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ENG2024-0184_Geotechnical Evaluation_4.23.2024_4.27.02_PM_4214718NELSON GEOTECHNICAL ASSOCIATES. INC. August 16, 2022 Sandra and Chris Olson VIA Email: sankolson(&hotmail.com Preliminary Geotechnical Engineering Evaluation Olson Slope Evaluation 9219 Olympic View Drive Edmonds, Washington NGA File No. 135941322 17311-1351h Ave. N.E. Suite A-500 Woodinville, WA 98072 (425) 486-1669 www.nelsongeotech.com RECEIVED MAY 07 2024 CITY OF EDMONDS DEVELOPMENT SERVICES DEPARTMENT Dear Mr. and Mrs. Olson, This preliminary report summarizes our initial opinions and recommendations regarding the site stability conditions located at 9219 Olympic View Drive in Edmonds, Washington, as shown on the Vicinity Map in Figure 1. INTRODUCTION We visited the site on June 1, 2022 to observe the site conditions. The property consists of an irregular - shaped parcel covering approximately 0.3 acres. It is occupied by a single-family residence and is bordered to the west by steep slopes descending towards the BNSF railroad tracks and Puget Sound. We understand you recently purchased the property and are interested in an evaluation and risk assessment of the slope stability conditions with respect to the existing residence. SCOPE The purpose of this study is to map the conditions on the slope, characterize the soil conditions within the vicinity of the residence structure and slope with limited hand tool explorations, and provide preliminary geotechnical recommendations for site improvements. Specifically, our scope of services included the following: 1. Reviewing available soil and geologic maps of the area. 2. Mapping the conditions on the site slopes using shallow, hand -tool explorations where necessary to construct geological cross sections and qualitatively evaluate slope stability. Preliminary Geotechnical Engineering Evaluation NGA File No. 13594B22 Olson Slope Evaluation August 16, 2022 Edmonds, Washington Page 2 3. Determining the presence of geologically hazardous areas in accordance with the City of Edmonds Community Development Code Chapter 23.80. 4. Evaluating the stability of the existing retaining wall at the top of the slope. 5. Documenting erosion control and drainage issues on the slope. 6. Providing preliminary, general recommendations to enhance the stability of the top of slope. 7. Providing general recommendations for improving site drainage and erosion control. 8. Documenting the results of our findings, conclusions, and recommendations in a written geotechnical report. SITE CONDITIONS Surface Conditions The site consists of a narrow, high -bluff lot occupied by a single-family residence constructed in 1933. The residence contains a partial basement extending approximately 3- to 4-feet below the existing ground surface. A paved driveway extends from Olympic View Drive towards the southeast corner of the residence from the southern point of the property. The remaining portions of the property are occupied by landscaping and grass yard areas. Stormwater runoff associated with the residence and driveway appears to be directed to a narrow swale running the length of the western property line. The swale slopes to drain towards the top of slope, at which point, stormwater is conveyed into a steel trough extending approximately 10- to 15-feet horizontally out and over the slope then into an approximately 25-foot long vertical 12-inch corrugated metal pipe (CMP), terminating at a makeshift catch basin along the slope surface. The catch basin outlets into another segment of 12-inch diameter CMP, which extends about one-third of the way down the slope and into a natural gully. Beyond the catch basin, the metal pipe was observed to be heavily corroded and broken apart in places. We reviewed the stormwater conveyance system on the City of Edmonds GIS web page. The mapping indicates the above -described drainage system within the property may also receive runoff from a network of storm ditches, facility lines, and detention facilities extending just beyond and to the south of Olympic View Drive, several hundred feet to the south of the site. NELSON GEOTECHNICAL ASSOCIATES, INC. Preliminary Geotechnical Engineering Evaluation NGA File No. 135941322 Olson Slope Evaluation August 16, 2022 Edmonds, Washington Page 3 Below the residence, a complex of ravines and steep west- to northwest -facing slopes descends towards the BNSF railroad tracks. The downhill northwestern portion of the residence is separated from the top of the steep slope area by approximately 15- to 17-feet of level to gently sloping yard area. The upper portions of the slope, below