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REV1 REVIEWED BLD2023-0055+Geotech Report+5.26.2023_1.59.14_PM+3572401REVIEWED • _EVISION BY May 26 2023 CITY OF EDMONDS y BUILDING DEPARTMENTS CITY OF EOMONDS - DEVELOPMENT SERVICES : ..................................... DEPARTMENT ll. BLD20223-0055 Mud Bay Geotechnical Services, LLC 1001 Cooper Point Road SW, Suite 140, PMB #108 I Olympia, WA 985021 360.481.97841 CHeathman@MudBayGeotech.com May 25, 2023 Subject: Additional Site Development Geotechnical Report 23107 100th Ave W, Edmonds, WA 98020 Parcel#00391400000200 Dear Autumn Snow, Project No: 2289-SNO Page 1 Per your request, Mud Bay Geotechnical Services, LLC is providing a geotechnical report for the additional site development work proposed at the situs address of 23107 100th Ave W, located in Edmonds, Washington. The scope for this project was to perform a site reconnaissance and subsurface investigation of the parcel and prepare a geotechnical report providing recommendations for the proposed additional site development work. This report provides our assessment of the geologic hazards on -site and recommendations for foundation bearing capacity for the new building area. The analyses, conclusions, and recommendations in this report are based on the information available. These informational resources include: one (1) shallow hand augured boring completed specifically for the subject project, down hole dynamic cone penetrometer testing, published geologic information for the site, remote data analysis, and our experience with similar soil conditions. The site exploration is assumed to be representative of the subsurface conditions where the work will occur. If during construction, subsurface conditions differ from those described in this report, we should be advised immediately so we may reevaluate our recommendations. 2289-SNO: 23107 100t" Ave W, Edmonds WA 98020 Page 12 SITE LOCATION AND PROJECT DESCRIPTION Parcel #00391400000200, designated situs address of 23107 100th Ave W, consists of approximately 0.25 acres within the jurisdiction of the City of Edmonds. Presently, the property features a large single -story daycare center with associated paved parking areas. The site is accessed from the western property boundary via 100t" Ave W. The approximate site location and parcel boundaries are shown in Figure 1, Site Map. The topography of the site is predominantly flat with no significant differences in grade across the site. Based on conversations with the contractor and architect, it is our understanding that a 338 square foot addition has been proposed along the northern side of the current structure. This new addition will be located where a landscaped garden bed area currently exists. The City of Edmonds has requested a Soils Report and Foundations Recommendation Letter to verify that the soil bearing capacity assumed by the structural engineer of record would be adequate or better to support this new addition. SITE GEOLOGY AND SOILS As part of this project, available geologic data from the Washington Department of Natural Resources (DNR) available at the 1:100,000-scale was reviewed, and a site -specific geologic map was prepared. The project vicinity geologic map is attached as Figure 2, WA DNR Geologic Map. Figure 2 indicates that the parcel is underlain by Pleistocene continental glacial drift (Qga). The WA DNR characterizes the Qga deposits as follows: Glaciofluvial sand and gravel and lacustrine clay, silt, and sand deposited during the advance of glaciers; sandy units commonly thick, well sorted, and fine grained, with interlayered coarser sand, gravel, and cobbles; locally contains nonglacial sediments and deposits mapped as transitional between glacial and nonglacial. The conditions observed on -site are generally consistent with the mapped geology at the site. Along with the site geology, soil data available from the United States Department of Agriculture, Natural Resources Conservation Service was also reviewed. This information is presented in the attached Figure 3, USDA Soil Map. The soils mapped within the northern half of the project area consists of No. 78 — Urban Land. The soils mapped within the southern half of the project area consists of No. 17— Everett very gravelly sandy loam, 0 to 8 percent slopes. The USDA has classified No. 17 as follows: a very gravelly sandy loam to extremely cobbly coarse sand derived from sandy and gravelly glacial outwash deposited over kames, moraines and eskers. Conditions observed at the site are generally consistent with the mapped soils at the site. It should be noted 2289-SN0: 23107 100th Av- W, Edmonds \^'A 98020 Page 13 that the slope percentages and composition associated with the mapped soil units are