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Home My WebLink About702273016 Geotech Assessment STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 September 21, 2005 Mr. Vanderweghe 4 ` '. ~_, 723 Discovery Ridge Road NOV ~ ~ ~~ ~' Port Townsend, WA 98368 ' f:.. Re: Geology Hazard Assessment . _. '' Tax Parcel 702273016 M~~ D ~" .`" _ , ~ _~ s-c. ~S Quilcene, Washington Dear Mr. Vanden~+eghe: In my previous geology hazard assessment of the above-described property, I assessed the geologic conditions of the slope above the proposed building site. It is my opinioa that the slope is not a geologically hazardous area. The slope stability was assessed per the erosion hazazd areas, landslide hazazd azeas and seismic hazard areas designation criteria of the Jefferson County Unified Development Code 3.6.7. The slope is not a landslide hazard area. No landslide areas have been mapped on the slope and the Relative Slope Stability in East-Central Jefferson County, Washineton (Birdseye, 1976) indicates the slope is a normally stable azea that may become unstable if modified by man. There are no indications on the slope of rockslides, earthflows, mudflows or landslides. The slope is not subject to erosion as the slope is underlain by well- drained gravel and cobbles. There is no indication of surface water flow on the site. The course grained material is not subject to soil liquefaction and the lack of landslide evidence anywhere on the slope is indicative that the slope is not subject to seismic hazards. No modifications are planned for the slope. I did recommend that the home should be setback at least 5 feet from the tce of the steep slope on the west side of the property in order to avoid engineering design of the home foundation. The purpose of the 5-foot setback is to allow for minor soil movement without requiring the construction of an engineered designed retaining wall or foundation wall. However, building on the slope or at the base of the slope would be acceptable from a geology perspective. If building does take place on the slope or within the 5-foot setback recommended at the toe of the slope, an engineer should be involved in the foundation design. Stratum Group appreciates 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 Dan McShane, L.E.G., M.Sc. Licensed Engineering Geologist STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 June 15, 2005 Mr. Vanderweghe 723 Discovery Ridge Road Port Townsend, WA 98368 Re: Geology Hazard Assessment Tax Parce1702273016 Quilcene, Washington Dear Mr. Vanderweghe: 4 Stratum Group is pleased to present the results of our geology evaluation of Tax Parcel 702273016 in Jefferson County, Washington. The purpose of this geology evaluation was to qualitatively evaluate the risk of slope failures from a slope on the east side of the property above the proposed building area. The property is located adjacent to the Big Quilcene River near the base of the northwest flank of Walker Mountain and approximately 2.8 miles southwest of Quilcene. The property. is located in the southwest quarter of Section 27, Township 27 North, Range 2 West. The location of the subject property vicinity is indicated on Figure 1. It is our understanding that the building must be set back at least 117 feet from the ordinary high water mark of the Big Quilcene River as per Jefferson County Critical Areas Ordinance Fish and Wildlife Habitat Areas requirements. SCOPE OF SERVICES The scope of our services included the following: 1) Conducted a site visit to visually inspect the subject property, including slope conditions and relevant conditions in the vicinity of the property. 2) Observed surface soil conditions on the slopes of the subject property and vicinity. 3) Reviewed geologic and slope stability maps for the subject property and vicinity. 4) Prepared this report summarizing our findings, including a qualitative evaluation of the stability in regards to the proposed residence on the property, flagging of the potential building site envelope, and recommendations for site development. June 15, 2005 ~~~ ) ~ ~J Tax Parce1702273016, Jefferson County, Washington Geology Hazard Assessment _ ~. _...._ _ GENERAL GEOLOGY Northwestern Washington has been occupied by continental glaciers at least four times during the Pleistocene Epoch (l.d milhion to 10,000 years ago). During these glacial and accompanying interglacial periods, the underlying bedrock was eroded and glacial related deposits and interglacial deposits were deposited over the bedrock in the vicinity of the subject property. The Geologic Maw of Surficial Deposits in the Seattle 30' x 60' Quadrangle, Washington (Yount, Minazd and Dembrof, (1993) indicates that the subject property is underlain by alluvial deposits associated with the Big Quilcene River on the west side of the property and bedrock in the eastern portion of the property. The Bedrock Geologic Ma~of the Seattle 30'x60' Quadrangle, Washington (Yount and Gower 1991) indicates the bedrock is the Crescent Formation. The Geologic Maw of East-Central Jefferson County, Washington (Birdseye, 1976) indicates the subject property is underlain by thin drift over basalt. Observations on the subject property are generally consistent with the mapping of the area. Alluvial deposits consisting of unconsolidated silt, sand, gravel, cobbles and boulders deposited in the channel and on the floodplain of the Big Quilcene River underlie the western portion of the property from the base of the slope to the river itself. No outcrops of bedrock aze present on the property. However, basalt bedrock is located immediately south of the property along the west bank of the Big Quilcene River. The exposed basalt is fairly massive with no significant jointing appazent. Flow features strike north-south and dip 30 to 40 degrees to the east. The slope above the level azea along the river is underlain by sand and gravel with cobbles. The cobbles are well rounded and the sediment is poorly sorted and not heavily compacted. We interpret this unit to be outwash sand and gravel likely deposited neaz the end of the last glacial period along the valley walls of the Big Quilcene River. The river has likely incised down through these deposits since the end of the last glacial period. The Relative Slo>7e Stability in West-Central Jefferson County, Washington (Birdseye, 1976) indicates the subject property is mapped as a normally stable area that may become unstable if modified by man. The stability mapping is broadly based on slope aspects and soil types underlying the area. Strgtum Group File: 6.3.OSC 2 June 15, 2005 ` r ;. Tax Parcel 702273016, Jefferson County, Washington - :,~+~~ Geology Hazard Assessment ~~~ ~ ?