the residence contained near vertical slope gradients in the range of approximately 65- to 85-degrees (214.5 to 1143 percent), providing approximately 35-feet of vertical relief. Below, the slope gradient shallowed to approximately 45 degrees (100 percent) for another approximately 30 vertical feet. The underlying mid to lower portions of the slope contained slope gradients in the range of approximately 15 to 31 degrees (27 to 60 percent) down to the railroad tracks. The overall vertical elevation of the slope underlying the residence is approximately 140 to 150 feet. The slope geometry is shown on Cross -Section A -A' in Figure 3. We should note that the vertical benching feature along the base of the upper slope, as shown on Cross -Section A -A', is reflective of the erosion and incision as a result of the corroded stormwater outfall pipe, while the average slope gradient on adjacent terrain in this area is approximately 45 degrees, consistent with the slope gradient just above the incised feature. A cast -in -place retaining wall measuring approximately 35-feet-long by 6-feet-high was observed along the top of slope below the residence with approximately 6-inches of the overall retaining wall extending above the yard area behind the wall. The retaining wall and top of slope area was heavily vegetated with ivy and other vine plants, such that the retaining wall and near vertical slopes are not easily seen from the backyard area. The material below the base of the retaining wall has partially eroded away, undermining the majority of the wall. The wall itself is perched on an approximately 12-foot-tall slope measuring approximately 85 degrees. The upper very steep to near vertical portions of the slope generally consisted of exposed highly compact sand and gravel, although ivy and other vine plants have grown over the surface of these areas. Lower on the slope vegetation consisted of a moderate density of young to mature deciduous trees, few coniferous trees, and thick underbrush. Groundwater seepage was observed emitting from the upper portion of the slope at an observable contact between the upper more permeable sand and gravel soils and the underlying massive silt/clay deposits. The approximate soil contact is noted on the attached cross section. Towards the lower portion of the slope, few exposures indicated compact, bedded, fine - to medium sands. Ongoing erosion was observed along the upper granular exposed soils and within the compact silt/clay deposits where active stormwater incision is occurring. NELSON GEOTECHNICAL ASSOCIATES, INC. Preliminary Geotechnical Engineering Evaluation Olson Slope Evaluation Edmonds, Washington NGA File No. 135941322 August 16, 2022 Page 4 Subsurface Conditions Geology: The geologic units for this site are shown on Geologic Map of the Edmonds East and Part of the Edmonds West Quadrangles, Washington by Minard, J.P. (US6S, 1983). The site is mapped as Olympia gravel (Qog) and is described as stratified, fluvial sand and gravel, deposited by the Olympia interglaciation. The unit is described as locally oxidized and weakly cemented, such that it stands vertically in fresh exposures. Other probable units mapped or inferred in the area consist of Possession drift (Qpd) and Whidbey formation (Qw). Possession drift is mostly a compact, gray, non -sorted lodgment till. Whidbey formation is described as bedded compact, medium- to coarse -grained sand. Based on our site reconnaissance, observation of the soil exposures, and review of the geologic mapping in this area, we anticipate the site is underlain by a sequence of Olympia gravel, Possession drift, and possible Whidbey formation along the lower portion of the slope. Explorations: We visited the site on June 1, 2022, to explore the subsurface conditions and delineate any undocumented fill soils present between the existing house and top of slope. We expect a more in- depth soils investigation will be warranted, depending on the future plans for the property. The approximate location of our exploration is shown on the Schematic Site Plan in Figure 2. A geologist from Nelson Geotechnical Associates, Inc. (NGA) completed the explorations, examined the soils and geologic conditions encountered, and maintained logs of the explorations. We performed one hand tool exploration along the central backyard area above the top of slope cast -in -place wall. We generally encountered gray -brown to dark brown, silty sand with varying roots and organics, in a loose to medium dense condition, which we interpreted as undocumented fill. Underlying the fill, we encountered orange -brown to light brown, fine