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Home My WebLink About702144024 Geotech AssessmentEnvirotech EngineeringEnvirotech EngineeringEnvirotech EngineeringEnvirotech Engineering,,,, PLLC.PLLC.PLLC.PLLC. Geotechnical ◦ Environmental ◦ Drainage ◦ Roadway PO Box 984 Belfair, Washington 98528 Off: 360-275-9374 Cell: 360-689-6045 envirotech@geotechnicalinfo.com September 21, 2022 101 General Contracting, LLC Sequim, Washington 98382 Reference: Geological Letter 513 W Columbia Street, Quilcene, Jefferson County, Washington Parcel 702144024 To Whom It May Concern: Envirotech Engineering, PLLC has completed this Geological Letter for the referenced property. Our geotechnical/ geological conclusions are based on the planned development, available geological/ landslide hazard mapping, and a site visit performed on September 12, 2022 in order to observe surface and subsurface conditions of the property. This Geological Letter per Jefferson County Code (JCC) 18.22 demonstrating our findings was prepared for the referenced property as we did not find that a hazard area exists within 200 feet of the proposed development. It is our understanding that the development is planned to consist of a single family home, and other ancillary features typical of this type of development. Surface and Subsurface Conditions The property has been cleared with past use. The topographic information provided herein was extrapolated from a public lidar source, and incorporated observations and field measurements where necessary. Average grades on the property are nearly flat to mild with a moderate 10-foot vertical relief descending to the east near the south end of the property. Runoff originating upslope of the development is mostly diverted away from the property by accommodating topography. Excessive scour, erosion or other indications of past drainage problems were not observed within the immediate vicinity of the planned development. There are no apparent water bodies or wetlands located upslope from the planned development that would significantly influence the project. A wetland is located offsite, and downslope from the development. Surface and subsurface conditions were observed for potential landslide or erosion hazards. Some indicators that may suggest past slope movements include: Page 2 of 5 • Outwash of sediments near the bottom of the slope, • Fissures, tension cracks, hummocky ground or stepped land masses on the face or top of the slope, and parallel to the slope, • Fine, saturated subsurface soils, • Old landslide debris, • Significant bowing or leaning trees, or, • Slope sloughing or calving. These slope instability indicators or other significant mass wasting on the property or within the general vicinity of the project were not observed or discovered during research. Indications of past landslides, current unstable slopes, deep-seated slope problems, or surficial slope failures were not observed during the site visit. In general, soils at the project are composed of materials from glacial advances. The geologic conditions as presented in the “Geologic Map of Washington,” compiled by J. Eric Schuster, 2002 indicates Quaternary sediments, Qg. Quaternary sediments are generally unconsolidated deposits, and dominantly deposited from glacial drift, including alluvium deposits. This project is located within the Puget Lowland. Typically, “lower tertiary sedimentary rocks unconformably overlie the Crescent Formation.” as revealed in the Geologic Map. Initial sedimentary rocks were formed from shales, sandstones and coal deposits from rivers. During the Quaternary period, the Puget Lowland was covered by numerous ice sheets, with the most recent being the Fraser glacier with a peak of approximately 14,000 years ago. Upon the glacial retreat, the landscape was formed by glacial erosion glacial drift deposits. The following Department of Natural Resources Excerpt is provided for the specific geology of this property: Geologic Unit Age: Pleistocene Geologic Unit Name: Vashon Stade till Unit Description: Unsorted, unstratified (but locally banded) mix of clay, silt, sand, and gravel; typically supported by a sandy matrix; mostly gray but locally ranging to tan, light brown, or orange; typically unweathered; lodgment till compact, with well- developed facies resembling concrete, but near the surface commonly hackly and (or) looser and covered by 1 to 6 ft of loose ablation till; deposited directly by glacial ice and commonly includes clasts or clumps plucked from underlying units. Clasts are commonly striated and faceted, with angular or rounded edges. Boulders are generally sparse within the till but large (erratic) boulders of plutonic or metamorphic rock are common on till surfaces. Some exposures include interbands and lenses of sand and gravel, locally with shears and joints. Till forms a locally patchy and seemingly randomly distributed cover up to a several tens of feet thick, with a thickness of 5 to 20 ft most common. It typically dominates, but is also locally discontinuous on, fluted surfaces, with individual drumlins measuring 0.1 to 0.3 mi wide by 0.8 to 1.3 mi long and the long axis aligned with the direction of ice flow. Till typically is in sharp, unconformable contact with underlying units, most commonly advance outwash (unit Qga and subunit). Unit Qgt lies stratigraphically below unit Qgo. It may include unrecognized exposures of older till. A map boundary mismatch between unit Qgt on this map and unit Qgos on the Vaughn quadrangle to the south (Logan and Walsh, 2007) may have resulted from a map-production error in the northwest corner of the Vaughn map (Josh Logan, Wash. Divn. of Geology and Earth Resources, oral commun., 2009). Age-Lithology: Pleistocene continental glacial till Page 3 of 5 Soils were observed to be a shallow layer of medium dense silty sand with gravel (SM) possibly overlying very dense SM hardpan soils (glacial till). According to the “Soil Survey of Jefferson County,” by the United States Department of Agriculture, Soil