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Home My WebLink About801282007 Geotech Assessment}~le.~3a ~y Jel'fse'ao~ Cou.~ly C~nli~J .-°.°.°.°.°.°.°.°.°~c,s~ ~IS rvt ? \ ...,, it · Subject Property GEOTECHNICAL REPORT Prepared For AMELIA LEE October 29, 2003 For the Property Described as Tax # 801282007, 1917 Dabob Road Section 28, Township 28 North, Range 1 West, W.M. Jefferson County, Washington Prepared by NORTHWESTERN TERRITORIES, INC. 717 S. Peabody Street Port Angeles, Washington 98362 Phone 360-452-8491 Fax 360-452-8498 Web Site www. nti4u.com E-mail info@nti4u.com NT/ JLS GROUP~ INC. NORTHWESTERN TERRITORIES, INC. A JLS GROUP COMPANY 717 SOUTH PEABODY STREET, PORT ANGELES, WA 98362 Engineers Land Surveyors Geologists Construction Inspection Materials Testing (360) 452-8491 FAX 452-8498 www.nti4u.com E-Maih info@nti4u.com Geotechnical Report Tax # 801282007 October 29, 2003 Amelia Lee 1917 Dabob Road Quilcene, WA 98376 Subject: Geotechnical report for Tax Cf 801282007 located in Section 28, Township 28 North, Range 1 West, W.M., Jefferson County, WA Dear Mrs. Lee: Background At your request, Bill Payton, Engineering Geologist with Northwestern Territories Inc. conducted a slope stability inspection at the above-mentioned property on October 16, 2003. The purpose of this inspection was to examine the slopes adjacent to the proposed homesite by visual means in order to determine the relative stability of the slope and make recommendations in regards to the proposed .construction of a single family residence with daylight basement. Site Description The subject property is located at 1917 Dabob Road on the west facing ridge of Tarboo Creek valley and is surrounded by wooded land (Figure 1). The hilly property is mostly wooded except for a small clearing for an existing garage and the future house (Figure 2 and Photo 1). Existing developments on the property include a garage, well, driveway, and septic system. The proposed house site is located south of the existing garage on a spur of land that slopes down in three directions (Figure 3). The southern sloPe dips at an angle of about 8 degrees from horizontal (14%), while the east and west slopes are about 28 to 30 degrees (53%-58%). The heights of the east and west slopes range from about 12' to 23' (Figure 4). The slopes around the proposed house site are shallower than the "angle of repose" for loose · cohesionless material. The "angle of repose" is defined as the maximum slope or angle at which loose, cohesionless material remains stable, and commonly ranges between 33 and 37 degrees on natural slopes. The cleared portion of the property was sparsely vegetated wi-th grass. The slopes around the proposed house site were vegetated with brush, grass, and a few small trees. Outside of the developed area, the vegetation consists of mature trees and brush. Site Geology The Department of Ecology's "Geology and Ground-Water Resources of Eastern Jefferson County, Washington" maps the property as (Qu) Undifferentiated glacial, fluvial, glaciofluvial, lacustrine and glaciolacustrine deposits. These deposits are older than the Fraser glaciation and include sediments from pre Fraser glaciations. This unit contains a variety of soil types and is generally mapped when more detailed data is not available. According to the Soil Survey of Jefferson County Area, Washington (United States Department of Agriculture, 1975), the subject site is in an area mapped as the Sinclair gravelly sandy loam (15%-30% slopes). This soil formed in glacial till on terraces adjacent to steep drainageways and canyons. The Survey states that there is .a seasonal perched water table at about 20" to 40" below the surface during the wet season. This soil has a slight frost action potential and a Iow shrink-swell potential. Runoff is medium to rapid and the hazard of water erosion is moderate to severe. Visual observations made in the area revealed the soil'to be silty sand/silty gravel which is consistent with the Soil Survey descriptions. Some of the spur material in the location of the proposed house is fill material from past grading operations that reportedly occurred more than 10 years ago. No evidence of landslides was noticed in the area around the proposed house site and no springs or seeps were noticed. However, during the site visit it was raining heavily and the area was saturated with surface water. Conclusions and Recommendations The area around the proposed house site appears grossly stable and the project seems feasible from a geotechnical perspective. However, measures will need to be taken to protect the slopes adjacent to the .house from erosion. The following recommendations should be considered with regards to the proposal' The fill material in the area of the proposed house should be removed and the house founded on clean firm undisturbed native soil or on properly placed and compacted clean structural fill. The on site native silty sand/silty gravel could be used for structural fill if it is clean and at the proper moisture content. , The sides of the house should be at least 15' from the east and west slopes. Some of this distance will be achieved once the site is cut down to the necessary grade for the basement. It may also be necessary to move the house location in