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CANOD.pdfr & ASSOCIATES, INC. . ... Geotechnical Engineering Mr. John Gilbert Gilbert Construction 23415 Timber Lane Woodway, WA 98020 Dear Mr. Gilbert: Engineering Geology July 10, 2016 Subject: Critical Area and Geotechnical Report Gilbert Residence 103 Edmonds Street Edmonds, Washington L&A Job No. 16-106 1�&_Cli6f -0693 Earth Science AP P1 LICANTCOPY, ,111112016 OF EDMONWR We understand that an addition to the existing house, located at the above address in Edmonds, Washington, is proposed. The existing house is a one-story above -grade, wood -framed structure. The proposed addition includes expanding the existing house laterally to the south and west up to several feet and constructing a second floor to the house. At your request, we have completed a critical areas report for the proposed development. 23.40.090 - CRITICAL AREAS REPORT D.1 Applicant: Mr. John and Julie Gilbert, 23415 Timber Lane, Woodway, WA 98020. D.2 a. Site Plan — Plate 1 attached. The proposed addition is shown on this plan. The addition will be an above -grade, two-story wood -framed structure expanded 19213 Kenlake Place NE - Kenmore, Washington 98028 Phone (425) 483-9134 - Fax (425) 486-2746 July 10, 2016 Critical Area / Geotechnical Report - Gilbert Residence L&A Job No. 16-106 Page 2 outward from the south and west sides of the existing house, to be supported on conventional footing foundations. Therefore, excavation. for construction of the addition will be limited to shallow footing foundation trenches. b. Storin Water Management Plan — The existing storm drain system of the house is to be protected and maintained. Storm runoff over the new roofs of the addition is to be collected and conveyed into this existing storm drain line system. UM Report prepared by: J. S. Liu, geotechnical engineer, P.E., with. Liu & Associates, Inc. This report is prepared based on review of a published geologic map, a site reconnaissance, and soil condition encountered in three test holes angered on the project site by the drain field designer, Whalen Designs LLC. DA — Geologic Hazard Identified The project site is nearly flat. Therefore, there is little or no erosion and landslide hazards for the site. According to City of Edmonds Critical Areas Map, the western portion of the subject project site is mapped in seismic hazard areas. D.5 -- The project site is nearly flat. The logs of the test holes excavated on the project site by Whalen. Designs LLC are included in their 6/10/2016 report (see attached). All three test holes encountered a glacially consolidated deposit of very -dense, gravelly, silty, fine sand at a depth. of about 2.25 to 2.5 feet below existing ground surface. This deposit is of high shear strength.. Groundwater was not encountered by the test holes. July 10, 2016 Critical Area / Geotechnical Report - Gilbert Residence L&A Job No. 16-106 Page 3 I "�'rorn the above description, the soils underlying the project site encountered by the test holes are not prone to liquefaction. Therefore, seismic hazard, such as landslide, soil lateral spreading, liquefaction, for the site 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 REQUIREMENTS GEOTECHNICAL INVESTIGATION REPORT Surface Condition The project site is almost flat with a very slight slope down to the west with an overall average grade estimated to be less than 3.0%. The project site and its surrounding areas within 200 feet to the north, south, and east, and the street fronting it to the west, are all either flat or very gently sloped. A single-family residence is currently occupying the site. The unpaved open areas on the site are mostly covered by lawn grass and shrubs. Geologic Setting The ('3'e(Aggic M 1) ,tit q,_1&drn0nds vs, 11art. of"' the Edmonds West . . ... ..... ... by James P. Minard, published by U. S. Geological Survey in 1983, was referenced for the geologic and soil conditions of the project site. The surficial soil unit at and in the vicinity of the project site is mapped as Whidbey formation (Q,) by this publication. 111 111��10,11 101 , 1111,111 July 10, 2016 Cr.itical.Area / Geotechnical Report - Gilbert Residence L&A Job No. 16406 Page 4 'rhe Whidbey formation soil unit on whicb 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 low permeability and tend to perch stormwater infiltrating into the overlying more permeable surficial soils. The perched groundwater may emerge out of natural slopes and man made cuts where the Whidbey fonnation is exposed. When the amount of.' groundwater seepage increases in wet winter months, the emerging groundwater can erode and cause the soil mass in and above the seepage zone to erode and slough. The ground within the properties, however, is nearly flat and, therefore, the possibility for groundwater seepage to occur within the site should he rather unlikely. Subsurface conditions of the site were previously investigated by Whalen Designs LLC with three test holes. The logs of these test pits are included in their 6/1.0/2016 report, (see attached). All. three test holes encountered a layer of sandy loam (loose organic topsoil) about 12 to 14 inches thick mantling the site. Underlying the topsoil is a layer of sandy loam (weathered soil of silty fine sand with gravel) from 13 to 18 inches thick.. This sandy loam layer is underlain to the depths explored is a serni.-compact gravelly sand (glacial soil of very -dense, gravelly, silty, fine sand). We pushed a steel T -bar into bottom of the test holes to get an indication of the density or consistency of the soil below bottom of the test holes. The probe bar penetrated a few inches of loose disturbed soils pushed to bottorn of the test holes and met with refusal. The probe results indicate the LIU & ASSOCIATES, INC. July 10, 2016 Critical Area / Geotechnical Report - Gilbert Residence L&A Job No. 16-106 Page 5 semi -compact gravelly sand deposit (glacial soil) is likely glacial consolidated soil. Groundwater was not encountered by the test holes. Groundwater Condition None of the three test holes angered on the project site has encountered groundwater. However, perched groundwater may exist on top of the underlying glacial till soil extremely low permeability. This perched groundwater is from stormwater infiltrating into the more permeable surficial soils. The amount of and the depth to the near -surface perched groundwater would fluctuate seasonally, depending on precipitation, surface runoff, ground vegetation cover, site utilization, and other factors. The near -surface perched groundwater may dry up completely during the dryer summer and fall months and accumulate and rise in the wet winter and early spring months. Seismic Hazard Evaluation The