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Geotech Letter 1NELSON GEOTECHNICAL ASSOCIATES. INC. October 1, 2020 Mr. Sean O'Donnell VIA Email: seanodonnelll@comcast.net VIA Phone: 206-380-1525 Geotechnical Engineering Evaluation O'Donnell Failing Retaining Wall 841 Elm Way Edmonds, Washington NGA File No. 1194720 17311-1351" Ave. N.E. Suite A-500 Woodinville, WA 98072 (425) 486-1669 www.nelsongeotech.com Dear Mr. O'Donnell: This letter presents the results of our geotechnical engineering investigation and evaluation of the O'Donnell Failing Retaining Wall located at 841 Elm Way in Edmonds, Washington, as shown on the Vicinity Map in Figure 1. INTRODUCTION The site consists of a roughly triangular -shaped parcel which is currently occupied by a single-family residence and associated concrete driveway within the central to southwest -central portions of the property, respectively. Vegetation within the site generally consists of grass -covered yard areas and scattered young to mature trees surrounding the residence. Topography within the site is generally relatively level to gently sloping from east to west with steeper west- to northwest -facing slopes bordering the western and a portion of the northwestern property lines. At the time of our initial site visit on July 10, 2020 we observed an approximately 3- to 5-foot tall timber retaining wall along the southwestern property line supporting a short steep cut descending from the western portion of the residence garage down to a level walkway between the subject house and the western neighbor's house. We observed that the wall was generally leaning and/or bulging westward with many of the timbers rotten or in poor condition. During our subsequent site visits on August 18 and 19, 2020 we observed that a footing trench had been excavated below and directly to the west of the subject failing timber wall, which appeared to undermine the subject failing timber. Geotechnical Engineering Evaluation NGA File 1194720 O'Donnell Failing Retaining Wall October 1, 2020 Edmonds, Washington Page 2 For our use in preparing this letter, we were provided with a previous geotechnical study for the attached garage addition prepared by Liu and Associates and dated May 12, 2010. Based on our review of the previous geotechnical report in support of constructing a new garage addition to the western portion of the residence, we understand approximately 3- to 7-feet of undocumented fill soils were encountered underlying the planned attached garage and as a result it was recommended that the garage foundations should be supported on 2-inch diameter steel pipe piles. We also understand that the construction and recommended underpinning was successfully completed. Based on our understanding of the planned development and site conditions, the services provided by NGA were to: 1. Review available soil and geologic maps of the area. 2. Explore the subsurface soil and groundwater conditions within the vicinity of the failing retaining wall with up to two drilled borings using a limited access portable drill rig. 3. Map the conditions on the sloping areas below the residence and evaluate current slope stability. 4. Provide design and construction recommendations for a replacement retaining system. 5. Document the results of our findings, conclusions, and recommendations in a written geotechnical letter. SITE CONDITIONS Surface Conditions The subject site consists of a roughly triangular -shaped parcel covering approximately 0.26 acres. The site is currently occupied by a single-family residence and associated concrete driveway area within the central and southwest -central portions of the site, respectively. Vegetation within the site generally consists of grass -covered yard areas and scattered young to mature trees surrounding the residence. The ground surface within the site generally slopes gently down from east to west with steeper slopes along the western to northwestern property lines. Within the vicinity of the subject timber retaining system the ground surface to the southwest of the residence consists of gentle west -facing slopes divided by an approximately 2-foot tall landscaping block wall and the recently removed timber wall, in a terraced manor, down to a level walkway on the neighboring property to the southwest, as shown on Cross Sections A -A' and B-B' in Figures 3 and 4. The steep timber wall -faced cut below and to the southwest of the residence garage is approximately 6- to 6.5-feet tall at the highest point adjacent to the southwest corner of the residence and tapers down to the northwest. The site is bound to the southwest and north by other existing residential properties, to the east by 100t" Avenue W, and to the southeast by Elm Way. We did not observe surface water within the site during our site visits on August 18 and 19. 