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20191014 Geotechnical Report Resub.pdfN G A NELSOT.¡ GEOTËCHNICAL AssocIATEs, INg. GEOÍECHNICAL ENCINT.ENS & GEOL()(IISTg Engineerin g-GeologY Blunc'h 5526 Inrlustry lane, #2 Eüst Wenrtchee, WA 98802 (509) 665-7696,FAX (509) 665-7692 Mrin Ofñce l73l I = l35th Ave NE, A-500 Woodinville, WA 980?2 (425) 486-1669 . FAX (42s) 48r-2510 December 21,2018 Ms: Lindell Graham 1.414 - 9th Avenue North Edmonds, WA 98020 lindell g@,hotmail. com MECENED ocÏ 14 2019 ÐEvEL0PñnE[l $m/rcEs Geotechnical Engineering Evaluation Sundstone Short Plat l4l4 - 9th Avenue North Edmonds, Washington NGAFileNo. 1074518 Dear Ms. Graham: 'We are pleased to submit the attached report titled ooGeotechnical Engineering Evaluation - Sundstone Short PIat - L4l4 - 9th Avenue North - Edmonds, Washington." This report summarizes our observations of the existing surface and subsurface conditions within the site, and provides general recommendations for the próposed site development. Our services were completed in general accordance with the proposal signed byyou on November 18, 2018. The level to gently west-sloping site is occupied by a multistory multifamily residential structure on the eastern poftiın of th" p.op"rly, and is vacant on the wester¡ portion. ' The properly is bordered to the north by existing residential properties, to the west by 9th Avenue North, to the south by an easement along Puget Drive (WA- 524), andto the east by commercial development. We understand that the 1.18- u"r" p*.ãl will undergo a short plat to create at least one additional residential parcel within the vacant westérn portion, and that the Sundstone Condominium structure is intended to remain in the east. We understand that stormwater generated within this site will be directed to on-site infiltration facilities if feasible. Specific grading and stormwater plans were not available when this report was prepared, but based on cónversations with you, lve understand that stormwater may be directed to on-site infiltration facilities, if feasible. In addiiion to providing recommendations for the development of the new single- family residence(s), we have been requested to evaluate the infiltration capacity of the site soils. The City of Eãmonds utilizes the 2014 WSDOE Stormwater Management Manual for Western Washington to determine the design of infiltration facilities. According to this manual, on-site infiltration testing consisting of the srnall Pilot Infiltration Test (PIT) is used to determine the long-term desigu infiltration rates. We performed four test pit explorations throughout the property, one of which we utilized for our small- scalé pilot infiltration tósting (PIT). Our explorations indicated that the site was underlain by surfioial undocumented fill with competent, native soils at depth. ruELsOM GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Sundstone Short Plat Edmonds, Washington NGA File No. 1074518 December 27,2018 Summary -Page? It is our opinion that the proposed site development is feasible from a geotechnical engineering standpoint, irovided that our'recommendations foisite development are incorporated into project plans. L, gËneraí,^the native soils underlying the site should adequately support the planned structures' Foundations should be advanced thróugh any loose soils down to the competent native bearing material interpreted to underlie the site, for bea=ring 'capacity and settlement considerations. These soils should g"noully be encountered approximately trvo to-three feet below the existing ground surface, based on our ãxplorations. If loose soili^or undocumented frll are encountered in unexplored areas of the site, they should be removed and replaced with structural filt for foundation and pavement support. Iinal stormwater plans have also not been developed, but we uncierstanci that on-site infiiiration is being considered for this site. Based on our,onsiie testing, it is our opinion that the onsite soils are not conducive to traditional methods of stormwater infiltrãtion, however low-impact design systems may be feasible. The subsurface soils generally consisted of surficial undocumented fill soils underlain by siþ fine to medium sand with lourying amounts of gravel and iron-oxide weathering that we interpreted as native glacial soils at relatively-shallow depths. We recommend that any low impact stormwater infiltrattn systems be designeã in accordance with the 2014 Department of Ecology Stormwater Management Manual for Western Washington. In the attached report, we have also provided general recommendations for site grading, slabs-on-grade, structural