to medium sand with silt and gravel in a medium dense or better condition, which we interpreted as competent native Olympia gravel deposits. The hand tool exploration met refusal at 6.5 feet below the existing ground surface. Hydrogeologic Conditions Groundwater seepage was not encountered any the upper hand tool exploration. However, we observed minor to moderate groundwater seepage emitting from the face of the slope at an apparent soil contact between the upper granular soils and underlying relatively impermeable silt/clay soils. We anticipate that groundwater conditions encountered within the site would be the result of perched water. Perched water occurs when surface water infiltrates through less dense, more permeable soils and accumulates on top of underlying, less permeable soils. 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 water to decrease during drier times of the year and increase during wetter periods. NELSON GEOTECHNICAL ASSOCIATES, INC. Preliminary Geotechnical Engineering Evaluation NGA File No. 135941322 Olson Slope Evaluation August 16, 2022 Edmonds, Washington Page 5 SENSITIVE AREA EVALUATION Seismic Hazard We reviewed the 2018 International Building Code (IBC) for seismic site classification for this project. Since very dense soils are interpreted to underlie the site at depth, the site best fits the IBC 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 two percent 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 Design Spectral Spectral Acceleration Spectral Acceleration Site Coefficients Response Site Class at 0.2 sec. (g) at 1.0 sec. (g) Parameters S, S1 Fa Fv SDs SD1 D 1.289 0.506 1.000 1.500 0.859 0.506 The spectral response accelerations were obtained from the USGS Earthquake Hazards Program Interpolated Probabilistic Ground Motion website (2008 data) for the project latitude and longitude. Hazards associated with seismic activity include liquefaction potential and amplification of ground motion. Liquefaction is caused by a rise in pore pressures in a loose, fine sand deposit beneath the groundwater table. ECDC 23.80.020(C) defines seismic hazard areas as: Seismic Hazard Areas. Seismic hazard areas are areas subject to severe risk of damage as a result of earthquake -induced ground shaking, slope failure, settlement, soil liquefaction, lateral spreading, or surface faulting. These areas are designated as having a "high" and "moderate to high" risk of liquefaction as mapped on the Liquefaction Susceptibility Map of Snohomish County by the Washington State Department of Natural Resources or areas located within landslide hazard areas. [Ord. 4026 § 1 (Att. A), 2016; Ord. 3527 § 2, 2004]. Review of the Liquefaction Susceptibility Map of Snohomish County by the Washington State Department of Natural Resources indicates the site contains a very low liquefaction susceptibility rating. Based on the soil conditions observed in the hand tool exploration and along the prominent soil exposures along the bluff slopes, it is our opinion that the dense glacial deposits interpreted to underlie the site have a low potential for liquefaction or amplification of ground motion. NELSON GEOTECHNICAL ASSOCIATES, INC. Preliminary Geotechnical Engineering Evaluation NGA File No. 135941322 Olson Slope Evaluation August 16, 2022 Edmonds, Washington Page 6 Erosion Hazard The criteria used for determination of the erosion hazard for affected areas include soil type, slope gradient, vegetation cover, and groundwater conditions. The erosion sensitivity is related to vegetative cover and the specific surface soil types, which are related to the underlying geologic soil units. The Natural Resources Conservation Service (NRCS) for this area of Snohomish County lists the two soil types for the upper gently sloping ground within the subject parcel and underlying steep slopes to the northwest. The units are listed as Alderwood gravelly sandy loam, 15 to 30 percent slopes and Alderwood-Everett gravelly sandy loams, 25 to 70 percent slopes, respectively. The erosion hazard rating was not available; however, we anticipate the erosion hazard is moderate to high within the upper granular soils where vegetative cover is not present. We expect the erosion hazard is low to moderate along areas where vegetative groundcover is present. Per Edmonds Community Development Code (ECDC) 23.80.020(A) erosion hazard areas are defined as: 1. Those areas of the city of Edmonds containing soils that may experience severe to very severe erosion hazard. This group of soils includes, but is not limited to, the following when they occur on slopes of 15 percent or greater: a. Alderwood