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HomeMy WebLinkAbout601061003 Geotech Assessment (2001) STRATUM GROUP 1451 Grant Street, B¢lling~ham, WA 98225 Phone (360) 714-9409 March 21,2001 Lynn Olsen 20017 SE 290th Place Kent, WA 98042 Re: Geotechnicai Report, Shoreline Bluff Parcel # 601061003 Township 26N, Range 1W, Section 6 Jefferson County, Washington Dear Ms Olsen: We are pleased to present the results of our geology_, evaluation of the above referenced property located on a shoreline bluff. The pu¢ose of this geolo_?_ 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 geolo~c 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 covered at least four times in the past two million years by ice lobes advancing out of the Coast Kange Mountains of Canada and the mountains of Vancouver Island. During these glacial and accompan_ving interglacial periods, the underlying, bedrock was eroded. A relatively thin layer of glacial related and interglacial fluvial sediments were deposited March21,2001 JUL 2 6 Tax Parcel # 601061003, Jefferson Count, WA Geotechnical Report JEFFERSON COUNTY DEPT. OF COMMUNITY DEVELOPMENT over the underlying bedrock in the vicinity of the subject property. However, a short distance from the subject property, the thickness of these glacial and interglacial deposits is significantly thicker~ The Geologic Map of Surficial Deposits in the Seattle 30' x 60' Quadrangle, Washin~on (Yount, Minard and Dembrof, 1993) and the Geologic Map of West-Central Jefferson County, Washington (Birdseye, 1976) indicate the subject property and the steep slopes in the vicinity of the subject property are underlain by the Twin River Formation, Vashon glacial till and pre-Fraser sediments. The Twin River Formation consists of feldspathic, fine-grained, thin- to thick-bedded sandstone and massive concretionary siltstone. The pre-Fraser sediments are described as consisting of interbedded oxidized brown, red-brown, and gray ~avel, sand, silt, and clay. The layers are moderately to well bedded and the unit contains minor amounts of ice-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 Olyrnpia nonglacial periods. The sediments have been overridden and consolidated by glacial ice. The Vashon glacial till is described as consisting of a mixture of clay, silt, sand and gravel and is in a vew compact condition. These sediments were deposited directly by glacial ice. Observations on the steep slope of the subject property and in hand dug test pits on the upland portion of the property indicate that the property is underlain by Twin River Formation and glacial till. The till was observed only in shallow hand dug test pits excavated on the upland portion of the subject property. Essentially the entire bluff is underlain by bedrock of the Twin Rivers Formation. Glacial till was observed near the top of the bluffto the west of the subject property and very. compact silt sandy gravel that are pre-Fraser sediments make up the bluff several hundred feet east of the subject property. The Twin Rivers Formation underlying the bluff face consists of approximately 90% concretionary layered siltstone. Layers of fine-grained sandstone ranging from a few inches up to several feet thick are present as well. The bedrock layers strike north 20 degrees west and dip 65 degrees to the northeast. 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 and the eroding shoreline. Stratum Group File: 2.21.01 2 March 21,2001 u - Tax Parcel # 601061003, Jefferson Count, WA Geotechnical Report JEFFERSON COUNTY DEPT. OF COMMUNITY DEVELOPMENT SPECIFIC SITE OBSERVATIONS The subject property is located at the southern end of the Bolton Peninsula. The location of the property is indicated on the attached Vicinity Map. The subject property consists of an approximately 3.2-acre parcel. The parcel consists of a gently sloping upland area and a very. steep shoreline bluff slope. Th width of the upland portion of the property ranges is approximately 160 feet on the east boundary of the parcel. The northwest comer of the lot is located essentially on the slope break such that the entire western boundary of the parcel is on the steep shoreline bluff slope. The upland portion of the property slopes to the south at an angle of 10 degrees or less. The upland portion of the property is covered with a mature stand of second growth Douglas fir with a few alder and red cedar with an understory consisting primarily of salal and evergreen huckleberry. The shoreline bluff on the property is approximately 200 feet high. The bluff on the eastern half of the property is a fairly consistent slope of approximately 55 degrees. The bluff on the western half of the property is approximately 60 degrees for the first 30 feet down the bluff then slopes at approximately 40 degrees for 100 feet, and the lowermost slope is approximately 65 degrees. Most of the shoreline bluff on the property is covered with Douglas fir and madrone trees with a thick brush understory. The lower 60 feet of the bluffon the west portion of the property has little vegetation and the bedrock is exposed on the bluff face. A drainage swale is located on the easternmost portion of the property. It appears that surface water may on rare occasions flow overland at this location. However, no water was or wet soils in the drainage swale at the time of our site visit on February 