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REVIEWED BLD BLD2023-1083+Geotechnical_Report+8.31.2023_9.16.41_AM+3757751 (2)December 15, 2022 NELSON GEOTECHNICAL ASSOCIATES. INC. .............................................. REVIEWED BY CITY OF EDMONDS BUILDING DEPARTMENT DarrenHopper ; ..............................................: c/o: Jacob Strobl Strobl Design, LLC VIA Email: jacob@strobldesign.com Geotechnical Engineering Evaluation Hopper Residence Development 1005 Olympic Avenue Edmonds, Washington NGA File No. 1406422 Dear Darren: 17311-1351h Ave. N.E. Suite A-500 Woodinville, WA 98072 (425) 486-1669 www.nelsongeotech.com RECEIVED Oct 20 2023 CITY OF EDMONDS DEVELOPMENT SERVICES DEPARTMENT BLD2023-1083 NELSON GEOTECHNICAL ASSOCIATES, INC. (NGA) is pleased to submit this letter for a subsurface geotechnical and infiltration evaluation for the Hopper Residence Development project located at 1005 Olympic Avenue in Edmonds, Washington, as shown on the Vicinity Map in Figure 1. Our services were completed in general accordance with the proposal signed by you on October 30, 2022. INTRODUCTION The property is irregular in shape and covers approximately 0.31 acres in area. It is currently vacant and covered with grass. The property is bordered by a private access drive to the south, by single-family residence to the east and west, and a vacant lot and single-family residence to the north. Topographically, the site slopes gently to the west. We understand that the plans for development include the construction of a single-family residence with associated driveway and utilities. We have been requested to provide this letter for determination of geological hazards affecting the site, as well as development considerations for the proposed residence. For our use in preparing this letter we were provided with Architectural plans titled "Hopper Residence," dated August 8, 2022, and prepared by Strobl Design. Specific grading and stormwater plans have not been developed, but we understand that stormwater may be directed to on -site infiltration systems, if feasible. Geotechnical Engineering Evaluation NGA File No. 1406422 Hopper Residence Development December 15, 2022 Edmonds, Washington Page 2 We have been requested to evaluate the infiltration capacity of the site soils. We will collect samples and determine the infiltration rate based on the Department of Ecology's 2019 Stormwater Management Manual for Western Washington (2019 SWMMWW), utilized by the City of Edmonds. According to this manual, we understand that long-term design infiltration rates for this site are to be determined by performing on -site infiltration testing consisting of the Small Pilot Infiltration Test (PIT). SCOPE The purpose of this study is to explore and characterize the site surface and subsurface conditions and provide general recommendations for site development. Specifically, our scope of services included the following: 1. Reviewing available soil and geologic maps of the area. 2. Exploring the subsurface soil and groundwater conditions within the site using trackhoe- excavated test pits. Excavation services were subcontracted by NGA. 3. Providing long-term design infiltration rates based on on -site Small Pilot Infiltration Testing (PIT) per the 2019 SWMMWW, if feasible. 4. Performing laboratory grain -size sieve analysis on soil samples, as necessary. 5. Determining the presence of Geologically Hazardous Areas in accordance with the City of Edmonds Code, as warranted. 6. Providing recommendations for mitigation or reduction of risk from present geologic hazards, as warranted. 7. Providing recommendations for earthwork and foundation support. 8. Providing recommendations for retaining walls. 9. Providing recommendations for temporary and permanent slopes. 10. Providing recommendations for subsurface utilities and pavement subgrade preparation. 11. Providing our opinion on stormwater infiltration feasibility. SITE CONDITIONS Surface Conditions The property is irregular in shape and covers approximately 0.31 acres in area. It is currently vacant and covered with grass. The property is bordered by a private access drive to the south, by single-family residences to the east and west, and a vacant lot and single-family residence to the north. Topographically, the site slopes gently to the west. There is a 2-foot-tall rockery on the west portion of the site that runs north to south. We did not observe standing water within the property during our site visit on December 1, 2022. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1406422 Hopper Residence Development December 15, 2022 Edmonds, Washington Page 3 Subsurface Conditions Geology: The geologic units for this area are shown on Geologic Map of the Edmonds East and Part of the Edmonds West Quadrangles, Washington, by James P. Minard (1983). The site is mapped as Advance Outwash (Qva). Advance Outwash underlies the till. The outwash typically is a thick section of mostly clean, gray, pebbly sand with increasing amounts of gravel higher in the section. Our explorations generally encountered surficial undocumented fill and topsoil underlain by silty fine to coarse sand with gravel which fits the description of the mapped Advance Outwash glacial soil. Explorations: We visited the site on December 1, 2022, to explore the subsurface conditions within the site by excavating four test pit explorations through the site, one which was used for infiltration testing. The approximate locations of our explorations are shown on the Schematic Site Plan in Figure 2. An engineer from Nelson Geotechnical Associates, Inc. (NGA) completed the explorations, examined the soils and geologic conditions encountered, and maintained logs of the explorations. The soils were visually classified in general accordance with the Unified Soil Classification System, presented in Figure 3. The logs of our explorations are presented as Figure 4. The following paragraph contains a brief description of the subsurface conditions encountered in the explorations. For a detailed description of the subsurface conditions, the hand auger logs should be reviewed. Topsoil/ Undocumented Fill: At the surface of Infiltration Test Pit One, Test Pit One, and Test Pit Two we encountered a 1.0-to-1.5-foot layer of topsoil. In Test Pit Three, which was located above the 2-foot-tall rockery, we encountered dark brown, silty, fine to medium sand with gravel and trace roots in a loose condition that we interpreted as undocumented fill soils. Advanced Outwash: Underlying the topsoil/undocumented fill soils in all our explorations at depth anywhere between 1 and 3 feet below existing grade we generally encountered silty, fine to coarse sand with gravel in a medium dense to dense condition which we interpreted as native advanced outwash soils. Test Pits One, Two, Three, and Infiltration Test Pit One terminated in the native glacial soils at depths of 5.0, 7.0, 8.0, and 6.0 feet below existing grade, respectively. Hydrogeologic Conditions Groundwater seepage was encountered in all explorations. We interpret encountered seepage to be perched water and not a regional groundwater table. 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. Geotechnical Engineering Evaluation Hopper Residence Development Edmonds, Washington SENSITIVE AREA EVALUATION Seismic Hazard NGA File No. 1406422 December 15, 2022 Page 4 We reviewed the 2018 International Building Code (IBC) for seismic site classification for this project. Since competent glacial soils were encountered at depth at the subject site, 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 ASCE 7-16, 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— ASCE 7-16 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 SD1 rC 1.291 0.455 1.0 - null 0.861 null The spectral response accelerations were obtained from the OSHPD Seismic Design Maps website for the project latitude and longitude. The site is located within the South Whidbey Island Fault Zone (SWIFZ): an active, shallow region of seismicity within central Puget Sound stretching from the Strait of Juan de Fuca to North Bend. Information published in 2013 by the Washington State Department of Natural Resources suggests the SWIFZ last ruptured less than 2,700 years ago, and that the fault zone can produce a M7.5 earthquake. The nearest mapped fault strand is located on the eastern property line and was constrained by the USGS based on subsurface geophysical techniques, not surface mapping. It is our opinion that the risk of surface rupture at the site is low, based on available information. 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 dense, generally fine-grained glacial deposits interpreted to underlie the site have a low potential for liquefaction or amplification of ground motion. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1406422 Hopper Residence Development December 15, 2022 Edmonds, Washington Page 5 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 Natural Resource Conservation Service (NRCS), has classified the east half of the site as Alderwood-Urban land complex, 8 to 15 percent slopes and the west half Alderwood-Urban land complex, 2 to 8 percent slopes. The erosion hazard for these soils is listed as moderate. On the City of Edmonds GIS map there is a small area on the east half of the site mapped as Erosion Hazard Areas 15 to 40 percent. Based on our experience in the area and our observations in the field, the native soils at the site present 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. CONCLUSIONS AND RECOMMENDATIONS General It is our opinion from a geotechnical standpoint that the planned single-family residence is feasible, provided the recommendations provided in this letter are followed. In general, the medium dense or better native soils interpreted to underlie the site at depth should provide adequate support for