2020. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File 1194720 O'Donnell Failing Retaining Wall October 1, 2020 Edmonds, Washington Page 3 Subsurface Conditions Geology: The geologic units for this area are shown on the Geologic Map of the Edmonds East and Part of the Edmonds West Quadrangles, Washington, by Minard, J.P. (USGS, 1983). The site is mapped as glacial till (Qvt). Till is described as a non -sorted mixture of clay, silt, sand, pebbles, cobbles, and boulders, all in variable amounts. Our explorations generally encountered undocumented fill soil behind and in the vicinity of the old timber retaining wall with competent native glacial material at depth, consistent with the description of till. Explorations: The subsurface conditions within the site were explored on August 18 and 19, 2020 by monitoring the drilling of two geotechnical borings. The approximate locations of our explorations are shown on the Site Plan in Figure 2. 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 explorations. A Standard Penetration Test (SPT) was performed on each of the samples during drilling to document 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. The soils were visually classified in general accordance with the Unified Soil Classification System, presented in Figure 3. The logs of our explorations are attached to this letter and are presented as Figures 4 and 5. We present a brief summary of the subsurface conditions in the following paragraphs. For a detailed description of the subsurface conditions, the exploration logs should be reviewed. At the surface of Boring 1, situated within the gently sloping ground between the existing residence attached garage and timber retaining wall, we generally encountered approximately 8- to 8.5-feet of light brown to dark brown, silty sand with varying amounts of gravel, roots, and scattered organics in a loose to medium dense condition, which we interpreted as undocumented fill. Underlying the fill we generally encountered medium dense or better light brown to gray -brown, fine to medium sand with silt, gravel, and iron -oxide staining, which we interpreted as native weathered till deposits. Boring 1 terminated at 10.0 feet below the existing ground surface. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File 1194720 O'Donnell Failing Retaining Wall October 1, 2020 Edmonds, Washington Page 4 At the surface of Boring 2, situated just north of the northwest corner of the residence we encountered approximately 11.0 feet of dark brown to light brown, silty sand with varying amounts of gravel, roots, trace asphalt and wood debris, which we interpreted as undocumented fill soils. Underlying the fill we encountered light brown to gray, silty fine to medium sand with gravel and trace iron -oxide staining in a medium dense to very dense condition, which we interpreted as native glacial till deposits. Boring 2 terminated at approximately 19 feet below the existing ground surface. We should note that Boring 2 was situated within the northern elevated yard area and was topographically three to four feet higher than the elevation of Boring 1. Hydrologic Conditions We did not encounter groundwater within our soil explorations. If groundwater is encountered during construction we would interpret this water to be perched. 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 consist of the topsoil/weathered soils. The low permeability soil consists of relatively silty glacial 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 groundwater to decrease during drier times of the year and increase during wetter periods. SENSITIVE AREA EVALUATION Seismic Hazard We reviewed the 2018 International Building Code (IBC) for seismic site classification for this project. Since competent glacial till soils are inferred to underlie the site at depth, the site conditions best fit 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 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 SDs Sol D 1.264 0.495 1.00 1.505 0.843 0.497 NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File 1194720 O'Donnell Failing Retaining Wall October 1, 2020 Edmonds, Washington Page 5 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. It is our opinion that the medium dense or better glacial deposits interpreted to underlie the site have a low potential for liquefaction or amplification of ground motion. 