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Revised Geotech Letter - 8-3-16 - Krazan File 09216010.pdf & A S S O C I A T E S, I N C. GEOTECHNICAL ENGINEERING ENVIRONMENTAL ENGINEERING CONSTRUCTION TESTING & INSPECTION July 15, 2016Krazan File No. 092-16010 Revised August 3, 2016 John and PamSwansonEmail: swansonpj1@gmail.com th 1305–11Place NorthPhone: 425-327-0212 Edmonds, WA 98020 RE:Limited Geotechnical Engineering Investigation Slope Evaluation for New Deck th 1305–11Place North Edmonds, WA98020 Greetings: This letter presents the results of our limited geotechnical engineering investigationfor the proposed deck th projectlocated at 1305–11Place North in Edmonds,Washington, as shown on the Vicinity Map in Figure 1.The scope of this limited geotechnical engineering letter was outlined in proposalno.G16-574WAL, dated July 11, 2016. INTRODUCTION The subject property is situated on moderatelysloping to nearly level ground along the top of a steep west- facing slope on the adjacent property to the west, which is classified as a geologically hazardous area.The project will include the construction of new decksnear the northwestern and northeastern corners of the existing house.It is our understanding that the City of Edmondsis requiring a geotechnical engineering assessmentas part of the building permit. A geotechnical assessment is required for this project due to the project being within 50 feet of a potential landslide hazard area (Edmonds City Code 23.80.050). For our use in preparing this geotechnical letter, we have been provided with photos of partial plans for the project.We have also reviewedavailable topographic and geologic maps of the site area, aerial photographs, as well as Edmonds City Code (ECC) Chapter 23.80 –“Geologically Hazardous Areas”. PLANNED DECK ADDITIONS The residence consists of a wood frame main floor over a daylight basement. Two deckadditions are planned extending from the main floor level of theresidence, one at the northwestern corner of the house and the other at the northeastern corner of the house. Current plans indicate that the northwesterndeck will be located at least25 feet from the top of the steep slope on neighboring propertyto the west.The northeastern deck will be located at least60 feet from the top of the steep slope. GEOLOGIC SETTING The “Preliminary Surficial Geologic Map of the Edmonds East and Edmonds West Quadrangles,Snohomish and King Counties, Washington,” prepared by Mackey Smith (State of Washington Department of Natural Offices Serving The Western United States • 4303–198 th Street SW Lynnwood, WA 98036(425) 485-5519; Fax (425)485-6837 KRAZAN & ASSOCIATES, INC. KA No. 092-16010 Slope Evaluation th 1305–11Place N Edmonds, WA July 15, 2016 Revised August 3, 2016 Page No. 2 ________________________________________________________________________________________ Resources –Division of Geology and Earth Resources,in cooperation with the USGS,1975) indicates that the property is underlain byQuaternary Vashon Recessional Outwash (Qvr), with Quaternary Vashon glacial till (Qvt)and Quaternary Esperance Sand (Qe) mapped nearby. Glacial recessional outwash generally consists of moderately sorted, medium dense sand and gravel with cobbles. Glacial tilltypically consists of an unsorted, very compact mixture of clay, silt, sand, gravel, cobbles and boulders.Esperance sand generally consists of moderately sorted, very compact sand with some interbedded layers of pebbles. LIMITED GEOTECHNICAL INVESTIGATION We visited the site on July 11, 2016to explore the shallow subsurface soil and groundwater conditions with hand auger excavationsin the proposed deck areasas shown on the Site Plan in Figure 2. A geologist from Krazan and Associates conducted the excavations, examined the subsurface soil and groundwater conditions encountered,and maintained logs of the explorations. Soil samples were collected and sealed in plastic bags. Surface Water Runoff: The property includes level areas and gentleslopes down to the west. Stormwater is interpreted to sheet-flow across lawn areas within the property. Muchof the stormwater runoff likely infiltrates into the grass lawn, topsoil and landscapingthat covers much of the site. Roof stormwater runoff is directed through downspoutsand into subsurface drainpipes.We did not see indications of runoff being directed onto the steep slope. Soil Types/Subsurface Conditions: The following paragraphsprovidedescriptions of the soils encountered in the hand auger borings. Hand Auger One (H-1)was located approximately 10 feet west of the northwest corner of the residence. H-1 encountered approximately 3 inches of sod and topsoil. Underlying the topsoil,H-1 encountered medium dense to dense, brown silty sand with gravel to a depth of about 6 inches. We interpreted the brown silty sand with gravel to be weathered soil. Underlying the weathered soil, H-1 encountered dense to very dense, gray silty sand with gravel to the depth explored of approximately 18 inches below the ground surface. We interpreted the dense to very dense soil to be native glacial till. Hand Auger Two (H-2)was located approximately 10 feet west and about 14 feet south of the northwest corner of the residence. H-2 