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REVIEWED PLN BLD2022-0877+GEO REPORT+7.7.2022_10.00.19_AM+2974968RECEIVED BLD2022-0877 Group Northwest, Inc. June 27th, 2022 John & Ruth Jewell c/o Scott Lloyd Norsk Design -Build Phone: (425) 881-3539 Email: scott.11oydknorskhome.com j ewellra916(a�live. com Subject: Geotechnical Engineering Investigation Proposed Addition & Remodel 946 Maple St Edmonds, Washington 98020 Dear Mr. and Ms. Jewell, Jul 07 2022 CITY OF EDMONDS DEVELOPMENT SERVICES DEPARTMENT Geolechnical Engineers, Geologists & Environmental Specialists G-5710 At your request, GEO Group Northwest, Inc., conducted a geotechnical engineering investigation of the above -subject property for the proposed addition and remodel in Edmonds, Washington. The scope of our services included review of the area geologic map; visual assessment of the existing surface expression at the site; characterization of the subsurface soil and groundwater conditions encountered; preparation of hand auger soil logs (attached); and preparation of this geotechnical report. SITE DESCRIPTION The project site is located in Edmonds, Washington, as illustrated in Plate 1— Site Location Map. The site is rectangularly shaped and consists of approximately 0.15 acres. The lot is currently developed with a two-story single-family residence, which is located in the central north region of the site. The site very gently slopes down northerly from the southern property line to the residence, where the ground surface is approximately level. North of the residence, the site continues to slope down at moderate inclinations to Maple Street. The site is bounded by 13705 Bel -Red Road • Bellevue, Washington 98005 Phone 425/649-8757 • Fax 425/649-8758 ATTACHMENT 6 June 27th, 2022 G-5710 John & Ruth Jewell 1946 — Maple St, Edmonds, Washington 98020 Page 2 residential developed properties to the west and east, by Maple Street to the north, and an alley to the south. PROPOSED DEVELOPMENT We understand that an addition to the first floor of the existing two-story single-family residence is proposed to be constructed. We understand that this addition is approximately 77 square feet and will replace the existing concrete slab patio with interior living space; the lower floor of the residence is to be converted into an accessory dwelling unit. For a figure illustrating the existing condition and the approximate proposed addition location, refer to Plate 2 — Site Plan. GEOLOGIC OVERVIEW Based on a review of the geologic map for the areal, the project site overlies Vashon Advance Outwash (Qva) deposits from the Fraser Glaciation time period. Advance outwash deposits were compacted by the overriding Puget glacial lobe after deposition and are typically dense to very dense. Advance outwash deposits typically are composed of sand with lesser amounts gravel and trace amounts of silt. SITE INVESTIGATION Surface Conditions On June 71h, 2022, Garrett Dean, Staff Engineering Geologist from our firm, visited the site to perform a visual reconnaissance of the site and investigate the subsurface soil conditions. We observed that the site configuration is generally as depicted on the public records, topographic mapping, and aerial photography available online. The site was observed to be landscaped with trees, shrubs, grass, and various other forms of vegetation. l Minard, J.P., 1983, Geologic map of the Edmonds East and part of the Edmonds West quadrangles, Washington, U.S. Geological Survey, Miscellaneous Field Studies Map MF-1541, 1:24,000. GEO Group Northwest, Inc. ATTACHMENT 6 June 27t`, 2022 G-5710 John & Ruth Jewell 1946 — Maple St, Edmonds, Washington 98020 Page 3 Subsurface Investigation In addition to our reconnaissance, two exploratory hand auger borings, HA-1 and HA-2, were drilled using a hand -operated auger as a means to investigate the subsurface conditions at the site. The boring locations are illustrated on Plate 3 — Site Plan. Soils encountered consisted of a thin layer of silty sand organic -rich topsoil to a depth of approximately 3-6 inches, underlain by weathered silty sand consisting of medium dense to dense sand with silt to silty sand containing trace amounts of subrounded gravel. Unweathered, very dense sand was