fill placement, retaining *ullr, erosion control, and drainage. We should be retained to review and comment on final developnient plans and observe the earthwork phase of construction. We also recommend that NGA be retained to provide monitoring and consultation services during construction- to confirm that the conditions encounterãd are consistent *ith thor" indicated by the explorations, to provide recommendations for design ohanges 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. It has.been a pleasure to provide service to you on this project. Please contact us if you have any questions regarding this report or require further information Sincerely, /vEtso,v G EorEcH NICÁ¿ ASSO CIATES, INC' Khaled M. Shawish, PE Principal Engineer TABLE OF CONTENTS Surface Conditions.....2 Subsurface Conditions Hydrogeolo gic Conditions ..J SENSITIVE AREA EVALUATION.......... Seismic Hazard...... Erosion Hazard...... CONCLUSIONS AND RECOMMENDATIONS............... General... Erosion Control Site Preparation and Grading Temporary and Permanent Slopes Foundations ................ Retaining Walls Structural Fill. Slab-on-Grade Pavements.10 10 ..... 10 ....,........2 3 J 4 4 4 5 5 6 7 I 9 9 Utilities........... CONSTRUCTION MONITORING USE OF THIS REPORT. LIST OF FIGURES Figure 1-VicinityMap Figure2-SitePlan Figure 3 - Soil Classification Chart Figure 4 - Test Pit Logs 12 12 Àfcr c.arr ^E^7E,âLt^tt/^^t ^êê/l^t^TEê tltfa Geotechnical Engineering Evaluation Sundstone.short Plat I4l4 - 9th Avenue North Edmonds, Washington INTRODUCTION This report presents the results of our geotechnical engineering investigation and evaluation of the planned Sundstone Short plat Development project in the Edmonds, Washington. The project site is located at1414 - 9th Avenue lVest, as shown on the Vicinity Map in Figure 1. The purpose of this study is to explore and charactenze the site's surface and subsurface conditions and to provide geotechnical recommendations for the planned site development. For our use in preparing this report, we have been provided with an undated and untitled preliminary site plan showing the existing and proposed conditions within the site. The property is currently occupied by a multi-level multi-family residential structure within tle eastern portion of the site, while the subject western portion of the site is undeveloped. We understand the proposed developments within the site will include short platting the properly and constructing a new residential structure within the lower western parcel. Topography within the subject portion of the site slopes gently from east to west. Vegetation generally consists of grass-covered yard areas and scattered young to mature trees. Final development and grading plans have not been prepared at the time this report was issued. Final stormwater plans have also not been developed, however, we understand that stormwater may be directed to on-site infiltration systems, if feasible. The existing and proposed site layout is shown on the Site Plan in Figure 2' SCOPE The purpose ofthis study is to explore and characterize the site surface and subsurface conditions, and provide general recommendations for site development. Specifically, o¡r scope of services includes the following: 1. 2. J 4. 5. 6. 8. Review available soil and geologic maps of the area. Explore the subsurface soil and groundwater conditions within the site with track- mounted backhoe-excavated test pits. Excavation services were provided by NGA' Perform laboratory gfain-size sieve analysis on soil samples, if necessary Provide recommendations for earthwork, foundation support, and slabs-on-grade. Provide recommendations for temporary and permanent slopes. Provide recommendations for pavement subgrade. Provide recommendations for site drainage and erosion control' Provide our opinion on the feasibitþ of infiltration for the onsite soils. ^rE' er'lÀ, t2 tr |1TF t: t¿ N I f2 Ä I ^¿l -q.salcr^¿l Tts rr\rc- Geotechnical Engineering Evaluation Sundstone Short Plat Edmonds, Washington NGAFileNo. 1074518 December 27,2018 Page2 9 Provide long-term design infiltration rates based on on-site Pilot Infilhation Testing (PIT) perthe 2014 WSDOE Manual. One testto be performed within each proposed properly. Provide recommendations for infiltration system installation. Document the results of our findings, conclusions, and recommendations in a written geotechnical report. 10. 