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HomeMy WebLinkAboutBLD2005-00638 Geotechnical Report • • STRATUM GROUP 145I Grant Street, Bellingham, WA 98225 Phone(360)714-9409 April 27, 2004 F 1*..N r Bill and Marlene Rich 9312 Kersey Drive Cheyenne, WY 82009 Re: Geology Evaluation Tax Parcel 947400070 Lot 70 Honeymoon Lane Jefferson County, Washington Dear Mr. and Ms Rich: 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 was limited to 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 Surficial Geologic Map of the Port Townsend 30 by 60 Quadrangle, Puget Sound Region, Washington(Pessl, Dethier, Booth and Millard, 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 subiect Property and overlies the advance outwash deposits. • April 27,2004 Tax Parcel 947400070,Jefferson County, WA Geology Evaluation SPECIFIC SITE OBSERVATIONS A site location map is provided on Figure I 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 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 a southeast area or 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 15 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 on 15 feet below. 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 60 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 apparent on the slopes above or below the former logging road. The level building area is bound by an approximately 35 to 40 degree slope on the south. This slope is tree-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. The slope then steepens to approximately 45 degrees down an approximately 25-foot high cut 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. Stratum Cimuo File:4.13.04 • • April 27,2004 Tax Parcel 947400070,Jefferson County, WA Geology Evaluation 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 bluff 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. If only the current existing level building area is to be used, the structure should be setback from the top of the slope on the southeast side of the level area 5 feet. The structure should be setback from the top of the slope on the south side of the level area 10 feet. These setback distances are indicated on the attached site plan sketch. It would also be acceptable to excavate downward at the southeast edge of the level building area and extend the base of the building area towards the southeast. The building area could be extended to the top of the 30-degree slope at the outer edge of the old logging road. If this approach is used,excess excavated soil should not be placed on any slopes greater than 30 degrees or at the top of any slopes greater than 30 degrees. I would suggest that the excess soil be placed either within the old logging road cut slope or on the level area away from the outer edge of the level area. 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 are followed. Roof drains and any other drainage should be discharged to a location that will not pose a risk of surface flow any of the steep slopes. Soils on the site are 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. The septic drain field should not cause problems for slope stability as long as it is not located on or within 30 feet of the south 40-degree slope or the southeast 45-degree cut slope above Honeymoon Lane. OCT - 4 2005 Stratum Groun j File:4.13.04 FMENT