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HomeMy WebLinkAbout947400070 Geotech AssessmentIL,9-~~ - 3q ~ STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 July 14, 2005 Rudy Prael 94 Woodhaven Lane Port Townsend, WA 98368 Re: Geology Evaluation Tax Pazce1947400070 Lot 70 Honeymoon Lane Jefferson County, Washington Dear Mr. Prael: __ _ ~ ~ ._ €' Jt1L 19 2005 ~ ~ .. , ~. L_..... ~ ~y., i ,, { ,~ z.; sJEi' a ~7f C~ ~ ,(~ ~ ukVELUF;Y~ENT We herein present the results of our geology evaluation of the above referenced property on a shoreline bluff. 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 an 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 Surficial Geologic M~ of the Port Townsend 30 b~60 Quadrangle Puget Sound Region, Washington (Pessl, Dethier, Booth and Minard, 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 aze 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. -' ~~ J U L l 9 2005 ,_ July 14, 2005 Tax Parce1947400071,Jeflerson County, WA ~ '""""~` Geology Evaluation r~~-, ~ ~ r ~;LV~~iiF ,:ENT ~t__.___ _._ _._._._. SPECIFIC SITE OBSERVATIONS A site location map is provided on Figure 1 and a general site plan sketch 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 azea 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 east-southeast. The level area is bounded on the east by an approximately 15 feet high slope with an overall slope of 25 degrees. This slope is a combination of native soils used as fill and a former small cut slope from a former logging road. The old logging road is overgrown with brush and is approximately 30 feet wide. The slope below the logging road slopes down towards the northeast at an angle of approximately 40 degrees for 100 feet. The slope ends on a gentle sloping azea near where the community water tank is located. This slope is covered by thick brush. No evidence of slope movement was apparent on the slope or below the slope. 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 landsliding will not threaten the structure within the expected life of the structure (120 years) as long as my recommendations are followed. 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. And should be set back from the 40 degree potentially unstable slope on the east side of the property at least 20 feet. Stratum Group File: 7. I.OSB __ ~~ July 14, 2005 Tax Parcel 947400071, Jefferson County, WA Geology Evaluation The 25 degree slope located to the east of the level building is stable and is not potentially unstable. Construction can take place on this slope as long as the support for the structure is set back from the slope at least 5 feet from the slope for purposes of avoiding differential settlement. It would also be acceptable to excavate downwazd at the east edge of the level building azea and extend the base of the building azea towazds the east. The excess soil should not be placed be placed within 20 feet of the top of the 40 degree slope on the east side of the property. I do not anticipate that the development of the subject property will cause any negative impacts on the stability of the slopes as long as our recommendations aze followed. Roof drains and. any other drainage should be dischazged in a dispersed manner to a location that will not pose a risk of surface flow on the 40 degree slope on the east side of the property. Soils on the site aze very well drained and drainage should be directed into the ground at locations away from the steep slopes and in a dispersed manner. That is drainage water should be dispersed as much as reasonable versus concentrating all the drainage into a single location. Septic drain fields should not cause problems for slope stability as long as the drain filed is not located on or within 30 feet of the 40-degree slope. 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 asrequired. Should you have any questions regazding our reconnaissance please contact our office at (360) 714-9409. Sincerely yours, Stratum Group G` ~~`' Mc han , .Sc., L.E.G. Licensed Engineering Geologist a>s h. °C !1~ . ~ Daniel McSh~ns f Stratum Group File: 7.1.056 ^i ~ ,~.A. '' y ,, ~ ~~~ "' - ~, r . ~ __ _ ~ ~, ;,, ' ~' ~ ~ 19 200 ~;~ ~;~, JUL t~~ ~ ~ ~ ~ ~ .~ ~ v J'~ -~ DEFi. C- CC.. _ ..,. , U~V" ~ ~. ,.EIS e ~ 3 ~ cn ~~ fi ~ ~ as ~ -~ o ; ~ (n ~. ! t ~ ~ ~ ~ ~ ` In -~' ~ .n b ( ~ ` `' Q t ~. ~ o ~~ ~~ - - ' ! ~`~°' ti~~; ~ ~~ ~ ~, ,..s~ _~~~ ~, A `~ ``. 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