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Home My WebLink About002133007 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 FAX 452-8498 E-Mail: info@nti4u.com ~'u'.n ti~u. corn March 13, 2001 Diane Macrae 2237 Minor Rd. East Seattle, WA 98102 Subject: Geotechnical Report for the Property Described as Parcel 3 of Government Lot 4, Section 13, Township 30 North, Range 2 West, W.M. Jefferson County, Washington; Tax parcel number 002133007. Dear Ms. Macrae: Introduction At your request, Bill Payton of NorthWestern Territories, Inc. conducted a geotechnical inspection of the above-mentioned property on March 6, 2001. The purpose of this inspection was to examine the marine bluff at the subject site by visual means and make recommendations regarding the development of the property in accordance with the Jefferson County Unified Development Code. it is our understanding that you plan to purchase the subject property for development as a residence and Bed and Breakfast. Site Conditions The subject property is located at the end of Goss Road and is reported to be 6.33 acres in size. The property is bounded on the north by residential property, on the south by undeveloped property, on the east by Goss Rd. and on the west by Discovery Bay. The property is undeveloped and heavily vegetated with native trees, brush and grasses. There are several very large trees on the property. The property does appear to have been logged in the recent past and there were several downed trees, possiblY brought down by .high winds. The property generally slopes towards DiScovery Bay with a broad drainage channel running through the middle of the property, which was wet with standing water at the time of the inspection. There is a culvert under Goss Road that allows surface water to flow onto the property from the east. There is a hill on the south side of the property roughly mid way between the marine bluff and Goss Road that would make a nice homesite. The marine bluff at the subject property is roughly 200 feet in elevation with a variable slope angle ranging from near vertical in some areas to around 35 - 40 degrees from horizontal. The majority of the slope face is well vegetated with trees, brush and grasses. A slide sca.,r was noticed on the bluff face near the north end of the property. This slide scar was vegetated mainly by grass with some small trees. Soils exposed at the head of the slide scar consisted of thinly stratified sandy silts with interbedded layers of gravelly soils at the base of the exposure. The soils at the base of the exposure were wet. The Soil Survey of Jefferson County classifies the soil at the subject property as the Cassolary sandy loam (CfD). The Survey states that the runoff is medium and the hazard of water erosion is moderate. The Coastal Zone Atlas of Washington classifies the soil at the property as the Vashon advance outwash (Qva). The Atlas describes these soils as being a member of the Vashon Drift that lies stratigraphically between the Esperance Sand and Vashon till. Vashon advance outwash typically consists of well sorted, well stratified sandy pebble-to cobble-sized gravel. In some places, sand predominates. The unit generally contains minor amounts of silt and clay. The Atlas also classifies the bluff in the area of the subject property as being unstable with old slide activity (Uos). Conclusions The subject property would make a nice homesite provided some precautions. are taken with regards to the bluff and drainage issues. A minimum 150 foot setback should be established from the top of the bluff measured perpendicular from the most landward portion of the bluff edge. The most landward portion of the bluff would correspond with the head of the slide scar mentioned above. It appears that the best location for a house would be on the hill on the south end of the property. This location would be much dryer than the central portion of the property, which is a natural drainage area. The following recommendations should also be considered with regards to development of the subject property: o It will be necessary to maintain ground cover to reduce erosion from surface runoff. Any bare areas that deVelop on the bluff or on the upland should be revegetated. Native vegetation that requires little or no irrigation would be the most beneficial. The native vegetation that is in place on the bluff face and upland should be disturbed as little as possible. o Surface water should not be allowed to flow over the face of the bluff and cause erosion of the bluff face. Likewise, it would be beneficial to reduce the amount of infiltration near the bluff. A formal drainage control plan should be developed for this property to address the issues of surface water runoff and infiltration through the drainage area in the center of the property. . 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 near the bluff. . Surface runoff from hard surfaces such as roofs, driveways, walkways and patios should be controlled and routed to the beach via tightline. Surface water discharge to adjacent properties should not exceed predevelopment conditions. . Silt fences or other sediment control devices may be needed during construction such that sedimentation to adjacent properties does not exceed predevelopment conditions. 6. All drainage control devices should be maintained in good working order and inspected at least once a year. 7. Septic systems and/or detention basins should not be constructed between the house and the bluff. Based on the findings, recommendations and limitations of this report: 1. The landslide hazard to the proposed home would be minimal during its economic lifespan due to the mitigation measures proposed in this report. 2. An analysis of slope stability indicates that the proposed home would not be subject to risk of landslide under normal conditions. o The proposal would not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions. 4. The proposal would not decrease slope stability on adjacent properties. 5. The proposed homesite 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 Washington State Department of Ecology 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". Information online can be found at: www. ecy. wa..q_ov/programs/sea/shorelan.html and at: www. wa. gov/dnr/htdocs/qer/index, htmi . Limitations This report is based on a visual inspection of the existing site conditions, 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 Ms. Diane Macrae. It may be used by others only with 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, NORTHWESTERN TERRITORIES, INC. J. R. Jerry Newlin, P.E. Principal Engineer Bill Payton Geologist G:\Gen\BilI\RepoHs\MACR0101 .bluff stability,3(30-2).Discovery Bay.doc 4 Engineer Health Dept. Official Backhoe Operator Others Present Properly Owner/Dev. SOIL HOLE LOGS Page / of ~/,,~-'"" z.~.g-./V.~.--/~..~ Date 7;Yl~Z¢~F.~.,~ooZ I~' cow -- / En . B,H. H.D.O. Stad Time ~ ¢> ~ ~'~" ~',,¢,'~,¢ Z:>/ Z:>¢',4¢.,~,',~ ~,¢'-"~--~.,,'¢~. 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