Conservation Service (SCS), the site soils are described as Grove very gravelly loamy sand (GoC) with 0% - 15% slopes. From the water well report(s) and knowledge of the general area, permanent groundwater is at least 50 feet directly below the property at the building pad location. Surface seepage or perched groundwater at shallow depths was not observed or indicated by mottling on-site. Landslide Hazard Assessment According to the Coastal Zone Atlas of Jefferson County, Washington, the project is within and near terrain labeled ‘Stable’ regarding potential landslide activity. Descriptions of these mapping units may be found in the aforesaid Atlas. According to the Resource Map from the Washington State Department of Natural Resources (DNR), the project is not within terrain labeled ‘highly unstable’ relating to soils. DNR labeled portions of this project as medium and high slope instability with relation to slopes. Historic landslides were not mapped on or near the property. DNR did not indicate historic landslide activity near the project. Mapped slope conditions, as delineated by the Departments of Ecology and/ or Natural Resources, were considered in our slope stability assessment. Based on the proximity and severity of mapped delineations with respect to the proposed development, observed surface conditions, and other pertinent information, it is our opinion that the proposed development may occur. Erosion Hazard Assessment Based on the USCS description of the project soils, the surface soils are considered low to moderately erodible. According to the Resource Map from the Washington State DNR, the project is not within terrain labeled ‘highly erodible.’ Problems with soil erodibility is not described within the SCS soil description. It is our opinion that this site is not within an erosion hazard area, and standard erosion control protocol is sufficient for the development of this project. Extents of temporary erosion control will mostly depend on the timeliness of construction, moisture content of the soil, and amount of rainfall during construction. Soil erosion typical to the existing site conditions and planned disturbance of the project include wind- borne silts during dry weather, and sediment transport during prolonged wet weather. Sediment transport could be from stormwater runoff or tracking off-site with construction equipment. Seismic Hazard Assessment There are no known faults beneath this project or within 1 mile for the property. The Debob Bay fault has a distal end that is approximately 2 miles to the southeast of the property. This information is based on the USGS Quaternary Fault and Fold Database for the United States. Page 4 of 5 Citation:Contreras, T. A.; Patton, A. I.; Legorreta Paulin, Gabriel; Hubert, I. J.; Cakir, Recep; Carson, R. J., 2014, Geologic map of the Quilcene 7.5-minute quadrangle, Jefferson County, Washington: Washington Division of Geology and Earth Resources Map Series 2014-03, 1 sheet, scale 1:24,000, 28 p. text. Dip Direction:unspecified Fault Code:664 Fault Description:dextral strike-slip fault Fault Detection Method:mapped trace Fault ID:2488 Fault Name:Dabob Bay fault (class B) Fault Source URL:Link Fault System:Dabob Bay fault zone (class B) Fault System Code:93 Fault Visiblilty:concealed fault trace Geologic Age (Years):unknown Geologic Age Description:insufficient data to determine age (class B) Soils immediately below the expected foundation depth are competent for this project are generally Type D or better, corresponding to the International Building Code (IBC) soil profiles. The potential for liquefaction is believed to be low for this project, and DNR mapping provides a low description for this property. This is based, in part, on the subsurface conditions such as soil characteristics and the lack of a permanent shallow water table. Subgrade characteristics that particularly contribute to problems caused from liquefaction include submerged, confined, poorly-graded granular soils (i.e. gravel, sand, silt). Although gravel- and silt-sized soil particles could be problematic, fine and medium grained sands are typically subjected to these types of seismic hazards. No significant saturated sand stratifications are anticipated to be within the upper 50 feet of the subsoil for this project. Liquefaction Map from DNR Interactive Mapping Page 5 of 5 Conclusions Areas of High Landslide Hazards or potential geologic hazards per the criteria identified in JCC 18.22 were not identified within 200 feet of the proposed development. Clearing vegetation on the property and site development is not expected to be problematic for slope stability. However, replanting or permanent erosion control per County standards should be completed in order to mitigate soil erosion. Drainage provisions may follow standard County protocol, and any drainage is suitable for the development without restrictions due to geologic hazards. From a geotechnical position, it is Envirotech’s opinion that the subject property and adjacent properties to the proposed development should not be significantly impacted. This opinion is based on the expected site development, existing topography, and our findings presented in this letter. Based on the project information provided by the owner, the proposed development, and site conditions as presented in this report, it is Envirotech’s opinion that additional geotechnical studies are not required to further evaluate this project. The services described in this report were prepared under the responsible charge of Michael Staten, a professional engineer with geotechnical emphasis. Michael Staten has appropriate education and experience in the field of geotechnical engineering in order to assess landslide hazards, earthquake hazards, and general soil mechanics. Thank you for the opportunity to work on this project. If you have any questions or need any further assistance, please contact us at 360-275-9374. Sincerely, Envirotech Engineering Michael Staten, P.E. Project Director 9/21/22