~' SPECIFIC SITE OBSERVATIONS The subject property consists of a neazly level azea adjacent to the Big Quilcene River on the west side of the property and awest-facing slope rising above the valley floor on the east side of the property. The level area along the river was partially cleared of trees a number of years ago and contains a mix of mature trees with grass and brush covered areas in a pazk like setting. This level azea is accessed by a gravel road from the north and a road that angles down the slope above from the east. The width of the level valley floor area from the ordinary high water mark (OHWM) to the toe of the west facing slope ranges from approximately 130 feet to 160 feet. The Big Quilcene River flows along the western boundary of the property.. The river is approximately 35 feet wide and flows to the north along the property boundary in a fairly straight path. Upstream of the subject property the river flows to the northeast and makes a bend to the north due to the presence of bedrock outcrops. This bedrock outcrop aligns the river to the north. There is no evidence of old river meanders along this stretch of river on the subject property. Approximately one-half mile down stream there is evidence of channel migration and certainly extensive channel migration takes place approximately one mile downstream. However, the river at the subject property appears to be still primarily bedrock controlled, is well incised into the valley floor, and has a steep enough gradient that channel migration is not likely. The western portion of the property slopes up to the west above the valley floor. The toe of the slope is not a sharp slope break, but gradually steepens to a slope of approximately 35 to 40 degrees. The slope is plainer and raises from the valley floor at an elevation of approximately 200 feet to a fairly broad gentle sloping bench area at an elevation of approximately 360 feet. This bench area is crossed by Highway 101 and then the slope continues steeply up the northwest flank of Mount Walker. The slope is tree-covered with a mix of mature Douglas fir, western red cedaz and hemlock with an understory consisting primarily of fern and Oregon grape. As noted in the geology section the slope is underlain by non-compact sand and gravel with well rounded cobbles. No evidence seasonal springs or seeps aze present on the slope. No indications of past slope failures are present on the slope. Except for very minor soil creep, the slope appears to be very stable and not subject to any slope movement. Essentially all the trees aze very straight, as aze the remaining stumps from past timer harvest on the slope. An access road angles down across the slope from the northeast corner of the property to the valley floor on the south side of the property. The road has been cut into the slope with slope cuts of approximately 5 to 8 feet in height. Except for minor ravelling on the cut slopes and soil creep in areas where sidecast material was placed, slopes above and below the road have not been impacted by the road construction. The soils along the road are very well drained and no evidence of surface water flow or erosion from surface water flow is present. File: 6.3.OSC Stratum Group 3 June 15, 2005 Tax Parcel 702273016, Jefferson County, Washington ~~ 1 ~ 2~ Geology Hazard Assessment EVALUATION OF GEOLOGIC RISK AND RECOMMENDATIONS. Based on our visual inspection of the subject property, it is our opinion that the site is geologically stable and a residence can be safely constructed on the property. It is our understanding that the home will be required to be setback from the OHWM at least 117 feet per Jefferson Country Critical Areas Ordinance Fish and Wildlife Habitat areas. Hence the ~tback from the river is 117 feet. As the risk of channel migration along this stretch of river is low, this distance will provide adequate protection from erosion hazazds posed by the river. The home should be setback at least 5 feet from the toe of the steep slope on the west side of the property. For purposes of this report the toe of the slope is defined as the location where the slope begins to exceed 30 degrees. There is a slight slope along the base of the slope before it becomes steep. The 5-foot setback will allow for minor slope movement without requiring the construction of an engineered designed retaining wall. It is possible to cut into the slope to site the home. However, this will require that an engineered designed foundation wall be constructed on the uphill side of the slope including a wall that would rise above the grade that could support the potential of minor soil creep accumulating against the uphill side of home. A site plan sketch in Figure 2 illustrates the building envelope and other pertinent information at the proposed building site area. In addition, we flagged the setback line along the base of the hill at the time of our site visit. Except for hazard tree removal, vegetation on the slope above the home site should be not be disturbed. Please note that there are inherent risks associated with lots neaz 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. Stratum Group appreciates the opportunity to be of service to you. Should you have any questions regarding please contact our office at (360) 7~~,~~ Sincerely yours, Str, tum Group Dan McS e, L.E.G., M.Sc. Licensed Engineering Geologist Stratum Group File: 6.3.OSC 4 Flgure 1. 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