encountered at approximately 2.5 feet below the ground surface (bgs) in the borings. A concrete fragment, likely from the initial construction of the residence, was encountered at a depth of approximately 1.5 feet bgs in boring HA-1. We interpret the encountered very dense sandy soils as native advance outwash soils at or near their approximate original grade. For additional information about the soils encountered, please refer to the boring logs attached as Appendix A to this report. GEOLOGIC CRITICAL AREAS REVIEW Based on criteria provided in Sections 23.80.020A and 23.80.020B of the Edmonds City Code and Community Development Code (ECDC), the project site meets the criteria for designation as an erosion geologic hazard area and the west adjacent project site meets the criteria for designation as a landslide hazard area. Our review and evaluation of these hazard areas are provided in the following sections. Landslide Hazard Area & Steep Slopes We understand that the west -adjacent lot to the project site contains areas which meet Edmonds City Code and Community Development Code (ECDC) criteria for designation as a landslide hazard area. In our opinion, the qualifying criteria as per ECDC 23.80.020.B.4., is the slope configuration at the site's northern margin adjacent to Maple Street. According to the topographic information available on the City of Edmonds GIS, the slope ranges in inclination between approximately 42 to 63 percent grade, and has a maximum vertical rise up to approximately 10 feet in some areas. The vast majority of the slope, however, is approximately 8 feet in vertical height or less. The local landslide hazard mapping as provided by the City of Edmonds GIS is illustrated on Plate 3 — Geologic Hazard Mapping. During our investigation, we encountered very dense sandy soil conditions at shallow depths in our exploratory borings on or near the slope. Groundwater was not observed in the borings. We GEO Group Northwest, Inc. ATTACHMENT 6 June 27t`, 2022 G-5710 John & Ruth Jewell 1946 — Maple St, Edmonds, Washington 98020 Page 4 observed the steep slopes at the northern region of the west -adjacent lot as well as the gentle to moderate slopes on the project site to be well vegetated with native trees, shrubs, and various other forms of groundcover. We find the existing vegetation and the dense to very dense soil conditions present at the site to be mitigating factors with regards to slope stability and potential landsliding of the slope areas near or on the site. We do not anticipate slope instability to arise from the proposed development processes contingent that the recommendations provided in this report, which are conformant to the criteria contained within ECDC 23.80.070, are followed. Erosion Hazard Area We understand that the project site contains areas which meet Edmonds City Code and Community Development Code (ECDC) criteria for designation as an erosion hazard area, based on the underlying soil type and topographic configuration. According to information obtained from the USDA Natural Resources Conservation Service (MRCS), surface soils on the property are mapped as Everett series soils. These soils typically consist of very gravelly sandy loam, and have a parent material of glacial outwash. Soils of this type, where present on slopes of 15 percent grade or steeper, have severe erosion potential according to NRCS. Therefore, the site meets the classification of erosion hazard areas, as described in Section 23.80.020.A of the ECDC. Earthwork for the proposed project is minimal. We do not anticipate an increase of soil erosion at the site from the proposed addition, in our opinion. No specific supplemental measures are required to mitigate the potential for landsliding associated with the proposed development. Mitigation of soil erosion hazard associated with the proposed development can be provided by revegetating areas where soils have been exposed, and by providing surface and subsurface drainage controls that prevent the concentrated flow of water on the ground surface. CONCLUSIONS AND RECOMMENDATIONS Foundations Soils that are anticipated to be acceptable for building support were encountered at a depth of approximately 2.5 feet bgs near HA-1 and HA-2. Based on