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REVIEWED BLD2023-0640+Geotechnical_Report+5.23.2023_2.06.59_PM+3564435NELSON GEOTECHNICAL ASSOCIATES. INC. May 23, 2023 Michael Koh Beng Chye c/o Nazim Nice Motionspace Architecture and Design VIA Email: nazim@motion-space.com BLD2023-0640 RECEIVED Jul 12 2023 CITY OF EDMONDS DEVELOPMENT SERVICES DEPARTMENT Geotechnical Engineering Evaluation Proposed Koh Residence 15604 — 751h Place West Edmonds, Washington NGA File No. 1360822 Dear Mr. Koh: 17311-135th Ave. N.E. Suite A-500 Woodinville, WA 98072 (425) 486-1669 www.nelsongeotech.com ............................................. REVIEWED BY CITY OF EDMONDS '• BUILDING DEPARTMENT'. ...............................................: We are pleased to submit the attached report titled "Geotechnical Engineering Evaluation — Proposed Koh Residence —15604 - 75th Place West — Edmonds, Washington." This report summarizes the existing surface and subsurface conditions within the project site, and provides geotechnical recommendations for design and construction of the proposed site improvements. Our services were completed in general accordance with the proposal signed by you on April 20, 2022. The site is situated on gently to steeply sloping ground that descends to the right-of-way for the Burlington Northern Santa Fe railroad and the shore of Puget Sound in the Meadowdale part of Edmonds. The proposal includes the construction of a multi level, single-family residence structure within ther upper eastern portion of the site. The site is mapped as part of the large historic/prehistoric North Edmonds Earth Subsidence and Landslide Hazard Area and is located in the designated "Zone A" within the lower portion of the slide complex. This report provides information and discussion to fulfill the requirements of the City of Edmonds for construction within this area of Edmonds. It is important to recognize that the site and overall vicinity is part of this ancient landslide, and that development plans and future activity on this property should take that into consideration. Based on our site reconnaissance and explorations, and our understanding of the project plans, we have concluded that development on this site as proposed should be feasible, from a geotechnical standpoint. We did not observe indications of recent deep-seated slope instability on the property during our visit. The recommendations presented in this report should aid in maintaining and/or improving the current stability of the site. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1360822 Proposed Koh Residence May 23, 2023 Edmonds, Washington Summary - Page 2 Because of the significant slide debris encountered in our explorations, we recommend supporting the entire planned residence on a deep foundation system consisting of drilled augercast piles in order to advance the structure loads through the loose/soft upper soils down to the more competent native deposits at depth. The augercast piles are intended to provide support for the residence and also provide an effective setback from the steep slopes within the lower portion of the property. Specific recommendations for foundation design and installation are provided in the attached report. The provided plans indicate the structure foundations will generally conform to existing topography to minimize cuts and fills. However, temporary or permanent shoring walls may be needed to support cut excavations for portions of the proposed development such as the proposed driveway. As such, recommendations for shoring measures are discussed herein. We recommend that all runoff generated within this site, including roof downspouts, driveways, yard and footing drains, and all runoff entering the property from the road, be collected in a tightline and routed to the bottom of the slope. If this alternative is not feasible, the collected water should be directed into the existing sewer system in the roadway. No water should be infiltrated or dispersed on or near the site slopes. Such activity may cause instability of the steep slopes. In the attached report, we have also provided general recommendations for site grading, structural slabs, structural fill placement, retaining walls, erosion control, and drainage. These recommendations are preliminary in nature. We should be retained to review and comment on final development plans and observe the earthwork phase of construction. We also recommend that NGA be retained to provide monitoring and consultation services during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during the work differ from those anticipated, and to evaluate whether or not earthwork and foundation installation activities comply with contract plans and specifications. We appreciate the opportunity to provide service to you on this project. Please contact us if you have any questions regarding this report or require further information. Sincerely, NELSON GEOTECHNICAL ASSOCIATES, INC. Khaled M. Shawish, PE Principal NELSON GEOTECHNICAL ASSOCIATES, INC. TABLE OF CONTENTS INTRODUCTION..........................................................................................................1 SCOPE........................................................................................................................ 2 SITECONDITIONS....................................................................................................... 3 SurfaceConditions....................................................................................................... 3 Subsurface Conditions.................................................................................................. 3 Hydrogeologic Conditions............................................................................................ 4 SENSITIVE AREA EVALUATION.................................................................................... 5 SeismicHazard............................................................................................................. 5 ErosionHazard............................................................................................................. 6 Landslide Hazard/Slope Stability.................................................................................. 6 Slope Stability Analysis............................................................................................... 11 CONCLUSIONS AND RECOMMENDATIONS................................................................ 12 General....................................................................................................................... 12 Erosion Control and Slope Protection Measures....................................................... 14 StructureSetbacks..................................................................................................... 15 Site Preparation and Grading..................................................................................... 15 Temporary and Permanent Slopes............................................................................. 16 Soldier Pile Shoring Wall............................................................................................ 17 DeepFoundations...................................................................................................... 19 StructuralFill.............................................................................................................. 20 RetainingWalls........................................................................................................... 21 StructuralSlab............................................................................................................ 22 PavementSubgrade................................................................................................... 22 SiteDrainage.............................................................................................................. 22 USE OF THIS REPORT................................................................................................ 24 LIST OF FIGURES Figure 1— Vicinity Map Figure 2 — Site Plan Figure 3 — Cross -Section A -A' Figure 4 — Soil Classification Chart Figures 5 and 6 — Boring Logs Figures 7 and 8 — Slope Stability Analysis NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Proposed Koh Residence 15604 — 75t" Place West Edmonds, Washington INTRODUCTION The property is roughly square in shape in shape and covers 0.6 acres in area. The parcel number for the property is 00513100002410. It is currently unoccupied and undeveloped. The property is bordered by 75th Place West to the east, by a residence to the south, by an undeveloped parcel to the north, and by a railroad right-of-way that separates the property from Puget Sound to the west. Topographically, roughly the northeastern half of the site slopes gently to moderately down to the west with a steep west -facing slope descending toward the railroad and then Puget Sound. We understand the proposed development will include the construction of a new single-family residence within the property. Critical areas including landslide hazard areas and erosion hazard areas have been mapped within the site. The site is also located within a designated Earth Subsidence and Landslide Hazard Area (ESLHA). We have been requested to provide this report for determination of geological hazards affecting the site, as well as development considerations for the proposed residence development. For use in preparing this report we were provided with the following documents: • Topographic Survey prepared by GroundMark Land Surveying, PLLC • Architectural plans, Sheets A1.1 — A10.1, dated May 15, 2023, and prepared by Motionspace Architecrure and Design, PLLC • Civil plan set, sheets C1.0 — C3.0, dated May 1, 2023, and prepared by BRH, Inc • Structural plan set, sheets S0.0 — S0.3, dated May 18, 2023, and prepared by TSE Eningeering This plan shows two-story residence in the northeast portion of the site. A garage will be at the northeast corner of the first floor, and we understand a driveway along the east side of the residence will access the garage. We anticipate that the west portion of the residence will be elevated above the ground surface. We also received a packet of several geotechnical documents concerning previously planned development within the property. The site is situated on gently to steeply sloping ground that descends from 75th Place West to the right- of-way of the Burlington Northern Santa Fe (BNSF) railroad and the shore of Puget Sound. We antipitate that the development will consist of constructing a multi -story, single-family residence within the upper, eastern portion of the property. Stormwater plans may include tightlining runoff down to the bottom of the slope. The current site layout is shown on the Site Plan in Figure 2. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Proposed Koh Residence Edmonds, Washington NGA File No. 1360822 May 23, 2023 Page 2 The site is mapped as part of the large historic/prehistoric North Edmonds Earth Subsidence and Landslide Hazard Area. Slide movement from both the large-scale slide complex and smaller localized slides within the complex can affect this property. The proposed residence design will have to take this into consideration. SCOPE The purpose of this study is to explore and characterize the site surface and subsurface conditions, and provide opinions and recommendations for the proposed site development. Specifically, our scope of services includes the following: 1. Review available soil and geologic maps of the area as well as other relevant geotechnical information, as provided. 2. Explore the subsurface soil and groundwater conditions within the proposed development areas with two geotechnical boreholes using a limited -access drill rig. Drilling services were subcontracted by NGA. 3. Install two inclinometer casings in the drilled holes. 4. Map the conditions on the site slopes using shallow, hand -tool explorations where necessary to construct geological cross sections and qualitatively evaluate slope stability. 5. Perform laboratory grain -size sieve analysis on soil samples, as necessary. 6. Provide recommendations for structure setbacks from geologic hazards, as necessary. 7. Provide recommendations for earthwork and foundation support. 8. Provide recommendations for retaining walls, as warranted. 9. Provide recommendations for temporary and permanent slopes. 10. Provide recommendations for subsurface utilities and pavement subgrade preparation. 11. Provide our opinion on stormwater infiltration feasibility. 12. Provide general recommendations for site drainage and erosion control. 13. Document the results of our findings, conclusions, and recommendations in a written geotechnical report. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Proposed Koh Residence Edmonds, Washington SITE CONDITIONS Surface Conditions NGA File No. 1360822 May 23, 2023 Page 3 The site is situated on a west -facing slope overlooking Puget Sound. The property is currently undeveloped, but we understand that a house was formerly located within the property. A driveway has been cut into the site's southeast corner to provide access from 75t" Place West. From 75t" Place West, the ground declines into the site up to about 25 vertical feet at an inclination of 35 degrees (70 percent), with the height of this slope taller to the north. Below this slope, the ground surface slopes gently down to the west at a gradients of approximately 10 degrees (18 percent). From this gently sloping area that is roughly 70 feet wide, the site steepens and declines to the existing railroad tracks and Puget Sound at gradients in the range of approximately 20 to 37 degrees (36 to 75 percent). The existing site conditions, site topography, and interpreted subsurface conditions are presented on Cross -Section A -A' in Figure 3. The site is thickly vegetated with blackberry vines, ivy, brush, and scattered deciduous trees. The site is bordered to the north and south by residential properties, to the east by 75th Place West and to the west by the BNSF railroad tracks and Puget Sound. The existing site conditions are shown on the Site Plan in Figure 2. Subsurface Conditions Geology: The geologic units for the site vicinity are shown on the Preliminary Geologic Map of the Edmonds East and Edmonds West Quadrangles, Snohomish and King Counties, Washington, by Mackey Smith (USGS,1975). The site is mapped as old landslides (Qols) with Whidbey Formation deposits mapped nearby. The old landslide deposits are described as large slumps that occurred during the ablation of the Puget Lobe of the Vashon ice sheet by lowering of water -table level. The Whidbey Formation is described as nonglacial river flood plain deposits consisting of bedded layers of clay, silt, and sand with a few lenses of small pebbles. Our subsurface explorations generally encountered landslide deposits underlain by silt and fine sand that we interpreted as native Whidbey Formation soils. Explorations: The subsurface conditions within the site were explored on May 18, 2022 by drilling two borings using a limited -access drill rig to depths of approximately 41.0 and 41.5 feet below the existing ground surface. The approximate locations of our explorations are shown on the Site Plan in Figure 2. A Standard Penetration Test (SPT) was performed on each of the samples during drilling to evaluate relative soil density at depth. The SPT consists of driving a 2-inch outer -diameter, split -spoon sampler 18 inches using a 140-pound hammer with a drop of 30 inches. The number of blows required to drive the sampler the final 12 inches is referred to as the "N" value and is presented on the boring logs. The N value is used to evaluate the strength and density of the deposit. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1360822 Proposed Koh Residence May 23, 2023 Edmonds, Washington Page 4 A geologist from NGA was present during the explorations, examined the soils and geologic conditions encountered, obtained samples of the different soil types, and maintained logs of the borings and hand augers. The soils were visually classified in general accordance with the Unified Soil Classification System, presented in Figure 4. The logs of our boring explorations are attached to this report and are presented as Figures 5 and 6. We present a brief summary of the subsurface conditions in the following paragraphs. For a detailed description of the subsurface conditions, the boring logs should be reviewed. Boring B-1 encountered silt with fine sand that was medium stiff at 2.5 feet, became medium stiff to stiff at 5 feet, stiff at 10 feet, and very stiff at 25 feet. We interpret these materials as landslide deposits, including blocks of intact materials. The boring continued into hard clayey silt at 30 feet, interpreted as native soils underlying the landslide deposits. Very dense sand was revealed at 36 feet, and continued to the base of the boring 41 feet below the surface. In Boring B-2, located in the upper part of the site, we observed loose sand with gravel to a depth of about 9.5 feet and then loose silty sand to sandy silt to about 13 feet. The upper loose soils were underlain by silt to sandy silt that was stiff to very stiff from 15 to 30 feet, and became hard at 35 feet. The hard silt continued to the explored depth of 41.5 feet. Inclinometer casing was installed in the lower boring, B-1, to a depth of 38 feet. The borehole was filled with cementitious grout after the inclinometer casing was installed. Hydrogeologic Conditions Groundwater seepage was not encountered in our explorations. However, some moist to wet zones were encountered, interpreted to result from groundwater collecting within sandier/less dense lenses. It is our opinion that there is also potential for a shallow perched water condition to develop. Perched water occurs when surface water infiltrates through less dense, more permeable soils and accumulates on top of relatively low permeability materials. The more permeable soils on this site would consist of the looser, surficial soils. The low permeability soil consists of the underlying silt. Perched water does not represent a regional groundwater "table" within the upper soil horizons. Perched water tends to vary spatially and is dependent upon the amount of rainfall. We would expect the amount of perched groundwater to decrease during drier times of the year and increase during wetter periods. However, there maybe areas of seepage and wet soils on the slopes even in the drier times of the year. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1360822 Proposed Koh Residence May 23, 2023 Edmonds, Washington Page 5 SENSITIVE AREA EVALUATION Seismic Hazard Older landslide debris and medium stiff to hard cohesive soils were encountered underlying the site at depth within the property. Based on the 2018 International Building Code (IBC), the site conditions best fit the description for Site Class D. Table 1 below provides seismic design parameters for the site that are in conformance with the 2018 IBC, which specifies a design earthquake having a 2% probability of occurrence in 50 years (return interval of 2,475 years), and the 2008 USGS seismic hazard maps. Table 1: 2018 IBC Seismic Design Parameters Site Class Spectral Acceleration Spectral Acceleration Site Coefficients Design Spectral at 0.2 sec. (g) at 1.0 sec. (g) Response Ss S1 Parameters Fa Fv Sys Soi D 1.33 0.473 1 null 0.887 null Hazards associated with seismic activity include liquefaction potential and amplification of ground motion by soft or loose geologic deposits. Liquefaction is caused by a rise in pore pressures in a loose, fine sand deposit beneath the groundwater table. It is our opinion that the medium dense/stiff or denser soils interpreted to underlie the site have a low potential for liquefaction or amplification of ground motion; however these materials could experience instability as a result of significant seismic activity. The competent cohesive soils interpreted to form the core of the site slopes at depth are considered stable with respect to deep-seated slope failures. However, the overlying loose surficial materials and landslide deposits have the potential for shallow sloughing failures during seismic events. Such events should not affect the planned residence provided our recommendations for foundation support are incorporated in the project designs. Hazards associated with seismic activity include liquefaction potential and amplification of ground motion by soft or loose geologic deposits. Liquefaction is caused by a rise in pore pressures in a loose, fine sand deposit beneath the groundwater table. It is our opinion that the medium dense/stiff or better native soils interpreted to underlie the site have a low potential for liquefaction or amplification of ground motion; however these materials could experience instability as a result of significant seismic activity. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1360822 Proposed Koh Residence May 23, 2023 Edmonds, Washington Page 6 Erosion Hazard The criteria used for determining the erosion hazard for the site soils includes soil type, slope gradient, vegetation cover, and groundwater conditions. The erosion sensitivity is related to the vegetative cover and the specific surface soil types, which are related to the underlying geologic soil units. The Soil Survey of Snohomish County Area, Washington, by the Soil Conservation Service (SCS), was reviewed to determine the erosion hazard of the on -site soils. The site surface soils were classified using the SCS classification system as Alderwood gravelly sandy loam, 15 to 30 percent slopes, and Alderwood-Everett gravelly sandy loams, 25 to 70 percent slopes. These soils are listed as having a severe erosion hazard. These soils should have a low to moderate hazard for erosion in areas that are not disturbed and where the vegetation cover is not removed. Landslide Hazard/Slope Stability The criteria used forthe evaluation of landslide hazards include soil type, slope gradient, and groundwater conditions. The proposed development area within the east -central portion of the site slopes gently down to the west, at a gradient of approximately 10 degrees (18 percent). Continuing to the west, the terrain continues to slope moderately to steeply down to the west to the existing railroad tracks and Puget Sound at gradients of approximately 30 to 37 degrees (57 to 73 percent). The existing site conditions, site topography, and interpreted subsurface conditions are presented on Cross -Section A -A' in Figure 3. We did not observe groundwater seepage emitting from the site slopes during our site visit. We also did not observe any indications of recent slope movement or sloughing during our site visit. This site and the surrounding vicinity are within the ancient North Edmonds Earth Subsidence and Landslide Hazard area. The 2007 Landau Associates "North Edmonds Earth Subsidence and Landslide Hazard Area Summary Report, Edmonds, Washington" describes the landslide as a massive block of land that extends from the shore of Puget Sound to the steep bluffs several hundred feet to the east. In the 1980's, the City of Edmonds improved drainage in and around the North Edmonds landslide area to increase stability of the landslide block by lowering groundwater levels. The Landau Associates' report summarizes that large-scale movements of the landslide mass have not been documented since the 1950's. We were provided with a preliminary geotechnical report for a previously planned development at the site and the adjacent northern parcel by AESI dated October 24, 1997. That report describes a 2 to 3 foot tall fresh landslide scarp near the north edge of the adjacent parcel to the north and states that the ground below that scarp likely shifted the previous winter. The report describes another, overgrown scarp that was a few inches tall near the west side of the gently -sloping portion of the subject site. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1360822 Proposed Koh Residence May 23, 2023 Edmonds, Washington Page 7 New development in the neighborhood has taken place in the form of single-family residences and roadways. Although the likelihood of the ancient slide to become active in the foreseeable future is low, extreme environmental conditions coupled with inadequate human practices could re -activate the ancient landslide. Such external factors could include severe and prolonged weather events and/or significant seismic activity. The site falls within "Zone A" of the North Edmonds Earth Subsidence and Landslide Hazard Area Report prepared by Landau Associates for the City of Edmonds and as such meets the definition of a landslide hazard area per ECDC 23.80.020 (13)(1). This designation requires that certain features be included (or excluded) in the design. Such features include the restriction of cuts and fills, the need for tightlining runoff into an approved system, the need to design foundations and retaining walls to withstand high lateral earth pressures and potential loss of soil beneath parts of the foundation, the need to vegetate slopes with deeply rooted drought -tolerant vegetation, and the elimination of any and all irrigation systems. We have addressed these requirements in the remainder of this report. We encountered soils interpreted as landslide deposits to depths of about 30 feet within our explorations. Competent hard silt and very dense sand soils were encountered below this material within our explorations and we interpret the core of the slope at depth to consist primarily of this relatively stable material. Localized areas of surface instability and surface sliding can occur on steep slopes, particularly where modified through grading activities. Backwasting (movement of near -surface soil) through soil erosion processes or local surface slides is common to slopes, particularly where the soils are exposed to weathering. Normal surface erosion and shallow sloughing failures should be expected to continue on the steeply sloping portions of the site, but we have provided geotechnical recommendations for deep - foundation support, shoring, erosion control, and other development considerations that should reduce the potential impact of site development on the site slopes and the steep slopes to the west and below the proposed development area. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Proposed Koh Residence Edmonds, Washington Specifically, ECDC 23.80.060 (A) (1-4) stipulates that: NGA File No. 1360822 May 23, 2023 Page 8 "Alterations of geologically hazardous areas or associated buffers may only occur for activities that: 1. Will not increase the threat of the geological hazard to adjacent properties beyond predevelopment conditions; 2. Will not adversely impact other critical areas; 3. Are designed so that the hazard to the project is eliminated or mitigated to a level equal to or less than predevelopment conditions; and 4. Are certified as safe as designed and under anticipated conditions by a qualified engineer or geologist, licensed in the state of Washington." Provided the recommendations in this report are included in the project design, it is our opinion that the proposal will not increase the threat of the geologic hazard beyond predevelopment conditions and will not adversely impact other critical areas. The mitigation measures provided in this report are intended to reduce hazards to a level equal to or less than predevelopment conditions. We certify the design recommendations included in this report account for the geologic hazards present and are safe as designed. Further, the specific hazard, Edmonds development standards applicable to the project, per ECDC 23.80.070, are addressed below: A. Erosion and Landslide Hazard Areas. Activities on sites containing erosion or landslide hazards shall meet the requirements of ECDC 23.80.060, Development standards — General requirements, and the specific following requirements: 1. Minimum Building Setback. The minimum setback shall be the distance required to ensure the proposed structure will not be at risk from landslides for the life of the structure, considered to be 120 years, and will not cause an increased risk of landslides taking place on or off the site. A setback shall be established from all edges of landslide hazard areas. The size of the setback shall be determined by the director consistent with recommendations provided in the geotechnical report to eliminate or minimize the risk of property damage, death, or injury resulting from landslides caused in whole or part by the development, based upon review of and concurrence with a critical areas report prepared by a qualified professional; As discussed in the Structure Setbacks subsection of this report, the proposed structure setback of 50- feet from the crest of the steep bluff slope is sufficient to mitigate hazards associated with potential shallow slope instability, erosional activity, and soil creep. The recommendations for deep foundation support will also increase the structures effective setback from the slope. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1360822 Proposed Koh Residence May 23, 2023 Edmonds, Washington Page 9 3. Alterations. Alterations of an erosion or landslide hazard area, minimum building setback and/or buffer may only occur for activities for which a hazards analysis is submitted and certifies that: a. The alteration will not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions; b. The alteration will not decrease slope stability on adjacent properties; and c. Such alterations will not adversely impact other critical areas, Provided the geotechnical recommendations documented in this reporty are incorporated in the project design and implemented during construction, it is our opinion that the proposal will not increase surface water discharge of sedimentation to adjance properties beyond predevelopment conditions; will not decrease slope stability on adjacent properties; and will not adversely impact other critical areas. 4. Design Standards within Erosion and Landslide Hazard Areas. Development within an erosion or landslide hazard area and/or buffer shall be designed to meet the following basic requirements unless it can be demonstrated that an alternative design that deviates from one or more of these standards provides greater long-term slope stability while meeting all other provisions of this title. The requirement for long-term slope stability shall exclude designs that require regular and periodic maintenance to maintain their level of function. The basic development design standards are: a. The proposed development shall not decrease the factor of safety for landslide occurrences below the limits of 1.5 for static conditions and 1.2 for dynamic conditions. If stability at the proposed development site is below these limits, the proposed development shall provide practicable approaches to reduce risk to human safety and improve the factor of safety for landsliding. In no case shall the existing factor of safety be reduced for the subject property or adjacent properties; Based on the provided slope stability analyses, the proposed development satisfies the minimum factors of safety for sliding. These minimum factors of safety were achieved through the recommended mitigation measures, which included supporting all foundation elements on deep foundations. b. Structures and improvements shall be clustered to avoid geologically hazardous areas and other critical areas, The proposal includes construction of a single-family residence and driveway area, aligned to the eastern portion of the overall site. c. Structures and improvements shall minimize alterations to the natural contour of the slope, and foundations shall be tiered where possible to conform to existing topography, The proposed foundation layout appears to satisfy this requirement and generally steps down to the west with existing topography. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1360822 Proposed Koh Residence May 23, 2023 Edmonds, Washington Page 10 d. Structures and improvements shall be located to preserve the most critical portion of the site and its natural landforms and vegetation; The proposed structure is to be setback approximately 50-feet from the steepest portion of the slope along the western portion of the site, which in our opinion represents the most geologically critical portion of the site. e. The proposed development shall not result in greater risk or a need for increased buffers on neighboring properties; Provided the recommendations and consturciton techniques discussed in this report are included in the project design, it is our opinion that the proposed residcne development will not result in greater risk or need for increased buffers on adjacent properties. 