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20140728164438371.pdfUU&ASSOCIATES, INC. Geotechnical Engineering Engineering Geology Earth Science July 18, 2014, Mi% Jima Henricks 336 Sunset Avenue Edmonds,'WA 98020 Dear Mr. Henricks: Subject: Critical Area and Geotechnical Report Single-faudly Residences ,3,3,6 and 340 Sunset Avenue Edmonds, Washington L Job No. 14-04 t Regarding the corntnent in City of Edmonds 4/9/2014 review letter by Mr. Clugston, we have the following responses: 23.40.090 - CRITICAL AREAS REPORT D. I Applicant: Mr. Jirn Henricks, 336 Sunset Avenue, Edmonds, WA 99020. UEN a, Site Plan — Plate 1. attached (covering both properties). A single-farnily residence is being or to be developed on each property, b, Storm Water Management Marr Storm runoff collected over impervious surfaces of both residences is to be discharged directly into city's steam sewer system per design plans, 19213 Keliflake Place NE - Kenmore, Washington 98028 Phone (426) 483-9134 - Fax (425) 486--2746 C�rJuly 18, 2014 itical Area / G'cotechnical Report --- TwoSingle-Farnily Residences L&A Job No. 14-041 Page 2 IM Report prepared by: L S. Liu, geotechnical engineer, PE, with Liu & Associates, Inc. Report prepared based on review of a ptiblished geologic rnap, a site reconnaissance, and soil condition encountered in a utility trench excavated on the east property (336) on which a residence is currently being constructed. DA — Geologic Hazard Identified. According to City of Edmonds Critical Areas Map, the western portion of the subJect pro.ject site is mapped in seismic hazard areas. D.5 -- The pro.ject sites are flat tovery- gently slope(]. The soils underlying the site are not prone to liquefaction. Therefore, seismic hazard, stieb as landslide, soil lateral spreading, liquefaction, for the sites is minimal and no mitigation is required (See Geotechnical Investigation report under 23.40.120 below). D.6 — No mitigation is required. 23.40.090 — MITIGATION REQUIREMEN'T'S GE077ECHNICAL INVESTIGAT ION' REPORT The two properties sit side by side in an east -west direction, The topo),),raphy survey plan with 2-I`(,)ot contour lines (see Plate 1) shows the properties are almost flat with a maximirtn 9 Ibet of drop Iron) its northeast coverer to its southwest corner over a horizontal distance of about 220 feet, The terrain within these properties generally slopes LJU &ASSOCIATES, INC. July 18, 2014, Critical Area / (1cotechnical Report -- Two Single-Fainily Residences L&A Jot) No. 14-041 Page 3 down 'very gently westward at in overall average grade of about 4,5% across the properties. The properties and their surrounding areas within 200 JIM to the north, south, and east, and the street fronting them to the west, are all either flat or very gently sloped. A single-family residence is currently being or to be developed oil each of the two properties. The unpaved area open areas on these properties will mostly be covered by lawn grass and shrubs. Geologic Setting The Geolo 'ie i alb of the E'drnonds East and Part of 'the Ednionds West , qL41r,,tt e, Wqsbjngq)jj, by Jarnes P. Minard, published by U. S. Geological Survey in 1983, was referenced for the geologic and soil conditions of the properties. The surficial soil unit at and in the vicinity of the properties is mapped as Whidbey formation (Qw) by this publication, The Whidbey formation soil unit on which the properties lie, consists of pre -Fraser Glaciation sediments composed of glacial and non -glacial flood -plain deposits of light - brown to gray, fine sand with silt, minor amount of clay, and occasional gravel and organics, These Whidbey formation deposits, generally underlying thin layers of topsoil and weathered soil, are normally very -stiff to hard in their native undisturbed state. They are also of extremely lo%v permeability and tend to perch storinwater infiltrating into the overlying more permeable surficial soils. The perched groundwater rnay ernerge out of natural slopes and man-made cuts where the Whidbey formation is exposed. When the amount of groundwater seepage increases in wet winter nionths, the emerging groundwater can erode and cause the soil mass in an(,] above the seepage zone to erode 1,11) & ASSOCIATES, INC. July 18, 2014 Critical Area / Geotechnical Report,--- Two Single-1,'arnily Residences L&A