soils (15 to 25 percent slopes); b. Alderwood/Everett series (25 to 70 percent slopes); c. Everett series (15 to 25 percent slopes); 2. Coastal and stream erosion areas which are subject to the impacts from lateral erosion related to moving water such as stream channel migration and shoreline retreat; 3. Any area with slopes of 15 percent or greater and impermeable soils interbedded with granular soils and springs or ground water seepage; and 4. Areas with significant visible evidence of ground water seepage, and which also include existing landslide deposits regardless of slope. Based on our site reconnaissance and review of the code, the subject site and steep slopes on the adjacent parcel to the northwest meet the criteria of an erosion hazard area. Landslide Hazard/Slope Stability The criteria used for evaluation of landslide and steep slope hazards include soil type, slope gradient, groundwater conditions, as well as vegetative and geomorphological indicators. The ground surface within the subject parcel generally slopes gently to moderately from southeast to northwest towards steep northwest -facing bluff slopes, which descend to the BNSF railroad tracks and Puget Sound below. NELSON GEOTECHNICAL ASSOCIATES, INC. Preliminary Geotechnical Engineering Evaluation Olson Slope Evaluation Edmonds, Washington NGA File No. 135941322 August 16, 2022 Page 7 Review of LiDAR imagery for the site and near vicinity indicates a broad, triangular -shaped erosional ravine is present below the subject parcel, which disrupts the relatively uniform bluff line present to the northeast and southwest of the site. The imagery also depicts a narrow erosional channel following the same apparent path as the observed stormwater system, previously described. ECDC 23.80.020 (B) defines landslide hazard areas as: 1. Areas of ancient or historic failures in Edmonds which include all areas within the earth subsidence and landslide hazard area as identified in the 1979 report of Robert Lowe Associates and amended by the 1985 report of GeoEngineers, Inc., and further discussed in the 2007 report by Landau Associates; 2. Coastal areas mapped as class u (unstable), uos (unstable old slides), and urs (unstable recent slides) in the Department of Ecology Washington coastal atlas; 3. Areas designated as quaternary slumps, earthflows, mudflows, or landslides on maps published by the United States Geological Survey or Washington State Department of Natural Resources; 4. Any slope of 40 percent or steeper that exceeds a vertical height of 10 feet over a 25 foot horizontal run. Except for rockeries that have been engineered and approved by the engineer as having been built according to the engineered design, all other modified slopes (including slopes where there are breaks in slopes) meeting overall average steepness and height criteria should be considered potential landslide hazard areas); 5. Any slope with all three of the following characteristics: a. Slopes steeper than 15 percent; b. Hillsides intersecting geologic contacts with a relatively permeable sediment overlying a relatively impermeable sediment; and c. Springs or ground water seepage; 6. Any area potentially unstable as a result of rapid stream incision or stream bank erosion; 7. Any area located on an alluvial fan, presently subject to, or potentially subject to, inundation by debris flow or deposition of stream -transported sediments; and 8. Any slopes that have been modified by past development activity that still meet the slope criteria. The Department of Ecology's Coastal Atlas map indicates the site contains unstable slope stability conditions. Our field review identified steep slopes greater than 15 percent, groundwater seepage emitting from the face of the slope, active erosional conditions, and impermeable over permeable soil conditions. Based on the above mapping and observations, it is our opinion that the steep slopes below the subject parcel meet the definition of a landslide hazard area. NELSON GEOTECHNICAL ASSOCIATES, INC. Preliminary Geotechnical Engineering Evaluation NGA File No. 135941322 Olson Slope Evaluation August 16, 2022 Edmonds, Washington Page 8 CONCLUSIONS AND RECOMMENDATIONS General Based on our office and field review of the site conditions, it is our opinion that the prevailing geologic hazard that has a moderate potential to impact the residence is ongoing erosion and natural bluff retreat, while the potential for deep-seated landslide activity is considered low under current conditions due to the presence of competent glacially consolidated soils underlying the site and forming the core of the steep slopes below