the planned structure. If structures are to be located in areas of loose soils or undocumented fill, foundations will need to extend through this material and rest on the underlying medium dense or better native soils or placed within reworked and engineered structural fill. The proposed structure should be founded on shallow spread footings extending down to the medium dense or better native soils. Such soils should be encountered approximately 1 to 3 feet below the existing ground surface. We recommend that the foundations for the planned structure extend through any loose or undocumented fill soils and bear wholly on the underlying native soils or approved structural fill. All foundations should generally be placed on subgrade soils or structural fill of the same type and density to reduce potential differential settlement. More information can be found in the Foundations and Structural Fill subsections of this report. Due to the presence of groundwater seepage in our explorations, it is our opinion that traditional stormwater infiltration is infeasible within this site. All runoff generated within this site should be directed to an approved discharge point most likely located in the adjacent streets. More detailed drainage recommendations are provided in the Site Drainage subsection of this report. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Hopper Residence Development Edmonds, Washington NGA File No. 1406422 December 15, 2022 Page 6 Some of the soil encountered on this site is considered very moisture -sensitive and may disturb easily when wet. We recommend that construction take place during the drier summer months, if possible. If construction is to take place during wet weather, the soils may disturb, and additional expenses and delays may be expected due to the wet conditions. Additional expenses could include the need for placing a blanket of rock spalls to protect exposed subgrades and construction traffic areas, and erecting silt fences and straw bales to prevent muddy water from leaving the site. Erosion Control The erosion hazard for the on -site soils is interpreted to be slight to moderate for exposed soils, but actual erosion potential will be dependent on 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 disturbed areas. Silt fences and straw wattles should be erected to prevent muddy water from leaving the site. Disturbed areas and stockpiles should be covered as soon as practical. Erosion potential of areas not stripped should be low. Site Preparation and Grading After erosion control measures are implemented, site preparation should consist of removing loose soils, topsoil, and any undocumented fill from foundations, slab, and pavement areas, to expose medium or better native bearing soils at depth. The stripped soil should be removed from the site. Based on our observations, we anticipate native, medium dense or better soil to be encountered at approximately one to three feet throughout explored areas of the site. We should note that additional deeper areas of unsuitable soils and/or undocumented fill could be encountered in unexplored areas of the site. This condition, if encountered, would require deeper excavations in foundation, slab, and pavement areas to remove the unsuitable soils. After site preparation, if the exposed subgrade is deemed loose, it should be compacted to a non -yielding condition and then proof -rolled with a heavy, rubber -tired piece of equipment. Areas observed to pump or weave during the proof -roll test should be reworked to structural fill specifications or over -excavated and replaced with properly compacted structural fill or rock spalls. If loose soils are encountered in the foundation areas, the loose soils should be removed and replaced with rock spalls. If significant surface water flow is encountered during construction, this flow should be diverted around work areas, and exposed subgrades should be maintained in a semi -dry condition. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Hopper Residence Development Edmonds, Washington NGA File No. 1406422 December 15, 2022 Page 7 If wet conditions are encountered, which is likely on this site, alternative site grading techniques might be necessary. These could include using large excavators equipped with wide tracks and a smooth bucket to complete site grading and covering exposed subgrade with a layer of crushed rock for protection. If wet conditions are encountered or construction is attempted in wet weather, the subgrade should not be compacted, as this could cause further subgrade disturbance. In wet conditions, it may be necessary to cover the exposed subgrade with a layer of crushed rock as soon as it is exposed to protect the moisture sensitive soils from disturbance by machine or foot traffic during construction. The prepared subgrade should be protected from construction traffic and surface water should be diverted around areas of prepared subgrade. Temporary and Permanent Slopes Cuts and fills should be used only where necessary and minimized in final grading plans for the development of the site. 