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Home My WebLink About701344031 Geotech AssessmentNORTHWESTERN TERRITORIES, INC. 717 SOUTH PEABODY STREET, PORT ANGELES, WA 98362 Engineers · Land Surveyors · Geologist Construction Inspection · Materials Testing (360) 452-8491 1-800-654-5545 FAX452-8498 E-Mail: Info@nti4u.com GEOTECHNICAL REPORT For 372 Beach Drive, Parcel # 701344031 Located in Section 34, Township 27 North, RanGe 1 VVest, W.M. Jefferson County, VVashington Prepared for DAVE AND .MARSHA NIELSEN Prepared by NORTHWESTERN TERRITORIES, INC. 717 S'. Peabody Street. Port Angeles, Washington..98362 ... July 12, 2001 · ~ NORTHWESTERN TERRITORIES, INC.~ 717 SOUTH PEABODY STREET, PORT ANGELES, WA 98362 , Engineers · Lend Surveyors [] Geologist Construction Inspection · Materials Testing (360) 452-8491 1-800-854-5545 FAX452-8498 E-Mail: info@nti4u.com Nil www. n ti4u. corn July 12, 2001 Dave and Marsha Nielsen 2207 207th Ave. SE Issaquah, WA 98029 Subject: Geotechnical report for the property located at 372 Beach Drive, Parcel # 701344031,' in Section 34, Township 27 North, Range 1 West, W.M., Jefferson County, WA. Dear Mr. And Mrs. Nielsen: Introduction At your request, Bill payton of Northwestern Territories, Inc. conducted a geotechnical inspection °fthe above mentioned property on July 5, 2001. The purpose of this inSpection, was to examine the proposed I~omesite at the subject property by. Visual means and make recommendations regarding the devel°pment of the. prOperty in accordance with the .JefferSon County Critical Areas Ordinance.' It is our understanding that you desire to construct a home on the subject property..Impr°vements to the property currently include a Shed, driveway, septic system, utilities and a pond.. Site COnditions The subject.property is located off of Rhododendron Rd. 'in the Dabob Cove Tracts~Subdivision on the Toandos peninsula. The property is 'hilly and.slopes to the north and northwest and is Predominantly vegetated .with native trees, brush and grass. Although'some areaS have been cleared for development. Most of the trees in the area of the proposed homesite have straight trunks. Many of these trees and stumps are quite mature. The prOposed home will be one stow with a loft and daylight basement, with a footprint of approximately 38 by 28 feet. The home will be cut into the edge of a bench on the south end of the property (Photo 1). 1 The average slope angle of the slope below the homesite is about 32% (--,18 degrees). The slope continues down toward the northwest at this angle to the head of a draw where the slope angle increases to about 58% (30 degrees). The distance from the proposed home to the head of the draw is about 150 feet. There were horsetail ferns at the head of the draw, but the area was relatively 'dry at the time of the site visit. It is possible that perched groundwater exits the slope at the head of the draw during the wet season. The bench south of the proposed homesite is relatively level and extends for about 70 feet toward the south. The property then slopes up towards Beach Drive. Site Geology The Soil Survey of Jefferson County maps four types of soil on the subject property: the Sinclair gravelly sandy loam (SnC), the Hoypus gravelly loamy sand (HuC), the Carlsborg gravelly loamy sand (CAD), and the Everett gravelly sandy loam (EvD). All of these soils are described as having slow to medium runoff with the hazard of water erosion being slight to moderate. Compact till is at a depth of about 48 inches in the Sinclair soil and water is perched above this till during the rainy season. The Coastal Zone Atlas of Washington maps two types of soil at the subject property. The Vashon advance outwash (Qva) is mapped along the southern edge of the property and is described as consisting of well sorted, well stratified sandy pebble - to cobble - sized gravel. The Undifferentiated stratified sediments (Qpf) cover the majority of the property and are described as mainly sand and gravel. The Atlas labels the slope stability of the property as Unstable (u). Visual observations of a test pit excavated on the bench near the proposed homesite revealed stratified'sandy gravel deposits, which is consistent with the above soil descriptions. Conclusions and Recommendations: Although many areas in the vicinity of the subject property have experienced landslides, it appears .that no recent slide activity has occurred at the subject propertY and that the subject prOperty is grossly stable. This is evidenced by the maturity and straightness of the trees and stumps, and the stratified, Undisturbed nature of the soils in the teSt pit. The following recommendations should be considered with regards to development of the subject property: . 