encountered approximately 3 inches of sod and topsoil. Underlying the topsoil, H-2 encountered dense, gray to brown silty sand with gravel to a depth of about 9 inches. We interpreted the dense, gray to brown soil to be undocumented fill. Underlying the undocumented fill, H-2 encountered dense to very dense, gray silty sand with gravel to the depth explored of approximately 14 inches below the ground surface. We interpreted the dense to very dense soil to be native glacial till. Krazan & Associates, Inc. Offices Serving The Western United States KRAZAN & ASSOCIATES, INC. KA No. 092-16010 Slope Evaluation th 1305–11Place N Edmonds, WA July 15, 2016 Revised August 3, 2016 Page No. 3 ________________________________________________________________________________________ Hand Auger Three (H-3) was located approximately 33 feet west of the northwest corner of theresidence. H- 3 encountered approximately 8 inches of topsoil. Underlying the topsoil, H-3 encountered medium dense to dense, gray to brown silty sand with gravel to the depth explored of about 16 inches below the ground surface. Hand Auger Four (H-4) was located approximately 4feet westand about 6feet northof the northeast corner of the residence. H-4 encountered approximately 6inches of topsoil. Underlying the topsoil, H-4 encountered pea gravel/drain rockto a depth of about 18inches.Underlying the drain rock, H-4encountered medium dense to dense, gray to brown silty sand with gravel to the depth exploredof approximately 24 inches below the ground surface. We interpreted the medium dense to dense materialto be native soil. Groundwater: Groundwater seepage was not observed in H-1 through H-3. However, we observed perched groundwater within test-hole H-4.The groundwater was observed at approximately 10inches below the ground surfacein a layer of drain rock. Perched groundwater occurs when surface water infiltrates through less dense, more permeable soils and accumulates on top of a relatively low permeability soil layer. 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 andincrease during wetter periods. Erosion Concern/Hazard: The Natural Resources Conservation Services (NRCS) map for Snohomish County classifies the site area as Alderwood-Urban Land Complex (8 to 15percent slopes)and Alderwood gravelly sandy loam (15 to 30 percent slopes). The NRCS indicates that these soils have a moderate to severe potential for erosion, respectively, when exposed at the surface.We do not anticipate that significant areas of the soil will be exposed for this deck project. It has beenour experience that the potential for soil erosion can be minimized through landscaping and surface water runoff control. Typically, erosion of exposed soils will be most noticeable during periods of rainfall and may be controlled by the use of normal temporary erosion control measures, such as mulching. Erosion control measures should be in place before the onset of wet weather. Slope Stability: The property is located in a residential neighborhood with the ground surface sloping down to the west. It appears that the neighborhood was graded with level benches and steep slopes during the original site development. Residential structures were constructed on the level benches, with steep slopes located between the residences. Asteep slope is located on the neighboring property to the west, with the top of the slope near the western property boundary.We observed slope conditions from our client’s property during the site visit. Most of the slope is covered with grass, with a few maple trees. The trees appear to have been repeatedly pruned, with tree heights of perhaps 20 to 30 feet. Berry bushes were observed on the northern portion of the Krazan & Associates, Inc. Offices Serving The Western United States KRAZAN & ASSOCIATES, INC. KA No. 092-16010 Slope Evaluation th 1305–11Place N Edmonds, WA July 15, 2016 Revised August 3, 2016 Page No. 4 ________________________________________________________________________________________ slope area. We measured the slope inclination using a hand-held clinometer. The average slope inclination is approximately 35 degrees (about 70 percent). Slope height is approximately 15 to 20 feet, according to the Snohomish County “SnoScape” website and our observations. We did not observe any tension cracks, sloughing, indications of soil creep,or any other signs of soil movement on the steep slope to the west of the propertyduring our site visit.We also did not observe indications of soil erosion on the neighboring steep slope. It is our opinion that the site is stable under current conditions, and the planned decksshould not change slope stability conditions.The planned setback of at least 25 feet from the top of the steep slope should provide an adequate buffer for the typical useful time period of the structure. In our opinion, there is some potential for relatively shallow sloughing and sliding in the surficial loose soils. The planned setback from the top of the steep slope should provide an adequate buffer for the anticipated natural slope recession over time. The control of surface water and near-surface groundwater is very important for the long-term stability of the steep