these findings, it is our opinion that new foundations for the project can consist of conventional concrete strip and column footings GEO Group Northwest, Inc. ATTACHMENT 6 June 27a`, 2022 G-5710 John & Ruth Jewell 1946 — Maple St, Edmonds, Washington 98020 Page 5 that bear directly on dense native soils or on compacted structural fill that has been placed on a subgrade of dense native soils. Our recommended design criteria for conventional footing foundations supported on native soils or structural fill are provided below. - Allowable bearing pressure, including all dead and live loads: Undisturbed, medium dense or dense soil = 2,000 psf Structural fill placed on medium dense or dense soil = 2,000 psf - Minimum depth to base of perimeter footing below adjacent exterior grade 18 inches - Minimum depth to bottom of interior footings below top of floor slab 12 inches - Minimum width of wall footings = 16 inches - Minimum lateral dimension of column footings = 24 inches - Estimated post -construction settlement = %2 inch - Estimated post -construction differential settlement across building width = %2 inch A one-third increase in the above allowable bearing pressures can be used when considering short-term transitory wind or seismic loads. Lateral loads against the building foundations can be resisted by friction between the foundation and the supporting subgrade or by passive earth pressure acting on the buried portions of the foundations. For the latter case, the foundations must be poured "neat" against the existing undisturbed soil or be backfilled with compacted structural fill. Our recommended parameters are as follows: - Passive Pressure (Lateral Resistance) 350 pcf, equivalent fluid weight, for structural fill or competent undisturbed native soil - Coefficient of Friction (Friction Factor) 0.35 for structural fill or competent undisturbed native soil GEO Group Northwest, Inc. ATTACHMENT 6 June 27t`, 2022 G-5710 John & Ruth Jewell 1946 — Maple St, Edmonds, Washington 98020 Page 6 Concrete Slabs -on -Grade Slab -on -grade floors should be constructed on a firm, unyielding subgrade. During preparation of the slab subgrade, any areas of the subgrade that have been disturbed by construction activity should be either re -compacted or excavated and replaced with compacted structural fill. We recommend that structural fill placed below slab -on -grade floors conform to the earthwork and grading recommendations provided in this report. To avoid moisture build-up on the subgrade, the floor slab should be placed on a capillary break, which is in turn placed on the prepared subgrade. The capillary break should consist of a 6"-minimum thickness layer of crushed rock or gravel that contains no more than five percent material finer than a No. 4 sieve. A vapor barrier, such as a 10-mil plastic membrane, should be placed over the capillary break and taped or sealed to minimize water vapor transmission upward through the slab, if post -construction vapor transmission is undesirable. Drainage In our opinion, additional drainage measures to facilitate drainage behind below grade walls and footing drains, such as a 4-inch minimum diameter, rigid perforated drain pipe, are not required for the development since the native soils near the proposed footings were observed to contain a high fraction of sand and are interpreted as free -draining material. Grading and Earthwork Erosion Control Temporary erosion and sedimentation controls (TESCs), such as silt fences, should be installed down -gradient of the areas to be disturbed to prevent sediment -laden runoff from being discharged off site. Surface runoff should not be allowed to flow over the top of slopes into excavations. During wet weather, exposed soils should be covered with plastic sheeting or straw mulch. Stockpiled soils should be covered with plastic tarps. For permanent erosion control disturbed soils should be landscaped and mulched upon completion of the site work. Excavations and Slopes Temporary excavation slopes should not be greater than the limits specified in local, state and federal government safety regulations. We recommend that temporary cuts greater than 4 feet in GEO Group Northwest, Inc. ATTACHMENT 6 June 27a`, 2022 G-5710 John & Ruth