the site. The upper portions of the steep slopes below the residence contain granular soils and very steep gradients on the order of approximately 45 to 85 degrees (100 to 1143 percent). These slopes appear to be over -steepened through ongoing erosion and is also made apparent where the upper retaining wall is undermined. Given that the residence and likely the top of slope retaining wall were constructed around 1933, we anticipate the bluff retreat rate to be a very slow process. Permanent stabilization of the steep slope below the residence is generally considered infeasible due to its overall height and gradient, as well as the tight access constraints between the residence and top of slope. In our opinion, mitigation of geologic hazards should consist of modifying the existing residence foundation systems to reduce the potential for the residence to be impacted by bluff retreat and/or shallow wedge failures along the top of slope. We recommend the existing house foundations are improved through the installation of closely spaced micropiles along the existing downslope western foundation line and along approximately one-half of the northern and southern foundation lines. Lateral restraint of the residence structure should also be improved through installation of drilled and grouted tie -back anchors, incorporated into the new micropile foundation grade beam along the western foundation line. These recommendations are discussed in detail in the Foundation Improvements subsection of this report. Along with these improvements, we recommend the cast -in - place retaining wall along the top of slope is proactively removed to reduce the potential of natural failure and likely greater impact to the top of slope buffer area. While we anticipate the slope retreat rate to be a slow process, severe precipitation or seismic events could rapidly accelerate slope recession. To additionally fortify the upper near vertical portion of the slope we recommend a combination of drilled and grouted soil nails and erosion control matting could be utilized to further slow the surface erosion process and reduce the potential for wedge failures along the top of slope. This treatment would generally apply to the upper 15- to 20-feet of near vertical slope underlying the residence, as discussed in the Erosion Control and Slope Protection subsection of this report. NELSON GEOTECHNICAL ASSOCIATES, INC. Preliminary Geotechnical Engineering Evaluation Olson Slope Evaluation Edmonds, Washington NGA File No. 135941322 August 16, 2022 Page 9 We also recommend the drainage picture within the site is improved. In place of the existing vertical stormwater fallout system, we recommend a new HDPE tightline system is devised to capture stormwater runoff from the site and be directed to the toe of the slope. Further discussion of drainage improvements is included in the Site Drainage subsection of this report. Foundation Improvements To reduce the risk of slope recession or shallow slope failures impacting the residence, we recommend the western perimeter foundation line and approximately one-half of the north and south foundation lines are underpinned by micro piles. A new concrete grade beam should be cast and doweled into the perimeter of the existing house foundation to enclose the new micro pile foundations. Grouted tie -back anchors should be installed along the new western grade beam foundation to laterally restrain the structure. A structural engineer licensed in the state of Washington should design the new foundation supports and determine the location of the supports based on the recommendations provided in this letter. A qualified contractor specializing in foundation retrofit should be retained to complete the repairs. Considering the limited site access conditions, and anticipated dense soils below the building foundations, it is our opinion that the most feasible foundation underpinning system is drilled and grouted micropiles. Extreme care should be taken during the proposed improvements as to not impact existing utilities within the vicinity of the structure, as well as limit further disturbance of the adjacent slope where possible. Since large gravels and cobbles are present in the glacial soils at the site, there is a possibility that this material may obstruct some piles at shallow depths. There should be contingencies in the budget and design for additional/relocated piles that may be obstructed by possible oversize clasts within the foundation alignment. The design values provided in this report are considered preliminary and should be verified with further geotechnical investigation, including