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Home My WebLink About701185002 Geotech Assessment~~ ~ ~~ STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 April 1, 2004 Dave Steelquist 16624 127' Ave SE Renton, WA 98058 Re: Geology Hazard Evaluation, Shoreline Bluff Tax Parcel 701185002 Quilcene, WA Dear Mr. Steelquist: ~~~~g~~~ ~~ ,~ $ ~~ ~E~~EB~~~COIt~ViY DCD Stratum Group herein presents the results of our geology hazard evaluation of the above referenced property. The purpose of this geology evaluation was to qualitatively evaluate the risk of slope failures impacting the proposed building area on the subject property. This evaluation was limited to a visual inspection of the property and vicinity, a visual inspection of the slopes on the property and vicinity, and a review of available geologic mapping in the area. The inspection of the site was focussed on the proposed building area. The property is located on a west and south facing slope approximately one mile northeast of F ~`~ Quilcene in the southeast quarter of Section 18, Township 27N Range 1 W. The location of the property is indicated on Figure 1. SCOPE OF SERVICES The scope of our services included the following: 1) Conducted a site visit to visually inspect the subject property including the slope conditions and relevant conditions in the vicinity of the property. 2) Observed surface soil conditions on the slopes and on the uplands above the slopes on and in the vicinity of the property. 3) Prepared this report summarizing our findings, including a qualitative evaluation of the slope stability in regards to the proposed residence on the property, recommendations for site development, and recommendations for further investigation, if necessary. April 2, 2004 Tax Parce1 701 1 85002, East Quilcene Road, Quilcene, WA Geologic Evaluation 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 vicinity of the subject property. The Geolo i~ c Mai of Surficial Deposits in the Seattle 30' by 60' Ouadran~le, Washington (Yount, Minard, and Dembroff, 1993} indicates the bluff is underlain by undifferentiated Pleistocene deposits. These deposits are described as consisting of poorly exposed gravel, sand, silt, and clay of uncertain provenance. The Geologic Map of East-Central Jefferson County, Washington (Birdseye, 1976) indicates the shoreline and bluff exposed on the subject property and vicinity is underlain Pre-Vashon glaciation stratified deposits. Observations on the subject property, slopes in the vicinity of the property indicate that the site is underlain by stratified sand and gravel deposits in a compact condition as well as glacial drift. The age of the drift is not known, but given its topographic relative to Vashon Till on the upland to the east of the subject property, I postulate that this glacial drift is apre-Vashon gIaciai deposit. The Relative Slope Stability in East-Central Jefferson County, Washington (Birdseye, 1976) map indicates the subject property and slopes in the vicinity of the subject property are unstable. However, Birdseye did not identify any areas of former or recent landslides on or in the vicinity of the subject property at the time the map was produced. Observations confirm that there are potential unstable slopes on the subject property and the vicinity. Recent shallow rapid slope failures have taken place in areas of convergent topography south of the subject property and at one small location on the subject property. SPECIFIC SITE OBSERVATIONS The subject property is accessed from East Quilcene Road via a gravel private shared road. The location of the property is indicated on the attached site vicinity map. The property is located on the west side of a north-south trending ridge. The property is located at an elevation ranging between approximately 180 feet and 240 feet. A sketch map of the property with pertinent features indicated is provided in Figure 2. The property slopes towards the west and southwest with a small area on the north side of the Strahim Group File: 3.5.04H Z April 2, 2004 Tax Parcel 701185002, East Quilcene Road, Quilcene, WA Geologic Evaluation property sloping towards the north. The slopes on the property are variable due to the presence of a stream valley on the southeast portion of the property as well as numerous road cuts from abandoned old logging roads. Most of the property is forested with a mix of Douglas fir, western red cedar, maple and alder and a thick understory of a variety of brush. The few open, unwooded areas are located along the roads. Soils underlying the site are also variable. As noted in the geology section, I observed stratified sand, gravel and silt/clays of variable thickness as well as glacial drift. The proposed building area is underlain primarily with glacial drift on the northern portion of the proposed building area and by sand and gravel on the southern portion of the building area. The proposed building area is essentially located on a gentle west sloping ridge with slopes to the south and north. The only steep slopes present in the vicinity of the proposed building area are the result of road cuts into the sides of the slopes. The south road cut is the only slope that is significantly high. The slope south of the building area slopes down at an angle of approximately 40 degrees and is approximately 25 feet high. A road has been cut into this slope near the property boundary. Portions of the road cut are vertical. A ditch has been excavated along the base of the slope in order to collect and direct water away from the slope below. A shallow rapid landslide is located east of the proposed building area and several years ago sent a flow of soil down the slope towards a home site on the adjoining property. The landslide area is underlain by silt and clay with sandy soil above and is located in an area of convergent topography. The silt/clay unit acts an impermeable layer and seeps and springs are present on the slope in this area. The ditch along with other drain lines have been constructed below the subject property in an effort to keep the sand layer from becoming saturated and failing. These conditions are not present on the steep slop below the building area. The entire steep slope and the base of the slope below the building area is underlain by well drained sand and no indications of wet soils or seasonal areas of saturation are present except in the area immediately within the landslide area. CONCI;USIONS AND RECOIVIl~vIEN1dA'I'IONS Based on my site observations and soils investigations, the proposed building area will not be at risk of future landsliding. Any structures should be set back from the top of the steep slope on the south side and west side of the gentle sloping building area a minimum of 25 feet. Brush and trees within 25 feet should be disturbed as little as possible. Trimming and limbing of trees for view purposes is acceptable within the buffer area as long as no debris is left on the Stratum Group File: 3.5.04H Apri12, 2004 Tax Parcel 701185002, East Quilcene Road, Quilcene, WA Geologic Evaluation steep slope or at the top of the steep slope. If trees are thinned, at least one mature tree per thirty- foot center should remain on the steep slope and on the buffer. Another vegetation management issue to consider will be to prevent spread of blackberries on the steep slope on the more exposed areas on the steep slope. Currently there are few blackberries on the slope. However, this invasive species should be kept in check to prevent it from crowing out more desirable native species for slope stability, as blackberries do not have strong root strength. Roof run off and any other storm water run off should be directed away from the steep slopes. Run off can be allowed to infiltrate into the well drained soils on the upland portion of the property as long it is not introduced into the ground in a concentrated area. CLOSURE Please note that there are inherent risks associated with lots near or adjacent 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 modify our recommendations asrequired. - We appreciate the opportunity to be of service to you. Should you have any questions regarding please contact our office at (360) 714-9409. Sincerely yours, St tum Group Dan McShane, M.Sc., L.E.G. 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