11. SITE CÛNDITIOIYS Surface Conditions The overall property consists of an approximately 1.18-acre parcel with a multi-family residential structure within the eastern portion of the site and vacant undeveloped land to the west. We understand the proposed improvements will consist of dividing the property into two parcels and constructing a residential structure within the westem parcel. The proposed western parcel is vegetated with grass lawn and few scattered young to mature trees. The ground surface slopes gently from east to west. The subject site is bound to the east by the multi-family structure, to the north by a residential properly, to the south by Puget Drive, and to the west by 9û Avenue North. We did not observe surface water throughout the site during our visit on Decembet 3,2018. Subsurface Conditions Geology: The site is mapped on the Geologic map of the Edmonds East and part of the Edmonds West quadrangles. Washington, by James P. Minard (US Geological Surve¡ 1983). The site is mapped as till and transitional beds (Qtu and Qtb, respectively). Till deposits generally consists of a compact non-sorted mixture of clay, silt, sand, pebbles, and boulders. Transitional beds are described as thin bedded clay, silt, and very fine to fine sand with some layers of peaty sand and gravel lower in the unit. Our. explorations typically encountered undocumented fill underlain by siþ fine to medium sand with varying amounts of gravel consistent with the descrþion of native till deposits at depth. Explorations: The subsurface conditions within the site were explored on December 3, 2018 by exoavating four test pits to approximate depths in the range of 4.0 to 5.5 feet below the existing ground surface using a mini-trackhoe. 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 test pits. The soils were visually classified in general accordance with the Unified Soil Classification System, presented in Figure 3. The logs ofour test pits are attached to this report and are presented as Figure 4. We present a brief summary of the subsurface conditions in the following paragraphs. For a detailed description of the subsurface conditions, the logs of the test pits should be reviewed. llst aatt ^F^]F^t t]tt^ ^, I ââ^^, a 7Fã t^t^ Geotechnical Engineering Evaluation Sundstone Short Plat Edmonds, Washington NGAFileNo. 1074518 December 27,2018 Page 3 At the surface of each exploration we generally encountered 1.7 to 2.5 feet of dark brown to brown, silty fine to medium sand with varying amounts of gravel, roots, and organics, which we interpreted as topsoil and/or undocumented fill soils. underþing the fill soils and topsoil we generally encountered medium dense or better orange-brown to gray, siþ fine to medium sand with varying amounts of gravel and iron- oxide staining, which we interpreted as native till deposits at depth. Infiltration Pit 1 and Test Pits 1 I L t-.L,^^-^^-L:--^r^-+L^^r/n <A << ^-/tÁ-frFaathelnrvfheer¿istïnsørotlndsurface.mrougn J lennllrateu aÙ lgsPçutrve uePLuù ul r.v, r'wt J'¿r 4r¡s 1'.v -Þ Þ- - - -- Hydrogeologic Conditions We did not encounter þoundwater within our explorations throughout the sitê. If gtoundwater is encountered during construction we would interpret this as a perched groundwater condition. Perched water oocurs when surface water infiltrates through less dense, more perlneable soils and accumulates on top of relatively low permeability materials. The more peÍneable soils consist of the topsoil/weathered soils and undocumented fill. The low permeability soil consists of relatively siþ native deposits. 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 201g International Building Code (IBC) for seismic site classification for this project. Since competent glacial 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, whioh specifies a design earthquake having a 2o/o probability of occurrence in 50 years (return interval of 2,475 years), and the 2008 USGS seismic hazard maps' Table I -20L8IBC Seismic Design Parameters Site Class Spectral Acceleration at 0.2 sec. (g) S, Spectral Acceleration at 1.0 sec. (g) Sl Site Coefücients Design Spectral Response Parameters F.Fo Sos Sor D 1.278 0.501 1.000 1.5 0,8s2 0.501 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. ^rFr QôI\r ê,EnrtreHMtcal ,¿lssoctATES. rNC, Geotechnical Engineering Evaluation Sundstone Short Plat Edmonds, Washington NGA File No. 1074518 December 21,20t8 Page 4 The site is located within the South Whidbey Island Fault Zone (SWIFZ): an active, shallow region of seismicþ 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 yearcago, and that the fault zone can produce aN.47.5 earthquake' In our opinion, the possibilþ of faulting ground rupture caused by this fault zone is considered low' 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 deposits interpreted to underlie the site have a low potential for liquefaction or