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Home My WebLink About502155010 Geotech Assessment~ , s GEOTECHNICAL REPQRT Prepared For Lorin Mustoe May 19, 2004 For the Property Described as Tax # 502155010 Section 15, Township 25 North, Range 2 West, W.M. Jefferson County, Washington - Prepared by ~ _ _ . NTI Engineering and Surveying APR - $ ~j 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 -.~ _ NORTHWESTERN TERRITORIES, INC. A JI.S GROUP COMPANY 717 SOUTH PFABODY STREET, PORT ANGELES, WA 98382 Engineers Land Surveyors Geologists NT/ Construction Inspection Materials Testing (360) 452-8491 FAX 452-8498 www.nti4u.COm E-Mail: info(~nli4u.com JLS GROUP, INC. May 19, 2004 Lori Mustoe 785 SW 67~' Place Portland, OR 97225-6156 Geotechnical Report Tax # 502155010 _.~ APR ' 8 2005 E - _ __ J Subject: Geotechnical Report for Tax # 502155010 located in Section 15, Township 25 North, Range 2 West, W.M., Jefferson County, WA Dear Mr. Mustoe: Background At your request, Bill Payton, Engineering Geologist with NTI Engineering and Surveying (NTI) visited your property at the above referenced location on May 5, 2004 to conduct a bluff stability inspection. The purpose of this inspection was to examine the marine bluff at the subject site by visual means in order to determine the relative stability of the bluff and make recommendations in regards to the proposed future construction of a single family residence in general accordance with Section 3.6.10 of the Jefferson County, WA Unified Development Code (UDC). Site Description The subject waterfront property is described as Lot 10, Pleasant Harbor Beach Tracts and is located at 304 Black Point Road, south of Brinnon. The property overlooks Pleasant Harbor to the north and is bounded on the east and west by undeveloped residential .property. The property extends south of Black Point Road, but this portion of the property was not observed during the .bluff stability inspection (Figures 1land 2). Also see Appendix for a series of aerial photos .that go back to 1977. Existing. improvements to the property include a driveway (Photo 1) and a benched homesite area (Photo 2) that was .reportedly constructed about 20 .years ago, probably in association with a logging operation. Test pits for septic design were present at the west end of -the property on the bench. Ifi is our understanding that plans call for the construction of asingle-family residence on the bench at the east side of the property. The upland portion of-the property is mixed wooded and cleared. The wooded areas consist of young to mature native trees and brush, while the cleared areas are predominantly unvegetated with scattered patches of grass and weeds. The bench area is roughly 50 feet wide and begins about 64' back from the top of the bluff, which is about 36' high. The upland slopes northward down toward the bluff at angles. ranging from approximately15degyees to approximately 25 degrees, except in the area of the bench (Figure 3). The bluff at the property is about 36 feet high with an average slope angle of about 40 degrees. The lower approximately 5' of the bluff is near vertical, probably a result of periodic wave erosion. The bluff is well vegetated with young to mature trees and brush (cedar and salal are common) (Photo 3). Some of the trees have curved trunks, indicating that shallow down slope creep of the surface soils is occurring. No springs or seeps were noticed on the bluff face. The beach is covered with large gravel and cobbles, which helps reduce erosion from wave action and long shore currents. Also, the property is situated in a cove, which helps protect the bluff from wave erosion. There is a trail and dilapidated stairs that lead down the bluff to the beach through an old slide scar (Photo 4). The slide area was revegetated with approximately 1 foot diameter cedars and brush. These cedars are roughly 20 to 25 years old, suggesting that the slide occurred more than 20-25 years ago. The face of the slide slopes at approximately 36 degrees, while the head of the slide, which is composed of compact till, is near vertical. Site Geology The Washington State Department of Ecology's Coastal Zone Atlas, 1978 maps the soils in the area of the subject property as Vashon till (Qvt) on the upland and Vashon advance outwash (Qva) on the bluff. The till is described as a very compact mix of poorly sorted, nonstratified gravel, sand, silt and clay with occasional boulders. This soil is excellent for foundation stability, good for seismic stability, and stands in steep natural and cut slopes for long periods. It may ravel and spall by wetting and drying, and freezing and thawing. The advance outwash is described as well stratified, well sorted sandy pebble to cobble sized graveh In some places, sand predominates. This soil is good for seismic stability and good to excellent for foundation stability, but may be poor on slopes that approach the angle of repose of the material. This soil is generally stable. in slopes up to the angle of repose, and may stand in steeper slopes for short periods. 