estimates and do not necessarily reflect the true on -site topography or soil characteristics. SUBSURFACE EXPLORATION As part of the geotechnical investigation, one (1) shallow hand augured boring was completed specifically for the subject project. The boring was performed from the existing ground surface just off the northwestern side of the structure at the approximate location shown in Figure 4, Site Exploration Map. The boring was completed using a Humboldt Manufacturing model H-4414QC hand auger with a 4-inch diameter bucket tube sampler. in situ testing was performed at selected depths using a Humboldt Manufacturing model H-4202A dynamic cone penetrometer to estimate the density of the soil. The dynamic cone penetrometer uses a 15-lbs steel mass falling a height of 20-inches onto an anvil to penetrate a 1.5-inch diameter 45-degree cone tip seated into the bottom of the hole. Upon excavating to the test depth, to be sure the cone is completely embedded, the cone point is seated 2-inches into the undisturbed bottom of the hole using the dynamic cone penetrometer. The cone point is further driven 1 3/4 inches using the ring weight hammer falling 20-inches. These blows for each interval are counted and recorded until one of the following occurs: A total of 50 blows occurs in less than 1 3/4 inches of penetration in any increment. The rod is advanced the complete test increment for a total of 3 3/4 inches (2-inch embedment, and 1 3/4 inch completed test increment). This recorded blow count is correlated to the Standard Penetration Test (SPT) field N-value blow count determined in accordance with ASTM D1586, which is the standard in situ test method for determining relative density of cohesionless soils and the consistency of cohesive soils. Samples were removed from the bottom of the hole after the dynamic cone penetration testing was performed to observe the soil material at the approximate depth the test was performed. The soil samples were classified visually in the field in general accordance with ASTM D2488, The Standard Practice for Description and Identification of Soils (Visual -Manual Procedure). Once transported back to the office, the samples were re-examined, and the field classifications were modified accordingly. A summary log of the boring is included in Appendix A. Note the soil descriptions and interfaces shown on the log are interpretive, and actual changes may be gradual. Upon completion, the hole was backfilled to the original ground surface using excavated material from the spoils pile. 2289-SN0: 23107 100th Ave W, Edmonds WA 98020 Page 14 SUBSURFACE AND GROUNDWATER CONDITIONS The geotechnical exploration completed for the additional site development project was designated BH-1-23. The boring was completed on April 10, 2023by Mud Bay Geotechnical Services LLC, personnel BH-1-23 was performed off the northwestern portion of the structure within a cleared and grubbed landscaping area at the location of the proposed addition. This boring was completed to a final depth of 60 inches below the existing ground surface. The soil conditions in BH-1-23 consisted of: moist, dark brown topsoil from grade to a depth of 9 inches below ground surface (bgs); loose, moist, tan -grey silt with sand (ML) with root debris and mottling to a depth of 48 inches bgs; and a final unit of medium dense to very dense, moist, grey -brown, well -graded sand with silt and gravel (SW-SM) to a depth of 60 inches bgs. Groundwater was not encountered within the hand augured boring. GEOLOGIC HAZARD ASSESSMENT Liauefaction Hazard A map of liquefaction susceptibility developed by the Department of Natural Resources is attached as Figure 5, Liquefaction Hazard Map. Soil liquefaction is a phenomenon whereby saturated soil deposits temporarily lose strength and behave as a viscous fluid in response to cyclic loading. These behaviors are most common in saturated, loose, sandy soils with lesser impacts occurring in silty soils. Figure 5 indicates that the glacial drift deposits mapped across the parcel are at Very Low risk of soil liquefaction. Landslide Hazard As part of the investigation of the site, we reviewed landslide hazard mapping and LiDAR imagery, available from the Washington Department of Natural Resources. The current landslide hazard mapping inventory available from the Washington State Department of Natural Resources (WA- DNR) shows no mapped landslides directly on -site. The nearest mapped landslides are positioned over 1 mile to the west of the project site. These nearest mapped landslides are likely the result of coastal and fluvial erosion processes that serve to decrease slope stability due to undermining the toe of slope. The DNR landslide data is attached to this report as Figure 