amplification of ground motion' Erosion Hazwd The criteria used for determination qf the erosion hazard for affected areas include soil type, slope gradient, vegetation cover, and groundwater conditions. The erosion sensitivþ is related to vegetative cover and the specific surface soil types, which are related to the underlying geologic soil units. The Soil Survey of Snohomish Countv Area. Washingon. by the Natural Resources Conservation Service (NRCS) was reviewed to determine the erosion hazard of the on-site soils. The surface soils for this site were mapped as Alderwood-Urban land complex ,2 to 8 percent slopes. The erosion hazard for this material is listed as slight. This site is relatively level to gently sloping and there are no steep slopes on the property' It is our opinion that the erosion hazard for site soils should be low in areas where the site is not disturbed. coNcLUsroNS AND RECOMMENDATTONS General It is our opinion that the site planned development is feasible from a geotechnical standpoint. Our explorations indicated that the site is generally underlain by competent native bearing soils at relatively shallow depths. The native soils encountered at depth should provide ¿dequate support for foundation, slab, and pavement loads. We recommend that the planned structure be designed utilizing shallow foundations. Footings should extend through any loose soil or undocumented fill soils and be founded on the underlying medium dense or better native bearing soil, or structural fill extending to these soils. The medium dense or better native glacial soils should typically be encountered approximately two to three feet below the existing surface, based on our explorations. We should note that localized areas of deeper unsuitable soils and/or undocumented fill could be encountered at this site. This condition would require additional excavations in foundation, slab, and pavement areas to remove the unsuitable soils. ArEr crì^, t?trñTtrn,tlMreil Á.ssocrATES. 'NC. Geotechnical Engineering Evaluation Sundstone Short Plat Ednnonds, Washington NGAFileNo. 1074518 December 27,2018 Page 5 Based on the results of our infiltration testing and soil explorations throughout the site, it is our opinion that the onsite native soils are not conducive for traditional methods of stormwater infiltration. This is further discussed in the Site Drainage section of this report' Erosion Control The erosion hazardfor the on-site soils is interpreted to be slight for exposed soils, but actual erosion potential will be dependent on how the site is graded and how water is allowed to concentrate' Best Management practices (BMps) should be used to control erosion. Areas disturbed during construction should be protected from erosion. Erosion control measures may include diverting surface water away from the strþped or disturbed areas. Silt fences and/or straw bales should be erected to prevent muddy water from leaving the site. Disturbed areas should be planted as soon as practical and the vegetation should be maintained until it is established. The erosion potential of areas not stripped of vegetation should be low. Site Preparation and Grading After erosion control measures are implemented, site preparation should consist of shipping the topsoil, undocumented fill and loose soils from foundation, slab, pavement areas, and other structural areas, to expose medium dense or better native bearing soils. The stripped soil should be removed from the site or stockpiled for later use as a landscaping fill. Based on our observations, we anticipate stripping depths of two to three feet, depending on the specific locations. However, additional strþing may be required if areas of deeper undocumented fill andlot loose soil are encountered in unexplored areas ofthe site' After site shipping, if the expoçed subgrade is deemed loose, it should be compacted to a ¡on-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 pavement areas, the loose soils should be removed and replaced with rock spalls or granular sJructural fill' If significant surface water flow is encountered during construction, this flow should be diverted around areas to be developed, and the exposed subgrades should be maintained in a semi-dry condition' If wet conditions are encountered, alternative site stripping and 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 ,\rtr, qrî^r êtrôTteþtM¡caL assocrAIES. 'NC. Geotechnical Engineering Evaluation Sundstone Short Plat Edmonds, Washington NGAFileNo. 