a northerly direction, where the spur gets wider, in order to achieve the 15' setback. o Due to the possible presence of a perched water table, .footing drains and a backdrainage system behind the basement retaining wall are recommended. These drains should be kept separate from roof drains, These drains should be tightlined away from the slopes surrounding the house. ° It will be necessary to maintain ground cover on the slopes and on the flat areas in order to reduce erosion from surface runoff. Existing bare areas and any future bare areas that develop should be revegetated. Native deep-rooted vegetation that requires 2 little or no irrigation would be the most beneficial. Please consult the online publications mentioned below for further information. . Surface water should not be allowed to flow down the face of the slopes from the flat area and cause erosion of the slopes. If this occurs, the water Should be directed away from the slopes and/or conveyed to a safe discharge area below the slopes via tightline drain. . . Heavy irrigation or other actiVities that would contribute large quantities of water to'the soil should be avoided. Surface runoff from hard surfaces such as roofs, driveways, walkways and patios should be controlled and routed to a drainage control device such that surface water discharge to adjacent properties does not significantly exceed predevelopment conditions. As with the footing drains, these drains should be conveyed away from the slopes around the house via tightline. 8. The outlets of all drains should be properly equipped with energy dissipaters in order to prevent erosion. o Silt fences or other sediment control devices may be needed during construction so that sedimentation to adjacent properties does not significantly exceed predevelopment conditions. 10. Drainage control devices should be maintained in good working order and inspected at least once a year. 11. All drainage control features should conform to the current regulations that apply to the development of this property and be constructed in accordance with standard engineering practice in order to insure that surface water discharge or sedimentation to adjacent properties does not exceed predevelopment conditions. Based on the findings, recommendations and limitations of this report: . . There should be minimal landslide hazard as suggested by a lack of evidence of recent landslide activity in the vicinity in the past. . Observations of slope stability indicate that the proposal should not be subject to risk of landslide. The proposal should not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions. The proposal should not decrease slope stability on adjacent properties. 5. The proposal should be stable under normal geologic conditions. For further information please review the three online publications published' by the Washington State Department of Ecology (DOE) entitled: "Slope Stabilization and Erosion Control Using Vegetation", "Vegetation Management: A Guide for Puget Sound Bluff Property Owners" and "Surface Water and Groundwater 'on Coastal Bluffs". These publications are now out of print but 3 can be obtained from the DOE webSite at: http://www, ecy. Wa..qov/biblio/sea.html under the.1993 and 1994 year heading. The DOE website also contains additional useful information regarding slope stability and site development; this reference is highly recommended. Limitations This report has been prepared for your exclusive use in conjunction with the above referenced project. The report has not been prepared for use by others or for other locations. It may be used by others only with the expressed written permission of the Engineer. : Within the limits of scope, schedule and budget, this report was prepared in general accordance with accepted professional engineering and geological principles and practices in this or similar localities at the time the report was prepared. No other warranty, expressed or implied, is 'made as to the conclusions and professional: advice included in this report. The observations', conclusions and .recommendations presented in this report were based on our visual observations of the Subject property at the time of our site visit; no laboratory tests were performed. Soil and geologic conditions can vary significantly between test holes and/or surface outcrops. If there is a substantial lapse of time, conditions at the site. have changed or appear different than those described in this report, we should be contacted and retained to evaluate the changed conditions and make modifications to our report if necessary. Sincerely, NORTHWESTERN TERRITORIES, INC. /'~,~ ~? //'~ Robert A. Leach, P.E., MBA Principal Engineer EXPIRES 12/3012004 Bill Payton, L.E.G. Engineering Geologist G:\Gen\Bill\Reports\LEEA0301 .slope stability.28(28-1 Expires 11-06-03 APPENDIX Map uutput ArclMS H m'ML Viewer Ma : '~i: ~' ' "~-* ' '~ ," /'.--. !''?', ' '" -" _' '" i~.')!, "',/'~-I':' ' ", " '.. ~;.'.~: (;':;7~;~' ' ' ..... ....~.-,-. ~,..,..~.. ,,, ,, _ 1. :.:1-';: .,.? . . :, '.--. ':"' '!~, ' , ' ! ': ?.'"' '- '~'-'~.';~-x:''-- ' '- ..... , ~; ' -- . . . '¢: :;'; ~? i~ .. ;; :;' .' ;" '- .?:,'i '.' .... :;;:;--}~i_ ' '- ~ . , '~ ;"- · ; ' T=' .' ,~i.' _~ .... 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