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REVIEWED RESUB1 BLD2023-0794+GEO REPORT 10.25.2023_12.08.10_PM+3860682OTTO ROSENAU & ASSOCIATES, INC. Geotechnical Engineering, Construction Inspection & Materials Testing 6747 M. L. King Way South, Seattle, Washington 98118-3216 USA Tel: (206) 725-4600 • Toll Free: (888) OTTO-4-US • Fax: (206) 723-2221 - WBE W21`5913684. WABO Registered Agency • Website: www.ottorosenau.com October 20, 2023 Janetta Lee 9309 23611 Street Southwest Edmonds, WA 98020 Re: Geotechnical Engineering Report Proposed Foundation Repair Lee Residence 9309 23611 Street Southwest Edmonds, Washington Snohomish County Parcel # 27033600112000 ORA Project Number: 23-0668 INTRODUCTION Otto Rosenau & Associates (ORA) evaluated the existing soil and groundwater conditions at the project site for the proposed foundation repairs to be located at the southwest portion of the residence. It is our understanding that Robbins and Company will be installing Grip-Tite piers to underpin the existing foundations along the west and southwest sides of the existing residence. Please see the attached Figure 1, Vicinity Map and Figure 2, Site Plan for an approximate location of the site and location of the foundation repairs. It is also our understanding that City of Edmonds Building Division sent a letter, dated July 10, 2023, for the plan review of the foundation repair plans requesting that a geotechnical report be submitted for the project. SITE CHARACTERIZATION AND PROJECT DESCRIPTION The project site consists of a rectangular -shaped parcel (11,326 square feet or 0.26 acres) with the existing residence located at the middle portion of the site. An attached garage (approximately 384 square feet in size) is located on the east side of the residence. It is our understanding that a one-story addition (approximately 448 square feet in size) was added in 2019 and is located on the northwest side of the residence. Concrete driveway is located south of the attached garage and on the southeast portion of the site. A concrete patio is located north of the attached garage. Otto Rosenau & Associates, Incorporated Geotechnical Engineering, Construction Inspection & Materials Testing Lee Residence Foundation Repair ORA Project No.: 23-0668 October 20, 2023 Page 2 of 6 The site grades gradually downwards from the southeast of the site (at an elevation of approximately of 447 to 448 feet) to the west side of the site with approximate elevations of 443 feet. See the attached Figure 2, Site Plan showing the site configuration and site features. We reviewed information on City of Edmonds online Geographical Information System (GIS), and determined that southwest corner of the residence is located within an erosion hazard area and tree code removal slope area. We reviewed the State of Washington Department of Natural Resource's online "Washington Interactive Geologic Map" at https://www.dnr.wa.gov/geology/. The soils at and near the site are mapped as "Pleistocene age, Vashon till deposits (Qgt) and "Pleistocene age, Vashon glacial drift or advance outwash deposits (Qga). The glacial till deposits (Qgt) generally consist of compact diamict of silts, sands, and gravels. These deposits were transported by the glaciers and deposited under the glacial ice. The advance outwash (Qga) deposits typically consist of dense sands and gravels with varying amounts of silt that were deposited by melt water streams that issued from the glaciers and were subsequently overridden by the glacier. The mapped soils at the site and in the vicinity of the site are shown on Figure 3, Geological Map. We also reviewed the Natural Resources Conservation Service's (NRCS) "SoilWeb" online mapping, which described the soils across the site as Alderwood-Urban land complex, 2 to 8 percent slopes (5). Please see the attached Figure 4, NRCS Web Soil Map, which shows the project site in referenced to the mapped soils. Alderwood-Urban land complex deposits generally consist of gravelly sandy loam that originated from basal till. These deposits are characterized as being "moderately well drained". CONCLUSIONS AND RECOMMENDATIONS It is our opinion that the settlement is most likely a result of concentrated foundation wall loads bearing on the underlying, poorly -consolidated, soils that appear to underlie the southwestern portion of the residence. The concentrated loads at the base of the foundation walls exceeded the bearing