exploratory soil borings. The micro pile foundations should consist of 4- to 6-inch diameter drilled and grouted reinforced shafts with spacing in the range of 3- to 4-foot on -center. The piles should be installed by a contractor with experience with such systems and using machinery capable of drilling into dense granular glacial deposits within relatively difficult access conditions. No grading or significant material removal should be attempted in the process of pile installation, aside from the minimum grading activity necessary to reach desired foundation elevations. Based on our preliminary findings we anticipate the overall depth of the piles will be approximately 25- to 30-feet below the existing ground surface. The holes should remain clear of any sloughing or water seepage prior to pumping grout. The contractor should be capable of casing the holes and removing any accumulated water as necessary. NGA should be on site during micro pile installation. NELSON GEOTECHNICAL ASSOCIATES, INC. Preliminary Geotechnical Engineering Evaluation NGA File No. 135941322 Olson Slope Evaluation August 16, 2022 Edmonds, Washington Page 10 The table below provides design axial comparison capacities for micro piles installed successfully as outlined above. Pile Minimum Design Axial Diameter Embedment Compression (inches) (feet) Capacity (tons) 4 25 12 6 25 20 The micro piles should be designed to carry all vertical loads applied by the existing residence while lateral loads should be transferred to the tieback anchors discussed below. We should be retained to review final plans, observe test anchor and pile installation prior to finalizing design, monitor installation of the piles, and evaluate pile capacity. One pile should be performance tested to 150 percent of the design capacity. The test location should be determined in the field, based on soil conditions observed during pile installation. NGA should oversee the load test. Tiebacks General: Drilled and grouted anchors should be installed along the downhill western portion of the residence to resist all lateral loads applied by the residence. We recommend a minimum of two anchors are tested to a minimum of 200 percent of the design loads to confirm design values. We recommend that measurements be made by the contractor in the field at the time of tieback installation to verify that tiebacks do not encounter any existing structures or underground utilities. No -Load Zone: The anchor portion of all tiebacks must be located a sufficient distance behind the wall face to develop resistance within a stable soil mass. We recommend the anchorage be obtained behind an assumed no-load zone. The no-load zone is defined by a line extending horizontally from the base of the wall back towards the residence a distance of six feet. The line should then extend up from the base elevation at an angle from the horizontal of 60 degrees. We expect that some undocumented fill may exist beyond the no-load zone. We therefore recommend that the anchors be a minimum of 20 feet long. We recommend that we monitor soil conditions during anchor installation in order to evaluate adequate penetration into competent soils. NELSON GEOTECHNICAL ASSOCIATES, INC. Preliminary Geotechnical Engineering Evaluation Olson Slope Evaluation Edmonds, Washington NGA File No. 135941322 August 16, 2022 Page 11 Soil Design Values: The tiebacks will likely terminate in glacial soils behind and below the residence. For use in design of the anchors, we estimate an allowable grout to soil adhesion of 2,000 pounds per square foot (psf). We recommend that we review anchor design and proposed installation methods. We should also observe anchor installation and testing. Tieback Installation and Testing: The contractor should be responsible for using equipment suited for the site conditions. We do not recommend the use of open -hole methods for the purpose of installing the tiebacks due to the potential for soil caving. Secondary grouting to increase soil adhesion may be used; however, if secondary grouting is used, the anchors should be tested using the methods outlined for the performance testing. A minimum of two anchors should be performance tested to 200 percent of the anchor design capacity. The performance test should consist of cyclic loading in increments of 25 percent of the design load, as outlined in the Federal Highways Administration (FHA) report No. FHWA/RD-82/047. The test location should be determined in the field, based on soil conditions observed during anchor installation. All other tiebacks should be proof -tested to at least 130 percent of design capacity. Erosion