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HomeMy WebLinkAbout001324031 Geotech AssessmentGEOTECHNICAL REPORT Prepared For Stig Osterberg March 28, 2003 For the Property Located at 150 Adelma Beach Road Section 32, Township 30 North, Range I West, W. Jefferson County, Washington APR 2 2 2OO3 JEFFERSON COUNTY DEPT. OF COMMUNI]-¥ DEVELOPMENT 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 a Construction Inspection [] Materials Testing (300) 452.8491 FAX 452-8498 www.nli4u.com E-Meil: Info@n[i4u.com Geotechnical Report 150 Adelma Beach Road March 28, 2003 Stig Osterberg 150 Adelma Beach Road Port Townsend, WA 98368 Subject: Geotechnical report for 150 Adelma Beach Road located in Section 32, Township 30 North, Range 1 West, W.M., Jefferson County, WA Dear Mr. Osterberg: Background At your request, Bill Payton, Engineering Geologist with Northwestern Territories Inc. conducted a bluff stability inspection at the above mentioned property on March 14, 2003. 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 construction of a cable tram down the face of the bluff (Please see attached drawing and photos). Site Description The subject waterfront property is located at Adelma Beach overlooking Discovery Bay. Developments on the property include a house on the bluff, a beach house and a stairway down the face' of the bluff. The bluff at the property is approximately 67 feet high with an overall slope of about 36 to 40 degrees. The upper ~9 feet of bluff is about 54 degrees and the toe of the bluff is vertical behind a wood piling bulkhead. The bluff face is vegetated with weeds and grass with some young trees. Some of the trees have curved trunks, which suggests that minor downslope creep of the surface soil is occurring. There is a large fir tree at the top edge of the bluff. No bare slide scars were noticed on the bluff face. No springs or seeps were noticed on the bluff face. The toe of the bluff is protected from erosion by the above mentioned bulkhead. The high tide line appears to be at the bulkhead. Site Geology The Washington State Department of Ecology's Coastal Zone Atlas maps the area of the subject property as Undifferentiated Pleistocene sediments (Qu) consisting of glacial and nonglacial deposits. In Jefferson County, the unit contains lake sediments, deposited mostly during the late Vashon Stade (ice receding), and partly during the early Vashon Stade (ice advancing). It includes sandy silt with clay and/or silty sand deposited during the Salmon Springs Glaciation. The Atlas also maps the stability of the bluff as Unstable. The Washington State Department of Ecology's "Geology and Ground-Water Resources of Eastern Jefferson County, Washington", April, 1981 maps the site as Recessional outwash and ice-contact stratified drift (Qvr). These late Pleistocene sediments were deposited at the end of the Fraser Glaciation and include deltas, alluvial fans, channel fills, kames, kame terraces, eskers and other sedimentary bodies deposited by streams from receding ice lobes of.the glacier. Typically, this unit consists of gravel with some sand, silt and clay, and is generally unconsolidated. 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 soil type Dick loamy sand (DcC) (See Appendix). This unit consists of somewhat excessively drained sandy soil, which formed in glacial outwash on plains and .terraces. This soil is classified as Silty Sand (SM) based on the Unified soil classification system. This soil is further described as having Iow shrink/swell potential, Iow corrosivity to uncoated steel and concrete, and no frost action potential. This soil profile only considers the upper 5 feet of soil. Visual observations made on the bluff face at the subject site were consistent with the above soil descriptions. Mechanics of Bluff Recession There are many forms of bluff recession that occur in the coastal regions of northwest Washington. Two common processes are the erosion of the toe of the bluff by wave action, and the sloughing of upper bluff soils due to saturation of the soil during the rainy season. When waves attack the toe of an unprotected bluff, the lower bluff soils are eroded away. Eventually, this erosion will oversteepen the bluff to a point where the' soil can no longer support itself at such a steep angle. Then the bluff soils will slough off, depositing material at the toe of the bluff. This will have the effect of temporarily reducing the angle of the bluff to a more stable angle, and then the whole process will start over again. This process has been suspended at the subject property due to the presence of the bulkhead at the toe of the bluff. If the bulkhead were ever damaged or removed, this form of erosion would begin again, thus destabilizing the slope and recession with landslides. . ~---~ Many of the landslides that occur in Our region happen in the winter or spring when the ground is saturated with water, and especially after heavy