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 Soil Survey of the Snohomish County Area, Washington, by the Soil Conservation Service (SCS) was reviewed to determine the erosion hazard of the on -site soils. The surficial soil on site is mapped as Alderwood-Urban land complex, 8 to 15 percent slopes. The erosion hazard for this material is listed as slight. Based on our experience in the area and our observations in the field, it is our opinion that the site would have a slight to moderate erosion hazard for areas where the soils are exposed. It is our opinion that the erosion hazard for site soils should be low in areas where vegetation is not disturbed. Landslide Hazard/Slope Stability The criteria used for evaluation of landslide hazards include soil type, slope gradient, and groundwater conditions. Steeper west- to northwest -facing slopes are present along the western property line and wrap around the northwestern corner of the property and continue to the northeast onto adjacent properties to the north. The subject failing timber retaining wall is generally facing a 2- to 6-foot tall cut slope adjacent to the western portion of the existing residence and directly along a portion of the southwestern property line, as shown on Cross Section A -A' and B-B' in Figures 3 and 4. We did not observe evidence of past erosion or sloughing on this slope during our site visit. We also did not observe seepage on the slopes during our visit. Based on our observations the existing timber retaining is in poor condition with many of the timbers deteriorating and leaning slightly to the southwest. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File 1194720 O'Donnell Failing Retaining Wall October 1, 2020 Edmonds, Washington Page 6 The core of the site slope is inferred to consist of dense glacial till soils. Relatively shallow failures as well as surficial erosion are natural processes and could be expected on the slope. In general it is our opinion that while there is potential for erosion, soil creep, and shallow failures within the loose surficial soils on the steeper slopes throughout the property, the potential for deep-seated slope failure under current site conditions is low. However, due to the overall condition of the existing timber wall and presence of loose undocumented fill behind the wall it is our opinion that the near vertical cut behind the timber retaining wall is currently unstable and will likely be subject to continued slope movement and/or slope failure, especially in the event of prolonged rainfall conditions or seismic activity. Proper site grading and drainage as well as a new slope retaining system as recommended in this letter should help maintain and improve current stability conditions. CONCLUSIONS AND RECOMMENDATIONS General It is our opinion from a geotechnical standpoint that the existing timber wall has deteriorated to a failing condition and has been undermined by recent grading activities on the adjacent property to the west, further destabilizing the cut slope and timber wall. It is also our opinion that the existing timber wall is likely failing due to natural deterioration and loose undocumented fill soils placed behind the walls creating an overloaded condition. Our explorations indicate that dense glacial till deposits generally underlie the site within the development area. However, approximately 8- to 11-feet of loose undocumented fill soils were encountered in the vicinity and directly behind the existing timber wall. To permanently stabilize the steep timber -faced cut we recommend the installation of a shotcrete-soil nail wall directly in front of the failing wall. The failing wall can remain in place. The overall extent of the wall and layout should be provided by a structural engineer, however we anticipate the wall will roughly span the length of the existing timber wall. We have provided recommendations for design and installation of a new shotcrete-soil nail retaining system along the base of the steep cut along the southwestern property line. We anticipate the exposed height of the proposed shotcrete wall will range from approximately 3- to 7- feet. Depending on the exposed height of the proposed retaining wall, tieback anchors will be needed to support lateral pressures associated with the retaining wall. We recommend that these anchors consist of grouted anchors. We have also provided recommendations for improvements on the slope above the wall to lessen the potential for future sliding on the slope within the vicinity of the retaining wall, as well as overall site drainage improvements. A schematic detail of the proposed shotcrete wall is shown on the Schematic Shotcrete Wall Detail in Figure 8. Detailed recommendations regarding the proposed retaining wall design and installation have been included in the following subsections of this letter. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File 1194720 O'Donnell Failing Retaining Wall October 1, 2020 Edmonds, Washington Page 7 The surficial soils encountered on this site are considered moisture -sensitive and may disturb easily when wet. To lessen the potential impacts of construction and to reduce cost overruns and delays, we recommend that construction take place during the drier summer months if possible. If construction takes place during the rainy months, additional expenses and delays should be expected. These extra expenses could include additional erosion control and temporary drainage measures to protect the proposed development area including placement of a blanket of rock spalls to protect exposed subgrades, and the need for importing all-weather materials for structural fill. All construction operations and drainage improvements planned as part of this project should be planned and completed in a manner that enhances the stability of the steep slope, not reduces it. Any excavation spoils generated during site improvements should not be stockpiled on site but rather promptly hauled away. Also, all current and future runoff generated within the site should be collected and routed to a permanent discharge location at the bottom of the slope, or to an approved drainage system. Under no circumstances should water be allowed to concentrate or flow uncontrollably over the slope. The vegetation cover on the slope should be evaluated for compatibility with desired slope stability conditions, and a vegetation management plan should be devised to enhance slope stability. We recommend that NGA be retained to review final project plans and provide consultation regarding the retaining wall construction and slope stabilization. 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 retaining wall installation activities comply with contract plans and specifications. Erosion Control The on -site soils can have a low to moderate potential for erosion, depending 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. Measures taken may include diverting surface water away from the stripped areas. Silt fences or straw bales should be erected to prevent muddy water from flowing over the site slopes or into the existing storm system. Disturbed areas should be replanted with vegetation at the end of construction. The vegetation should be maintained until established. Final grading should incorporate appropriate erosion control measures to route stormwater runoff away from the top of slope and to appropriate discharge locations. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File 1194720 O'Donnell Failing Retaining Wall October 1, 2020 Edmonds, Washington Page 8 Runoff generated within the site, including roof downspouts, yard areas, and any other hard surfaces should be collected into catch basins and yard drains and tightlined into an approved stormwater management system such as the existing drainage system within the roadway. Under no circumstances should runoff be allowed to flow over the slope either during construction or on a permanent basis after construction has been completed. Vegetation on the slope above the wall should be reestablished after proposed wall construction has been completed. Vegetation should consist of deep-rooted, drought -resisting vegetation. The ground surface should be covered with erosion control matting prior to placement of the vegetative cover. Irrigation systems within the yard areas above steep slopes should be avoided. Future yard waste, grass clippings, or any waste material should never be cast over the slope or piled near the walls. Such activity could lead to future instability. Additionally, drainpipes associated with the new walls, should all be tightlined to an approved stormwater management system. Site Preparation and Grading Plans for site grading should be devised such that cuts and fills are kept to a minimum if possible. Site preparation should consist of excavating the retaining wall to planned elevations and removing the existing timber wall and any loose surficial soils from the cut face, however the existing timber wall can remain in place. 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 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. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File 1194720 O'Donnell Failing Retaining Wall October 1, 2020 Edmonds, Washington Page 9 Soil Nail Wall Design and Construction General: Soil nailing is an in -situ technique for reinforcing, stabilizing, and retaining steep slopes through the introduction of relatively small, closely spaced inclusions (usually steel bars) grouted into a soil mass, the face of which is then locally stabilized. A zone of reinforced ground is the result, which functions as a soil retention system. Soil nail walls are retaining walls which are built from the top downwards in situations where the soil has enough apparent cohesion that it can stand up on its own during construction. The walls consist of soil nails spaced three to six feet on center with a reinforced shotcrete facing. The retaining wall and soil nail layout should be designed by an experienced structural engineer licensed in the State of Washington, and we should be retained to review the design. Our explorations encountered approximately 8- to 8.5-feet of relatively loose undocumented fill soils directly behind the existing timber wall, however because we did not have access to the base of wall area due to it being situated on the adjacent neighboring property we do not have specific soils explorations at the base of wall. Due to the overall depth of fill soils behind the wall we anticipate undocumented fill soils may be encountered at desired subgrade elevations for the proposed shotcrete retaining wall base. As such, we recommend that the proposed retaining wall be supported on a deep foundation system consisting of hydraulically driven pin piles to transfer the retaining wall loads through the looser surficial soils and down to the more competent native outwash soils at depth. Supporting the proposed retaining wall on a deep foundation is intended to reduce any additional deeper excavations to reach the more competent native glacial soils at depth within the proposed retaining wall alignment. Due to the rigid anchor support, friction between the foundation and subgrade soil should not be considered for resisting lateral pressures on this structure. Also, passive resistance acting on the below - grade portion of the foundation should not be used to resist lateral pressures. We recommend that all lateral loads be transferred to soil nails terminated behind the new wall. Alternatively, the foundation for the proposed shotcrete wall may be supported on competent native soils or structural fill extending to these soils, if feasible. NGA should be retained in the field to observe foundation excavations and determine if the soils at desired foundation subgrade elevation are suitable for supporting the wall. Pin Piles: For the proposed shotcrete foundation support, the piles should consist of 2-inch diameter galvanized extra strong (Schedule 80) steel pipe sections driven using a hand-held, 140-pound jackhammer. For 2-inch diameter pipe piles driven to refusal using a hand-held, 140-pound jackhammer, we recommend NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File 1194720 O'Donnell Failing Retaining Wall October 1, 2020 Edmonds, Washington Page 10 a design axial compression capacity of three tons for each pile. The refusal criterion is defined as less than one inch of movement during 60 seconds of continuous driving. Final pile depths should be expected to vary somewhat and will depend on the nature of the underlying soils. The piles should advance through any fill and organic -rich soils, reach a minimum embedment of 5 feet into competent native material, and meet the minimum refusal criteria as described in this letter. Piles that do not meet this minimum embedment criterion should be rejected, and replacement piles should be driven after consulting with the structural engineer on the new pile locations. The structural engineer should design the new foundation supports and determine the location of the supports based on the recommendations provided in this letter. Foundations: Alternatively, the retaining wall foundation should extend into native competent soils but at least 18 inches below the lowest adjacent finished ground surface for frost protection and bearing capacity considerations. Foundations should be designed in accordance with the current IBC. Footing widths should be based on the anticipated loads and the allowable soil bearing pressure. If pin pile foundation supports are not utilized for the shotcrete retaining wall and foundations are supported on competent native material or structural fill extending to native soils we recommend using a soil bearing pressure of 2,000 psf. Water should not be allowed to accumulate in footing trenches. All loose or disturbed soil should be removed from the foundation excavation prior to placing concrete. Soil Nails: The soil nail wall proposed for protection of the existing timber -faced cut should consist of a grid of soil nail anchors drilled and grouted into the cut and attached to a reinforced shotcrete facing. We anticipate this wall will be up to approximately 4- to 