24, 2001 despite a recent heavy snow and melt. The slope on the shoreline bluff below this drainage swale appears to slightly less stable and a debris fan approximately 20 feet across is located at the bottom of the bluff at this location. Bedrock on the bluff consists primarily of siltstone with a lesser amount of fine sandstone. The rock is deeply weathered and breaks into small angular gravel fragments. Where exposed the slope below is covered with this _gravel. The bluff is eroded by a combination of wave action that undermines the base of the bluff and raveling of material off the exposed bedrock and soils on bluff face. The material eroded from the bluff face is removed from the base of the bluff and transported to the west along the shore by wave action. Bluff erosion is episodic at this location. A combination of high tides with high south Stratum Group File: 2.21.01 3 March21,2001 jili ? 6 Tax Parcel # 601061003, Jefferson Com~, WA Geotechnical Report J E FFERS ON C 0O NTY DEPT. OF COMMUNITY DEVELOPMENT winds will cause ve.ry high-enemy wave action at the base of the bluff because of the long fetch of water towards the south down Hood Canal from the subject property. The beach at the base of the bluff consists of a bedrock platform partially covered with gravel from eroding bluffs to the east. Landslide debris covers the base of the bluff at several locations, but at most locations in the immediate vicinity of the subject property the base of the bluff consists of exposed bedrock. No ground water seeps were observed during our site visit on Februa~ 24, 2001. CONCLUSIONS AND RECOMMENDATIONS Based on our visual inspection of the subject property and vicinity, we conclude that a residence can be located on the property such that shoreline erosion and 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 distancq for the residence from the tho of the steed slope. It is our opinion from an engineering geology standpoint that this distance will be adequate to allow for expected bluff failures. The presence of mature trees and the presence of bedrock underlying the bluff indicate that the bluff is~fairly~stable~despite its exposure to occasional high wave energy from the south. The strike and dip angle of the bedrock is at an angle such that the slope will not be effected by inherent weaknesses in the bedrock. We recommend a 3 O-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 let~ on slopes steeper than 30 degrees as this material may become saturated and slide down the slope. 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. Wastewater System We have reviewed the proposed wastewater system for the proposed property desi~_med by John Scott Fleming Engineering, Inc. (JSF) for the site. JSF has indicated that the peak flow to the Stratum Group File: 2.21.01 4 March 21,2001 Tax Parcel # 601061003, Jefferson CounB,, WA Geotechnical Report wastewater system will be 240 gallons/day (gpd) with an average capacity of 160 gpd. The application rate for the drainfield is 0.9 gpd per square foot. This is equivalent to approximately 1.3 inches of rain added to the area over the drain field. This additional water discharge represents a very small percentage of the total water that infiltrates the ground surface on the slope above the steep slope. We do not anticipate that the septic drainfield will have any adverse affects on the stability of the steep slopes on the subject property or elsewhere as long as our recommendations are followed. We recommend that the drain field is located at least 50 feet.from the tod of the steer) slope, tad_ that t~he following storm water control recommendations are followed. . Stormwater Three areas of storm water will generated from development of the site: 1) roof runo~, 2) run off from the access road construction and 3) intercepted perched ground water from the building foundation and access road constmction~ ~torm wa~er...from the roof should be collected and foundation-footing drainq ~qhould he collected and dlsc~arged vi_~_ tightline tO the base of the bluff on the pr0pertv. In addition, we recommend Shq construction of a drainage ditch on the ut)hill side of the property access road_ and water from thi~ ditch be collected in a tightline pipe _a_t,d _¢r,~ted to, .t.he base of the bluff_ The collection of water from the ditch should include the collection of water,. ~om the, existing ditch on Gufstavson Road..This ditch currently occasionally discharges water to the steep slope of the subject property via a natural swale. It is our opinion that the interception of drainage and runoff water will more than offset the water discharged to the wastewater treatment system. 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. JEFFERSON COUNTY DEPT. OF COMMUNITY E)EVELOPMENT Stratum Grour~ File: 2.21.01 5 March 21,2001 Tax Parcel # 601061003, Jefferson Cotmty, WA JEFFERSON COUNTY Geotechnical Report DEPT. OF COMMUNITY DEVELOPMENT CLOSURE Please note that there are inherent risks associated with building on lots near or a4iacent to steep slopes. These are risks that the building owner should recognize and be willing to accept. 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 modi~ our recommendations as required. We appreciate the opportunity to be of service to you. Should you have any questions regarding our reconnaissance please contact our office at (360) 714-9409. Sincerely yours, Stratum Group Dan McShane, M.S. Senior Geologist Stratum Group File: 2.21.01 6 : ~. 