slopes. Maintenance of vegetation cover on the steep slope and the maintenance of drainage systems to direct water away from the steep slope should reduce the potential of slope movement. Deck Foundation Support: We interpretedthe native subsurface soils in the planned deck locations at the northwestern and northeastern corners of the house to consist of dense to very dense glacial till, which should provide adequate soil bearingcapacity for the planned structures.At a minimum, topsoil should be removed from the areas ofthe planned deck foundations. However, the surficial soils (approximately 18 inches deep, but may vary) near the northeastern corner of the house are interpreted to consist of topsoil and loose/soft fill materials,including drain rock,and would not likely provide adequate support for the planned northeastern deck. We recommend that the foundation excavations extend to medium dense or firmernative soils. Thedeck footings should be placed on the native soils. Response to City of Edmonds Plan Review Comments We received a copy of the City of Edmonds Plan Review Comments, dated August 2, 2016. The plan review comments indicated that additional information is needed to specifically address City of Edmonds code ECDC 23.80.060.A and ECDC 23.80.070.A.3 and A.4. The following information includes our response regarding the City code items. Krazan & Associates, Inc. Offices Serving The Western United States KRAZAN & ASSOCIATES, INC. KA No. 092-16010 Slope Evaluation th 1305–11Place N Edmonds, WA July 15, 2016 Revised August 3, 2016 Page No. 5 ________________________________________________________________________________________ Code Requirement ECDC 23.80.060.A: This code requirement pertains to general requirements to development standards, particularly to alterations of geologically hazardous areas or associated buffers. In our opinion, from a geotechnical standpoint, the proposed deck structures will not increase the risk of geologic hazard to neighboring properties with respect to pre-development conditions. Regarding other potential critical areas, it is our opinion that the project should notadversely impact other critical areas to the best of our knowledge. It is our opinion that the deck is designed so that the hazard to theproject is equal to the pre-development conditions,primarily due to the underlying very dense glacial soils and setbackdistance from the steep slope. In our opinion, the proposed activities (ie. the proposed deck construction project) should be considered as a safe activity, provided that our recommendations are followed as indicated in this geotechnical engineering letter. Code Requirement ECDC 23.80.070.A.3 and A.4: These code requirements pertain to development standards regarding specific hazards. A.3 –Alterations Regarding surface water and the proposed deck project, it is our opinion that the new decks should not increase surface water discharge or sedimentation to adjacent properties to a level any greater than pre- development conditions. It is our opinion that the deck project will not decrease slope stability on the neighboring properties. With respect to potential other critical areas, it is our opinion that the deck project should not adversely impact other critical areas, to the best of our knowledge. A.4 –Design Standards Regarding slope stability and factors of safety, it is our opinion, from a geotechnical standpoint, that the proposed deck project will not decrease the current factor of safety for both static and dynamic (ie. seismic) conditions for the subject property or neighboring properties. The underlying very dense glacial soils are interpreted to have high internal strength, with a slope factor of safety interpreted to be above 1.5 for static conditions and above 1.2 for dynamic conditions. Krazan & Associates, Inc. Offices Serving The Western United States KRAZAN & ASSOCIATES, INC. KA No. 092-16010 Slope Evaluation th 1305–11Place N Edmonds, WA July 15, 2016 Revised August 3, 2016 Page No. 6 ________________________________________________________________________________________ Based on project drawings, the proposed decks are to be connected to the existing residence and located at least 25 feetfrom the top of the steep slope, and therefore should aid in avoiding geologically hazardous and other critical areas. The proposed deck structures will be located at least 25 feet from the top of the steep slope, and therefore will not change the existing topographic features of the slope. The proposed deck will not be located on ornear the most critical portions of the subject property or neighboring properties, and therefore the natural landforms and vegetation should be preserved.Grading is not planned on or near the steep slope on the adjacent property. In our opinion, the deck project should not have a negative impact on the neighboring properties, and therefore should not result in a need for increased buffers. Regarding retaining walls and slope alterations, the deck project does not include retaining wall construction or slope modification. The proposed decksare elevated, and to the best of our knowledge, should not significantly increase impervious coverage at the property. LIMITATIONS