west fringe of the properties is mapped by City of Edmonds in seismic hazard areas, Seismic hazards include rupture of fault lines, soil liquefaction, lateral soil spreading, and landslide, The properties are flat to very -gently sloped; therefore, the potential for lateral soil spreading and landslide to occur on the properties should be minimal. The properties are not in close proximity of known fault lines; therefore, their hazard from fault line rupture should also be minimal. Liquefaction is another form of seismic hazard. The type of soils most susceptible to liquefaction during a strong earthquake is saturated, loose, -fine sand to silty fine sand deposits. Such. loose fine-grained deposits, when subjected to strong ground shaking, can LIU & ASSOCIATES, INC. July 10, 2016 Critical Area / Geotechnical Report - Gilbert Residence L&A Job No. 16-106 Page 6 be densified and decrease in volume. If water in the deposits is unable to drain quickly, pore water pressure in the deposits would increase. When the pore water pressure continues to build up by prolonged ground shaking, a "quick" condition will be reached when the pore water pressure equals the effective overburden soil pressure at some depths in the deposits. Under this condition, the sand deposits would to into a liquid state and lose its load bearing capacity. The Whidbey formation deposits of hard 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 chance for pore water pressure in these soils to build up and lead to soil liquefaction is nil. Therefore, in our opinion, the liquefaction hazard of the properties should also be minimal. Cut Slopes The project site is nearly flat and is underlain by dense glacial soils at shallow depth. Therefore, temporary cut of trenches to reach dense soils for construction of footing foundations to support the proposed addition will be shallow. Under no circumstance should cut slopes be steeper than the limits specified by local, state and federal safety regulations if workers have to perform construction work in excavated areas. Unsupported temporary cuts greater than 4 feet in height should be no steeper than I H - IV in surricial weathered soils and no steeper than l/2H:lV in underlying glacially consolidated gravelly, silty, fine sand deposit. July 10, 2016 Critical Area / Geotechnical Report - Gilbert Residence L&A Job No. 16-106 Page 7 The above recormnended cut slopes are under the assumption that groundwater seepage would not be encountered during construction. If groundwater is encountered, excavation work should be immediately halted and slope stability re-evaluated. 'rhe slopes may have to be flattened and other measures taken to stabilize the slopes. Stormwater should not allowed to flow uncontrolled over cut slopes. Disturbed areas should be seeded and vegetated as soon as possible for erosion protection and long-term stability, and should be covered with clear plastic sheets, as required, to protect them from erosion until the vegetation is fully established. Foundation Support Conventional footing foundations may be used to support the proposed addition. The footing foundations should be constructed on or into the underlying very -dense glacially consolidated gravelly silty fine sand deposit. Water should not be allowed to accumulate in excavated footing trenches. Disturbed soils in footing trenches should be completely removed down to native, undisturbed, till soil prior to pouring concrete for the footings. If the above recommendations are followed, our recommended design criteria for footing foundations are as follows: `rhe allowable soil bearing pressure for design of footing foundations, including dead and live loads, should be no greater than 3,000 psf. The footing bearing soil should be verified by a geotechnical engineer after the footing trenches are excavated and before the footings poured. The minimum depth. to bottom of perimeter footings below adjacent final exterior grade should be no less than 18 inches. The minimum depth to bottom of the interior footings below top of floor slab should be no less than 12 inches. July 10, 2016 Critical Area / Geotechnical Report - Gilbert Residence L&A Job No, 16-106 Page 8 The minimum width should be no less than 16 inches for continuous footings, and no less than 24 inches for individual footings, except those footings supporting light -weight decks or porches. A one-third increase in the above recommended allowable soil bearing pressure may be used when considering short-term, transitory, wind or seismic loads. For footing foundations designed and constructed per recommendations above, we estimate that the maximum total post -construction settlement of the buildings should be 1/2 inch or less and the differential settlement across building width should be 3/8 inch or less. Lateral loads on the proposed addition may be resisted by the friction force between the foundations and the subgrade soils or the passive earth pressure acting on the below -grade portion of the foundations. For the latter, the -foundations must be poured "neat" against undisturbed soils or backfilled with a clean, free -draining, compacted structural fill. We recommend that an equivalent fluid density (EFD) of 275 pef (pounds per cubic foot) for the passive earth pressure be used for lateral resistance. The above passive pressure assumes that the backfill is level or inclines upward away from the foundations for a horizontal distance at least twice the depth of the foundations below the final grade. A coefficient of friction of 0.55 between the foundations and the subgrade soils may be used, The above soil parameters are unfactored values, and a proper factor of safety should be used in calculating the resisting forces against lateral loads on the buildings. Based on the above analysis, we concluded that: A — No construction activities from development would degrade critical areas, B — No requirement specific to critical area type is needed. LIU & ASSOCIATES, INC. July 10, 2016 Critical Area / Geotechnical Report - Gilbert Residence L&A Job No. 16-106 Page 9 1. No construction activities from development would degrade critical areas. 2. No steps would be necessary to minimize impact on critical areas. 3. No rectifying measures are required. 4. No restoring or stabilizing actions required. 5. No special preservation or maintenance measures required. 6. No compensating measures required. 7. No monitoring programs required. We are pleased to be of service to you on this project. Please contact us if you have questions. J. S. (Julian) Liu, Ph.D., P.E. Principal Attached: 1. Plate 1 — Site Plan 2.6/10/2016 Report by Whalen Designs LLC LIU & ASSOCIATES, INC. 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