1074518 December 21,2018 Page 6 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 ofprepared subgrade' The site soils are considered to be moisture-sensitive and will disturb easily when wet' \Me recommend that construction take place during the drier summer months if possible' However, if construction takes place during the wet season, additional expenses and delays should be expected due to the wet conditions' Additional expenses could include the need for placing a blanket of rock spalls on exposed subgrades' construction haffic areas, and paved areas prior to placing structural fill. Wet weather grading will also require additional erosion control and site drainage measures. Some of the on-site soils may be suitable for use as shuctural fill, depending on the moisture content of the soil at the time of construction' NGA should be retained to evaluate the suitability of all on-site and imported structural fill material during construction. Temporary and Permanent SloPes Temporary cut slope stability is a function of many factors, including the type and consistency of soils' depth of lhe 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 conshued to imply that Nelson Geotechnical Associates, Inc. assumes responsibility for job site safety. Job site safety is the sole responsibility ofthe project contractor' For planning pufposes, we recommend that temporary cuts in the upper undocumented fill soils be no steeper than 2Horizontal to 1 vertical (2H:1V). Temporary cuts in the competent native soils at depth should be no steeper than 1.5H:1V. If siguificant 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. The slope protection measures may include covering cut slopes with plastic sheeting and diverting surface runoff away from the top of cut slopes. we do not recommend vertical slopes for cuts deeper than four feet, if workef access is necessary' We recommend tliat cut slope heights and inclinations conform to appropriate OsHA/\MISHA regulations. Permanent cut and fitl stopes 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' 't' Geotechnical Engineering Evaluation Sundstone Short Plat Edmonds, Washington NGAFileNo. 1074518 December 21,2018 Page 7 Foundations conventional shallow spread foundations should be placed on medium dense or better native bearing soils, or be supported on structural fill or rock spalls extending to those soils. Medium dense soils should be encountered approximately two to three feet below ground surface based on our explorations' Where undocumented fill or less dense soils are encountered at footing bearing elevation, the subgrade should be over-excavated to expose suitabie bearing soil. The over-excavation rnav be filled with structura! fil!, or the footing may be extended down to the competent native bearing soils. If footings are supported on structural fill, the fill zone should extend outside the edges ofthe footing a distance equal to one halfof the depth of the over-excavation below the bottom of the footing. Footings should extend at least 18 inches below the lowest adjacent finished ground surface for frost protection and bearing capacþ considerations. Foundations should be designed in accordance with the 201g IBC. Footing widths should be based on the anticipated loads and allowable soil bearing pressure' \Mater 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. For foundations constructed as outlined above, we recommend an allowable desigu bearing pressure of not more than 2,000 pounds per square fooi (psf) be used for the design of footings founded on the medium dense or better native bearing soils or structural fill extending to the competent native bearing material. The foundation bearing soil should be evaluated by a representative of NGA' We should be consulted if higher bearing pressures are needed. Current IBC guidelines should be used when considering increased allowable bearing pfessure for short-term transitory wind or seismic loads' potential foundation settlement using the recommended allowable bearing pressure is estimated to be less than l-inch total and yr-inchdifferential between adjacent footings or across a distance ofabout 20 feet, based on our experience with similar projects' Laterul 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 densþ 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 ,.neaf, against the native medium dense soils or compacted fill should be used as backfill against the front of the footing. we recommend that the upper one foot of soil be neglected when calculating the passive resistance. ^tE' (r¡.tÀ' t2trî1Ttrr.I1NILAt Á.s.socrÁTEs- r rc. Geotechnical Engineering Evaluation Sundstone Short Plat Edmonds, Washington NGA File No. 1074518 December 2L,20L8 Page 8 Retaining'Walls specific grading plans for this project were not