6, WA DNR Landslide Map. In addition to WA-DNR landslide hazard mapping, the geomorphology (shape of the land) was analyzed during the site evaluation and compared to the Light Detection and Ranging images 2289-SN'0: 23107 1001h A- W, Edmonds \^'A 98020 Page 15 (LiDAR) from the Washington State LiDAR portal. LiDAR is a remote sensing method where light is pulsed down to the surface of the Earth and back to a sensor. This methodology enables bare earth images of the surface to be analyzed for the presence of geologic landforms. The most recent available LiDAR images of this site are from 2021. LiDAR imagery can be found attached to this report as Figure 7, WA DNR LiDAR Map. Figure 7 indicates that the topography across the project area is flat to gently sloping. Based on the absence of known landslides in the region and the relatively flat topography of the project location, it is our interpretation that the project site is at Very Low risk of landslide and the proposed work will not increase the site or neighboring lot risk of slope instability. GEOTECHNICAL RECOMMENDATIONS Shallow Foundation Suaaort Shallow strip footings and shallow pier pads can be used to support the structure. We recommend a depth of embedment of 1 foot below the final ground surface for shallow foundations. Prior to placement of concrete, the subgrade should be cleared and grubbed, and the exposed native subgrade soils should be compacted in place. The subgrade should be inspected for any pockets of loose material. Loose material should be removed and replaced with a minimum of 6-inches of Crushed Surfacing Base Course (CSBC) meeting the requirements of Section 9-03.9(3) of the WSDOT Standard Specifications, or an equivalent material. The CSBC should be placed in layers no greater than 6-inches and compacted to at least 95 percent of the maximum dry density. Footings bearing on a subgrade prepared as described above can be designed using a maximum allowable bearing pressure of 2,000 psf. We recommend a minimum footing width of 16 inches be used in the design. The maximum allowable bearing pressure may be increased by up to one- third for short-term transient loading conditions such as wind and seismic loading. We anticipate that total settlement will not exceed one inch, and differential settlement along an equivalent 50- foot length of footing will not exceed half of the total settlement. The settlement is expected to be elastic and will occur as the footings are loaded. We recommend the footing subgrade preparation be evaluated by Mud Bay Geotechnical Services, LLC prior to placement of concrete. Foundation subgrade preparation should not be performed during periods of wet weather. We recommend staging the foundation subgrade excavation, compaction of native subgrade soils, and placement of CSBC to limit the time the foundation subgrade is exposed to weather. 2289-SNO: 23107 100t" Ave W, Edmonds WA 98020 Page 16 Seismic Design Based on the explorations performed and included in the geotechnical report for the project, we recommend using the adjustment factors for Site Class D soils to determine the site class adjusted seismic design accelerations at the site location. Material Backfill We recommend all material used as backfill for footings and stem walls be placed in horizontal layers no more than 6 inches thick with each layer compacted to 95 percent of the maximum density. The backfill material should be comprised of Gravel Backfill for Walls material meeting the requirements of Section 9-03.12(2) of the WSDOT Standard Specifications, or an equivalent free -draining material. Prior to backfilling, a perimeter footing drain system, consisting of a 4-inch diameter, perforated, or slotted, rigid plastic pipe placed at the base of the structure excavations wherever existing footings are exposed as part of the work. The drain should be embedded in a clean, free -draining sand and gravel meeting the requirements of Section 9-03.12(4) of the WSDOT Standard Specifications for Gravel Backfill for Drains. The drains should be sloped slightly to drain to an appropriate discharge area. Excavations Temporary cuts will be stable at a vertical angle up to 4 feet in height and may be used in the design where temporary excavations are less than or equal to 4 feet will be necessary to construct the project. We anticipate that temporary excavation cuts greater than 4 feet in height will be stable at a maximum slope angle of 1 H:1 V. The ground surface at the top of the temporary cuts should be periodically monitored for vertical movement, cracks, and other signs of instability. If signs of instability are observed, we should be contacted immediately so that we can assist and provide additional geotechnical recommendations. RECOMMENDED