions Factor of Safety Static Factor of Safety Seismic Factor of Safety 1.15 1.98 1.23 We modelled the soldier piles with a shear strength of 104 kips. This value is equal to 0.4*Fy*A, where Fy is the yield strength of steel (assumed to be 50 kips per square inch) and A is the cross - sectional area of a W 14x61 section. Sections 23.80.050(C)6, 23.80.070(A)1, and 23.80.070(A)2 of the ECDC indicate that minimum setback recommendations for avoiding landslide hazard must be provided. For this Project, a setback recommendation is not applicable due to the nature of the design. The purpose of the designed soldier pile wall is to protect the existing residence and property immediately upland of the wall by stabilizing the slope. By design, the wall is located at the crest of the slope with zero setback to maximize the amount of property retained with the slope stabilization. Aspect Consulting, LLC Page 6 Raphael Rodriguez October 4, 2023 Project No. 230007 Comment 13 - Development standards — Specific hazards, ECDC 23.80.070(A)3. Response 13 The proposed soldier pile wall will not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions, will not decrease slope stability on adjacent properties, and will not adversely impact other critical areas. References American Society of Civil Engineers (ASCE), 2017, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, ASCE Standard 7-16. Aspect Consulting, LLC (Aspect), 2023a, Geotechnical Report — Rodriguez Residence — 18418 Olympic View Drive — Edmonds, WA, Prepared for: Raphael Rodriguez. Project No. 23007, August 28. Aspect Consulting, LLC (Aspect), 2023b, Geotechnical Engineering Basis of Design Report — Rodriguez Property Slope Stabilization, Edmonds, WA, Prepared for: Raphael Rodriguez. Project No. 23007, August 30, 2023. Rocscience, 2022, Slide2 9.024 Analysis Program, build date September 6, 2023. Washington State Department of Natural Resources (DNR), 2023, Washington LiDAR Portal, Division of Geology and Earth Resources, https://Iidarportal.dnr.wa.gov/, accessed September 26, 2023. Washington Department of Ecology, 2019, Stormwater Management Manual for Western Washington, Publication Number 19-10-021. Limitations Work for this project was performed for Raphael Rodriguez (Client), and this report was prepared consistent with recognized standards of professionals in the same locality and involving similar conditions, at the time the work was performed. No other warranty, expressed or implied, is made by Aspect Consulting, LLC (Aspect). Recommendations presented herein are based on our interpretation of site conditions, geotechnical engineering calculations, and judgment in accordance with our mutually agreed -upon scope of work. Our recommendations are unique and specific to the project, site, and Client. Application of this report for any purpose other than the project should be done only after consultation with Aspect. Variations may exist between the soil and groundwater conditions reported and those actually underlying the site. The nature and extent of such soil variations may change over time and may not be evident before construction begins. If any soil conditions are encountered at the site that are different from those described in this report, Aspect should be notified immediately to review the applicability of our recommendations. Aspect Consulting, LLC Page 7 Raphael Rodriguez October 4, 2023 Project No. 230007 Risks are inherent with any site involving slopes and no recommendations, geologic analysis, or engineering design can assure slope stability. Our observations, findings, and opinions are a means to identify and reduce the inherent risks to the Client. It is the Client's responsibility to see that all parties to this project, including the designer, contractor, subcontractors, and agents, are made aware of this report in its entirety. If project developments result in changes from the preliminary project information, Aspect should be contacted to determine if our recommendations contained in this report should be revised and/or expanded