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HomeMy WebLinkAbout701311019 Geotech Assessment (2005)i STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 January 21, 2005 Mr. Vanderweghe 723 Discovery Ridge Road Port Townsend, WA 98368 Re: Geology Hazard Assessment Report, Shoreline Bluff Parcel # 701311019 Township 27N, Range 1 W, Section 31 Jefferson County, Washington Dear Mr. Vanderweghe: ~~:y=; ~ ~ I `~ ~~ ~' ~ ~ .,~-;, J U L 1 8 2Q05 E . ~ ~~?' ;~J~~~a~Cl' This geology hazard assessment concludes that the upland area east of East Quilcene Road and the level graded area adjacent to the west side of Quilcene Road are suitable building areas from a geology standpoint. The purpose of this geology assessment 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 included 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. Two Jefferson County Ordinances are potentially applicable to development of this property: the Shoreline Management Master Program and the Critical Areas Ordinance. The Shoreline Management Program is applicable to all areas of the property within 200 feet of the ordinary high water mark. The portion of the steep shoreline bluff within 200 feet of the shoreline is within the shoreline management area. Based on four different estimates, the top of the steep. slope where the building site is located is greater than 200 feet from the shoreline. A tape measure estimate measured the distance to be 236 feet. A line of site measurement combined with the measured average slope of 40 degrees indicated a distance of 232 feet. Measuring off of the digital terrain model map of the area indicated a distance of 228 feet. Estimating the bank height from the topographic map as 180 feet and using a measured average slope angle of 40 degrees indicated a distance of 214 feet. Based on these estimates, the top of the steep shoreline bluff is outside of the shoreline rules by between 14 and 36 feet. Jefferson County Critical Areas Ordinance establishes a standard 30-foot buffer between the top a landslide hazard area and the proposed structure. However, the buffer can be less if established by an individual or firm qualified to do so. Depending on the site a 30-foot buffer may or may not be adequate. Based on my assessment of the property detailed further below, it is my opinion that the residence can be located on the nearly level area on the top of the slope. The slope below this level area is initially only approximately 20 degrees before gradually steepening to 40 January 21, 2005 Tax Parcel # 701311019, East Quilcene Road, Jefferson County, WA Geology Hazardous Area Assessment degrees. The setback for-the residence should be 5 feet from the outer edge of the nearly level area. At the time of my visit, I flagged the outer edge of the level area by tying pink tape to a few trees located on the outer edge of the nearly level area. The 5-foot setback will be to avoid differential settlement along the outer edge of the building pad area. 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 glacial ice advancing out of the Coast Range Mountains of Canada and the mountains of Vancouver Island. During these glacial and accompanying interglacial periods, the underlying bedrock was eroded. A relatively thin layer of glacial related and interglacial fluvial sediments were deposited 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 last glacial period ended approximately 13,000 years ago. The Geolo imp of Surficial Deposits in the Seattle 30' x 60' Quadrangle, Washington (Yount, Minard and Dembrof, (1993) and the Geolo ig c Map of West-Central Jefferson County, Washington (Birdseye, 1976) indicate the subject property and the steep slopes in the immediate vicinity of the subject property are underlain by the Pre-Vashon sediments and the uppermost eastern portion of the property is underlain by Vashon-age glacial till. Yount, Minard, and Dembrof (1993) and Birdseye (1976) indicate shale and siltstone is present on the southwest end of the Bolton Peninsula south of the subject property. Yount, Minard and Dembrof (1993) indicate that the siltstone and shale is part of the Twin River Formation. Observations on the property and vicinity are consistent with above described mapping. Stratum Group file: 12.10.04 January 2l, 2005 Tax Parcel # 701311019, East Quilcene Road, Jefferson County, WA Geology Hazardous Area Assessment f ~ tJ t 4 I('1F( JUL 18 2005 However, I observed that the entire steep slope is underlain by bedrock consisting of layered silt stone and sandstone. Hence, the bedrock readily visible to the south of the subject property extends further to the north than the previous mapping indicated. The bedrock underlies the steep shoreline slopes for at least 300 feet to the north and south of the subject property. The Twin River Formation consists of feldspathic, fine-grained, thin- to thick-bedded sandstone and massive concretionary siltstone. Observations of exposed bedrock in the drainage on the south side of the property, in hand dug test pits on the slope and along the base of the shoreline bluff are consistent with this description. The bedrock observed was primarily layered siltstone and fine grained sandstone. The bedrock layers dip into the slope with a strike of north 20 degrees west and a dip 65 degrees to the northeast. Observations on the slope between along East Quilcene Road and on the upland area east of the road indicate that these areas are underlain by sand and gravel that I interpret to be either advancing glacial outwash or reworked glacial till. Soils in the ditch on the east side of East Quilcene Road appear to be glacial till. 