Jewell 1946 — Maple St, Edmonds, Washington 98020 Page 7 height be sloped at inclinations up to 1H:1V (Horizontal: Vertical) in loose to medium dense soils. Temporary excavations into the very dense advance outwash soils can be sloped near vertical under the observation of the geotechnical engineer. Permanent cut and fill slopes should be inclined no steeper than 2H:IV. Fill slopes should consist of granular material compacted to a minimum of 90 percent of the material's maximum dry density. Based on the subsurface findings, groundwater seepage is not anticipated. If water seepage or other adverse conditions are encountered, excavation should be halted, and the geotechnical engineer should be contacted to review the site conditions. LIMITATIONS The findings and recommendations stated herein are based on field observations, our experience on similar projects and our professional judgment. The recommendations presented herein are our professional opinions derived in a manner consistent with the level of care and skill ordinarily exercised by other members of the profession currently practicing under similar conditions in this area and within the project schedule and budget constraints. No warranty is expressed or implied. In the event that site conditions are found to differ from those described in this report, we should be notified so that the relevant recommendations in this report can be reevaluated and modified if appropriate. GEO Group Northwest, Inc. ATTACHMENT 6 June 27a`, 2022 John & Ruth Jewell 1946 — Maple St, Edmonds, Washington 98020 CLOSING G-5710 Page 8 We appreciate the opportunity to provide you with geotechnical engineering services for this project. Please do not hesitate to contact us if you have any questions regarding this report. Sincerely, GEO GROUP NORTHWEST, INC. 3 201 �(S FGIsT��9�� S'ONAI- Eta Garrett Dean, G.I.T. Staff Engineering Geologist Attachments: William Chang, P.E. Principal Engineer Plate 1 — Site Location Map Plate 2 — Site Plan Plate 3 — Geologic Hazard Mapping Appendix A — USCS Soil Classification Legend & Hand Auger Boring Logs GEO Group Northwest, Inc. �Mi Imo. E v ere t k_ .11 ' L • +-F ti d� I f mIII Creek I � -� f f i lie �• i PROJECT SITE pr ...0-' 00 1 I' Edr i wooer l �• I * I ■ � I � I f . 1 � d r- ■ ` t I T f r r p i - MLckRied �� #� ■fit �� r rail. I' r { Terp ace I PIOP A k I•VAft IL I Bothell i SITE LOCATION MAP Group Northwest, Inc. PROPOSED ADDITION - Geotechnical Engineers, Geologists, & 946 MAPLE ST Environmental Scientists EDMONDS, WASHINGTON 98020 SCALE NONE DATE 6/27/2022 1 MADE GD CHKD WC I JOB NO. G-5710 I PLATE 1 - ,t!A . A--. M .} . i 3- , }. 4 PROPOSED ADDITION AREA HA-1 (APPROXIMATE) 1t � Hand Auger Number & Approximate Location Source: City of Edmonds GIS, 2022. +�m4l SITE PLAN Group Northwest, Inc. PROPOSED ADDITION - Geotechnical Engineers, Geologists, & 946 - MAPLE ST Environmental Scientists EDMONDS, WASHINGTON 98020 SCALE 1" = 10' I DATE 6/27/2022 1 MADE GD CHKD WC JOB NO. G-5710 PLATE 2 City of Edmonds Leger10E Seismic Hazard Areas Earth Suhsidence and Landslide) Cantaur Lines — 2 — 10: 5U 100 Larwdsllide Hazard Area 40% 9evete Erosion #ianard 153,40% MErosiari Hazard Areas 159�rd�% AM50E.GISSTREEf_CENTE RLI NE -ell all7ei Yalr,ee} 7 5; 4 g;7t_7_8 0 31.57 53.1 feet 564 47.i] Tina map i3 a user generated staCir• Output trOrn an Internet rnappmg xi[e and is fbY r4eiprerbw only Data layers that appear on this Map may or may riot be accurate, WGS_1984_Web_Mereator_Aµxi9jary_Sphere current, or otherwise renable. C. city of Edmonds tH15 MAP IS NG7 70 BE IYSLD rOR DE5tGN O1i CQNSTRtcTION ATTACHMENT 6 SOIL CLASSIFICATION & PENETRATION TEST DATA EXPLANATION MAJOR DIVISION CLEAN GRAVELS GRAVELS (little or no COARSE- (More Than Half fines) GRAINED SOILS Coarse Fraction is Larger Than No. 4 Sieve) DIRTY GRAVELS (with some fines) SANDS CLEAN SANDS (More Than Half More Than Half Coarse Fraction is (little or no by Weight Larger Smaller Than No. fines) Than No. 200 4 Sieve) Sieve DIRTY SANDS (with some fines) SILTS Liquid Limit (Below A -Line on < 50% Plasticity Chart, Liquid Limit FINE-GRAINED Negligible SOILS Organics) > 50% CLAYS Liquid Limit (Above A -Line on < 50% Plasticity Chart, Liquid Limit Negligible Organics) > 50% Less