122~5231. W~ Nfi.D'27 1Z~4701" ~g ! ..JEFFERSON COUN~ DEPT. OF COMMUNITY DEVELOPMENT ~ON DAN McSHANE, M.S. GEOLOGIST JEFFERSON COUNTY DEPT. OF COMMUNITY DEVELOPMENT Dan McShane is the senior geologist and oWner of Stratum Group: a geology, geotechnical and environmental consulting firm in Bellingham, Washin.gton. Mr. McShane fotmded Stratum Group in 1997. He has 12 years of geologic and geotechnical project management experience. He routinely conducts geologic evaluations on shoreline pr~ects and property throughout Washington State. Mr. McShane is the current chair of the Northwest Washington Association of Earth Scientists, and serves on the Whatcom County Flood Zone District Citizens Advisory Committee including positions on the project review committee and the sediment committee. Mr. McShane was elected to the Whatcom County Council in 1999. As a council member, he serves on the Natural Resources, Planning and Development, and Public Works and Capitol Projects Committees. Examples of Mr. McShane's recent shoreline project experience include: PROJECT EXPERIENCE: Benson Property, Shine Road, Port Ludlow, WA: Conducted an engineering geology evaluation of a proposed building site adjacent to a deep-seated landslide on Hood Canal. Delineated the extent of the failure and determined the slide mechanism. The property owner was able to obtain a building permit based on site development recommendations and setting the structure back an appropriate distance. The evaluation was conducted to satisfy Jefferson County Shoreline and Critical Areas Ordinance permitting requirements. Walton Property, Sandy Point, Whatcom County, WA: Conducted a geology evaluation of a shoreline property on a shoreline spit. The evaluation indicated that the property was at risk due to the construction of shoreline bulkheads both north and south of the subject property. The client was able to demonstrate the need for a btflkhead due to the damage caused by a~oining shoreline armoring. Curtis Property, Acme, Whatcom County, WA: Evaluated the effects of an tmpermitted wood piling riverbank deflector. Concluded that the deflector was not causing any damage to doWnstream or upstream properties and that the deflector would not withstand winter high flow events. The river destroyed the deflector two weeks after the evaluation was completed. Gattinella Residence, Marysville, WA: Conducted a test pit investigation and slope evaluation at a steep shoreline bluffnear Marysville, WA. The test pit investigation provided data for the engineering and design of a sixteen-foot high retaining wall. The slope evaluation was conducted to determine the overall slope stability in regard to the placement of the retaining wall as well as providing recommendations for slowing erosion on the steep bluff face. Tapparo Property., Beckett Point Road, Port Townsend, WA: Conducted an engineering geology evaluation of a very steep eroding bluff west of Port Townsend, WA. The evaluation was conducted to satisfy Jefferson County Shoreline and Critical Areas Ordinance permitting requirements. Provided an assessment of the proposed building site and provided recommendations for slope revegetation and infiltration of storm water nmoff and footing drain drainage. Dan McShane, M.S. Page 2 Olympus Beach Property, Port Ludlow, WA: Conducted an engineering geology evaluation of a very steep eroding bluff near Port Ludlow WA. The evaluation was conducted to satis~ Jefferson County Shoreline and Critical Areas Ordinance pen~tting requirements. Provided an assessment of the proposed building site setback and provided recommendations for infiltration of storm water runoff and footing drain drainage. Larson Bridge, South Fork Nooksack River, Crown-PacifiC, Skagit Count, WA: Conducted a geotechnical investigation for a bridge abutment for crossing the South Fork Nooksack River. The investigation involved a bedrock evaluation of the north abutment, test pit and boring investigation to determine bearing capabilities of soil and depth to bedrock at the proposed south abutment, and an evaluation of river bank stability ~ostream of the pro_posed bridge location. EDUCATION M.S. Geology Westem Washin~on University, 1991 B.S. Geology Western Washington University, 1983 B.S. Education Western Washin~on University, 1984 PROFESSIONAL MEMBERSHIPS American Society of Civil Engineers, Geotechnical Group Northwest Washington Engineers Club Northwest Washington Association of Earth Scientists, Chair PUBLICATIONS McShane, D. and Brown, E.H., 1991, Age of loading of the Ska~t Gneiss and implications for orogeny in the North Cascades crystalline core: Geologic Society of America Abstracts with Programs, v. 23, no. 2, p. 78. Miller, R.B., Brown, E.H., McShane, D., and Whitney, D.L., 1993, Intra-arc crustal loading and its tectonic implications, North Cascades cry_ stalline core, Washin~on and British Columbia: Geology, v. 21, p. 255-258. Brown, E.H., Cary, J.A., Dougan, B.D., Dragovich, B.D., Fluke, S.M., McShane, D., 1994, Tectonic evolution of the Cascades Crystalline Core in the Cascade River Area, Washington: Regional Geology Of Washington State, Washin~on Division of Geology and Earth Resources, Bulletin 80, p. 93-113. JEFFERSON COUNTY DEPT. OF COMMUNITY DEVELOPMENT