Geotechnical engineering is one of the newest divisions of Civil Engineering. This branch of Civil Engineering is constantly improving as new technologies and understanding of earth sciences improves. Although your site was analyzed using the most appropriate current techniques and methods, undoubtedly there will be substantial future improvements in this branch of engineering. In addition to improvements in the field of geotechnical engineering, physical changes in the site either due to excavation or fill placement, new agency regulations or possible changes in the proposed projectafter the time of completion of the geotechnical lettermay require the geotechnical letterto be professionally reviewed. In light of this, the Owner should be aware that there is a practical limit to the usefulness of this letterwithout critical review. Although the time limit for this review is strictly arbitrary, it is suggested that two years be considered a reasonable time for the usefulness of this letter. The conclusionsmade in this letterare based on the assumption that soil and groundwater conditions do not vary significantly from those disclosed during our field investigation. If any variations or undesirable conditions are encountered during construction, the geotechnical engineer should be notified so that supplemental recommendations can be made. Krazan & Associates, Inc. Offices Serving The Western United States KRAZAN & ASSOCIATES, INC. KA No. 092-16010 Slope Evaluation th 1305–11Place N Edmonds, WA July 15, 2016 Revised August 3, 2016 Page No. 7 ________________________________________________________________________________________ The conclusions of this letterare based on the information provided regardingtheproposed construction. If the proposed construction is relocated or redesigned, the conclusions in this lettermay not be valid. The geotechnical engineer should be notified of any changes so that the recommendations can be reviewed and re- evaluated. This letter is a limited investigation with the purpose of evaluating the soil and groundwater conditions. The scope of our services did not include any environmental site assessment for the presence or absence of hazardous and/or toxic materials inthe soil, groundwater or atmosphere, or the presence of wetlands. Any statements, or absence of statements, in this letteror on any explorationlog, regarding odors, unusual or suspicious items, or conditions observed are strictly for descriptive purposes and are not intended to convey engineering judgment regarding potential hazardous and/or toxic assessments. The information presented herein is based upon professional interpretation utilizing standard engineering practices and a degree of conservatism deemed proper for this project. It is not warranted that such information and interpretation cannot be superseded by future developments. We emphasize that this letteris valid for this project as outlined above, and should not be used for any other site. Properties on Hillsides: During our visit, we did not observe indications of current land sliding within the property. Relatively shallow failures as well as surficial erosion are natural processes and should be expected to occur within sloping ground. It is our opinion that there is the potential for erosion, soil creep, and shallow failures within the loose surficial soils on the steep slopes. Although indications of erosion and current significant land sliding were not observed on the steep slopes during our site visit, it is our opinion that there is always some potential for soil movement on sloping ground. All people who own or occupy homes on or near hillsides should realize that landslide movements are always a possibility. The landowner should periodically inspect the slope, especially after a winter storm. If distress is evident, a geotechnical engineer should be contacted for advice on remedial/preventative measures. The probability that landsliding will occur is substantially reduced by the proper implementation and maintenance of drainage control measures. Therefore, the homeowner should take responsibility for performing such maintenance. Krazan & Associates, Inc. Offices Serving The Western United States KRAZAN & ASSOCIATES, INC. KA No. 092-16010 Slope Evaluation th 1305–11Place N Edmonds, WA July 15, 2016 Revised August 3, 2016 Page No. 8 ________________________________________________________________________________________ Weappreciate the opportunity to provide service to you on this project. If you have any questions, or if we may be of further assistance, please do not hesitate to contact our office at (425) 485-5519. Respectfully submitted, KRAZAN & ASSOCIATES, INC. 08/03/16 Michael D. Rundquist, P.E.Jeffrey G. League, L.G. Senior Project ManagerProject Geologist JGL:MDR Attachments: Vicinity Map, Site Plan, and Cross Section Krazan & Associates, Inc. Offices Serving The Western United States Puget Sound Approximate SiteArea 1305 - 11th Place North, Edmonds,WA Reference:The Vicinity Map is based on atopographic map titled,USGS “Edmonds East Quadrangle - Washington - 7.5-Minute Series,”dated 2014. Swanson Deck Project, Edmonds,WA Project Number: 092-16010 Date: June 2016 Drawn By:JGL