available at the time this report was prepared, but retaining walls may be incorporated into project plans. In general, the lateral pressure acting on subsurface retaining walls is dependent on the nature arid 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 backÍìli. For walis that are froe to yiei,J at the top at least cne 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. A seismic design loading of 8H should also be included in the wall design, where H represents the total height of the wall' 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 height of the wall. This would include the effects of surcharges such as haffic loads, floor slab loads, slopes, or other surface loads' We could consult with the structural engineer regarding additional loads on retaining walls during final design, ifneeded. The lateral pressules 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 we! 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 ofthe wall backfill. This can be accomplished by placing wall backfill in f-inch loose lifts and compacting the backfill with small, hand-operated compactors within a distance behind the walt 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 backfitl material and observe installation of the drainage systems. ^rE' eflÄr ntrnrFCrlweAt ÁssocrÁTEs. ,'VC, Geotechnical Engineering Evaluation Sundstone Short Plat Edmonds, Washington NGAFileNo. 1074518 December 2I,20I8 Page 9 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 r1egree 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 filI placement' Materials: Structural fill should consist of a good qualþ, 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-inchsieve). 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' X,ill placement: Following subgrade preparation, placement of structural fîll 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 densþ' Maximum dry density, in this report, refers to that density as determined by the ASTM D-1'557 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 Slabs-on-grade should be supported on subgrade soils prepared as described in the Site Preparation and Grading subsection of this report. we recommend that all floor slabs be underlain by at least six inches of free-draining gravel 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 additional2-inch-thick moist sand layer may be used to cover the vapor barrier. This sand layer may be used to protect the vapor barrier membrane and to aid in curing the concrete' ^rE' Qfì^r nÉnrFerlMtcaÍ assocrÁTEs. 'ruC. Geotechnical Engineering Evaluation Sundstone Short Plat Edmonds, Washington NGAFileNo. 1074518 December 21,2018 Page 10 Pavements Pavement subgrade preparation and structural frlling where required, should be completed as recommended in the site prepar¿tion and Grading and structurat Fill subsections of this report' The pavement subgrade should be proof-rolled with a heavy, rubber-tired piece of equipment, to identiff soft or yielding areas that require repair. The pavement section should be underlain by a minimum of six inches of clean gtanuiar pit run. -We shouiti be retai¡red to observe the prooÊrolling and recommenel repairs prior to placement of the asphalt or hard surfaces' 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 compacte d to 95yoof 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. Site Drainage Infïltration: We conducted a Small pIT within lnfiltration Pit 1, located within the northwest portion of the site as shown on the attached schematic site Plan in Figure 2. The test was conducted within a pit that measured 4.O-feet rong by 3.0-feet wide by 4.O-feet deep. The pit was filled with l2-inches of water at the beginning of the day and we began the soaking period of the pIT for approximately 6 hours. At this time, the water flow rate into the hole was monitored with a Great Plains lndustries (GPÐ TM 075 water flow meter for the Pre-soak Period. After the 6-hour soaking period was completed, the water level was maintained at approximately 12- inches for one hour for the steady-state period. The flow rate for Infiltration Pit 1 st¿bilized at 0'02 gallons per minute (1.20 gallons per hour). This equated to an approximate infilhation rate of 0'182 inches perhour. The water was shut off after the steady-state period.and monitored at least every 15 minutes for one hour. After 60 minutes, the water level within the pit dropped approximately 0'125 inches, resulting in a measured infiltration rate of 0.125 inch per hour. In accordance with the Table 3.5 of the Department of Ecology 2014 SWMMWW, correction factors of 0.8, 0.5, and 0.9 for cFv, cFt, cFm, respectively were applied to the field measured infiltration rate of 0.125 inches per hour, obtained from the falling-head portion of the testing in Infiltration Pit 1' A total correction factor of 0.36 was applied to the measured field infiltration rate obtained from the falling head portion of the test to determine the long-term design infiltration rate. Geotechnical Engineering Evaluation Sundstone Short Plat Edmonds, Washington NGAFileNo. 