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Home My WebLink About701344049 Geotech Assessment (2001)STRATUM GROUP • 1451 Grant Street, Bellingham, WA 98225 ~ ~~ y~~ Phone (360} 714-9409 ~~~ ~~/ August 2a, 2001 pUG 2 12006 Paul and Jan Coover JEFFF~SD~ ~~~~ ~~~, 1255 Nancy Court r ~ Cffi Upland, CA 91786 Re: Geotechnical Report, Shoreline lull Kennard Lot 2, Tax Parcel 70134049 Township 27N, Range 1W, Section 34 Jefferson County, Washington Dear Mr. and Ms Coover: We are pleased to present the results of our geology evaluation of the above referenced property located 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 a potentially unstable steep slope. This evaluation was limited to a visual inspection of the property, the steep slope on the property, other steep slopes in • the vicinity of the property, available geologic mapping in the area, and the shoreline at the base of the bluff. SCOPE OF SERVICES The scope of our services included the following: 1) Conducted a site visit to visually inspect the subject property including the steep slope conditions and shoreline conditions in the vicinity of the property. 2) Observed surface soil conditions on the steep slope and at the top of the slope. 3) Prepared this report summarizing our findings, including an evaluation of the feasibility of building a residence on the subject property, a qualitative evaluation of the slope stability, recommendations for site development, and.. recommendations for further investigation, if necessary. 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 a relatively thick layer of glacial • related and interglacial fluvial sediments were deposited over the underlying bedrock in the August 21, 2001 • Kennard Lot 2, Dabob Cove, Tax Parcel # 1013~~049, Jefferson County, WA Geotechnical Report vicinity of the subject property. The Geolo ig c Map of Surficial Deposits in the Seattle 30' x 60' Ouadrang_le, Washin tg_on (Yount, Minard and Dembrof, 1993) and the Geologic Malt ofWest-Central Jefferson County, Washington (Birdseye, 1976) indicate the subject property and vicinity is underlain Vashon glacial till and advance outwash deposits. The slopes below the subject property are mapped as being underlain by undifferentiated pre-Fraser glaciation deposits. The Vashon glacial till is described as consisting of a mixture of clay, silt, sand and gravel and is in a very compact condition. These sediments were deposited directly by glacial ice. The glacial till was observed in hand dug test pits on the upland portion of the property near Beach Drive. The advance outwash deposits consist of sand and gravel in a dense condition due to consolidation by glacial ice. However, lack of silt or clay results in portions of the advance outwash not having cohesive strength. The upland portion of the property and the steep slope below the property to the west are underlain by sand and gravel outwash with little silt or clay. Advance outwash deposits are present on the upland portion of the property and underlie the upper approximately 150 feet of the steep slope below the property. The pre-Fraser sediments underlie the advance outwash deposits. The pre-Fraser sediments are described as consisting of interbedded oxidized brown, red-brown, and gray gravel, sand, silt, and clay. The layers are moderately to well bedded and the unit contains minor amounts ofice-contact deposits and outwash gravel and sand. Generally the unit is nonglacial and has abundant peat and woody debris. These sediments were deposited sometime prior to the last glacial event and include deposits of the Whidbey and Olympia nonglacial periods. The sediments have been overridden and consolidated by glacial ice. Sand and gravel units within the pre-Fraser sediments without silt or clay can lack cohesive strength even though they have been consolidated by the weight of glacial ice. The pre-Fraser deposits an the steep slopes below the subject property consist primarily of sand and gravel. However, silt and clay layers are present as well, and at least one. glacial till layer from an older glacial advance is located on the lower slopes. The silt and clay content within the sand and gravel units is variable; hence, the cohesive strength of these units is variable as well. The Relative Slope Stability in West-Central Jefferson County. Washington {Birdseye, 1976) indicates the subject property is located in an area of unstable slopes and identified active landslides. The unstable slopes and active landsliding in the area is a result of the steepness of the shoreline bluff, the loose condition of the advance outwash deposit sands, possible structural • Stratum Group File: B.S.O1B 2 August 21, 2001 • Kennard Lot 2, Dabob Cove, Tax Parcel # 70134049, Jefferson County, WA Geotechnical Report weaknesses within the compact silt layers because of the removal of confining pressure and the eroding shoreline. SPECIFIC SITE OBSERVATIONS The subject property is an approximately 2.7 acre parcel that extends from