7. Point Discharges. Point discharges from surface water facilities and roof drains onto or upstream from an erosion or landslide hazard area shall be prohibited except as follows: a. Conveyed via continuous storm pipe downslope to a point where there are no erosion hazard areas downstream from the discharge; b. Discharged at flow durations matching predeveloped conditions, with adequate energy dissipation, into existing channels that previously conveyed storm water runoff in the predeveloped state; or c. Dispersed discharge upslope of the steep slope onto a low -gradient, undisturbed buffer demonstrated to be adequate to infiltrate all surface and storm water runoff, and where it can be demonstrated that such discharge will not increase the saturation of the slope; and The proposed development plans appear to satisfy the code requirement. The civil plans indicate all stormwater generated from the proposed development will be conveyed to a diffuser tee and rock spall splash pad via a 4-inch diameter surface -mounted tightline. 8. Prohibited Development. On -site sewage disposal systems, including drain fields, shall be prohibited within erosion and landslide hazard areas and related buffers. The provided civil plans indicate the proposed residcne will be connected to the existing sdanitary sewer system within 75th Place West. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1360822 Proposed Koh Residence May 23, 2023 Edmonds, Washington Page 11 Slope Stability Analysis The bluff slope within the proposed development area was analyzed for stability along Cross -Section A -A' for the proposed conditions using the computer program Slope/W, by Geo-Slope International. Slope/W is a two-dimensional, limit equilibrium slope stability program that generates random potential failure surfaces or specific failure surfaces and determines their corresponding factors of safety with respect to failure. By generating a large number of random surfaces, a critical failure surface with the minimum factor of safety can be identified. The slope stability analyses were performed using information gathered from the field explorations and soil properties were assigned to the soil layers to reasonably reflect their engineering characteristics. Stability analyses were performed localized to the areas along the cross section. Stability analyses were performed for non -seismic and seismic conditions for the existing and proposed conditions. We utilized the ASCE 7 Hazard Tool to determine the site specific peak ground acceleration (PGA) and its mean value (PGAm), which are 0.572 and 0.629, respectively. A pseudostatic coefficient of horizontal acceleration equal to % of the PGAm value was utilized in the analysis and equates to 0.31g. The soil parameters used in our analyses, along with the results of the analyses, are presented in Figures 7 and 8. Soil engineering properties assigned for the specific geologic units for the analyses can be found in Table 2 below. Table 2: Soil Properties for Slope Stability Analyses Colluvium Disturbed Native Deposits Undisturbed Native Deposits Internal Friction Angle, Degrees ((p) 30 34 36 Unit Weight, PCF (p) 128 133 135 Cohesion, PSF (C) 0 100 300 Our slope stability analyses indicated numerous potential shallow landslides within the steep slope. The most critical slip surface under proposed static and seismic conditions resulted factors of safety of 2.7 and 1.2, respectively. The values indicate the proposed residence supported on deep foundation extending 40-feet below the finished ground surface is adequate to mitigate slope stability issues. The drilled pier configuration was applied to the model based on the provided structural plan set. The properties applied to the drilled piers were 7-foot on -center spacing and a 140 kip shear resistance. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1360822 Proposed Koh Residence May 23, 2023 Edmonds, Washington Page 12 CONCLUSIONS AND RECOMMENDATIONS General It is our opinion, from a geotechnical standpoint, that the site is compatible with the proposed development of a single-family residence provided that the geotechnical recommendations presented in this report are incorporated into project plans and followed during construction. However, it should be understood that there is a small potential for movement of the historic landslide block that underlies and surrounds the site. Such movement could be triggered by extreme natural conditions and/or poor grading and/or drainage activities within the site and by other human activity in the general site vicinity. We recommend that foundation loads from the entire residence be supported on a deep foundation system consisting of augercast piles extending through the upper landslide debris/colluvium and into the underlying undisturbed dense/hard soils at depth. This is further described in the Deep Foundations subsection of this report. We should be retained to review geotechnical aspects of the finalized project plans, and to monitor earthwork and foundation system installation during construction. Additionally, we recommend that we be retained to periodically monitor the recently -installed inclinometer casing, which would detect slope movement within the site. In general, based on our review of historic documents and our subsurface explorations, the site currently appears fairly stable with respect to deep-seated movement. However, the site vicinity is mapped within an older landslide complex and our explorations encountered evidence of past ground movement. The potential for landslide and erosion hazards will depend on how the site is graded and how surface water and near surface water are controlled. We recommend that site grading be kept to a minimum and that the finalized grading and site drainage plans be subjected to geotechnical engineering review prior to construction. Based on the provided plans, we understand the proposed residence will be setback approximately 50- feet away from the top of the steep western slope. It is our opinion that this setback distance will reduce the potential for shallow sloughing events on the steep slope to impact the proposed residence, provided that our recommendations for deep foundations and erosion control management are followed. We further discuss the setbacks in the Structure Setbacks subsection of this report. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1360822 Proposed Koh Residence May 23, 2023 Edmonds, Washington Page 13 The grading plans for this project appear to minimize cuts and fills by stepping the residence foundations downslope with existing site contour and as such expect temporary excavations to be minimal. However, if the cuts cannot be sloped back to a safe inclination due to site constraints, we recommend that the cuts be shored with a soldier pile retaining wall or a similar system. The need for a shoring system should be determined during final design. We provide recommendations for temporary and permanent cut slopes in the Temporary and Permanent Slopes section of this report. We also provide recommendations for soldier pile shoring walls in the Soldier Pile Shoring Wall subsection of this report. We recommend that any floor slab be designed as a structural slab and be supported on the deep foundation system. For other hard surfaces, such as paved areas or walkways that are supported on the existing soil, some risk of future settlement, cracking, and maintenance should be expected. To reduce this risk, we recommend over -excavating a minimum of two feet of the upper soil from those areas and replacing this material with compacted pit run or crushed rock structural fill. This recommendation is only for hard surfaces to be supported on grade and is not intended for the lower floor structural slab. Even with the recommended treatment, some settlement of the underlying loose material should be anticipated, which could result in distress of site improvements. The control of surface and near -surface water is very important for the long-term stability of the site and steep slopes. We highly recommend that temporary and final site grading be designed to direct surface water away from the structures and away from the steep site slopes. Final drainage plans have not been developed at this time, but we anticipate that all stormwater generated on the site will be collected in tightlines and transported to the bottom of the slope to the west of the property via a pipe anchored to the slope. No water should be infiltrated or dispersed within the site. We discuss general site drainage in the Site Drainage subsection of this report. The soils encountered within our explorations are considered extremely moisture sensitive and will disturb easily when wet. We recommend that construction take place during extended periods of dry weather if possible. If construction takes place during wet weather, additional expenses and delays should be expected due to the wet conditions. Additional expenses could include the need to export on -site soil, the import of clean, granular soil for fill, and the need to place a blanket of rock spalls or crushed rock in the construction traffic areas and on exposed subgrades prior to placing structural fill or structural elements. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1360822 Proposed Koh Residence May 23, 2023 Edmonds, Washington Page 14 In this report, we have also provided recommendations for drainage, erosion control, and other development considerations intended to reduce the potential impact of development on the site and the steep slope to the west. We should be retained to review final project plans prior to construction. We also recommend that NGA be retained to provide monitoring and consultation services during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during the work differ from those anticipated, and to evaluate whether or not earthwork and foundation installation activities comply with contract plans and specifications. Erosion Control and Slope Protection Measures The erosion hazard for disturbed on -site soils is considered severe, but the actual hazard will be dependent on how the site is graded and how water is allowed to concentrate. Best Management Practices (BMPs) should be used to control erosion. Areas disturbed during construction should be protected from erosion. Erosion control measures may include diverting surface water away from the stripped or disturbed areas. Silt fences and/or straw waddles should be erected to prevent muddy water from leaving the site or flowing over the site slopes and the steep slope to the west of the property. Disturbed areas should be planted as soon as practical and the vegetation should be maintained until it is established. The erosion potential for areas not stripped of vegetation should be low to moderate. Also, irrigation systems should not be installed within the site. Protection of the slopes should be performed as required by the City of Edmonds. Specifically, we recommend that the slopes not be disturbed or modified through placement of any fill or future structures outside the planned development areas. No additional material of any kind should be placed on the steep slope or any portion of sloping ground, such as excavation spoils and soil stockpiles. Trees may be cut down and removed from the slopes as long as a mitigation plan is developed for maintaining slope stability, such as the replacement of vegetation for erosion protection. A vegetation cover should be preserved on the slopes outside the proposed development area. Replacement of vegetation should be performed in accordance with the City of Edmonds code. Under no circumstances should water be allowed to concentrate on the slopes. Any sloping areas disturbed during construction should be planted with vegetation as soon as practical to reduce the potential for erosion. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1360822 Proposed Koh Residence May 23, 2023 Edmonds, Washington Page 15 Structure Setbacks Uncertainties related to building along the top of steep slopes are typically addressed by the use of building setbacks. The purpose of the setback is to establish a "buffer zone" between the structure and the top of the slope so that ample room is allowed for normal slope recession during a reasonable life span of the structure. In a general sense, the greater the setback, the lower the risk of slope failures to impact the structure. From a geological standpoint, the setback dimension is based on the slope's physical characteristics, such as slope height, slope gradient, soil type, and groundwater conditions. Other factors such as historical slope activity, rate of regression, and the type and desired life span of the development are important considerations as well. Based upon the conditions described above, it is our opinion that there is a potential for shallow sloughing - type failures and small-scale landslides to take place on the steeper site slopes, especially during wet weather. This condition can be exacerbated where water is present or where the slopes become locally very steep. Backwasting through sloughing of steep slopes can occur up the slope, such that a loss of ground could occur. Currently, the new residence is proposed to be setback approximately 50-feet from the top of the steep bluff slope within the western portion of the property. In our opinion, this setback distance is adequate; however, due to the potentially unstable debris found on the site and the history of the area, we have recommended that the residence be supported entirely on deep foundations in the form of augercast piles. We recommend that the piles extend a minimum of 5 feet into the competent hard or dense soils encountered below the landslide deposits. We anticipate that the piles will need to extend approximately 40 feet below the existing ground surface. Soil should not be stockpiled in any area between the top of the slope and the residence footprint. Site Preparation and Grading General: Plans for site grading should minimize cuts and fills to reduce the potential to detrimentally impact site stability. Site preparation should consist of excavating the residence footprint and driveway areas down to planned elevations. If shoring walls are needed to support cut excavations, we recommend that the excavation for the building should only be attempted after the shoring systems are installed. Site preparation should also consist of stripping any organic topsoil and/or loose/soft soils in areas that will support pavement or structural fill. The stripped material should not be stockpiled in any area between the top of the slope and the residence footprint. If the exposed soils are loose/soft, they should be compacted to a non -yielding condition. Areas observed to pump or weave during compaction should be over -excavated and replaced with rock spalls. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1360822 Proposed Koh Residence May 23, 2023 Edmonds, Washington Page 16 If significant surface water flow is encountered during construction, this flow should be diverted around areas to be developed and the exposed subgrade maintained in a semi -dry condition. In wet conditions, the exposed subgrade should not be compacted, as compaction of a wet subgrade may result in further disturbance of the soils. A layer of crushed rock may be placed over the prepared areas to protect them from further disturbance. The site soils are considered extremely moisture sensitive and will disturb easily when wet. We recommend that earthwork construction take place during periods of extended dry weather, and suspended during periods of precipitation if possible. If work is to take place during periods of wet weather, care should be taken during site preparation not to disturb the site soils. This can be accomplished by utilizing large excavators equipped with smooth buckets and wide tracks to complete earthwork, and diverting surface and groundwater flow away from the prepared subgrades. Also, construction traffic should not be allowed on the exposed subgrade. A blanket of rock spalls should be used in construction access areas if wet conditions are prevalent. The thickness of this rock spall layer should be based on subgrade performance at the time of construction. For planning purposes, we recommend a minimum one -foot thick layer of rock spalls. Temporary and Permanent Slopes Temporary cut slope stability is a function of many factors, including the type and consistency of soils, depth of the cut, surcharge loads adjacent to the excavation, length of time a cut remains open and the presence of surface or groundwater. It is exceedingly difficult under these variable conditions to estimate a stable, temporary, cut slope angle. Therefore, it should be the responsibility of the contractor to maintain safe slope configurations since he is continuously at the job site, able to observe the nature and condition of the cut slopes, and able to monitor the subsurface materials and groundwater conditions encountered. The following information is provided solely for the benefit of the owner and other design consultants and should not be construed to imply that Nelson Geotechnical Associates, Inc. assumes responsibility for job site safety. Job site safety is the sole responsibility of the project contractor. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1360822 Proposed Koh Residence May 23, 2023 Edmonds, Washington Page 17 For planning purposes, we recommend that temporary cuts in the on -site material be no steeper than 2 Horizontal to 1 Vertical (2HAV). If groundwater seepage is encountered, we would expect that flatter inclinations would be necessary. We should be retained to specifically review proposed geometry for significant cuts planned on this site. We recommend that cut slopes be protected from erosion. Erosion control measures may include covering cut slopes with plastic sheeting and diverting surface water runoff away from the top of cut slopes. We do not recommend vertical slopes for cuts deeper than four feet, if worker access is necessary. We recommend that cut slope heights and inclinations conform to appropriate OSHA/WISHA regulations. Permanent cut and fill slopes should be no steeper than 3HAV. However, flatter inclinations may be required in areas where loose soils or seepage are encountered. Permanent slopes should be covered with erosion control matting and vegetated. The vegetative cover maintained until established. We should specifically review all plans for grading on this project. We do not recommend grading or filling on the remaining steep slopes outside the proposed building footprint, or placing irrigation systems near the slopes. Soldier Pile Shoring Wall General: Depending on overall heights of the proposed excavation, a solider pile shoring wall could be utilized to support cut excavations around the proposed structure. In particular, a portion of the driveway extending down and into the site may require retention of fill and recommend this is done through installation of a soldier pile wall. A solider pile wall typically consists of a series of steel H-beams placed vertically at a certain spacing from one another (typically six to ten feet). The beams are usually placed in drilled shafts that are filled with structural concrete or a lean mix. The concrete shafts are typically embedded below the bottom of the planned excavation a distance equals one to two times the exposed height of the wall. The steel beams extend above the finished ground surface to provide shoring capabilities for the area to be retained. The beams are typically spanned by pressure treated timber lagging or concrete panels. The H-beam size, shaft diameter, shaft embedment, and pile spacing are dependent on the nature of the soils anticipated to be retained by the wall and the soils at depth, wall height, drainage conditions, and the final geometry. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1360822 Proposed Koh Residence May 23, 2023 Edmonds, Washington Page 18 Wall Design: The shoring wall should be designed by an experienced structural engineer licensed in the State of Washington. The lateral earth pressure acting on the shoring wall will be dependent on the nature and density of the soil behind the wall, structure and traffic loads on the wall, and the amount of lateral wall movement that may occur as material is excavated from the front of the wall. If the shoring wall is free to yield at least one -thousandth of the retained height, an "active" loading condition develops. If the wall is restrained from movement by stiffness or bracing, the wall is considered in an "at -rest" loading condition. Active and at -rest earth pressure can be calculated based on equivalent fluid densities. Shoring walls supporting a horizontal surface should be designed to resist a lateral load resulting from a fluid with a unit weight of 50 and 70 pounds per cubic foot (pcf) for the active and at -rest loading conditions, respectively. Higher values would be appropriate if the ground surface slopes behind the shoring. A uniform surcharge of 8H (in psf) should be applied to the wall design to account for seismic loading, if the shoring walls are intended to provide permanent support. H in this case, is the exposed height of the wall. These loads should be applied across the pile spacing above the excavation line. These loads can be resisted by a passive pressure of 150 pcf. The passive pressure should be applied on two - pile diameters under the excavation line. These values of the passive pressure incorporate a factor of safety of 2.0. The upper ten feet of pile embedment should be neglected when calculating the passive resistance for the permanent condition. If the proposed soldier pile walls are designed as temporary, we recommend that the embedded portion of the pile be a minimum of 1.5 times the height of the exposed portion of the wall. The above loads should be applied on the full center -to -center pile spacing above the base of the exposed portion of the wall. A fifty percent reduction of the active pressure could be applied for the purpose of designing the wall lagging. The above pressures assume that the on -site soils retained by the shoring wall are not significantly disturbed. These values do not include the effects of surcharges other than what is described above. The retained soils should be readily drained and collected water should be routed into a permanent storm system. Adequate gaps should be maintained between the lagging elements to allow for any potential water seepage buildup to flow through the wall. All voids behind the lagging should be filled with drain rock. The wall designer should calculate the predicted wall deflection, including deflection resulting from the below -grade movement of the piles. The predicted deflection values should be confirmed in the field through a survey monitoring program. Also, surrounding structures should be monitored for any adverse effects resulting from shoring wall installation. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1360822 Proposed Koh Residence May 23, 2023 Edmonds, Washington Page 19 Shoring Wall Installation: The shoring wall should be installed by a shoring contractor experienced with this type of system. We recommend that the soldier piles be installed similarly to the augercast piles as described in the Deep Foundations subsection of this report. We should be retained to monitor on site activities during the shoring wall installation on a full-time basis. Deep Foundations We recommend that the entire residence be supported on 16- to 24-inch diameter augercast piles extending a minimum of five feet into competent native hard or dense soils to transfer foundation loads past the loose, near -surface landslide deposits. Based on our boring explorations, we anticipate that the piles will need to extend approximately 40 feet below the existing ground surface to achieve the recommended pier embedment into the competent native fine-grained soils. Augercast piles are installed with a hollow -stem auger advanced to the desired pile depth. After reaching a minimum recommended penetration into bearing soils, a pressure head is created when grout is pumped into the hollow stem of the auger before starting auger withdrawal. After the grout head is developed, withdrawal of the auger is timed to maintain the grout pressure head and avoid intrusion of loose soil into the sides of the pile excavation or discontinuity or "necking" of the pile. The actual volume of the concrete pumped into each pile is recorded and compared with the theoretical volume of the pile. Piles with a ratio of actual to theoretical great volume less than 1.1 should be re -drilled. The augercast piles should provide the necessary vertical support for the structure as well as some lateral resistance. The success of this method will depend, in part, on site access for the drill rig and other equipment needed for pile installation. Obstructed piles should be relocated and/or additional piles installed. Some discussion on relocation of piles should be made with your structural engineer prior to start of drilling. It is usually most economical to make any changes while the drill rig is on site. We anticipate that the piles will need to extend approximately 40 feet below the existing ground surface to achieve the recommended embedment into the competent native soils that underlie the landslide materials. Design capacities for an 18-inch diameter augercast pile installed to a depth 40 feet below the existing ground surface can be 80, and 20 tons for axial compression and uplift capacities, respectively. If the piles will be utilized to resist uplift forces, the buoyant unit weight and adequate safety factors should be considered. Lateral resistance of the piles could be calculated based on an equivalent fluid density of 150 pounds per cubic foot (pcf) applied to two pile diameters. The upper 10 feet of loose soils should be neglected for the purpose of calculating the lateral resistance. We should be retained to review the augercast pile design and observe pile installation. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1360822 Proposed Koh Residence May 23, 2023 Edmonds, Washington Page 20 Structural Fill General: Fill placed beneath pavements and other settlement -sensitive structures, or behind retaining walls should be placed as structural fill. Structural fill, by definition, is placed in accordance with prescribed methods and standards and is monitored by an experienced geotechnical professional or soils technician. Field monitoring procedures would include the performance of a representative number of in -place density tests to document the attainment of the desired degree of relative compaction. The area to receive the fill should be prepared as outlined in the Site Preparation and Grading subsection of this report. Sloping areas to receive fill should be benched prior to fill placement. The benches should be level and at least four feet wide. Materials: Structural fill should consist of a good quality, granular soil, free of organics and other deleterious material and be well graded to a maximum size of about three inches. All-weather fill should contain no more than five -percent fines (soil finer than U.S. No. 200 sieve, based on that fraction passing the U.S. 3/4-inch sieve). We do not anticipate placement of significant volumes of structural fill for this project. The on -site soils consist of moisture -sensitive silty materials and slide debris. We recommend that the on -site material not be used as structural fill. We should be retained to evaluate the suitability of proposed structural fill materials at the time of construction. Fill Placement: Following subgrade preparation, placement of structural fill may proceed. All filling should be accomplished in uniform lifts up to eight inches thick. Each lift should be spread evenly and be thoroughly compacted prior to placement of subsequent lifts. All structural fill underlying building areas and pavement subgrade should be compacted to a minimum of 95 percent of its maximum dry density. Maximum dry density, in this report, refers to that density as determined by the ASTM D-1557 Compaction Test procedure. The moisture content of the soils to be compacted should be within about two percent of optimum so that a readily compactable condition exists. It may be necessary to over -excavate and remove wet soils in cases where drying to a compactable condition is not feasible. All compaction should be accomplished by equipment of a type and size sufficient to attain the desired degree of compaction. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1360822 Proposed Koh Residence May 23, 2023 Edmonds, Washington Page 21 Retaining Walls Retaining walls may be incorporated into project plans to support below grade portions of the structure. The lateral pressure acting on subsurface retaining walls is dependent on the nature and density of the soil behind the wall, the amount of lateral wall movement which can occur as backfill is placed, wall drainage conditions, the inclination of the backfill, and other possible surcharge loads. For walls that are free to yield at the top at least one thousandth of the height of the wall (active condition), soil pressures will be less than if movement is limited by such factors as wall stiffness or bracing (at -rest condition). We recommend that walls supporting horizontal backfill and not subjected to hydrostatic forces be designed using a triangular earth pressure distribution equivalent to that exerted by a fluid with a density of 50 pcf for yielding (active condition) walls, and 70 pcf for non -yielding (at -rest condition) walls. These recommended lateral earth pressures are for a drained granular backfill and are based on the assumption of a horizontal ground surface behind the wall for a distance of at least the subsurface height of the wall, and do not account for surcharge loads. Additional lateral earth pressures should be considered for surcharge loads acting adjacent to subsurface walls and within a distance equal to the subsurface height of the wall. This would include the effects of surcharges such as traffic loads, floor slab and foundation loads, slopes, or other surface loads. Also, hydrostatic and buoyant forces should be included if the walls could not be drained. We could consult with the structural engineer regarding additional loads on retaining walls during final design, if needed. All wall backfill should be well -compacted; however, care should be taken to prevent the buildup of excess lateral soil pressures, due to over -compaction of the wall backfill. This can be accomplished by placing wall backfill in thin loose lifts and compacting it with small, hand -operated compactors within a distance behind the wall equal to at least one-half the height of the wall. The thickness of the loose lifts should be reduced to accommodate the lower compactive energy of the hand -operated equipment. Retaining walls that are part of the residence should be supported on augercast piles as described above. Retaining walls up to 3 feet tall should be supported on a minimum of two feet of rock spalls to reduce the potential for differential settlement of the walls. The active pressure on the wall can be resisted by friction on the bottom of the wall footing and passive resistance on the below -grade portion of the footing. Frictional resistance should not be considered for pile - supported walls. We recommend using a design soil bearing pressure of no more than 1,000 pounds per square foot (psf) along with a friction coefficient and passive resistance of 0.30 and 150 pcf, respectively. NGA should be retained to review the proposed retaining wall designs prior to finalizing plans. Permanent drainage systems should be installed for retaining walls. Recommendations for these systems are found in the Subsurface Drainage subsection of this report. We recommend that we be retained to evaluate the proposed wall drain backfill material and drainage system installation. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1360822 Proposed Koh Residence May 23, 2023 Edmonds, Washington Page 22 Structural Slab As mentioned earlier, we recommend that the lower floor slab be designed as a structural slab fully supported on the deep foundation system. We recommend that slabs be underlain by at least six inches of free -draining gravel with less than three percent by weight passing the Sieve #200 for use as a capillary break. A suitable vapor barrier, such as heavy plastic sheeting (6-mil minimum), should be placed over the capillary break material. An additional 2-inch-thick moist sand layer may be used to cover the vapor barrier. This sand layer may be used to protect the vapor barrier membrane and to aid in curing the concrete; however, this sand layer is optional and is intended to protect the vapor barrier membrane during construction. Pavement Subgrade Pavement subgrade preparation should be completed as recommended in the Site Preparation and Grading and Structural Fill subsections of this report. Depending on the tolerance to pavement cracking, we recommend that the upper two feet of the existing material be removed and replaced with granular structural fill or crushed rock. The pavement subgrade should be proof -rolled with a heavy, rubber -tired piece of equipment, to identify soft or yielding areas that may require repair prior to placing any structural fill and prior to placing the pavement base course. We should be retained to observe the proof -rolling and recommend repairs prior to placement of the asphalt or hard surfaces. If the existing soil is left in place, the pavement section should be thickened to further reduce the effects of settlement. Site Drainage Surface Drainage: The finished ground surface should be graded such that stormwater is directed to an appropriate stormwater collection system. Water should not be allowed to collect in any area where footings, slabs, or retaining walls are to be constructed. Final site grades should allow for drainage away from the structure and away from the steep slopes. We suggest that the finished ground be sloped at a minimum gradient of three percent, for a distance of at least 10 feet away from the structure and slopes. Surface water should be collected by permanent catch basins and drain lines, and be discharged into an appropriate discharge system. Under no circumstances should water be allowed to flow uncontrolled over the site slopes or excavation walls. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1360822 Proposed Koh Residence May 23, 2023 Edmonds, Washington Page 23 We recommend that all stormwater generated on the site, including roof downspouts, footing drains, pavement and yard drains, and any water flow from the road, be tightlined to the bottom of the slope to the west. This should entail directing all collected runoff into a main catch basin placed at least 10 feet above from the top of the steep slope, from which a HDPE pipe should be extended to the bottom of the slope along the BNSF right-of-way. A concrete collar should be placed around the pipe between the catch basin and top of the slope to help anchor the pipe. The pipe should be laid on the slope surface but should be anchored using T-posts and metal straps. The pipe should end with a perforated tee section approximately eight feet long that is capped on both ends and embedded onto a crushed rock pad at the toe of the steep slope along the railroad tracks. The project civil engineer should design the size and alignment of the HDPE pipe, as well as other aspects of the stormwater system. Subsurface Drainage: If groundwater is encountered during construction, we recommend that the contractor slope the bottom of the excavations and collect water into ditches and small sump pits where the water can be pumped out of the excavations and routed into an appropriate outlet. We recommend the use of footing drains around the planned structure and behind retaining walls. Footing drains should be installed at least one -foot below planned finished floor elevation. The drains should consist of a minimum four -inch -diameter, rigid, slotted or perforated, PVC pipe surrounded by free -draining material, such as washed rock, wrapped in a filter fabric. We recommend that an 18-inch-wide zone of clean (less than three -percent fines), granular material be placed along the back of the walls above the drain. Washed rock is an acceptable drain material, or drainage composite may be used instead. The free -draining material should extend up the wall to one -foot below the finished surface. The top foot of backfill should consist of low permeability soil placed over plastic sheeting or building paper to minimize the migration of surface water or silt into the footing drain. Footing drains should discharge into tightlines leading to an appropriate collection and discharge point with convenient cleanouts to prolong the useful life of the drains. Roof drains should not be connected to footing drains. Roof drains should also be installed around the structure. Roof drains should consist of gutters and downspouts collecting stormwater runoff from the roof. The downspouts should discharge to catch basins and 4-inch minimum diameter, rigid, PVC tightline pipes. The drains should be directed into catch basins and then into the controlled drainage system. The footing and roof drains should discharge via independent (separate) tightlines into catch basins/cleanouts leading to the stormwater system. Surface water from the driveway and yard areas should also be collected in a catch basin and tightlined separately to the stormwater system. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Proposed Koh Residence Edmonds, Washington NGA File No. 1360822 May 23, 2023 Page 24 USE OF THIS REPORT NGA has prepared this report for Michael Koh Beng Chye and associated agents, for use in the planning and design of the development planned on this site only. The scope of our work does not include services related to construction safety precautions and our recommendations are not intended to direct the contractors' methods, techniques, sequences, or procedures, except as specifically described in our report for consideration in design. There are possible variations in subsurface conditions between the explorations and also with time. Our report, conclusions, and interpretations should not be construed as a warranty of subsurface conditions. A contingency for unanticipated conditions should be included in the budget and schedule. We recommend that we be retained to review final project plans and provide consultation regarding specific structure placement, site grading, foundation support, and drainage. We also recommend that NGA be retained to provide monitoring and consultation services during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during the work differ from those anticipated, and to evaluate whether or not earthwork and foundation installation activities comply with contract plans and specifications. We should be contacted a minimum of one week prior to construction activities and could attend pre -construction meetings if requested. All people who own or occupy homes on or near hillsides should realize that landslide movements are always a possibility. The landowner should periodically inspect the slope, especially after a winter storm. If distress is evident, a geotechnical engineer should be contacted for advice on remedial/preventative measures as soon as possible. The probability that landsliding will occur is substantially reduced by the proper maintenance of drainage control measures at the site (the runoff from the impervious surfaces should be led to an approved discharge point). Therefore, the homeowner should take responsibility for performing such maintenance. Within the limitations of scope, schedule, and budget, our services have been performed in accordance with generally accepted geotechnical engineering practices in effect in this area at the time this report was prepared. No other warranty, expressed or implied, is made. Our observations, findings, and opinions are a means to identify and reduce the inherent risks to the owner. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Proposed Koh Residence Edmonds, Washington NGA File No. 1360822 May 23, 2023 Page 25 We appreciate the opportunity to provide service to you on this project. If you have any questions or require further information, please call. Sincerely, NELSON GEOTECHNICAL ASSOCIATES, INC. ex . inaldi - --] Alex Rinaldi, LG, EIT Project Geologist )23 Khaled M. Shawish, PE Principal ABR:KMS:dy Eight Figures Attached NELSON GEOTECHNICAL ASSOCIATES, INC. 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Meadowdale PlayfieldsQ ' J a < n � m Harbour Pointe Electric D 0 T f f � f N N N d N ds Gdc�5 A m <u� Cp :E E D 0 F U ® Meadowda/e9 Meadowdale 16ath St sw t Lynnwood "d�sga Middle School © Meadowdale High School Q Meadowdale stewater Treatment Neighborhood _ Park 16em PI SW Augusta Edmonds, WA Project Number IIELSOti GEOTECHIIICAL No. Date Revision By CK Is 1360822 Koh Residence Development 1111CM11111pSSOCIpTES, inc1 6/8/22 Original DPN ABR Wenatchee Vicinity Map " -,j Wootlinville Office Office U Figure 1 ""1 135th Ave. NE, A-500 105 Palouse St. _ Woodinville, WA 98072 Wenatchee, WA 98801 www.nelsongeotech.co1 (425) 486-1669 / Fax. 481-2510 (509) 665-7696 / Fax: 665-7692 C. �W CD Schematic Site Plan Co z rQ N N 3 ) / N NIII F ' I 3— . w ° °'f I „� n J r' I IIII II� *. Cn CDI m �'I \. a� I Proposed ci m' t1 I }�,} Py ;;!, Residence C/) I \ n CD CD CD �\ I 01 CDV M� N c-� m C'7 LEGEND 0 20 40 N ° — Property line N N o B Scale: 1 inch = 20 feet Number and approximate location of boring A A' Approximate location of cross-section T T W C/) Z D D 0 M X I Reference- Site plan based on a plan dated September 28. 2022 titled "Boundary & Tonoaranhic Survev." orenared by Ground Mark Land Survevina. PLLC. C:\Users\FaithStelter\Nelson Geotechnical Associates, Inc\Nelson Geotechnical Associates, Inc - Company\2022 NGA Project\13608-22 Koh Residence Development Edmonds\Drafting\SP.dwg A A' T ,o CD 05 CD 0z N m West East 200 400 0 n0 0 0 c Q cn CD v m o 150 D 75th Place W 350 ° < a�0i D m D -0 o Proposed m > Residence 34° w 100 B"2 15 300 a)20' B-1 6 C L 25 m x L 5 8 8 9 ,6 28 3 Q 37° „ 37 76 = a s Q 50 50 Ft. 23 38 78 250 , 0 a o 33° R.O.W. 50-fi n�o N m� o a o cmn r�r7 0 200 C-) I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I >� n 0 50 100 150 200 250 300 350 Exploration Distance (feet) d w m Boring Designation B-1 23 p ° y 23 <-- SPT N-value Groundwater Level 1 NOTES: During Exploration 23 1) Stratigraphic conditions are interpolated between 23 Geologic Contact ? — —? 2) the explorations. Actual conditions may vary. Elevations are approximate. W U (approximate) Cross Section is based on topographic map dated September 28, 2022 titled "Michael Koh Beng Chye" prepared by Groundmark Land Surveying, PLLC. AReference: 01 C:\Users\FaithStelter\Nelson Geotechnical Associates, Inc\Nelson Geotechnical Associates, Inc - Company\2022 NGA Project\13608-22 Koh Residence Development Edmonds\Drafting\CS.dwg UNIFIED SOIL CLASSIFICATION SYSTEM GROUP MAJOR DIVISIONS GROUP NAME SYMBOL CLEAN GW WELL -GRADED, FINE TO COARSE GRAVEL COARSE- GRAVEL GRAVEL GP POORLY -GRADED GRAVEL GRAINED MORE THAN 50 % GRAVEL GM SILTY GRAVEL OF COARSE FRACTION RETAINED ON SOILS NO.4 SIEVE WITH FINES GC CLAYEY GRAVEL SAND CLEAN SW WELL -GRADED SAND, FINE TO COARSE SAND SAND SP POORLY GRADED SAND MORE THAN 50 % RETAINED ON MORE THAN 50 % NO. 200 SIEVE OF COARSE FRACTION SAND SM SILTY SAND PASSES NO. 4 SIEVE WITH FINES SC CLAYEY SAND FINE - SILT AND CLAY ML SILT INORGANIC GRAINED LIQUID LIMIT CL CLAY LESS THAN 50 % SOILS ORGANIC OL ORGANIC SILT, ORGANIC CLAY SILT AND CLAY MH SILT OF HIGH PLASTICITY, ELASTIC SILT INORGANIC MORE THAN 50 % PASSES LIQUID LIMIT CH CLAY OF HIGH PLASTICITY, FAT CLAY NO. 200 SIEVE 50 % OR MORE ORGANIC OH ORGANIC CLAY, ORGANIC SILT HIGHLY ORGANIC SOILS PT PEAT NOTES: 1) Field classification is based on visual SOIL MOISTURE MODIFIERS: examination of soil in general accordance with ASTM D 2488-93. Dry - Absence of moisture, dusty, dry to the touch 2) Soil classification using laboratory tests is based on ASTM D 2488-93. Moist - Damp, but no visible water. 