Control and Slope Protection The exposed, on -site soils have a moderate to high potential for erosion, depending on how water is allowed to concentrate. To additionally fortify the near vertical, exposed portions of the slope below the residence, an array of drilled and grouted soil nails spaced at approximately 6-foot on -center could be installed along the face, along with heavy duty erosion control matting. We recommend Spider Hub baskets are affixed to the head of each soil nail as a means of applying pressure to the erosion control material. These systems consist of a circular steel hub with rebar strands extending radially out from the hub, spanning approximately 5- to 6-foot in diameter. Soil nails are attached to the central hub such that the diameter of the spider cage makes contact with the slope surface. The spider hub system is generally accompanied by revegetation to restore over steepened or disturbed slopes. This system is considered an additional protection measure and its application will depend on acceptance of risks associated with bluff recession. As previously mentioned, the bluff retreat rate appears to be a very slow process; however, extreme atmospheric conditions or seismic events could accelerate the process or result in rapid loss of ground in the event of a shallow wedge failure. NELSON GEOTECHNICAL ASSOCIATES, INC. Preliminary Geotechnical Engineering Evaluation NGA File No. 135941322 Olson Slope Evaluation August 16, 2022 Edmonds, Washington Page 12 The introduction of reinforced concrete shafts (soil nails) along the near vertical slope face will reduce the risk of shallow sloughing events along the top of bluff, while the erosion control material applied to the face of the exposed slope will slow the freeze -thaw and other erosional processes. The erosion matting will also provide a medium for vegetative growth to be established along the slope surface. After successful installation of the soil nails and erosion control matting, we recommend the surface is hydroseeded to establish a bio-fascia along the slope surface. These measures should be considered surficial in nature and will require on -going maintenance, especially during the wet season. We should be retained to provide further details on these systems if warranted. Long -Term Maintenance: No material of any kind, such as excavation spoils, lawn clippings, debris, and soil stockpiles, should be placed on or near the slope. Any additional sloping areas disturbed during the work should be planted as soon as practical to reduce the potential for erosion. All new vegetation should be maintained until it is established. Under no circumstances should water be allowed to concentrate on the slopes. Site Drainage and Slope Improvements The existing drainage systems around the structure should be investigated to ensure that all stormwater is collected and routed into a permanent discharge system away from the slope. We recommend the existing vertical stormwater outfall system is replaced with a surface -mounted stormwater tightline. All stormwater runoff generated from the residence and other hard surfaces within the site should be directed to a new main catch basin positioned near the top of slope, from which a new surface -mounted HDPE pipe should extend down towards the toe of slope. The HDPE pipe should terminate at a diffuser tee placed on a rock pad consisting of 2- to 4-inch quarry spalls or similar. USE OF THIS REPORT This preliminary report has been prepared for Mr. and Mrs. Chris and Sandra Olson and their agents, for use in the planning and design of the new deck 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 letter. There are possible variations in subsurface conditions between the explorations and also with time. Our letter, conclusions, and interpretations should not be construed as a warranty of subsurface conditions. A contingency for unanticipated conditions should be included in the project budget and schedule. NELSON GEOTECHNICAL ASSOCIATES, INC. Preliminary Geotechnical Engineering Evaluation Olson Slope Evaluation Edmonds, Washington NGA File No. 135941322 August 16, 2022 Page 13 We 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 differ from those anticipated, and to evaluate whether or not foundation support installation complies with our recommendations. We should be contacted a minimum of one week prior to construction activities. All people who own or occupy homes on or near hillsides should realize that erosion and 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. The probability that landsliding will occur is substantially reduced by the proper maintenance of drainage control measures at the site. 