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 at all times as indicated in OSHA guidelines for cut slopes. 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. For planning purposes, we recommend that temporary cuts in the site soils be no steeper than 1.5 Horizontal to 1 Vertical (1.51-1:1V). If significant groundwater seepage or surface water flow were encountered, we would expect that flatter inclinations would be necessary. We recommend that cut slopes be protected from erosion, as necessary. The slope protection measures may include covering cut slopes with plastic sheeting and diverting surface runoff away from the top of cut slopes, if such potential exists. 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 2H:1V. However, flatter inclinations may be required in areas where loose soils are encountered. Permanent slopes should be vegetated, and the vegetative cover maintained until established. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1406422 Hopper Residence Development December 15, 2022 Edmonds, Washington Page 8 Foundations Foundations should be designed in accordance with the 2018 IBC. Footing widths should be based on the anticipated loads and allowable soil bearing pressure. We recommend an allowable design bearing pressure of not more than 2,000 pounds per square foot (psf) be used to evaluate the design of footings founded on the medium dense or better native, bearing soils exposed at depths of 1.0 to 3.0 feet in our explorations. We should be retained during construction to observe and evaluate foundation subgrade. If higher bearing pressures are needed, current IBC guidelines should be used when considering increased allowable bearing pressure for short-term transitory wind or seismic loads. Potential foundation settlement using the recommended allowable bearing pressure is estimated to be less than 1-inch total and %-inch differential between adjacent footings or across a distance of about 20 feet, based on our experience with similar projects. Lateral loads may be resisted by friction on the base of the footing and passive resistance against the subsurface portions of the foundation. A coefficient of friction of 0.35 may be used to calculate the base friction and should be applied to the vertical dead load only. Passive resistance may be calculated as a triangular equivalent fluid pressure distribution. An equivalent fluid density of 200 pounds per cubic foot (pcf) should be used for passive resistance design for a level ground surface adjacent to the footing. This level surface should extend a distance equal to at least three times the footing depth. These recommended values incorporate safety factors of 1.5 and 2.0 applied to the estimated ultimate values for frictional and passive resistance, respectively. To achieve this value of passive resistance, the foundations should be poured "neat" against the soils or compacted fill should have been used as backfill against the front of the footing. We recommend that the upper one foot of soil be neglected when calculating passive resistance. Retaining Walls Plans have not been finalized for the new single-family residence but retaining walls may be incorporated into the design. 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, and the inclination of the backfill. 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 40 pcf for yielding (active condition) walls, and 60 pcf for non -yielding (at -rest condition) walls. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Hopper Residence Development Edmonds, Washington NGA File No. 1406422 December 15, 2022 Page 9 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 height of the wall, and do not account for surcharge loads. Additional lateral earth pressures should be considered for surcharge loads acting adjacent to walls and within a distance equal to the subsurface height of the wall. This would include the effects of surcharges such as floor slab loads or other surface loads. We could consult with the structural engineer regarding additional loads on retaining walls during final design, if needed. The lateral pressures on walls may be resisted by friction between the foundation and subgrade soil, and by passive resistance acting on the below -grade portion of the foundation. Recommendations for frictional and passive resistance to lateral loads are presented in the Foundations subsection of this report. All wall backfill should be well compacted as outlined in the Structural Fill subsection of this report. 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 8-inch loose lifts and compacting the backfill 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. The recommended level of compaction should still be maintained. 