7-feet high; however, the final extent and heights of the soil nail wall should be determined by the structural engineer. The soil nails should extend a minimum of 15 feet into the cut face and should be battered at an angle of 15 degrees below horizontal, but the batter may have to be steeper to avoid the existing building foundation and any utilities. The grouted soil nails should be a minimum of three inches in diameter, but should be sized to provide resistance for an active pressure resulting from a fluid of equivalent density of 55 pounds per cubic foot (pcf), acting along the back of the wall. The wall should also be designed to resist a seismic load of 8H pcf applied uniformly along the entire back of the wall, where H is the total wall height in feet. Soil Design Values: For use in design of the soil nails, we recommend an allowable grout to soil adhesion of 1,500 psf. This value is presented for planning purposes only and should be confirmed or modified using the data obtained from performance testing of at least two nails prior to production soil nail installation. The no-load zone should be defined by a horizontal line extending three feet from the bottom of the wall NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File 1194720 O'Donnell Failing Retaining Wall October 1, 2020 Edmonds, Washington Page 11 towards the building and then inclining at 60 degrees from horizontal up towards the ground surface. We recommend that we monitor soil conditions during anchor installation in order to evaluate adequate penetration into competent soils. An acceptable form of bond breaker (such as plastic sheathing) should be applied to the soil nail within the length of the no-load zone. Soil Nail Installation and Testing: The contractor should be responsible for using equipment suited for the site conditions. The existing cut should be cleared of all vegetation and loose materials, including the existing timber wall, however, extreme care should be taken to not further destabilize the slope and/or undermine the residence foundations. The building should be closely monitored during this process and operations adjusted should any damage to the building be overserved. Five percent of the soil nails, but a minimum of two nails, should be performance tested to 200 percent of the design capacity. The performance tests should consist of cyclic loading in increments of 25 percent of the design load. Final soil adhesion design values will be based on these tests. The test location should be determined in the field, based on soil conditions observed during construction. All production soil nails should be proof tested to at least 130 percent of design capacity. The soil nail testing program should be reviewed and monitored by NGA. After the soil nails are installed and tested, the face of the soil slope should be covered with a composite drain material, and rebar for reinforcement of the shotcrete should be placed. We recommend that two inch diameter weep holes be placed at a maximum four foot on center at the base of the shotcrete retaining wall to allow drainage through the facing. We recommend that the weep holes be designed to flow directly onto the ground surface below of the proposed wall, or tightlined to flow into an existing drainage system. The shotcrete facing can then be installed, and after the shotcrete is cured, the soil nails can be locked off. Structural Fill General: Fill placed beneath foundations, pavement, or other settlement -sensitive structures 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 suitably prepared as described in the Site Preparation and Grading subsection prior to beginning fill placement. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File 1194720 O'Donnell Failing Retaining Wall October 1, 2020 Edmonds, Washington Page 12 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). Some of the more granular on -site soils may be suitable for use as structural fill, but this will be highly dependent on the moisture content of these soils at the time of construction. We should be retained to evaluate all proposed structural fill material prior to placement. 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 letter, 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 and should be tested. ADDITIONAL SERVICES We recommend that we be retained to review final residence underpinning plans, monitor pin pile installation, and provide additional consultation as needed for the project. USE OF THIS LETTER NGA has prepared this letter for Mr. Sean O'Donnell, and his agents, for use in planning and reconstruction of the retaining wall 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 for consideration in design. 