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 letter 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. Preliminary Geotechnical Engineering Evaluation Olson Slope Evaluation Edmonds, Washington NGA File No. 135941322 August 16, 2022 Page 14 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. Alex B. Rinaldi, LG Project Geologist 2022 Khaled M. Shawish, PE Principal ABR:KMS:dy Five Figures Attached NELSON GEOTECHNICAL ASSOCIATES, INC. VICINITY MAP Not to Scale Stamm d Overlook Park Ieanedmonds19 o` 3 Hutt Park 3 U F� 0 _ Imr Renovations a f D ae ca 0 \`'� F P of Cherry St 3 1';00 �y 5 von �r Just Flowers Portofino `-9*1, Restaurant & Bar Olympic View Deli Pugm Dr Project Site 9 Q gto�1� Comfort and Q Joy Longarm Quilting... �Soundview Design Studio Frederick PI Olympic V'tevi Dr Northstar Carpenters 184th St SW 0 _ n A Pr N S�stSW v F'roiecTion 172nd Street Full Service Elect Emily D. Austin, LMTQ a 1751h St SW D' A 1g,,,st Southwest County Park 180th S1 SW M co 3 a � Q : n A Wei ri � `fry 1841h St SW e Bistro 76 Cafe & Cal Seaview Park ©Lynndale Dog Area 186th St SW i5 v W r— � � U = S SeavleW = 1871h St SW :2:2 v 2 Elementary School a = = s a a' PERRINVILLE < y Z 00 N m Lynndale D V 189lhPIS,,, PicnicSl'. m o a 19oth St SW o U Lynndale r97st St sw Elementary School I Ti 192nd St SW m 00 � P _ < D F Edmonds Adventist Church 196th St SW 52a Edmonds, WA 192nd PI SW 3 Or � J Royal India Fine Dining & Catering T dill sza Project Number Olson Property Slope , II son GEOTECHICAL No. Date Revision By cK 13594B22 111 Evaluation ASSOCIATES, incI1C 1 6/13/22 Original DPN Al t Vicinity Map - Woodinville Office Wenatchee Office Figure 1 311-135th Ave. NE, A-500 05 Palouse St l Woodinville,WA98072 Wenatchee, WA 98801 ww.nelsongeotech.con (425) 486-1669 / Fax 481-2510 (509) 665-7696 / Fax 665-7692 C Schematic Site Plan HA-1 tence LEGEND for-. rroper y line 1= HA-1 Number and approximate 0 60 120 0 location of hand auger U Approximate Scale: 1 inch = 60 feet A A' Approximate location L� of cross-section 0 N Reference: Site Plan based on field measurements, observations, and aerial parcel map review. Z Project Number IlELS0t1 GEOTECHIIICAL No. Date Revision By cK y Olson Property Slope , ryry 135941322 Evaluation ASSOCIATES, inc 1 6/13/22 Original DPN ABR o Schematic Site Plan Wo dinvill. Office Wenatchee OHic. Figure 2 11311-135th Ave. NE, A-500 105 Palouse St y Mod l mill., WA 98072 Wenatchee, WA 98801 7 w.nelsongeotech.com (425)486-16691 Fax. 481-2510 (509) 665-76961 Fax: 665-7692 U South 150 T 120 ti01 0 30 Exploration Hand Auger Designation --> HA-1 Groundwater Level 1 During Exploration Geologic Contact (approximate) t�eoiecnnicai Hssociaies, incuveison k�eoiecnnicai Hssociaces, inc - A' 60 90 120 150 180 210 240 270 300 Distance (feet) Reference: Cross Section is based on field measurements using a hand-held clinometer and 100-ft tape measure. iv,A r-ro eci\ iaoaves-« uison aio a tamonaswramn \w.aw Y U � z 0- 0 .y 5 >m �o (� M North 6 0 z 150 N Czu S. ice/ N O W � >Q N O L O N O C N U r 120 O C 6L1 cn ° 1'— W ® I— L1J C o o CM W 0 0 N N Q �sU2 iC u m �wm� OZ¢ ° 90 Cn -_> O > Q d m � p M r G N > O _M1,6 0 LLA M > N N iC E O 60 s U N � �\ O N O W 0 330 NOTES: 1) Stratigraphic conditions are interpolated between the explorations. Actual conditions may vary. 2) Elevations are arbitrary. 0 Q � C Q y� L O .O (o U � � m aM U) w 0 0 C U O N E N co =5 m z U 2) N M IL O L a 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 OF COARSE FRACTION SILTY GRAVEL 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 CH 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 IIELSOn GEOTECHnICAL No. Date Revision By cK 13594B22 Olson Property Slope e Evaluation ASSOCIATES, inc , 6/13/22 Original DPN ABR Figure 4 Soil Classification Chart Woodinville Office Wenatchee Office 311-135th Ave. NE, A-500 105 Palouse St. Woodinville, WA 98072 Wenatchee, WA 98801 ww.nelsongeotech.com (425) 486-1669 / Fax: 481-2510 (509) 665-7696 / Fax: 665-7692 LOG OF EXPLORATION DEPTH (FEET) USCS SOIL DESCRIPTION HAND AUGER ONE 0.0 - 1.0 GRASS UNDERLAIN BY TOPSOIL 1.0 - 5.5 GRAY -BROWN TO DARK BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, ROOTS, AND ORGANICS (LOOSE, MOIST) (UNDOCUMENTED FILL) 5.5-6.5 SP-SM ORANGE -BROWN TO LIGHT BROWN, FINE TO MEDIUM SAND WITH SILT AND GRAVEL (MEDIUM DENSE TO DENSE, MOIST) SAMPLES WERE NOT COLLECTED GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED HAND AUGER CAVING WAS NOT ENCOUNTERED HAND AUGER WAS COMPLETED AT 6.5 FEET ON 06/01/2022 DPN:ABR NELSON GEOTECHNICAL ASSOCIATES, INC. FILE NO 135941322 FIGURE 5