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 observe installation of the drainage systems. Erosion Control The erosion hazard for the on -site soils is interpreted to be slight to moderate for exposed soils, but actual erosion potential will be dependent on 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 disturbed areas. Silt fences and straw wattles should be erected to prevent muddy water from leaving the site. Disturbed areas and stockpiles should be covered as soon as practical. Erosion potential of areas not stripped should be low. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1406422 Hopper Residence Development December 15, 2022 Edmonds, Washington Page 10 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. Sloping areas to receive fill should be benched using a minimum 8-foot-wide horizontal benches into competent soils. Materials: Structural fill should consist of 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; however, this will be highly dependent on the moisture content of the soil during construction. The use of the on - site soils as structural fill during wet weather will be very difficult, if not impossible. 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. Slab -on -Grade We recommend that all floor slabs be underlain by at least six inches of free -draining material with less than three percent by weight of the material passing Sieve #200 for use as a capillary break. We recommend that the capillary break be hydraulically connected to the footing drain system to allow free drainage from under the slab. 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 NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1406422 Hopper Residence Development December 15, 2022 Edmonds, Washington Page 11 used to cover the vapor barrier. This sand layer is optional and is intended to be used to protect the vapor barrier membrane and to aid in curing the concrete. Pavements The pavement subgrade should be prepared as recommended in the Site Preparation and Grading and Structural Fill subsections of this report, including proof -rolling the subgrade with a loaded dump truck and repairing areas observed to pump or weave during the proof -roll test. Also, all fill placed within the pavement areas, including utility trench backfill, should be compacted to 95 percent of the Maximum Dry Density (Modified Proctor). We should be retained to observe the proof -roll test. Any areas observed to pump or weave under the wheels of the loaded dump truck should be over -excavated and replaced with crushed rock. Utilities We recommend that underground utilities be bedded with a minimum six inches of pea gravel prior to backfilling the trench with on -site or imported material. Trenches within settlement sensitive areas should be compacted to 95% of the modified proctor as described in the Structural Fill subsection of this report. Trenches located in non-structural areas should be compacted to a minimum 90% of the maximum dry density. The trench backfill compaction should be tested. Stormwater Infiltration Small-scale pilot infiltration tests were performed within Infiltration Test Pit 1(ITP1) on December 1, 2022. ITP1 measured approximately 4.0-feet long by 3.0-feet wide by 6.0-feet deep. During the presoak period, the water level was unable to be maintained at 12 inches. The sides of the excavation were caving and perched water was filling the hole faster than it was infiltrating. It is our opinion that onsite infiltration is not an option for this site. Site Drainage Surface Drainage: The finished ground surface should be graded such that stormwater is directed to an approved stormwater collection system. Water should not be allowed to stand in any areas where footings, slabs, or pavements are to be constructed. Final site grades should allow for drainage away from the structures, if possible. 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 structures. Surface water should be collected by permanent catch basins and drain lines and be discharged into an approved discharge system. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1406422 Hopper Residence Development December 15, 2022 Edmonds, Washington Page 12 Subsurface Drainage: Groundwater seepage was observed as shallow as 2.0-feet below the existing ground surface in our explorations. If groundwater is encountered during construction, we recommend that the contractor slope the bottom of the excavation and collect the water into ditches and small sump pits where the water can be pumped out and routed into an approved location. We recommend the use of footing drains around the structure. Footing drains should be installed at least one foot below planned finished floor elevation. The drains should consist of a minimum 4-inch-diameter, rigid, slotted or perforated, PVC pipe surrounded by free -draining material wrapped in a filter fabric. We recommend that the free -draining material consist of an 18-inch-wide zone of clean (less than three -percent fines), granular material placed along the back of walls. Pea gravel is an acceptable drain material. The free - draining material should extend up the wall to one foot below the finished surface. The top foot of backfill should consist of impermeable soil placed over plastic sheeting or building paper to minimize surface water or fines migration into the footing drain. Footing drains should discharge into tightlines leading to an approved collection and discharge point with convenient cleanouts to prolong the useful life of the drains. Roof drains should not be connected to wall or footing drains. USE OF THIS LETTER NGA has prepared this letter for Darren Hopper and associated agents, for use in the planning and design of the development 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 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. 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 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. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Hopper Residence Development Edmonds, Washington NGA File No. 1406422 December 15, 2022 Page 13 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 Hopper Residence Development Edmonds, Washington NGA File No. 1406422 December 15, 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. Jonathan D. Klevan, PE Project Engineer � 7 J!9 Nr. s�i `i tic, i x 35215 S�CISTE4�g �FVt �NNAL Khaled M. Shawish, PE Principal JDK:KMS:dy Four Figures Attached 12.15.2022 NELSON GEOTECHNICAL ASSOCIATES, INC. rk m U a D o " PUg0 a IDS Project Q Site `.,nnSt 6OK'pOrry W ey VICINITY MAP Not to Scale -AN �G F� D -- O ii 176th St SW D Seaview Park J\or Q��` Lynnwood . � Costco BusinesswQenter V Recreation Center 9 Fred Meyer sza Lynnwood 24 V p. D � 208th St SW 1 N 212th St SW :' 212th St SW 71?th cr cw r WinCo Foods Edmonds, WA go n lake Terrace High School Project Number Hopper Residence � mson GEOTEnnim No. Date Revision By cK 1406422 Development ASSOCIATES, inc 11 1 12/1/22 Original FKS JDK Figure 1 Vicinity Map Wo°dinvIIIeOffce Wenatchee Office ` 17311-135th Ave. NE, A-500 105 Palouse St. Wood-ille, WA 98072 Wenatchee, WA 98801 www.nelsongeotech.com (425) 486-1669 / Fax. 481-2510 (509) 665-7696 / Fax: 665-7692 C o� CD z N N 3 CT CD 2 0-0 U! CD CD N CD -0-0 CD ET CD CDD Q CD CD 3 M o�o � a_a � a O O o O m —I cn M C7 c- T��� n i— 1 � 1 '* INF-1—�— TP-2 I TP-1 i I � n Reference: Site , 1 LEGEND Property line TP-1 —�— Number and approximate location of test pit TP-3 Site Plan N a r t � INF-1 —�— Number and approximate location of infiltration test pit based on field measurements. observations. and aerial mao review. 0 50 100 Scale: 1 inch = 50 feet Al "AO 1< c>' D CD v c CD 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 Hopper Residence �. 1406422 Development ASSOCIATES, inc , 1 12/1/22 Original FKS JDK Figure 3 Soil Classification Chart ui " 135th,"'°e CA Wenatchee Office 17311-1351h Ave. ffi A-500 105 Palouse St. Woodinville, WA 98072 Wenatchee, WA 98801 www.nelsongeoiech.com (425) 486-1669 / Fax 481-2510 (509) 665-7696 / Fax: 665-7692 LOG OF EXPLORATION DEPTH (FEET) USCS SOIL DESCRIPTION INFILTRATION TEST PIT ONE 0.0 — 1.0 TOPSOIL / FILL 1.0 — 6.0 SM BROWN, SILTY, FINE TO COARSE SAND WITH GRAVEL (MEDIUM DENSE TO DENSE, MOIST TO WET) (OUTWASH) SAMPLES WERE NOT COLLECTED GROUNDWATER SEEPAGE WAS ENCOUNTERED AT 2.0 FEET TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT WAS COMPLETED AT 6.0 FEET ON 12/01/2022 0.0 — 1.5 TOPSOIL / FILL 1.5-3.0 SM GRAY BROWN, SILTY, FINE TO MEDIUM SAND WITH GRAVEL AND IRON -OXIDE STAINING (MEDIUM DENSE TO DENSE, MOIST TO WET) (OUTWASH) 2.0 — 5.0 SM BLUE -GRAY, SILTY, FINE TO COARSE SAND WITH GRAVEL (MEDIUM DENSE TO DENSE, MOIST TO WET) (OUTWASH) SAMPLE WAS COLLECTED AT 4.0 FEET GROUNDWATER SEEPAGE WAS ENCOUNTERED AT 1.5 FEET TEST PIT CAVING WAS ENCOUNTERED TEST PIT WAS COMPLETED AT 5.0 FEET ON 12/01/2022 TEST PIT TWO 11111m l llo1:'iol l lwm y llIq 1.0 — 4.5 SP LIGHT BROWN, GRAVELLY, FINE TO COARSE SAND WITH TRACE SILT (MEDIUM DENSE TO DENSE, MOIST TO WET) (OUTWASH) 4.5 — 7.0 SM BROWN, SILTY, FINE TO COARSE SAND WITH TRACE GRAVEL (MEDIUM DENSE TO DENSE, MOIST TO WET) (OUTWASH) SAMPLES WERE COLLECTED AT 3.0 AND 5.0 FEET GROUNDWATER SEEPAGE WAS ENCOUNTERED AT 1.5 FEET TEST PIT CAVING WAS ENCOUNTERED TEST PIT WAS COMPLETED AT 7.0 FEET ON 12/01/2022 TEST PIT THREE 0.0 — 3.0 DARK BROWN, SILTY, FINE TO MEDIUM SAND WITH GRAVEL AND TRACE ROOTS (LOOSE, MOIST TO WET) (FILL) 3.0-4.0 SM LIGHT BROWN, SILTY, FINE TO COARSE SAND WITH TRACE GRAVEL, AND IRON -OXIDE STAINNING (MEDIUM DENSE, MOIST TO WET) (OUTWASH) 4.0 — 8.0 SM BROWN TO GRAY BROWN, SILTY, FINE TO COARSE SAND WITH GRAVEL, (MEDIUM DENSE TO DENSE, MOIST TO WET) (OUTWASH) SAMPLE WAS COLLECTED AT 6.0 FEET GROUNDWATER SEEPAGE WAS ENCOUNTERED AT 3.0 FEET TEST PIT CAVING WAS ENCOUNTERED TEST PIT WAS COMPLETED AT 8.0 FEET ON 12/01/2022 FKS:JDK NELSON GEOTECHNICAL ASSOCIATES, INC. FILE NO 1406422 FIGURE 4