ADDITIONAL SERVICES Before construction begins, we recommend a copy of the draft plans and specifications prepared for the project be made available for review so that we can ensure that the geotechnical recommendations in this report are included in the Contract. Mud Bay Geotechnical Services, LLC is also available to provide geotechnical engineering and construction monitoring services throughout the remainder of the design and construction of the project. The integrity of the 2289-SNO: 23107 1001h Ave W, Edmonds WA 98020 Page 17 geotechnical elements of a project depends on proper site preparation and construction procedures. In addition, engineering decisions may need to be made in the field if conditions are encountered that differ from those described in this report. During the construction phase of the project, we recommend that Mud Bay Geotechnical Services, LLC be retained to review construction proposals and submittals, perform inspections of foundation subgrade, excavations, backfill placement and compaction, drainage installation, verify setbacks, and provide recommendations for any other geotechnical considerations that may arise during construction. INTENDED USE AND LIMITATIONS This report has been prepared to assist the client and their consultants in the engineering design and construction of the subject project. It should not be used, in part or in whole for other purposes without contacting Mud Bay Geotechnical Services, LLC for a review of the applicability of such reuse. This report should be made available to prospective contractors for their information only and not as a warranty of ground conditions. The conclusions and recommendations contained in this report are based on Mud Bay Geotechnical Services, LLC understanding of the project at the time that the report was written and on -site conditions that existed at time of the field exploration. If significant changes to the nature, configuration, or scope of the project occur during the design process, we should be consulted to determine the impact of such changes on the recommendations and conclusions presented in this report. Parcel boundaries reflected in this report and attached maps are interpreted from public Geographic Information Systems portals from your local jurisdiction, and do not reflect surveyed property boundaries. Digitized parcel boundaries reflected in this report are intended to assist in visualization and report comprehension and are not for legal interpretation. Site exploration and testing describes subsurface conditions only at the sites of subsurface exploration and at the intervals where samples are collected. These data are interpreted by Mud Bay Geotechnical Services, LLC rendering an opinion regarding the general subsurface conditions. Actual subsurface conditions can be discovered only during earthwork and construction operations. The distribution, continuity, thickness, and characteristics of identified (and unidentified) subsurface materials may vary considerably from that indicated by the subsurface data. While nothing can be done to prevent such variability, Mud Bay Geotechnical Services, LLC is prepared to work with the project team to reduce the impacts of variability on project design, construction, and performance. 2289-SNO: 23107 100t" Ave W, Edmonds WA 98020 Page 18 We appreciate the opportunity to serve your geotechnical needs on this project and look forward to working with you in the future. Please contact us at your earliest convenience if you have any questions or would like to discuss any of the contents of this report. Sincerely, Chris Heathman, P.E. Mud Bay Geotechnical Services, LLC 5/25/2023 r , r- Legend O Approximate Site Location Approximate Parcel Boundary "Parcel boundaries derived from Snohomish County Parcel Search • Mud Bay Geotechnical Services, LLC Job #:2289-SNO I Date:A ril 2023 Figure 1: Site Map 23107 100th Avenue W Edmonds, WA 98020 Geotechnical Report "Sources: I \ ' yam- — � S - Esri, Sources: Esri, USGS WA State Parks GIS, Esri, HERE, Garmin, FAO, NOAA, USGS, Bureau of Land Management, EPA, NPS I Washington Geological Survey, 2019, Surface geology, 1:24,000--GIS data, November 2019: Washington Geological Survey Digital Data Series DS-10, version 3.1. wagon— I rr I law �UPa pAi;�nomsR g Ir- r po Q47 _ —ice li L 100 m 1 zoo n Legend / Approximate Parcel Boundary Geologic Units 100k ❑ Pleistocene continental glacial till Pleistocene continental glacial ❑ drift Mud Bay Geotechnical Services, LLC Job #:2289-SNO I Date:April, 2023 Figure 2: WA DNR Geologic Map 23107 100th Avenue W Edmonds, WA 98020 Geotechnical Report Soil Map --Snohomish County Area, Washington dF6p Sde.11,ZA fpmmlmANrzGp{Ie r�rs rea o ,m mo am ®o f't�lia)a]m-w�wamror