1074518 December 21,2018 Page l1 using the above correction factor, we calculated a long-term design infiltration rate of approximately 0.045 inches per hour. In our opinion, a long-term design infiltration rate of 0.045 inches per hour could be utilized to desigu low-impact infiltration systems, such as rain gardens, bioswales' or permeable pavements within the native, siþ fine to medium sand with gravel found on this site at depth' we recommend these systems be sized and designed in accordance with the 2014 Department of Ecolog-v stormwater Management Manuai for 'westeni washingfon and in conjunction with the provided long- term design infiltration rate of 0.045 inches per hour. Additionally, due to the very low infiltration rate we recommend incorporating an overflow component into any infiltration systems within the site' which should be directed towards an approved point of discharge' We recommend that any proposed infiltration systems be placed as to not negatively impact any proposed or existing nearby structures and also meet all required setbacks from existing properly lines, structures' and sensitive areas as discussed in the drainage manual. In general, infiltration systems should not be located withq proposed fill areas within the site associated with site grading or retaining wall backfill as such condition could lead to failures of the placed fills and/or retaining structures. we should be retained to evaluate the infiltration system design and installation during construction' 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 ¡esidences. 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 residences. Surface water should be collected by permanent catch basins and drain lines, and be discharged into an approved discharge system' Subsurface l)rainage: 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 route.d into a permanent storm drain. we recommend the use of footing drains around the structures. 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 ^rE, c,.r^, êtrñÍÉt:,HMlei,/. ÁSSOC'AIES. tilC. 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' CONSTRUCTION MONITORING We should be retained to provide construction monitoring services during the earthwork phase of the project to evah:ate sr-rbgrade conditions. temporary cut conditions, fill compaction, and drainage system installation. USE OF THIS REPORT NGA has prepared this report for Ms. Lindell Graham and her 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 report, conclusions, and interpretations should not be construed as a warranty of subsurface conditions. A contingency for unanticipated conditions should be included in the budget and schedule. we recommend that 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' 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 ãtthe time this report was prepared. No other warranty, expressed or implied, is made. our observations, findings, and opinions are a means to identiff and reduce the inherent risks to the owner. Geotechnical Engineering Evaluation Sundstone Short Plat Edmonds, Washington NGAFileNo. 1074518 December 21,2018 Page 12 o-o-o A'F' QôflI I}trÔTFEHNICALASSOC'ATES, INC. Geotechnical Engineering Evaluation Sundstone Short Plat Edmonds, Washington NGA File No. 1074518 December 21,201'8 Page 13 It has been a pleasure to provide service to you on this project. If you have any questions or require further information, please call. Sincerely, ,vEtso| GEOTECHNICAL ASSO CIATES, lNC. ,lø,ú AlexB. Rinaldi, GIT Staff Geologist tr Maher A. Shebl, PhD, PE, M.ASCE Senior Engineer ABR:MAS:dy Four Figurês Attached ruELSO'V GEOTECHNICAL ASS OCIATES, INC. /\ VICINITY MAP N Not to Scale I :lj :i Ð:;ii:iJ t:j s:r tjìijiy ti I t fr rÞ ¡r¡!n" rÌ,'þ, li. , i?or,,".o il, : .$ "i.*¡;:i. '11. , .ll .c', tt PÍiif1 lb . fì; 'iì1. Pugrlr\$ Maoltlv¡çud flat:k ¡ç¡ & Gom CIub Y <l -llrltilt I {i Edmr:nds 6A ËlenrÊtlary $t:!¡ooi ? 'll'iÌtu l-¡I::....ı Vì111¡n!ì{.frf !r ìt e ìJl))l:.¡¡:iri *i ':ì üJ.iijùr6 iíi PI :.! -s s . . , 1¡:iit, Si ,iiv Cri$Þ{}rc gl C.li' r.J:r: .¡t Fdrlr{tn(ls tirrilÈdrlr Mr-.r:¡ar!ist fl i¡rirch j,.l é'2- Tl.iä t3Õ"1t1 ü:Ë Ë.t)M0í¡Ð.s lntãâi' iit iinly RÕ$'-ry Otiurçh ü a,,xt.:i l¡¿s 2 t ;r!;..i f; :;$ ç!,:| 1ìr i1 ,...