capacity of the underlying soils and resulted in a bearing capacity failure. The result of the bearing capacity failure was settlement. Temporary Excavations We anticipate that the maximum depth of excavations for this project will be less than 2'/z feet at the location of underpinning to be installed at the exterior of the residence. Excavations that are 2'/2 feet in depth may be made with vertical cut slopes. We recommend that the temporary cut slopes be flattened to an inclination no steeper than 1.5H:1 V (horizontal to vertical), if the vertical cut slopes are observed to be unstable. Otto Rosenau & Associates, Incorporated Geotechnical Engineering, Construction Inspection & Materials Testing Lee Residence Foundation Repair ORA Project No.: 23-0668 October 20, 2023 Page 3 of 6 All temporary cut slopes and excavations must comply with the provisions of Washington Administrative Code (WAC) Chapter 296-155, Part N, "Excavation, Trenching and Shoring." The contractor performing the work has the primary responsibility for protection of workers and adjacent improvements. Erosion Hazard Area Considerations It is our opinion that the proposed foundation repairs for the residence will not adversely affect the mapped erosion hazard area located at the southwest corner of the residence if the work is completed in accordance with our recommendations. The area located along the exterior of the west side of the residence consists of grassy area which will minimize any amount of sediment - laden water from migrating from the disturbed area and onto the adjacent property. Also, the onsite site soils are considered to be moderately well draining. Erosion and Sedimentation Control The migration of sediments from the site must be controlled in accordance with City of Edmonds requirements. We recommend that the following minimum erosion control measures be employed at the site during the foundation repair: • Provide silt fencing or compost socks around the construction area to reduce offsite transportation of sediments and to delineate the construction limits. No construction or soil disturbance should take place outside of the construction limits. • Any stockpiled soils of the excavated soils should be covered with carefully secured plastic sheeting. • Prevent any surface water from flowing from the disturbed area and onto the adjacent property. If you have any questions, please contact us. Sincerely, OTTO ROSENAU & ASSOC/A TES, INC. Scott Hoobler, P.E. 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Date: October 20, 2023 Figure: 1 o� I L6 ON F3 I I N 88'19'30" W 90,00, 22' 8' ; EX DECK ---- - EX I PATIO oo - I // 10 00 LC 44 I i 9309 236th ST SW I N RESIDENCE # 27033600112000 I a I N ENTRANCE --- 1 0 L I z 25' 15' 16' 19, —41 1' LAREA OF WORK WITHIN EXISTING FOOTPRINT I EX DRIVE N 8 ' 19' 30" W 90,00' Note: The location of all features shown is approximate. Reference: Robbins's Preliminary Site Plan SITE PLAN APPROXIMATE SCALE 1" = 20' vroject Name: Lee Kesiaence For: Janetta, Lee Foundation Repair - Edmonds Location: 9309 236th Street SW, OTTO ROSENAU & ORA Project No.: 23-0668 Edmonds, Washington ASSOCIATES, INC. Date: October 20, 2023 Figure: 2 PROJECT SITE 9309 236th ST. SW Qgt Qgt Qga WE . E.• LEGEND Qgt - Continental Glacial Till, Fraser -Age Qga - Advance Continental Glacial Outwash, Fraser -Age Qgo - Continental Glacial Outwash, Fraser -Age Note: The location of all features shown is approximate. Reference: Washington Interactive Geologic Map online mapping service by Washington State Department of Natural Resources GEOLOGIC MAP Project Name: Lee Residence Foundation Repair - Edmonds For: Janetta, Lee Location: 9309 236th Street SW, —� OTTO ROSENAU & ORA Project No.: 23-0668 Edmonds, Washington ASSOCIATES, INC. Date: October 20, 2023 Figure: 3 4A 4TYN Soil Map —Snohomish County Area, Washington 51e030 54= 5490 54M 518110 wix 5 150 5 im 5 im 5 10 3 Map 5¢1e: 1:891 If printed on A 6rKEaae (11" x 8.5') sheet 4 N Maas 0 10 1D 40 50 Few 0 40 80 180 140 mapprDladJDn: Web Mwtor Camerooadrates: WC,584 FdgetlM UrM Zane SON WG584 Note: The location of all features shown is approximate. Reference: USDA, NRCS Web Soil Survey (WSS) online soil mapping service Project Name: Lee Residence Foundation Repair - Edmonds Location: 9309 236th Street SW, Edmonds, Washington Date: October 20, 2023 NRCS SOIL MAP I OTTO ROSENAU & ASSOCIATES, INC. For: Janetta, Lee ORA Project No.: 23-0668 Figure: 4