Job No, 14-041 Page 4 and slough. The ground within the properties, however, is very gently sloped and, therefore, the possibility for groundwater seepage to occur within the properties stiould be ratherunlikely. A. sewer line trench has been excavated along the south side of the east property. This trench encountered a thin layer of top soil and a layer of weathered soil of light -brown, inedium-dense, gravelly, silty, fine sand. This layer of weathered soil is underlain to the bottom of the trench by a light -brown to fight -gray deposit of hard, fine -sandy to clay silt, which appears to be of Whidbey forniation. This soil condiliori is comparable with our experience of the soil condition in the ininiediate vicinity of the su�jcct properties. Groundwater Condition The sewer line trench did riot encountered groundwater. However, perched groundwater may exist on top of 'the underlying Whidbey formation of extremely low perineability. This perched groundwater is from storinwater infiltrating into the more permeable surficial soils. The arriount of and the depth to the Dez'u,-surfitcc perched groundwater would flrnCtUatC seasonally, depending on precipitation, stirl"ace rirnoft'l ground vegetation cover, site utilization, and other fiactors. The near -surface perched groundwater may dry tip cornpletely during the dryer summer and fall months and accumulate and rise in the wet winter and early spring months. Seismic Hazard Evaluation The west fi-inge of the properties is rnapped by City of Edmonds in seismic hazard areas. Seismic hazards include rupture of fault, lines, soil liqUCfi1Cti0D, lateral soil spreading, and landslide, "t"he properties are flat, to very -gently sloped; therefore, the potential for lateral L,JU & ASSO(""IXTES, INC. July 18, 2014 Critical Area / Geotechnical Report - Two Single -14 amily lkesidences L.&A Job No, 14-041 Pagc 5 soil spreading and landslide to occur oil the properties should be minimal, The properties are not in close proxiniity of known fai.ilt lines; therefore, their hazard firorn fhult line rupture should also be minirnal. 11(luefiaction is another form or seismic hazard. The type of soils most susceptible to liquet.'action during a strong earthquake is saturated, loose, thle sand to silty fine sand deposits. Such loose fine-grained deposits, when suhiected to strong ground shaking, can be densifted and decrease in volume. If water in the deposits is unable to drain quickly, pore water presstire in the deposits would increase. When the pore water pressure continues to build up by prolonged ground shaking, a "quick" corWition will be reached when the pore water pressure equals the effiective overburden soil pressure at some depths in the deposits. Under this condition, the sand deposits would turn into a liquid state and lose its load bearing capacity. ",ate Whidbey formation deposits of had fine -sandy to clayey silt underlying the properties at shallow depth are of high cohesion and shear strength. They are not susceptible to decrease in volume during ground shaking from strong seismic activities. Also, there is a lack of" constant and extensive groundwater table under the properties at shallow depth. Therefore, the chane for pore water pressure in these soils to build up and lead to Soil li(IlletactiOD is Ili]. Therefore, in our opinion, the liquefaction hazard of the properties should also be rninirnat Based on the above analysis, we concluded that: A— No constructior) activities ftonl development wotild degrade critical areas, B —bier requirement specific to critical area type is need( -A. LIU & ASSOCIATES, IN(1­1. July 19, 2014 Critical Area / Geotechriical Repor(--Two Single -Family Residences IAA Job No. 14-041 Page 6 1. No construction activities firom development would degrade critical areas, 2, No steps would be necessary to jrfinimize inrpact on critical areas. 3. No rectiyin g measures are required, 4. No restoring or stabilizing actions require(], 5. No special preservation or maintenance measures required. 6, No compensating measures required. 7. No inonitoring prograerrs required. We are pleased to be of service tca you or) this project. Please contact us if you have questions. Attaclied: Plate I --, Site Plan Yout,s very truly, 1,11 L& 3SOCIA",",IES", INC. 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