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 Atlas maps the stability of the bluff as Unstable and the stability of the upland as Stable (See Appendix): The Department of Ecology's °Geology and Ground-Water Resources of Eastern Jefferson County, Washington" map of the soils in the area of the subject property is generally consistent with the Coastal Zone Atlas descriptions. According to the Soii Survey of Jefferson County Area, Washington (United States Department of Agriculture, 1975), the subjecYsite is in an area mapped as the Hoodsport-Grove very gravelly sandy loam (HrD). This soil formed in glacial till or glacal outwash and consists of silty sandy gravel. The Survey states that there is a seasonal perched water table at a depth of 20"- 36" below the surface during the rainy season. The Survey also states that this soil has a moderate to high corrosivity to concrete. Visual observations made at the site are generally consistent with the above soil descriptions. .. _ ~ APO - 8 2005 2 a Conclusions and Recommendations It appears that the bluff at the subject property is currently grossly stable, judging by the size of the mature trees on the bluff. Periodic slides may occur as a result of undercutting of the toe of the bluff by wave action, as evidenced by the old slide scar. Judging by the bluff vegetation, this appears to be a very infrequent occurrence. The proposed house will be located on the bench area, which is about 64' back from the top of the bluff. Based upon our investigation, we conclude that this setback distance should be ample considering that typical average rates of bluff recession for the Puget Sound area are usually only an inch or two a year. The following .recommendations should also be considered with regards to the proposal: 1. It will be necessary to maintain ground cover on the bluff and upland in order 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. Please consult the enclosed publications for further information. 2. Vegetation on the bluff face provides stabilization to the bluff face soils and helps remove water from the soil. Existing established vegetation should be left in as natural state. as possible. If a better view is desired, minor thinning and pruning should. be done in such a way that minimizes disturbance to the soil and root zone and that insures the continued health of the vegetation. Trees should not be topped. 3. Heavy irrigation or other activities that would contribute large quantities of water to the soil should be avoided. 4. 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. Based on the 2000 Department of Ecology guidelines, as adopted by Jefferson County, it seems that a tight line to the base of the bluff without detention would provide the best solution, given the very slow permeability of the site soils as mentioned by the Soil Survey. 5. Silt fences or other sediment control devices may be needed- during construction. such that sedimentation to adjacent properties does not significantly. exceed predeveiopment conditions. 6. Drainage control devices should be maintained in good working order and inspected at least once a year. 7. An engineered drainage and erosion control plan will be required by Jefferson County and should be developed for this property to address items 4; 5, and 6 above. NTI can perform this task if requested. 8. We anticipate that improvements and maintenance to the existing. trail down the face of the bluff would not significantly decrease slope stability. The trailahould be APR - 8 2005 3 i i ..~ maintained such that erosion of the bluff is not exacerbated. It may be beneficial to consult someone who specializes in this type of construction. 9. Fill material will likely be found in the pad area. The house should not be founded 'on fill, but rather on native undisturbed soil below the fill. 10. If impermeable or slowly permeable soils are encountered in the foundation soils of the house, footing drains should be utilized. 11. Sloughing or raveling of the cutslope on the south side of the bench may occur in the future, a retaining wall could be constructed to mitigate this if necessary. Based on the findings, recommendations and limitations of this report: 1. There should be minimal landslide hazard to the proposed home based on the setback distance recommended. 2. Observations of slope stability indicate that the proposal should not be subject to risk of landslide under the current conditions that exist at the site. 3. The proposal should not increase surface water discharge or sedimentation to adjacent properties .beyond predeveiopment conditions. 4. 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 publications .(included with the original of this report) 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 can- also be viewed on the DOE website at: http://www.ecy.wa.gov/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 exdusive 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 for other purposes 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' condusions and professional advice included in this report. a The observations, conclusions and recommendations presented in this report were based an 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 s ~ `. Bill Payton, L.E.G. 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