Than Half by Weight Larger Liquid Limit Than No. 200 ORGANIC SILTS < 50% Sieve (i.e., fines) g CLAYS (Below A -Line on Plasticity Chart) Liquid Limit > 50% HIGHLY ORGANIC SOILS UNIFIED SOIL CLASSIFICATION SYSTEM (USCS) GROUP TYPICAL DESCRIPTION LABORATORY CLASSIFICATION CRITERIA SYMBOL WELL GRADED GRAVELS, GRAVEL -SAND Cu = (D60 / D10) greater than 4 GW MIXTURE, LITTLE OR NO FINES CONTENT Cc = (D30)2 / (1310 " D60) between 1 and 3 OF FINES BELOW POORLY GRADED GRAVELS, AND GRAVEL -SAND 5% CLEAN GRAVELS NOT MEETING ABOVE GP MIXTURES LITTLE OR NO FINES REQUIREMENTS GM SILTY GRAVELS, GRAVEL -SAND -SILT MIXTURES GM: ATTERBERG LIMITS BELOW "A" LINE. CONTENT or P.I. LESS THAN 4 OF FINES EXCEEDS CLAYEY GRAVELS, GRAVEL -SAND -CLAY 12% GC: ATTERBERG LIMITS ABOVE "A" LINE. GC MIXTURES or P.I. MORE THAN 7 WELL GRADED SANDS, GRAVELLY SANDS, Cu = (D60 / D10) greater than 6 SW LITTLE OR NO FINES CONTENT Cc = (D30)2 / (1310 " D60) between 1 and 3 OF FINES BELOW POORLY GRADED SANDS, GRAVELLY SANDS, 5% CLEAN SANDS NOT MEETING ABOVE SP LITTLE OR NO FINES REQUIREMENTS SM SILTY SANDS, SAND -SILT MIXTURES ATTERBERG LIMITS BELOW "A" LINE with P.I. LESS THAN 4 CONTENT OF FINES EXCEEDS12% SC CLAYEY SANDS, SAND -CLAY MIXTURES ATTERBERG LIMITS ABOVE "A" LINE with P.I. MORE THAN 7 ML MH CL CH OL OH Pt SOIL PARTICLE SIZE U.S. STANDARD SIEVE FRACTION Passing Retained Sieve Sieve mm (mm) ( ) SILT / CLAY #200 0.075 SAND FINE #40 0.425 #200 0.075 MEDIUM #10 2.00 #40 0.425 COARSE #4 4.75 #10 2.00 GRAVEL FINE 0.75" 19 #4 4.75 COARSE 3" 76 0.75" 19 COBBLES 76 mm to 203 mm BOULDERS > 203 mm ROCK > 76 mm FRAGMENTS ROCK >0.76 cubic meter in volume INORGANIC SILTS, ROCK FLOUR, SANDY SILTS OF SLIGHT PLASTICITY 60 INORGANIC SILTS, MICACEOUS OR 50 - DIATOMACEOUS, FINE SANDY OR SILTY SOIL INORGANIC CLAYS OF LOW PLASTICITY, 40 GRAVELLY, SANDY, OR SILTY CLAYS, LEAN X w CLAYS z 30 INORGANIC CLAYS OF HIGH PLASTICITY, FAT >_ CLAYSU 20 ORGANIC SILTS AND ORGANIC SILTY CLAYS OF LOW PLASTICITY a 10 7 ORGANIC CLAYS OF HIGH PLASTICITY 4 0 sells slogan I WAW"15" IN11111 mm 5 0 10 20 30 40 50 60 70 80 90 100 PEAT AND OTHER HIGHLY ORGANIC SOILS LIQUID LIMIT (%) GENERAL GUIDANCE FOR ENGINEERING PROPERTIES OF SOILS, BASED ON STANDARD PENETRATION TEST (SPT) DATA SANDY SOILS SILTY & CLAYEY SOILS Unconfined Blow Counts Relative Friction Angle Blow Counts N Density, % AI, degrees Description N Strength Clu, Description tsf 0-4 0 -15 Very Loose 12 < 0.25 Very soft 4-10 15 - 35 26 - 30 Loose 2-4 0.25 - 0.50 Soft 10-30 35 - 65 28 - 35 Medium Dense 4-8 0.50 - 1.00 Medium Stiff 30-50 65 - 85 35 - 42 Dense 8 - 15 1.00 - 2.00 Stiff > 50 85 - 100 38 - 46 Very Dense 15 - 30 2.00 - 4.00 Very Stiff > 30 > 4.00 Hard Group Northwest, Inc. Geotechnical Engineers, Geologists, & Environmental Scientists 13705 Bel -Red Road Bellevue, WA 98005 Phone (425) 649-8757 E-mail: info@geogrourpnw.com PLATE Al ATTACHMENT 6 HAND -AUGER BORING: HA-1 LOGGED BY GD LOG DATE: 6/7/2022 GROUND ELEV. 288 feet +/- DEPTH ft. USCS SOIL DESCRIPTION SAMPLE No. Water % OTHER TESTS/ COMMENTS SM Silty SAND, brown, medium dense, damp; Probe 22" at 0' with minor subrounded gravel, small roots, plastic fabric at 3" 1 (topsoil/disturbed outwash) S1 -concrete fragment at 1.5' Probe 5" at 1.4' 2 3 - --- -------------------------------- SAND, gray, very dense, damp; S2 SP with minor silt and subrounded gravel Probe 0.5" at 2.8' (glacial outwash) 4 Total depth = 2.8 feet No groundwater encountered. 5 6 7 I HAND -AUGER BORING: HA-2 I LOGGED BY GD LOG DATE: 6/7/2022 GROUND ELEV. 285.5 feet +/- DEPTH ft. USCS SOIL DESCRIPTION SAMPLE No. Water % OTHER TESTS/ COMMENTS Silty SAND, brown, medium dense, damp; Probe 26" at 0' SM with minor subrounded gravel, small roots, plastic fabric at 3" S1 1 (topsoil/disturbed outwash) — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — Probe 9" at 1.5' 2 SAND, gray, dense to very dense, damp; SP with minor silt and subrounded gravel (glacial outwash) S2 Probe 0.5" at 2.5' 3 S3 Probe 0.5" at 3.5' 4 Total depth = 3.5 feet No groundwater encountered. 5 6 7 Group Northwest, Inc. Geotechnical Engineers, Geologists, & Environmental Scientists HAND AUGER BORING LOGS PROPOSED ADDITION 946 - MAPLE ST EDMONDS, WASHINGTON 98020 JOB NO. G-5710 I DATE 6/27/22 (APPEND. A2 ATTACHMENT 6