1. it will be necessary to maintain ground cover to reduce erosion from surface runoff. Any bare areas that develop should be revegetated. Native deep-rooted vegetation that requires little or no irrigation would be the most beneficial. '.2. Heavy irrigation or other activities that would contribute large quantities of water to the soil should be avoided. One cause of landslides is the presence of excessive groundwater in the soil. 3. Slopes should not be destabilized with activities sUch as over steePening, cutting at the toe or vegetation removal. 4. Surface runoff from hard surfaces such as roofs, driVeways, walkways and patios should be controlled and .routed t° a drainage control device such that surface water discharge to adjacent properties does not exceed predevelopment conditions. A formal drainage control plan should be developed for this .property to address these issues. 5. It will be necessary to maintain proper drainage of the roadway above the 'homesite to ensure that runoff does not flow through the area of the proposed homesite. 6. Although it appears that the majority of the runoff is already directed away from the proposed homesite, the drainage and runoff patterns through the property should be monitored and corrected as necessary. 7. Silt fences or.other sediment control devices may be needed during construction so that sedimentation onto adjacent'properties does not exceed predevelopment conditi°ns. 8. Ali drainage control devices should be maintained in good working order and inspected at least once a year. 9. The Soil Survey mentions the possibility of a Shallow Perched water table during 'the wet season. Thus it wOuld be.beneficial to allow for wetness under the home in the building design. One measure would be' the use of footing drains. Based on the findings, recommendatiOns'and limitations of this report: · 1. There is minimal landslide hazard as proven by the lack of evidence of landslide activity at the subject property in the recent past. 2. An analysis of slope stability indicates that the ProPosed homesite is not subject to risk of landslide under normal conditions. 3. The proposal would not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions. 4. Th'e proposal would not decrease slope stability on adjacent properties. 5. The proPosed home site would be stable under normal geologic conditions.. ·. · . .. For. further infOrmation please review the three pamphlets, .which are enclosed with the Original of this report, pUblished by the Washingto'n'State Department of .Ec°logy entitled: "Slope Stabilization and Erosion. Control Using Vegetation", "Surface Water and Groundwater on Coastal Bluffs" and "Vegetation Management: A Guide' for Puget Sound Bluff Property, oWners''. Limitations This report, is based on a visual inspection of the existing site'cOnditi°ns, barring majOr Catastrophic events such as earthquakes or abnormal weather conditions. No scientific measurements, tests or calculations Were perfOrmed. The findings and report'are'limited to the normal standard of care of the'industry for an investigation without benefit of scientific data and calculatiOns.. The sole objective of this investigation has been to diagnose the geotechnical conditions at the site. The 'report has been Prepared for the exclusive use of Dave and Marsha Nielsen. It may be used by others Only w. ith expressed written permission of. the Engineer. The 'report has not been prePared for use by others or other'uses. The observations, interpretations, and'concluSions herein are based On generally accepted professional 'engineering and geologic princiPles and practice. This warranty is in lieu of all others, either expresSed or implied. . SincerelY,. : ' >R je'rryN~wlin, P.P ""' ' " · .., . . ._. Principal Engineer ~'.~~ Bill .~ayton GeolOgist' W¢~:mej G:\Gen~Bill\Roports~llfil-0101 .slope stability. 34(27-1 ).Toando$ Pea. J¢ounty cao.doc