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 budget and schedule. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation O'Donnell Failing Retaining Wall Edmonds, Washington NGA File 1194720 October 1, 2020 Page 13 We recommend that we be retained to provide construction monitoring services to evaluate conditions encountered in the field with respect to anticipated conditions, to provide recommendations for design changes should the conditions differ from anticipated, and to evaluate whether earthwork activities comply with contract plans and specifications. 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. Geotechnical Engineering Evaluation O'Donnell Failing Retaining Wall Edmonds, Washington NGA File 1194720 October 1, 2020 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. wz1-U/V/- Alex B. Rinaldi, GIT Project Geologist 1 9 M. �O�VWA J q� n 4' 35215 W w ��S�CISTE¢�gCy�$ 10NAL Khaled M. Shawish, PE Principal ABR:KMS:sg Attachments: Figures 1 through 8 NELSON GEOTECHNICAL ASSOCIATES, INC. VICINITY MAP N Not to Scale r Spruce St - N Mom Na�Or y � Edmonds City Park > vi Hemlock Way"gnl°" �` N Pacific Northwest Gold Buyers n n n 8•amorrt Ln Laurel St 1A5 _ D m N 216Ih St SW 216th S[ SW Pacific Salmon Q °II F •", 217tn st sw w Forsyth In 21 H y Project D D n n ro in ,n Fir St 218th St SW Site PINE PARK v 8 NEIGHBORHOOD Lill ,1 220th St SW 220th Westgate 3 a Llmtr " Elementary School T = s t Q r' Q N < y rr Birch St 4 •, w 13In wey sw 104 14th SI ,;A, 22111,SI S',P 22etn St SW 0 0 1 � n D FIVE © f t,t+9 Sit, `� a q1 a�9y,� 226th St SW ,o f o © Edmonds Memorial Q s s Cemetery a - V D L ❑ WESTGATE 228th St o° SW Goodwill Edmonds Q 228th St SW E Lu Lu ® QFC °nds II'ay Q Westgate Chapel at a� �a Grocery store Q _ m Sherwood - ,oa Q Elementary School m 0 0 b 0 Edmonds, WA Project Number NELSON GEOTECHNICAL No. Date Revision By CK o O'Donnell Failing Retaining ASSOCIATES INC. N G A 1194720 Wall 1 8/26/20 Original DPN ABR GEOTECHNICAL ENGINEERS & GEOLOGISTS E Figure 1 Vicinity Map Woodinville Office East Wenatchee Office OU J ('1 •`� 17311-135th Ave. NE, A-500 5526 Industry Lane, #2 Woodinville, WA 98072 East Wenatchee, WA 98802 J (425) 486-1669 / Fax: 481-2510 w.nelsonyeotech.com (509) 665-7696 / Fax: 665-7692 = �0 4�1z m - N) o 3 0- CD Cn o n CD CD m ^^ z A �Dm o r } F z Ar Z O m 0 Z m n � 3 N A m Ro 'i O Or 2 Z �A 0 0 mD� -wi 0 r Ib Schematic Site Plan • ` ` R J Approximate alignment of failing timber wall `1 Neighboring House LEGEND — • — Property line B-1 Number and approximate location of boring W f f Approximate location Z of cross-section s Existing House W, 0 ,A aria 0 20 40 Approximate Scale: 1 inch = 20 feet D (7 7 Reference: Site plan based on field measurements, observations, and aerial parcel map review. o A A' T 11 � 0 CD N m Southwest Northeast I I 40 40 O n 0 o v cn m cn = & -n CD * v.0 a) 30 30 o cQ Neighboring Existing House > CDm Residence D v � - Block Wall `° 0 20 Timber a� Wall 20 m Lu n L M � M x DDOZ ` /d y_ L o a r Z Q 10 — — — 10 Z yr Z (n 0 m 0 Z m n 3 N A m �, -► 0 v dm cmnm 0 0 2, aF o TmZ Z L 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 0 10 20 30 40 50 60 70 -t A � r Exploration Distance (feet) o m v m o m Boring Designation B_1 23 p u; 23 <-- SPT N-value Groundwater Level 1 NOTES: 23 During Exploration 23 1) Stratigraphic conditions are interpolated between 23 Geologic Contact --> ? 2) the explorations. Actual conditions may vary. Elevations are arbitrary. o W z `� (approximate) Reference: Cross Section is based on field measurements using a hand-held clinometer and 100-ft tape measure. D X 0 7 \\HILL\company\2020 NGA Project Folders\11947-20 O'Donnell Failing Retaining Wall Edmonds\Drafting\CS.dwg o B B, T 11 � 0 _�1 V N) m Southwest I I Northeast 40 r — 40 O n 0 0 v cn m cn = & m CD * L 30 30 o cQ Neighboring Existing House � X W M Residence W v 0 20 Timber Wall B-1 20 p �E, m O Z Lu "a '- n ^ L 24 xnmox o oyq Z a "= Fill L r n > Z Q 10 10 F z >r 39 Z (n 0 m 0 Z m n 3 N A m �, -► 0 v dm cmnm 0 0 F o ,o zZ m D 0 �-�, 0 0 10 20 30 40 50 60 70 -t > L r Exploration Distance (feet) o m v m o m Boring Designation B_1 23 p 23 SPT N-value y' Groundwater Level 1 23 NOTES: During Exploration 23 1) Stratigraphic conditions are interpolated between 23 the explorations. Actual conditions may vary. o W Geologic Contact 2) Elevations are arbitrary. z (approximate) > 0 X 7 Reference: Cross Section is based on field measurements using a hand-held clinometer and 100-ft tape measure. \HILL\comoanv\2020 NGA Proiect Folders\11947-20 O'Donnell Failina Retainina Wall Edmonds\Drafting\CS.dwa 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 GM OF COARSE FRACTION OF 