¡r:'i ' a Edmonds, WA Project is¡te By CKDateRevisionNo.Project Number 1074518 DPN ABR12t5118origlnal¡Graham Short Plat Vicinity Map NE N G l\ GEOTECHNICAL ENGf NEERS & GEOLOGISTS LSON GEOTECHNICÄL AssocrATES, lNC. hdlnvill. onè Figure 1 I 731 l-1 35lh Âv.. NE, ^.500r¡Vood¡nvillr, WA 9t072 (¡251 ¡8e166S ¿ Fatr 181-251 0 w.ñal6onlaoLdr.@m Éâ!l \l¡.n¡þh¡ offo. 5528 lnû¡ry b., {2 EdW6¡.bh.., WA 88802 (509) 60t7€98 , Fd: €8t7692 l I ¡ N ite Plan /\ 20.00'I tI I I .*.:]-"Ê-=i I -t- I Number and approximate location of infiltration test pit 0 30 I I IIt I qí ,$ tI, ¿ t I t I I I ¡ II I t I I I t ì ¡ I I II I f ¡ I i I t I t s t ì i' I, li II -ri!l ï ! I I I LEGEND 196th St NW I,TP.3trI40.00' I-r-I Number and approximate location of test pit Property line (o) oz INF-1 TP-.I Approximate Location of Proposed Residence II I Scale: 1 inch = 30 feet tr ox z.o o 0t .D Ðo s.JL o5 Jo\¡À(,t J æ oof.! (o L ı l\) G)a 0):tu) s, ntDOJ:¡ .E d z FI zo Ê¡ G)iÉE- EBÉIÈ OåãüË { g=ã= f, * >zo ¡ iëiã 3 5"Ë IrÉFrlzåoázPrfElÍ'8ã 3O ^Pëg-" o8?$Ë nr äÈãE Ê È.:Ëå ö Eå.d I olËsF IgÉ o à[ 8ä Qa> frl{o f{{om'o7!o3'o |- Þ 6 @ o q. lo (fTz @v UNIFIED SOIL CLASSIFICATION SYSTEM MAJOR DIVISIONS GROUP SYMBOL GROUP NAMI COARSE GRAINED SOILS MORE THAN 50 o/o RETAINED ON NO.2OO SIEVE GRAVEL MORÊ THAN 50 % OF COARSE FRACTION RETAINED ON NO.4 SIEVE CLEAN GRAVEL GW WELL-GRADED, FINE TO COARSE GRAVEL GP POORI.Y-GRADED GRAVEL GRAVEL WTH FINES GM SILry GRAVEL GC CLAYEY GRAVEL SAND MORE THAN SO % OF COARSE FRACTION PASSES NO.4 SIEVE CLEAN SAND SW WELL-GRADED SAND, FINE TO COARSE SAND SP POORLY GRADED SAND SAND WITH FINES SM SILÏY SAND SC CLAYEY SAND FINE. GRAINED SOILS MORE THAN 50 % PASSES NO. 200 STEVE SILT AND CLAY LIQUID LIMIT LESS THAN 50 % INORGANIC ML SILT CL CLAY ORGANIC OL ORGANIC SILT, ORGANIC CLAY SILTAND CLAY LIAUID LIMIT 50 o/o OR MORE INORGANIC MH SILT OF HIGH PLASTICITY, ELASTIC SILT CH CLAY OF HIGH PLASTICITY, FAT CLAY ORGANIC OH ORGANIC CLAY, ORGANIC SILT HIGHLY ORGANIC SOILS PT PEAT NOTES: 1) Field class¡fication is based on v¡sual examination of soil in general accordance with ASTM D 2488-93. SOIL MOISTURE MODIFIERS: 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 consistency are based on interpretation of blowcount data, visual appearance of soils, and/or test data. Wet -Visible free water or saturated, usually soil is obtained from below water table Project N 1074518 NE LSON GEOTECI.INICAL AssoctÄ,TEs, lNc.N G A GEoTEcHN¡cAL ENGTNEERS & GEOLOGISTS $¡ædlnv¡llê oñcc t7ô l l-1 35h Av.. NE, A-sm Woodlñvi[., WA 08072 (1251 1fÊ166e , F.r { at25| 0 w.ò.lrong6oM.coñ 5526 lnô¡¡¡y þô., 12 EstW.¡À¡ch.., WA 08802 No.Date Revision By CK 'l 12t5t18 Original DPN A8RGraham Short Plat Soil Classification Chaft Figure 3 DEPTH (FEET)USC LOG OF EXPLORATION SOIL DESCRIPTION INFILTRATION PIT ONE 0.0 - 0.8 0.8 - 2.0 t^ A^ TEST PIT ONE 0.0 - 2.5 2.5 - 5.0 TEST PIT TWO 0.0 - 1.7 'l.7 - 5.5 TEST PIT THREE 0.0 - 1.0 1.0 - 4.0 9¡Vr SM SM SM GRASS UNDERI.A¡N BY DARK BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, ROOTS, AND ORGANTCS (LOOSE TO MEDTUM DENSE, MOTST) (TOPSO|L) ORANGE-BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, WOOD DEBRIS, AND ROOTS (LOOSE TO MEDTUM DENSE, MOTST) EILL) .¡t(ÂY, ötL ¡ Y i-tNE ¡o MEÐ¡iituî SAND vútri-i GRAVEL 0liED¡ijM DENSË To DENSE, Motsr) SAMPLE WAS NOT COLLECTED GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED TEST PIT CAVING WAS NOT ENCOUNTERED TEST PITWAS COMPTETED AT4.O FEET ON 121312018 DARK BROWN, ORGANIC-RICH SILTY FINE TO MEDIUM SAND W¡TH GRAVEL AND ROOTS (LOOSE TO MEDTUM DENSE, MOTST) (TOPSOIL) GRAY, SILry HNE TO MEDIUM SAND WTH GRAVEL AND TRACE IRON-OXIDE STAINING (MEDTUM DENSE TO DENSE, MOTST) SAMPLE WAS COLLECTED AT 4,0 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT WAS COMPLETED AT 5.0 FEET ON 12I3I2O1B DARK BROWN TO BROWN, ORGANIC-RICH SILTY FINE TO MEDIUM SAND WITH GRAVEL AND ROOTS (LOOSE TO MEDTUM DENSE, MOTST) (TOPSOIL) ORANGE.BROWN TO GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL, TRACE IRON.OXIDE sTArNtNc, AND ROOTS (MEDIUM DENSE TO DENSE, MOTST) SAMPLE WAS NOT COLLECTED GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED TEST PIT CAVING WAS NOT ENCOUNTERED TEST PITWAS COMPLETED AT 5.5 FEET ON 121312018 DARK BROWN TO ORANGE-BROWN, ORGANIC-RICH SILTY FINE TO MEDIUM SAND WITH GRAVELAND ROOTS (LOOSE TO MEDTUM DENSE, MOTST) (TOPSO|UEILL) LIGHT BROWN TO GRAY, SILTY F¡NE TO MEDIUM SAND WITH GRAVEL AND TRACE IRON-OXIDE sTAtNtNc (MED|UM DENSE TO DENSE, MOTST) SAMPLE WAS NOT COLLECTED GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT WAS COMPLETED AT 4.0 FEET ON IZ3l2018 ABR:KMS 'VE¿SO'V G EOTECHNICA¿ ASSO CIATES, INC. F|LE NO 1074518 FIGURE 4