rainfall events. When the soil becomes saturated, there is a decrease in the cohesion between the soil grains and an increase in the pore-water pressure. This condition can trigge'r landslides and debris flows on slopes. For this reason, it is important to control on-site drainage and runoff in order to minimize the infiltration into and resulting erosion of the soil. It is also important to maintain vegetation on the bluff face in order to reduce erosion of the bluff soils. The bluff at the property is vegetated and no bare areas were noticed, which suggests that erosion from surface water runoff has been controlled or is not sufficient enough to cause erosion. Another factor to consider with regards to bluff stability is the "angle of repose" of the soil. The "angle of repose" is defined as the maximum angle at which loose, cohesionless material remains stable (assuming no destabilizing forces). This angle commonly ranges between 33. and 37 degrees. The majority of the bluff at the property is between 36 and 40 degrees, which is just over the typical angle of repose. However this may be the angle of repose for this particular soil type and its state of compaction. However, the upper 9 feet or so of bluff is steeper than the rest of the bluff and may slough to a more stable angle in the future. Conclusions and Recommendations The bluff at the subject property appears grossly stable at present and the proposal seems feasible from a geotechnical perspective. This assumes that the bulkhead at the toe of the bluff remains intact and that erosion of the bluff face does not become a problem. If the bluff does become destabilized, bluff stabilization measures will be necessary in order to protect the cable lift from being undermined. If the upper 9 feet or so of the bluff were to slough off to the same angle as the majority of the bluff, approximately four feet of bluff top could be lost. This possibility should be taken into account with respect to the design of the cable lift and has been reported to the tram design/construction company. The design has been modified to address this concern and is reflected on the attached drawing. The following recommendations should also be considered with regards to the proposal: 'It will be necessary to maintain ground cover to reduce erosion from surface runoff. Any bare areas that develop on the upland or on the bluff 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. Any drainage control devices should be maintained in good working order and inspected at least once a year.3 ~i~APR 2 2 ~2003~~~J~,~L~ JEFFERSON COUNTY DEPT. OF COMMUNITY DEVELOPMENT 4. The bulkhead at the toe of the bluff should be maintained and kept in good condition. Any additional surface runoff created by the proposal should be controlled and routed to a drainage control device such that surface water discharge to adjacent'properties does not significantly exceed predevelopment conditions. Silt fences or other sediment control devices may be needed during construction such that sedimentation to adjacent properties does not significantly exceed predevelopment conditions. Based on the findings, recommendations and limitations of this report: Coastal bluffs are inherently unstable, however there appears to be minimal landslide hazard to the proposal as suggested by the lack of evidence of landslide activity at the subject property in the recent past, in large part because of the suspended erosional forces acting on the toe due to the presence of the bulkhead. Observations of slope stability indicate that the proposal would not be subject to risk of landslide under the current conditions that exist at the site. 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 proposal would 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 can be obtained from the DOE Website at: http://www.ecy.wa.gov/biblio/sea.html under the 1993 and 1994 year heading. The DOE website also contains much more useful information regarding slope stability and site development; this reference is highly recommended. Limitations This report has been prepared for the exclusive use of our client 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. APR 2 2 2003 4 JEFFERSON COUNTY DEPT. t ...... 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 12/30/2004 Bill Payton, L.E.G. Engineering Geologist Expire. s 11-06-03 Appendix: 1. site photos, 3,14-03 2. tram drawing G:\Gen\Bill\Reports\OSTE0301 .bluff stability.32(30-1).Adelma Beach.doc 5 APR 2 2 2001 JEFFERSON COUNTY _ DEPT. OF COVV N TV 3SV~LOPMEP,! APPENDIX APR 2 2 2003 JEFFERSON COUNTY DEPT. OF CO~,'~[',,~.~J?.~!-f¥ DE'VELOPM~ APR 2 2 2003 JEFFERSON" '"'" Photo 1. View of top of bluff looking towards Discovery Bay Photo 2. View of bluff from beach with bulkhead in APR 2 2 2003 JEFF,_P, SON CO,jNTY ,.- / DEPT. OF C(})?is.;uNII-? tT.:(!VELOPME,~ ! J Photo 3. View of beach showing bulkhead and beach house Photo 4. View of tree on bluff with curved trunk