Beach Drive to partially down the steep slope on the west side of the property. The location of the property is indicated on the attached Vicinity Map. The upland portion of the property slopes very gently towards the west. The steep slope on the west side of the property breaks sharply downward towards the west. Except for a relatively small cleared area on the west side of the property, the property istree-covered primarily with Douglas fir with an understory of salal, ferns and evergreen huckleberries. The property is located at an elevation of approximately 500 feet. The steep slope to the west drops down to Dabob Bay. The uppermost portion of the slope slopes at an angle of approximately 45 degrees for a height of approximately 100 feet. The slope then steepens for approximately 30 feet, then averages approximately 30 degrees to the shore. However, the lower slopes are hummocky. Recent landslides have occurred on the slopes below the subject property. The slopes below the subject property consist of a large landslide complex. Both debris flows and rotational failures are present. The steep slope on the west side of the subject .property defines the eastern extent of the landslide complex. Ground water seeps and springs are present on the lower slopes below the subject property. The highest seeps observed on the slope are approximately 150 feet in elevation down the slope. Most of the slopes below the subject property are covered with a stand of mature second growth Douglas fir and maple. However, due to recent slide activity portions of the lower slopes have few trees and trees are just beginning to reestablish on the failed soil slopes. CONCLUSIONS AND RECOMMENDATIONS Based on our visual inspection of the subject property and vicinity, and our assessment of the landslide area, we conclude that a residence can be located on the property such that landsliding will not threaten the proposed residence within the expected life of the structure {75 years) as long as our recommendations are followed. We recommend a set back distance of 150 feet from the top of the steep slope. It is our opinion from a geology standpoint that this distance will be • Stratum Group File: 8.5.O1B 3 August 21, 2001 • Kennard Lot 2, Dabob Cove, Tax Parcel # 701331049, Jefferson County, WA Geotechnical Report adequate to allow for expected bluff failures. No indications of recent slope failures are present on the subject property; however, topographic features and the geology of the site indicate that the steep slope on the west side of the property is the boundary of a large landslide complex. Over time the landslide area may be expected to expand further to the east as the lower slopes below the subject property continue to fail. The presence of mature trees and the fact that even the lower slopes have been well vegetated until the most recent sliding in the winter of 1998-1999, indicates that the active sliding that took place two winters ago is probably not typical. However, the upper part of the steep slope is potentially very unstable and may be subject to a future slope failure. It is our opinion that the 150-foot setback will provide a safe distance from the slope in the event of a slope failure. It may be possible to safely locate a residence closer to the steep slope; however, we would recommend a geotechnical boring to determine the depth to impervious soil horizons to more accurately determine a safe setback distance. Given the distance of the proposed residence from the shoreline, we do not anticipate that shoreline erosion poses a threat to a residence set back 150 feet from the top of the steep bluff. In addition, the shoreline at the base of the bluff is only being eroded during periods of very high • tides as a sand and gravel spit is actively forming on the shoreline below the property. Soils on the west side of the property are very well drained, and therefore we do not anticipate discharging of footing drains will be necessary. Roof drainage and any storm water should be discharged into the ground near the building. We recommend a 50-foot native vegetation buffer consisting of tress and low brush at the top of the bluff. The top of the bluff should be disturbed as little as possible, and we recommend that no grading be performed on the steep slopes of the bluff face. Trimming of tree branches for view purposes will not have any negative impacts to the steep slope. However, no trimming debris should be left on slopes steeper than 30 degrees as this material may become saturated and slide dawn the slope. We do not anticipate that the development of the subject property will cause any negative impacts on the stability of the slopes in the vicinity of the subject property as long as our recommendations are followed. Site grading soils or debris, landscape debris, or any other material should not be disposed of over the steep slope on the property or placed at the top of the slope. . 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