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 NELSON GEOTECHNICAL No. Date Revision By CK 1194720 O'Donnell Failing Retaining N �G ASSOCIATES, INC. Wall �+A GEOTECHNICAL ENGINEERS & GEOLOGISTS 1 8/26/20 Original DPN ABR Figure 5 Soil Classification Chart Woodinville Office East Wenatchee Office 311-135th Ave. NE, A-500 5526 Lane, 88 Woodinville, WA 98072 East Wenatchee, WA 98602 (425) 486-16691 Fax: 481-2510 w.nelsongeotech.com (509) 665-76961 Fax: 665-7692 BORING LOG B-1 Approximate Ground Surface Elevation: ?? Soil Profile Sample Data Penetration Resistance (Blows/foot - ■) 10 20 30 40 50 50+ c Piezometer Installation - Description 2 n 0) a o 3 3 °' o w �'m o Ground Water Data Moisture Content m J o f a O m U m U� (Percent ■) o (Depth in Feet) C9 O U) � iv 10 20 30 40 50 50+ � J Light brown, silty fine to medium sand with gravel (medium dense, moist) (FILL) 24 -becomes loose, dry to moist 8 -becomes medium dense, no recovery 11 -becomes dark brown, silty fine to medium sand with gravel and trace organics, dry 11 Light brown to gray -brown, fine to medium sand with silt, gravel, and trace iron -oxide staining (dense, moist) SP-SM 39 10 Boring met refusal at 10.0 feet below existing grade on 8/18/20. Groundwater seepage was not encountered during drilling. 15 .................................................... 15 20 .................................................... 20 25 .................................................... 25 LEGEND Solid PVC Pipe Concrete M Moisture Content 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 IDS Direct Shear to Piezometer PP Pocket Penetrometer Readings, tons/ft Depth Driven and Amount Recovered ❑ Silica Sand P �c 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 NELSON GEOTECHNICAL No. Date Revision By CK O'Donnell Failing Retaining N C7 A ASSOCIATES, INC. 1194720 Wall Boring Log GEOTECHNICAL ENGINEERS & GEOLOGISTS 8)26/20 Original oPN ABR Figure 6 Woodinville Office East Wenatchee Office 17311-135th Ave. NE, A-500 5526 Industry Lane, #2 Woodimill., WA Page 1 of 1 (425)48611669IFax94812510 w.nelsongeotech.com (509)65-76961 Fax' 6657692 BORING LOG B-2 Approximate Ground Surface Elevation: ?? Soil Profile Sample Data Penetration Resistance (Blows/foot - ■) 10 20 30 40 50 50+ c Piezometer Installation - Description 2 n 0) a o 3 3 °' o w �'m o Ground Water Data Moisture Content m J o f a O m U m U� (Percent ■) o (Depth in Feet) C9 O U) � iv 10 20 30 40 50 50+ � J Dark brown to light brown, silty fine to medium sand with gravel and trace asphalt grindings (medium dense, moist) (FILL) 19 -becomes loose 6 1 5 ....... ............................................ 5 -becomes gray to dark brown, loose to medium dense, 10 with trace wood debris 14 10 ................................................... 10 Light brown to gray -brown, silty fine to medium sand with — gravel and trace iron -oxide staining — (medium dense, moist) — -becomes gray, very dense — 15 ........................................... ........ 15 = SM 50-6 50-5 .................................................... 20 Boring met refusal at 19.0 feet below existing grade on 8/18/20. Groundwater seepage was not encountered during drilling. 25 .................................................... 25 LEGEND Solid PVC Pipe Concrete M Moisture Content 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 IDS Direct Shear to Piezometer PP Pocket Penetrometer Readings, tons/ft Depth Driven and Amount Recovered ❑ Silica Sand P �c 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 NELSON GEOTECHNICAL No. Date Revision By CK O'Donnell Failing Retaining N C7 A ASSOCIATES, INC. 1194720 Wall Boring Log GEOTECHNICAL ENGINEERS & GEOLOGISTS 8)26/20 Original oPN ABR Figure 7 Woodinville Office East Wenatchee Office 17311-135th Ave. NE, A-500 5526 Industry Lane, #2 Woodimill., WA Page 1 of 1 (425)48611669IFax94812510 w.nelsongeotech.com (509)65-76961 Fax' 6657692 Schematic Soil Nail Wall Detail Steel Reinforing Mesh Shotcrete Facing Lock -off plate and nut 2-in diameter weep hole (4ft on center max) 2-inch Diameter Pin Pile NOT TO SCALE Yard Area Existing Residence Native Soil 4 a a 20-ft minimum Drainage Composite Steel Bar Cement Grout Retaining Wall Footing; to be supported on 2-inch diameter pin piles or competent native soils. Subgrade to be approved by NGA. Project Number O'Donnell Failing Retaining NELSON GEOTECHNICAL No. Date Revision By I CK 1194720 Wall NGA ASSOCIATES, INC. 1 10/1/20 Original DPN ABR Schematic Soil Nail Wall GEOTECHNICAL ENGINEERS & GEOLOGISTS I Figure 8 Detail 17311-135th Ave. NE, A-500 Snohomish County(425) 337-1669 Woodinville, WA 98072 Wenatchee/Chelan (509) 665-7696 (425) 486-16691 Fax 481-2510 www.nelsongeotech.com