upon. The scope of work does not include services related to construction safety precautions. Site safety is typically the responsibility of the contractor, and our recommendations are not intended to direct the contractor's site safety methods, techniques, sequences, or procedures. The scope of our work also does not include the assessment of environmental characteristics, particularly those involving potentially hazardous substances in soil or groundwater. All reports prepared by Aspect for the Client apply only to the services described in the Agreement(s) with the Client. Any use or reuse by any party other than the Client is at the sole risk of that party, and without liability to Aspect. Aspect's original files/reports shall govern in the event of any dispute regarding the content of electronic documents furnished to others. Please refer to Appendix B titled "Report Limitations and Guidelines for Use" for additional information governing the use of this report. We appreciate the opportunity to perform these services. Sincerely, Aspect consulting, LLC $,n;ineering Geologist c2 \2931 �� sad Ge,01 10/4/2023 Matthew A. von der Ahe Matthew von der Ahe, LEG Project Geologist mvonderahe@aspectconsulting.com )/4/2023 Erik O. Andersen, PE Principal Geotechnical Engineer eandersen@aspectconsulting.com Aspect Consulting, LLC Page 8 Raphael Rodriguez October 4, 2023 Project No. 230007 Attachments: Figure 1 — Site and Exploration Map Appendix A — Slope Stability Analysis Appendix B — Report Limitations and Guidelines for Use V:\230007 Rodriguez Property Geotech\Deliverables\Addendum\230007 - Geotech report addendum_FINAL.docx Aspect Consulting, LLC Page 9 FIGURES r � 1 J AB-01 - t AB-02 c * t, , U c A a, , s t f sM T .yt v ...1, v 41v- Approximate Boring Location Cross Section Subject Property King County Property Parcel 0 50 100 Feet Site and Exploration Map N Geotechnical Engineering and Basis of Design Report Addendum Rodriguez Residence Edmonds, Washington BY: FIGURE NO. Aspect SEP-2023 MH/SCC PROJECT NO. REVISED BY: CONSULTING 230007 MAV/SCC Basemap Layer Credits I I EagleView Technologies, Inc. APPENDIX A Slope Stability Analysis Material Unit Weight Strength Cohesion Phi Water Hu Name Color (Ibs/ft3) Type (psf) (deg) Surface Type Hu Mohr- Water Fill ❑ 115 Coulomb 0 28 Surface Custom 1 Transitional Mohr- Water Beds ❑ 125 Coulomb 200 32 Surface Custom 1 Olympia Mohr- Water Gravel ❑ 130 Coulomb 500 36 I Surface Custom 1 0 Legend Search Grid Search Limits 1Z Modeled Groundwater Level IB-zl Boring Location and Depth SLIDEINTERPRET 9.026 House Ibs/ft2 W 20 40 60 80 Existing Conditions Before Soldier Pile Wall SCALE: 1:240 C:\Users\jmartz\Desktop\Rodriguez SSA 2023.09.28.slmd 100 120 140 160 180 Slope Stability Analysis Geotechnical Engineering and Basis of Design Report Addendum Rodriguez Soldier Pile Wall 18418 Olympic View Drive, Edmonds, WA BY 9/28/2023 APPENDIX: %Aspect ,MM PRe,Er Ne. RE1IEwED BY A-1 /CONSULTING 230007 Material Unit Weight (Ibs/ Strength Cohesion Phi Water Hu Name Color ft3) Type (psf) (deg) Surface Type Hu Water Fill 115 Coulomb 0 28 Surface Custom 1 Transitional Mohr- Water Beds ❑ 125 Coulomb 200 32 Surface Custom 1 Olympia Mohr- Water Gravel p 130 Coulomb 500 36 Surface Custom 1 Structural Mohr- Water Backfill ■ 140 Coulomb 0 I 40 I I Surface Custom 1 1 Legend nSearch Grid N Search Li-te 1Z Modeled Groundwater Level B-z Boring Location and Depth SLIDEINTERPRET 9.026 House 00 Ibs/ft2 a Support Force Out -Of -Plane Failure Pile Shear Strength Force Name Color Type Application Spacing (ft) Mode (Ibs) Orientation Pile/Micro Active Parallel to Soldier Pile ■ Pile (Method A) $ Shcar 104000 surface Soldier Pile Wall 40 60 80 Static with Soldier Pile Wall SCALE: 1:240 C:\Users\jmartz\Desktop\Rodriguez SSA 2023.09.28.slmd 100 120 140 160 18C Slope Stability Analysis Geotechnical Engineering and Basis of Design Report Addendum Rodriguez Soldier Pile Wall 18418 Olympic View Drive, Edmonds, WA %9/28/2023 BY APPENDIX: Aspect JMM 2 PROTECT NO. REVIEWED BY: A--/ /CONSULTING 230007 0 ► 0.278 0 