care mon.,ors vxsso t sm Natural Resources Web Soil Survey .01101 Conservation Service National Cooperative Soil Survey USDA Natural Resources Conservation Service Legend / Approximate Parcel Boundary Mud Bay Geotechnical Services, LLC Job #:2289-SNO Date:A ril, 2023 Figure 3: USDA Soil Map 23107 100th Avenue W Edmonds, WA 98020 Geotechnical Report Map Unit Symbol Map Unit Name 3 Alderwood gravelly sandy loam, 15 to 30 percent slopes 4 Alderwood-Everett gravelly sandy looms, 25 to 70 percent slopes 5 Alderwood-Urban land complex, 2 to 8 percent slopes 6 Alderwood-Urban land complex, 8 to 15 percent slopes 17 Everett very gravelly sandy loam, 0 to 8 percent slopes 78 Urban land Sources: Esri, USGS I WA State Parks GIs, Esri, HERE, Garmin, FAO, NOAA, USGS, Bureau of Land Management, EPA, NPS I Palmer, Stephen P.; Magsino, Sammantha L.; Bilderback, Eric L.; Poelstra, James L.; Folger, Derek S.; Niggemann, Rebecca A., 2007, Liquefaction susceptibility and site nATURAL class maps of Washington State, by county: Washington Division of Geology and Earth Resources Open File Report 2004-20, [78 plates, 45 p. text]. DIVISION OF GEOLOGY AND EARTN RESOURCES Legend Approximate Parcel Boundary o� iaom -Mm� Liquefaction Susceptibility Ej Q ❑ High S � ❑ Moderate to High ❑ Moderate mum ❑ Low to Moderate p fl �� 13 ❑ Low fl awt,amo© a�maa�� ❑ Very Low to Low B ❑ Very Low ❑ Bedrock g roam ❑ Peat fl Mud Bay GEotechnical Services, LLC Job #:2289-SNO I Date:A ril, 2023 a ' B �<� Figure 5: Liquefaction Hazard Map 202 m 3 soo ft e a 23107 100th Avenue W N P © Edmonds, WA 98020 8�� Geotechnical Report coo sources: Esri, USGS I WA State Parks GIS, Esri, HERE, Garmin, FAO, NOAA, USGS, Bureau of Land Management, EPA, NPS I Washington Geological Survey, 2022, Washington State Landslide Inventory Database-GIS data, July 2022: Washington Geological Survey Digital Data Series 29nRTURAL RESOURCES DIVISION OF GEOLOGY AND EARTH RESOURCES 01 r ''' i L=TnF3 W... %3 irJ7 !flQFT.7 _ fLII�E3Q e Legend p,. Approximate Site Location ,N Landslides ' © 'j ' ' ✓ g i El fl Tel Scarps ,I �cm\ 'k�m Elu Scarps and Flanks /MMe� � � f J a fl Landslide Deposits E3 �_M f High (30-40) r _ •p''� B _ Moderate (11-29) o flD3 �,; �'�c j. _ q ' , .�•+' Low (1-10) y rr c f r¢A1Qv 1 €j r Fans High (23-30) Moderate (8-22) Low (1-7) • ��`� ,, �,/�"` 'r'►^arr�`J � r )' Imt�llaf3ET�7. (� Mud Bay Geotechnical Services, LLC Job #:2289-SNO Date:A ril, 2023 _ AP i Figure 6: WA DNR Landslide Map -am I' 'z-- -----— 23107 100th Avenue W Edmonds, WA 98020 Geotechnical Report Sources: Esri, USGS I WA State Parks GIS, Esri, HERE, Garmin, FAO, NOAA, USGS, Bureau of Land Management, EPA, NPS I Washington Geological Si ' I loom N 200 ft Legend Approximate Parcel Boundary I� Mud Bay Geotechnical Services, PLC Job #:2289-SNO I Date:A ril, 2023 Figure 7: WA DNR LiDAR Map 23107 100th Avenue W Edmonds, WA 98020 Geotechnical Report APPENDIX A - FINAL BORING LOGS �Additional ® Project: Site Develo ment Client: Autumn Snow Bore No. 1 of 1: BH-1-23 Project Number: Contractor: Equipment: Mud Bay Geotechnlcal Services, LLC 2289-SNO n/a Humboldt H-4414QC Auger Address: Started: Bit Type: Diameter: 23107 100th Ave W, 4/10/23 Bucket Tube Auger 4 inches Edmonds, WA 98020 Completed: Hammer Type: Fluid: 0 4/10/23 Humboldt H-4202A n/a Logged By: Backfilled: Hammer Weight: Hammer Drop: Hannah Anderson 4/10/23 15 Ibs 20 inches Helper: Groundwater Depth: Elevation: Total Depth of Boring: Calie Rose n/a Existing Surface 60 inches GPS Method: GPS Coordinates: GPS Elevation: n/a n/a n/a Lithology .. T E O Q H _ M Soil Group Name: modifier, color, moisture, density/consistency, grain size, other descriptors. o U Z o U o 0 d p E M E m !n iRock Descr' top: modifier, color, hardness/degree of concentration, bedding andjoint u w y to characteristics, solutions, void conditions. Q 9" 01, `t 1 t 1 Moist, dark brown, topsoil. 24„ — ® S-1 g Loose, moist, tan -grey, silt with sand (ML). Root debris and mottling present in sample. Coarseness increasing with depth. 48" ® S-2 21 Medium dense, moist, grey -brown, well -graded sand with silt and gravel (SW-SM). 60" ® S-3 REF °� Very dense, moist, grey -brown, well -graded sand with silt and gravel (SW-SM). Refusal for 1 5/8" embedment. Large gravel obstruction. Test Pit and Boring Log Symbols Soil Density Modifiers 0 Standard Penetration Slit Spoon Sampler (SPT) Gravel, Sand, Non -Plastic Silt Elastic Silts and Clays California Sampler Blows/1 3/4" Density Blows/1 3/4" Consistency ® Shelby Tube 0-4 Very Loose 0-1 Very Soft ® CPP Sampler 5-10 Loose 2-4 Soft 1 Stabilized Ground water 11-24 Medium Dense 5-8 Medium Stiff 7 Groundwater At time of Drilling 25-50 Dense 9-15 Stiff ® Bulk/ Bag Sample REF Very Dense 16-30 Very Stiff 31-60 Hard >60 Very Hard