3) Descriptions of soil density or Wet - Visible free water or saturated, consistency are based on usually soil is obtained from interpretation of blowcount data, below water table visual appearance of soils, and/or test data. Project Number nELSOn GEOTEnninl_ No. Date Revision By cK 1360822 Koh Residence Development Soil Classification Chart ' ASSOCIATES, inc ui W.o mvllle—.. Wenatchee—.. 1 1/8122 Original DPN ABR Figure 4 311 135th Ave. NE, A-500 105 Palouse St. Woodinville,WA98072 Wenatchee, WA 98801 www.nelsongeotech.com (425) 486-1669 / Fax: 481-2510 (509) 665-7696 / Fax: 665-7692 BORING LOG B-1 Soil Profile Sample Data Penetration Resistance (Blows/foot -) 10 20 30 40 50 50+ I I I I 0) N Piezometer Installation - Ground Water Description E o> J 0- �° o E 3 Q m Moisture Content (Percent - �) Data o COv L 0 O (Depth in Feet) c� U) co 10 20 30 40 50 50+ p J Light gray silt with fine sand and roots (medium stiff, moist) 5 -becomes medium stiff to stiff 8 5 1 ................................................... 5 -becomes gray, no wood content 8 ' 10 ....... ............................................ 10 -becomes stiff g ' -becomes clayey silt ML 11 ' 15 ........ ........................................... 15 11 ' 20 .................................................... 20 -becomes very stiff 23 ' 25 ................................................... 25 LEGEND ❑ Solid PVC Pipe Concrete M Moisture Content Limits Depth Driven and Amount Recovered 0 Slotted PVC Pipe Bentonite A AtterbergGrain-size P G Grain -size Analysis with 2-inch O.D. Split -Spoon Sampler Monument/ Cap Native Soil DS Direct Shear to Piezometer PP Pocket Penetrometer Readings, tons/ft Depth Driven and Amount Recovered ❑ Silica Sand P �k Liquid Limit P Sample Pushed with 3-inch Shelby Tube Sampler + Plastic Limit 1 Water Level T Triaxial NOTE: Subsurface conditions depicted represent our observations at the time and location of this exploratory hole, modified by engineering tests, analysis and judgement. They are not necessarily representative of other times and locations. We cannot accept responsibility for the use or interpretation by others of information presented on this log. Project Number IIELSOtI GEOTECHnICAL No. Date Revision By CK 6/8/22 Original DPN ABRFigure 1360822 Koh Residence Development Log nASSOCIATES, II1C1 ,P-70 °,Woodinville Office Wenatchee Office 17311-135th Ave. NE, A-500 105 Palouse 51. Woodinville, WA 98072 Wenatchee, WA 98801 5Boring Page1 2 g of www.nelsongeolech.wm (425) 486-1669 / Fax: 481-2510 (509) 665-7696 / Fax: 665-7692 N N O N 1 0 O 0 0 Q BORING LOG B-1 (cont.) Soil Profile Sample Data Penetration Resistance (Blows/foot - 10 20 30 40 50 50+ Piezometer Installation - �, Ground Water Description Q � �° o 3 Q m Moisture Content (Percent - �) Data @ o m v L o (Depth in Feet) c� U) co 0 CL 10 20 30 40 50 50+ p J -becomes hard 36 ML 78 ' 35 ................................................... 35 Gray -brown, fine to medium sand (very dense, moist) SP :.'. :•. " 40 ................................................... 40 50-6 Boring terminated at 41.0 feet below existing grade on 05/19/2022. Groundwater seepage was not encountered during drilling. 45 .................................................... 45 50 .................................................... 50 55 .................................................... 55 LEGEND ❑ Solid PVC Pipe Concrete M Moisture Content 0 Slotted PVC Pipe Bentonite A Atterberg Limits Depth Driven and Amount Recovered G Grain -size Analysis with 2-inch O.D. Split -Spoon Sampler Monument/ Cap Native Soil DS Direct Shear to Piezometer PP Pocket Penetrometer Readings, tons/ft Depth Driven and Amount Recovered ❑ Silica Sand P * Liquid Limit P Sample Pushed with 3-inch Shelby Tube Sampler + Plastic Limit 1 Water Level T Triaxial NOTE: Subsurface conditions depicted represent our observations at the time and location of this exploratory hole, modified by engineering tests, analysis and judgement. They are not necessarily representative of other times and locations. We cannot accept responsibility for the use or interpretation by others of information presented on this log. Project Number IIELSOtI GEOTECHnICAL No. Date Revision By CK 1 6/8/22 Original DPN ABR 1360822 Koh Residence Development Boring Log SSOCIATES, incI1 ,P-70 °,Woodinville Office Wenatchee Office 17311-135th Ave. NE, A-500 105 Palouse 51. Woodinville, WA 98072 Wenatchee, WA 98801 Figure 5 Page2 2 g of www.nelsongeolech.wm (425) 486-1669 / Fax: 481-2510 (509) 665-7696 / Fax: 665-7692 N N O N 1 0 O 0 0 Q BORING LOG B-2 Soil Profile Sample Data Penetration Resistance (Blows/foot -) 10 20 30 40 50 50+ I I I I 0) N Piezometer Installation - Ground Water Description E o> J 0- �° o E 3 Q m Moisture Content (Percent - �) Data o CO v 0 L O (Depth in Feet) 0 U) cn 10 20 30 40 50 50+ p J Topsoil Brown, fine to coarse sand with gravel (loose, moist) . 5 -becomes very moist :... ,.. SW 5 5 .......................................... .... 5 9 — — — — — — — — — — — — — — — Red -brown to gray -brown, silty fine sand to fine, sandy ' — 10 10 silt (loose, moist) 6 .'•' SM Gray silt to fine, sandy silt with a 2 inch seam of wet fine sand (stiff to very stiff, moist) 18 -becomes fine, sandy silt to clayey silt, very stiff 25 ' 20 ................................................... 20 ML -becomes clayey silt, stiff 16 ' 25 ................................................... 25 LEGEND ❑ Solid PVC Pipe Concrete M Moisture Content Limits Depth Driven and Amount Recovered 0 Slotted PVC Pipe Bentonite A AtterbergGrain-size P G Grain -size Analysis with 2-inch O.D. Split -Spoon Sampler Monument/ Cap Native Soil DS Direct Shear to Piezometer PP Pocket Penetrometer Readings, tons/ff Depth Driven and Amount Recovered ❑ Silica Sand P �k Liquid Limit P Sample Pushed with 3-inch Shelby Tube Sampler + Plastic Limit 1 Water Level T Triaxial NOTE: Subsurface conditions depicted represent our observations at the time and location of this exploratory hole, modified by engineering tests, analysis and judgement. They are not necessarily representative of other times and locations. We cannot accept responsibility for the use or interpretation by others of information presented on this log. Project Number IIELSOtI GEOTECHnICAL No. Date Revision By CK 1 6/8/22 Original DPN ABR 1360822 Koh Residence Development Boring Log SSOCIATES, incI1 ,P-70 °,Woodinville Office Wenatchee Office 17311-135th Ave. NE, A-500 105 Palouse 51. Woodinville, WA 98072 Wenatchee, WA 98801 Figure 6 Page1 2 g of www.nelsongeolech.wm (425) 486-1669 / Fax: 481-2510 (509) 665-7696 / Fax: 665-7692 N N O N 1 0 O 0 0 Q BORING LOG B-2 (cont.) Soil Profile Sample Data Penetration Resistance (Blows/foot - 10 20 30 40 50 50+ Piezometer Installation - �, ` Ground Water Description Q o Q o o o c E m Moisture Content (Percent - �) a o m U L 0 O (Depth Feet) c� U) co 10 20 30 40 50 50+ -p J -becomes silt to clayey silt, very stiff 28 35 .................................................... 35 -becomes clayey silt, hard ML 37 ' 40 ............................................ ....... 40 76 Boring terminated at 41.5 feet below existing grade on 05/19/2022. Groundwater seepage was not encountered during drilling. 45 .................................................... 45 50 .................................................... 50 55 .................................................... 55 LEGEND ❑ Solid PVC Pipe Concrete M Moisture Content 0 Slotted PVC Pipe Bentonite A Atterberg Limits Depth Driven and Amount Recovered G Grain -size Analysis with 2-inch O.D. Split -Spoon Sampler Monument/ Cap Native Soil DS Direct Shear to Piezometer PP Pocket Penetrometer Readings, tons/ft Depth Driven and Amount Recovered ❑ Silica Sand P * Liquid Limit P Sample Pushed with 3-inch Shelby Tube Sampler + Plastic Limit 1 Water Level T Triaxial NOTE: Subsurface conditions depicted represent our observations at the time and location of this exploratory hole, modified by engineering tests, analysis and judgement. They are not necessarily representative of other times and locations. We cannot accept responsibility for the use or interpretation by others of information presented on this log. Project Number IIELSOtI GEOTECHnICAL No. Date Revision By CK 1 6/8/22 Original DPN ABR 1360822 Koh Residence Development Boring Log SSOCIATES, incI1 ,P-70 °,Woodinville Office Wenatchee Office 17311-135th Ave. NE, A-500 105 Palouse 51. Woodinville, WA 98072 Wenatchee, WA 98801 Figure 6 Page2 2 g of ww.nelsongeolech.wm (425) 486-1669 / Fax: 481-2510 (509) 665-7696 / Fax: 665-7692 N N O N 1 0 O 0 0 Q -n o CD 05a CD 0� rQ -4 N 3 is CD O 0 3 C) Cn (n Cn CD o CD Cn. -0 0 Un o CD >CD LT = D 0 < O3 cn. m CD cn Q •-• N - ti - C O � (6 M > r N can o W m= o a o m —1 cn m c=-) Z O N Cl N W T ♦D y y. O 3 Slope Stability Analysis - Cross Section A -A' Static Conditions Morgenstern -Price most critical surface with minimum FOS = 2.659 75th Place West Drilled Piers (typ) 120 40-Ft. embedment Proposed 120 7-Ft. on -center 110 140,000 IV shear demand Residence 110 100 Colluvium 100 Moist Unit Weight: 128 pcf 90 Friction Angle: 30° 90 Effective Cohesion: 0 psf 80 80 70 Disturbed Native Deposits 70 Moist Unit Weight: 133 pcf 60 Friction Angle: 34° 60 Effective Cohesion: 100 Psf / 50 Undisturbed Native Deposits 50 Moist Unit Weight: 135 pcf 40 Friction Angle: 36° 40 Effective Cohesion: 300 psf 30 / 30 Critical Slip Surface 20 20 10 10 0 0 -10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 Distance (Feet) W NOTES: Not to scale 0 Elevations are Approximate iStelter\Nelson Geotechnical Associates, Inc\Nelson Geotechnical Associates, Inc - W -n 05a 0� rQ 00 N 3 07 CD CD Fn. O O 3 r� m Cn. o 0 Q- o' D cn m D W D 0 T(D � _ m o E3 m n Slope Stability Analysis - Cross Section A -A' Seismic Conditions Morgenstern -Price most critical surface with minimum FOS = 1.233 with a seismic coefficient of ground acceleration = 0.31 g 75th Place West 120 Drilled Piers (typ) 120 40-Ft. embedment Proposed 110 7-Ft. on -center Residence 110 140,000 Ibf shear demand 100 100 colluvium .-. 90 Moist Unit Weight: 128 pcf 90 a) Friction Angle: 30° 80 Effective Cohesion: 0 psf 80 LL 1_­1 70 Disturbed Native Deposits 70 C: Moist Unit Weight: 133 pcf 60 Friction Angle: 34' 60 co Effective Cohesion: 100 psf > a) 50 is Native Deposits 50 W Moist Unit Weight: 135 pcf 40 Friction Angle: 36' 40 Effective Cohesion: 300 psf 30 Critical Slip Surface 30 20 20 10 10 0 0 -10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 Distance NOTES: Not to scale 0 Elevations are Approximate iStelter\Nelson Geotechnical Associates, Inc\Nelson Geotechnical Associates, Inc - N N C O N W