House 1.23 250.00 Ibs/ft2 0 Soldier Pile Wall CD 0 .0 0 o 0 0 -20 0 20 40 60 80 100 120 140 160 180 Legend n Search Grid Slope Stability Analysis Seismic with Soldier 14 Search Limits Geotechnical Engineering and Basis of Design Report Addendum = Modeled Groundwater Level Pile Wall Rodriguez Soldier Pile Wall Sri Locatien and.epth 18418 Olympic View Drive, Edmonds, WA SCALE:1:240 9/28/2023 6V. APPENDIX: SLI DEINTERPRET 9.026 'CONSULTING spect A-3 C:\Users\ ma tz\Desktop\Rodriguez SSA 2023.09.2S.slmd aa8/20 230007 R EIVIM APPENDIX B Report Limitations and Guidelines for Use REPORT LIMITATIONS AND GUIDELINES FOR USE Geoscience is Not Exact The geoscience practices (geotechnical engineering, geology, and environmental science) are far less exact than other engineering and natural science disciplines. It is important to recognize this limitation in evaluating the content of the report. If you are unclear how these "Report Limitations and Guidelines for Use" apply to your project or property, you should contact Aspect Consulting, LLC (Aspect). This Report and Project -Specific Factors Aspect's services are designed to meet the specific needs of our clients. Aspect has performed the services in general accordance with our agreement (the Agreement) with the Client (defined under the Limitations section of this project's work product). This report has been prepared for the exclusive use of the Client. This report should not be applied for any purpose or project except the purpose described in the Agreement. Aspect considered many unique, project -specific factors when establishing the Scope of Work for this project and report. You should not rely on this report if it was: • Not prepared for you; • Not prepared for the specific purpose identified in the Agreement; • Not prepared for the specific subject property assessed; or • Completed before important changes occurred concerning the subject property, project, or governmental regulatory actions. If changes are made to the project or subject property after the date of this report, Aspect should be retained to assess the impact of the changes with respect to the conclusions contained in the report. Reliance Conditions for Third Parties This report was prepared for the exclusive use of the Client. No other party may rely on the product of our services unless we agree in advance to such reliance in writing. This is to provide our firm with reasonable protection against liability claims by third parties with whom there would otherwise be no contractual limitations. Within the limitations of scope, schedule, and budget, our services have been executed in accordance with our Agreement with the Client and recognized geoscience practices in the same locality and involving similar conditions at the time this report was prepared. Property Conditions Change Over Time 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 a change in property use or occupancy, or by natural events, such as floods, earthquakes, slope instability, or groundwater fluctuations. If any of the described events may have occurred following the issuance of the report, you should contact Aspect so that we may evaluate whether changed conditions affect the continued reliability or applicability of our conclusions and recommendations. ASPECT CONSULTING Geotechnical, Geologic, and Environmental Reports Are Not Interchangeable The equipment, techniques, and personnel used to perform a geotechnical or geologic study differ significantly from those used to perform an environmental study and vice versa. For that reason, a geotechnical engineering or geologic report does not usually address any environmental findings, conclusions, or recommendations (e.g., about the likelihood of encountering underground storage tanks or regulated contaminants). Similarly, environmental reports are not used to address geotechnical or geologic concerns regarding the subject property. We appreciate the opportunity to perform these services. If you have any questions, please contact the Aspect Project Manager for this project.