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 and inactive landslides. The unstable slopes and active landsliding in the area is a result of the steepness of the shoreline bluff and the eroding. shoreline at the base of the bluff. SPECIFIC SITE OBSERVATIONS The subject property is located on the southwest shore of the Bolton Peninsula. The location of the property is indicated on the attached Vicinity Map. The subject property consists of an approximately 5-acre parcel. The parcel is bisected by East Quilcene Road. The upland area east of the road slopes gently towards the west. This area has been cleared of large trees in the past and is currently covered with young trees and brush. The upland area east of the road is underlain by glacial till. West of the road the land slopes down at an angle of approximately 25 degrees to a relatively flat graded area approximately 10 feet in elevation below the road. This slope is covered with mature second growth Douglas fir and big leaf maple and is underlain by glacial till and very compact coarse sand and gravel. The slope appears to be stable with only minor surface soil creep. The relatively flat graded area is grass and brush covered. The area is accessed by a grass- covered access road. The level graded area is bounded on the west by the steep shoreline bluff. The graded area is approximately 40 feet wide. Stratum Group File: 12.10.04 January 21, 2005 Tax Parcet # 701311019, East Quilcene Road, Jefferson County, WA Geology Hazardous Area Assessment r, ~~ 3 1~ `~~;~ `~ ~ ~1~ ~0. a ~t i i : <~"1 ~~ ''r JUL 1 8 2005 _k , ,~ ,;~^i ~ r~^ The steep shoreline bluff slope on the west side of the property is approximately 160 feet high and has an average slope angle of 40 degrees. The slope is underlain by bedrock covered in most areas with colluvium consisting of weathered bedrock and topsoil. Mature second growth Douglas fire and western red cedar cover most of the slope. Shallow topsoil failures of various ages are apparent at several locations on the slope. The depth of soil failure is generally on the order of three to four feet and involves the slope colluvium and topsoil layer. A drainage Swale is located on the south side of the property. Surface water flows overland from the uplands at this location during periods of heavy rainfall or snowmelt. A trickle of water was flowing in the drainage at the time of my site visit in December 2004. I had observed no water in this same drainage in late November 2002 when mapping this section of the shoreline on another project. Bedrock was exposed within the drainage throughout most of the drainage. Some additional water has been directed to this drainage by ditches along East Quilcene Road, but due to the bedrock no significant erosion increase is apparent. The bluff is eroded by a combination of wave action that undermines the base of the bluff and occasional slope failures of built up colluvium and topsoil on the steep slope. Past slope failure soils and debris is present on the beach at the base of the bluff. Trees are growing within these old slope. failure debris piles, and these areas of old slope debris are effectively naturally protecting the base of the bluff from erosion. Due to the orientation of the shoreline, the size of waves impacting the- shoreline at this location will be relatively small even during storm events. The beach at the base of the bluff consists of a bedrock platform partially covered with gravel from eroding bluffs to the south. No ground water seeps were observed during my site visit in late December 2004 except for a small seep in the drainage on the south side of the property. No vegetation was observed on the site that indicated any areas of seeps on the steep 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 shoreline erosion and landsliding will not threaten the proposed residence within the expected life of the structure (75 years). The residence should be set back 5 feet from the outer edge of the level building pad. The slope below the level building area initially slopes at a slope of less. than 20 degrees and then gradually steepens to 40 degrees. The slope does not become steeper than 30 degrees for a distance of approximately 35 feet from the outer edge of the building pad area. It is my opinion from an engineering geology standpoint Stratum Group File: 12.10.04 January 21, 2005 Tax Parcel # 701311019, East Quilcene Road, Jefferson County, WA Geology Hazardous Area Assessment a ~ ~ ~ ~ _~.`t L7 ., I ~~.` { i ,,.11, J U L 1 8 2005 , ti~ ( ! i`~1~~'? °`( that the setback described above will be adequate to allow for expected shallow soil and slope failures on the steep portion of the bluff to have no impact on the residence. The presence of mature trees and the presence of bedrock underlying the bluff indicate that the bluff is fairly stable. The strike and dip angle of the bedrock is at an angle such that the slope will not be effected by inherent weak plains between layers in the bedrock. The top of the bluff should be disturbed as little as possible, and we recommend that no grading be performed on the slope between the current building pad and the steep 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 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. 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 30 feet from any portion of the 40-degree slope. Three areas of storm water will generated from development of the site: 1) roof runoff, 2) run off from the access road construction and 3) intercepted perched ground water from the building foundation. Storm water and drainage should be discharged in a manner such that water is not concentrated on the ground surface within 30 feet of any portion of the 40-degree slope. This can be done by level infiltration spreaders or by routing water onto the forest floor away from the steep slope. Alternatively storm water and drainage may be routed to the steep bedrock drainage Swale on the south side of the property or via tight line to the base of the shoreline bluff. If these alternatives are selected, it is important that the tightline pipes carrying the water are routinely inspected and maintained to prevent discharging of water onto the steep bluff face. 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. CLOSURE Please note that there are inherent risks associated with building on lots near or adjacent to steep slopes. These are risks that the building owner should recognize and be willing to accept. If Stratum Group File: 12.10.04 January 21, 2005 Tax Parcel # 701311019, East Quilcene Road, Jefferson County, WA Geology Hazardous Area Assessment 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. 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, St tum Group Dan McShane, M.Sc., L.E.G.' 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