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HomeMy WebLinkAbout821355011 Geotech AssessmentSTRATUM GROUP P.O. Box 2546, Bellingham; WA 98227 Phone {360) 714-9409 October 15, 2008 Jared Enzler 1633 Water Street Port Townsend, WA 98368 , Re: Geology Hazard Assessment Tax Parce1821355011 Port Ludlow, WA Dear Mr. Enzler: Stratum Group is pleased to present the results of our geology hazard evaluation of the above referenced property. The purpose of this geology. evaluation was to evaluate the risk of slope failures impacting a proposed building area on the northwest portion of the property. This assessment also addresses a potential access route to the northwest portion of the site via a driveway route off of Paradise Bay Road. This evaluation included a visual inspection of the property and vicinity, a visual inspection of the slopes on the property and vicinity, and review of available geologic mapping in the area. The proposed building area on the northwest portion of the property is not at risk from landslides. A feasible access route from .Paradise Bay Road would angle across the slope to the southwest and switchback up the slope one and possible two times. This route will require the least grading and cut slopes would be minimal as well. The route does not cross any unstable or potentially unstable slopes. The property is located on an east to southeast sloping property above the west side of Paradise Bay Road just north of State Route 104. 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 slope and on the uplands above the slope.. 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 sit e~~~d recommendations for further investigation, if necess 0 C T 3 12QOQ JE~FE~SON ~DU~~ ~~;n October 15,.2008 Parcel # 821355011, Paradise Bay Road, Jefferson County, WA Geologic Hazard Assessment 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 Geologic Man of Surficial Deposits in the Seattle 30' by 60' Quadrangle Washin, tg_on (Yount, Minard, and Dembroff, 1993) and the Geolo ig_c Map of the Uncas-Port Ludlow Area Jefferson County, Washin on (Hanson, 1976) indicates the bluff is underlain by Vashon glacial till. The glacial till consists of nonsorted, nostratified clay, silt, sand and gravel up to boulder size. The till was deposited directly by glacial ice. Observations on the subject property, slopes inthe vicinity of the property and upland areas. above the subject property are consistent with the mapping described above. The till deposits on the subject property and in the vicinity of the property are highly compacted and consist of poorly sorted silty clay with sand and gravel. The ,Slope Stability Man of the Uncas-Port Ludlow Area Jefferson County Washin on (Hanson, 1976) map indicates that slopes on the subject property are areas believed to be stable under normal conditions, but may become unstable if modified and areas that are unstable. Jefferson.. County CAO maps designate the property as-being located in an area of a slight landsliderisk. Based on observations on the property, there are areas of the property that have a low risk of landslide and alteration of slopes in those areas may cause minor slope stability problems.. SPECIFIC SITE OBSERVATIONS A sketch map of the property with pertinent features indicated is provided in Figure 2. Paradise Bay Road bounds the southeast side of the property. An access point to the subject property is located at the northeast portion of the site. from Paradise Bay Road. This access point leads to a rough road that parallels Paradise Bay Road and connects. with the State right-of--way just to the south of the property. Besides this road there are a few old- logging skid roads on the site. Most of the property is tree-covered with a mix forest of second growth trees. An area cleared of trees is located on the northwest portion of the site. This area is brush covered with portions covered with thick blackberry. The cleared area can be accessed at present via steep. skid roads, ---•, r~ ,,--a ,--a ~ t~- ~ T-~ ._ ~, .i~ Stratum Crroup File: 8.6.08 d. ~ 2 ~. ' a~~.*6 ~t~~ it t~U~~~"?~ ~~ ~%'. October 15, 2008 Parcel # 821355011, Paradise Bay Road, Jefferson County; WA Geologic Hazard Assessment Slopes on the property appear to be stable with only minor soil creep in a few limited areas, One small convergent area is located on the north side of the property and minor soil creep is present in this area. Seasonal water appears to be located south of this area. The old road described above crosses this seasonal water area via an existing culvert. Otherwise most of the slopes on the site are stable and there are no landslide hazard areas on the property.. Soils on the site consist of very :compact silt and clay with some sand and gravel. The soils are consistent with the site being underlain by glacial till. CONCLUSIONS AND RECOMMENDATIONS Based on my visual inspection of the subject property and vicinity, I conclude that a proposed building area on the northwest portion of the property is not at risk from landslides. Development on this portion of the property will not be at risk from landslides and would not cause slope stability problems elsewhere on or off the site. The slopes on the site are such that erosion hazards are present in areas where the soils may be disturbed. Because of slopes on the property access to the building area on the northwest portion of the lot -will require angling the road across the slope and switching back up the lope likely with two switch backs. The most feasible road route up to the building site is indicated on the attached site plan sketch.. The route does not cross any unstable or potentially .unstable slopes. The access road following this route will be the least steep and will require the least amount of cutting and grading on the slope. Other routes may be possible, but will. require significantly more grading work and the proposed route avoids the convergent steep slope on the north portion of the property. The recommended route has not been surveyed and determining the grade on the uppermost leg of the route was not possible with the thick blackberry growing in that area. Otherwise, we estimate that the route indicated will have grades under 10% and cut. slopes will be less than 4 feet high. No debris should be placed on the slopes above the convergent topography area on the north portion of the property. Storm water run off from the roof gutters and the access road should be dispersed as much as practical using level infiltration/dispersion trenches. ..The septic drain field should not be located within the convergent topography area on the north portion of the property. 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 as Stratum Group t = ,+ ° ~ -File: 8.b.08 3 October 15, 2008 Parcel # 821355011, Paradise Bay Road, Jefferson County, WA Geologic Hazard Assessment required. 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, Stratum Group cam-, an McShane, M.S., L.E.G. Licensed Engineering Geologist Stratum Group ~~ ~ ~ ~ `~' aCT 3 1208 4 ~~~~~~~Q~ ~~~~~ ~~~ File: 8.6.08 Figure 1. Site Vi~Inity Map 0 O O O O O~ n tt Z 0 M M M m m P. 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