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Home My WebLink About947400070 Geotech Assessment AddendumSTRATUM GROUP 1451 Gtant Street, Bellingham, WA 98225 Phone (360)714-9409 November 23, 2005 Mark Getzendaner 5825 Old Gardner Road Port Townsend, WA 98368 Re: Geology Evaluation, Rich Property Tax Parcel 947400070 Lot 70 Honeymoon Lane Jefferson County, Washington Dear Mr. and Ms Rich: ,~~~~~~~ ~~~~ ~~~ s;~ y~''' Hav ~a ~5 ~``~+` We herein present the results of our geology evaluation of the above referenced property on a steep slope area above the west side of Discovery Bay. The purpose of this geology evaluation was to 1) determine the suitability of the property for the siting of a residence, 2) qualitatively evaluate the risk of slope failures, and 3) provide general site development and maintenance recommendations for development of the property adjacent to potentially unstable slopes. This evaluation included a visual inspection of the property and vicinity, a visual inspection of the steep slopes on the property and vicinity, review of available geologic mapping in the area, and inspection of the other slopes in the area. GENERAL GEOLOGY Northwestern Washington has been occupied by continental glaciers at least four times during the Pleistocene Epoch (1.6 million to 10,000 years ago}. During these glacial and accompanying interglacial periods, the underlying bedrock was eroded and glacial related and interglacial fluvial sediments were deposited over the underlying bedrock in the vicinity of the subject property. The Su~ciat Geolotgic Map of the Port Townsend 30 by 60 Ouadnanngle. Puget Sound Region. Washingtton (Pesst, Dethier, Booth and biinazd, 1989) and Geologic Map of Northwestern Jefferson County (Gayer 1976) indicate the glacial till and advance outwash deposits underlie the area where the subject property is located. Site observations are consistent with the mapping. The subject property is underlain by compact silty, sandy gravel and cobbles consistent with the interpretation of advance outwash deposits. The advance outwash deposits were deposited by melt water rivers and streams from the advancing glacial ice at the onset of the last glacial period. Glacial till was observed up slope from the subject property and overlies the advance outwash deposits. ~~i~~G~~~C~l, November 23, 20115 ~ ~ i 4 f ~%J;', Tax Parce194740~70, Jefferson County, wn NOV 3 Q 2005 , , Geobgy Evaluation SPECIFIC STI'E OBSERVATIONS A site location map is provided on Figure 1 and a general site plan sketch of existing predevelopment conditions is provided on Figure 2. The property located west of U.S. Highway 101 on the west side of Discovery Bay in Jefferson County. The property is accessed from Highway 101 via Honeymoon Lane, Fager Hill Road and Woodhaven Lane. The property is located at an elevation of approximately 300 feet on the east facing slope above Discovery Bay. Woodhaven Lane crosses the west end of the property. A steep cut slope is located immediately west of the Woodhaven right-of--way. The cut slope is approximately 30 feet high at its highest. The cut slope is underlain by compact silty sandy gravel and cobbles. A small area of glacial till is exposed near the top of a portion of the cut slope. Portions of the cut slope are vertical with the highest vertical section being approximately 20 feet high. The base of the slope is underlain by loose gravel and cobbles that have raveled off of the cut slope and accumulated at the base of the slope. Overall the cut slope appears to have receded little since it was cut and a few small trees have begun to grow even on the vertical portions of the slope. No seeps or areas indicating seasonal saturation were observed on the cut slope. The cut slope is the result of cutting through an east-west trending, east plunging ridge of divergent topography. A grass-covered level building area extends from the base of the cut slope 80 feet towards the southeast. The level area is bounded on the southeast by an approximately 1 S feet high slope with an overall slope of 45 degrees. This slope is a combination of native soils used as fill and a small cut slope from a former logging road located I S feet below the level building area The old logging road is overgrown with brush and is approximately 30 feet wide. The slope below the logging road slopes down towards the southeast at an angle of approximately 30 degrees for 6fl feet. This slope is tree and brush covered although some of the trees have been removed to maintain a view towards Discovery Bay. No evidence of slope movement was appazent on the slopes above or below the former logging road. The slope then steepens to approximately 45 degrees down an approximately 25-foot high cuk slope above Honeymoon Lane. The cut slope is grass-covered with a few small trees. The slope is underlain by sandy silty gravel and cobbles. Except for minor soil creep and ravelling of loose material the slope appears stable. The level building area is bound by an approximately 35 to 40 degree slope on the south. This slope istree-covered and except for minor shallow surface soil creep no areas of instability were observed. The base of this slope ends in an area of convergent topography approximately 100 feet southwest of the subject property. SUatmn Group Fik: 4.13.P31t November 23, 2005 Tax Parcel 947400070, Jefferson County, WA Geology Evaluation 1 ~~' ~1 ~~' ~NQV 3 Q 2fl05 I ° l1 CONCLUSIONS AND RECOMMENDATIONS Based on my visual inspection of the subject property and vicinity, I conclude that a residence can be located on the property such that slope erosion or landsflding will not threaten the structure within the expected life of the structure (120 years) as long as my recommendations are followed. It is my understanding that the proposed home will be located on the site as indicated in the attached site plan sketch provided by the applicant. I have reviewed the proposed building elevation drawings provided by Linwood Custom Cedar Homes. Based on the proposed building locations and the elevations provided by Linwood, it is my opinion that the home as proposed will not be at risk from landslides and will not cause any off-site slope stability problems as long as the recommendations described below are followed. There are four steep potentially unstable slopes on the subject property. The following describes my conclusions and recommendations for each potentially unstable area. Steep cut slope on the west side of the building area: Any structure should be set back from the base of the very steep cut slope on the west side of the property at least 25 feet. The proposed home is set back more than 25 feet. 45-degree potentially unstable cut slope above Honeymoon Lane on the east side of the property: Any structure should be set back atleast 30 feet from the top of the 45-degree slope above Honeymoon Lane. The proposed home is set back well over 30 feet from the top of the slope. 40-degree slope on the south side of the property: The 35- to 40-degree slope south of the Level area is not unstable. However, due to the risk of differential settlement, any structure should be setback from the top of the slope on the south side of the level area 10 feet. The proposed home is located 30 feet from the 35- to 40-degree slope. 1 ~ foot high 45-degree slope: The manmade 45 degree slope on the east side of the level building area is potentially unstable due to the steep slope. The slope was created by the placement of fill soils on the slope and the old logging road cut at the base of the slope. The proposed home will be built into this slope by the constntction of a daylight basement. Construction of the daylight basement will remove the majority of the 45- degree slope such that the slope will no longer be potentially unstable. The excavated soil for the basement should be placed on the old logging road bringing the base of the slope up to the original slope configuration. A schematic cross-section of the excavation and new slope configuration is provided in the attached Figure 3. The foundation support should be set back at least 5 feet horiwntally from the final slope for purposes of avoiding differential settlement. sv~w,c~ r~~ a.~s.oau Novemtxr 23, 2005 Tae Parce1447400070,Defferson County, WA Geology Evaluation Roof drains and any other drainage should be discharged to a location that will not pose a risk of surface flow on any of the steep slopes. Soils on the site are very well drained and drainage should be directed into a dispersion trench at locations shown on Figure 4. Stratum Group appreciates the opportunity to be of service to you. If conditions appear different than those described in this report, or other concerns arise, we request that we be notified so we can review those areas and modify our recommendations as required. Should you have any questions regarding our recannaissance please contact our office at (360) 714-9409. Sincerely yours, Stratum Group ~°.' , Dan McShaneJj~E.G. 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