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HomeMy WebLinkAbout602283011 Geotech Assessment STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 ' Phone (360) 714-9409 May 7, 2001 Stan Johnston Johnston Realty P.O. Box 88 Brinnon, WA 98320 Rez Geotechnical Report Tax Parcel 602283011 Township 26N, Range 2W, Section 28 Jefferson County, Washington ECEI] OE , m mm JEFFERSON COUNTY DEPT. OF COMMUNITY DEVELOPMENT Dear Mr. Johnston: We are pleased to present the results of our geology evaluation of the above referenced property located in a potential landslide hazard area. The purpose of this geology evaluation was to determine the suitability of siting residence on each of the lot. This evaluation was limited to a visual inspection of the property, the slopes on the property, other steep slopes in the vicinity of the property, and available geologic mapping in the 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 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 Range Mountains of Canada and the mountains of Vancouver Island. During these glacial and accompanying interglacial periods, the underlying bedrock was eroded. Glacial related sediments were deposited over the underlying bedrock in the vicinity of the subject property. May 7, 2001 Ta~ Parcel 602283011. Jefferson County.. WA Geotechnical Report The Geologic Map of Surficial De osits in the Seattle 30' x 60' uadran le Washin on (Yount, Minard and Dembrof, 1993) indicates the subject property and the vicinity of the subject property is underlain by the Vashon outwash deposits and Vashon till. The Vashon glacial till is described as consisting of a mixture of clay, silt, sand and gravel and is in a very compact condition. These sediments were deposited directly by glacial ice. Both advance and recessional Vashon outwash deposits are indicated to be located in the vicinity of the subject property. The outwash deposits Consist of a wide variety of glacial outwash sediments deposited while the Vashon ice sheet advanced and retreated from the area. Basalt of the Crescent Formation is mapped along the Dosewallips River. We observed glacial till and ice contact deposits on the uppermost portion of the lot. The middle portion of the subject property including the proposed building area on the property is underlain by sand and gravel deposits. Hand dug test pits and observations made at several septic drain field test pits revealed that the soils in the proposed building area (located in the middle portion of the property) are in a relatively loose to medium dense condition indicative of recessional glacial deposits. Road cuts on the into glacial deposits on the Dosewallips Road have exposed very compact silts and sand and gravel deposits that would be indicative of advance glacial deposits that have been compacted by overriding glacial ice. However, we did not observe any indication of advance outwash deposits on the subject property. The lowermost portion of the property is located in a steep bedrock canYon. Bedrock exposed in the canyon walls consists of basalt of the Crescent Formation. SPECI~IC SITE OBSERVATIONS The subject property is located approximately two miles northwest of Brinnon within the lower Dosewallips River canyon at the southwest base of Green Hill. The location of the property is indicated on the attached Vicinity Map. The subject property is approximately 5.2 acres in size. The uppermost portion (east side) of the property is at an elevation of approximately 400 feet, and the lowermost portion of the property adjacent to the D°sewallips River (west side) is at an elevation of approximately 100 feet. Two roads cross the property: an access road that provides access to the subject property and adjacent properties, and the Dosewallips Road, a paved county road that crosses the western portion of the property. The proposed building site is located on the upper slope west of the acceSs road. The slope between the access road and the Dosewallips Road ranges from nearly flat to 100 percent. The slope is covered with a mixed stand of mature big leaf maple, alder, cedar and Douglas fir. A sloping wetland drainage is located on the northern portion of this slope. This wet slope appears to be subject to soil creep, but no evidence of significant landslide activity or potential is present Stratum Group File: 4.16.01B 7, 2001 Tax Parcel 602283011, Jefferson County., WA Geotechnical Report on this wet slope. The slope between the Dosewallips Road and the Dosewallips River slopes towards the fiver at an angle of approximately 3 5 degrees before the slope becomes essentially vertical approximately 40 feet above the river. The vertical slope is underlain by massive basalt bedrock. The upper slope above the vertical slope is primarily covered with alder trees. Trees are present on the cliff including mature old growth cedar trees. The overall slope of the property is variable and somewhat hummocky. In addition, there are areas of unstable slopes in the area where the Dosewallips River is under cutting unconsolidated glacial sediments or simply because of the extreme steepness of slopes in the area. Except for the wetland drainage on the northern portion of the property between the access road and the Dosewallips Road, we did not observe any unstable slopes. The soils in the wetland drainage area appear to be creeping due to the saturated condition of the soils on a steep slope. It is our opinion that the hummocky topography on the subject property and the immediate vicinity are the result of ice margin terrace deposits. That is sand and gravel was deposited between remnant ice within the Dosewallips valley and the sides of the valley. Obvious kame terraces are present at several locations along the sides of the Dosewallips valley. We did not observe any evidence of past or potential landslides on the subject property. The proposed building area consists of a 15-degree slope from the access road down to and including a 50-foot wide nearly flat area west of the access road on the south half of the property. The flat area is bound on the west by a 20-foot high 45 degree slope that appears to have been at least in part created by the pushing of fill soils over the slope. Except for the area within 5 feet of this slope break, the flat area is underlain by native sand and gravel that we interpret to be recessional outwash deposits. CONCLUSIONS AND RECOMMENDATIONS Based on our visual inspection of the subject property and vicinity, we conclude that a residence can be sited on the subject property without risk of being impacted by landsliding as long as our recommendations are followed. The residence should be sited west of the access road on the southern portion of the property. The residence should be set back at least 30 feet from the slope break between a nearly flat 50- foot wide area and a steep 45 degree slope. This slope appears to have been partially created by fill soils being pushed over the steep such that the slope becomes less steep approximately 20 feet down the slope. File: 4.16.0lB Tax Parcel 602283011, Jefferson County, WA Geotechnical Report No debris or fill soils should be added to the steep 45-degree slope or to the wet slope on the north portion of the property. Storm water drainage should infiltrated via a level dispersion spreader or infiltration ditch and should be directed away from the steep slope west of the building site. Vegetation on slopes steeper than 30 degrees should be lef~ in place unless specifically evaluated by a geologist. Vegetation on the wet slope on the northern portion of the property should be lef~ in place as well. Based on our understanding of the geology of the site, the construction of a residence on the property will not be at any risk from geologic hazards posed by erosion or landslides as long as our recommendations are followed. We do not anticipate that development will cause any negative impacts on the stability of the slopes in the vicinity of the subject property. 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 Senior Geologist Stratum Group File: 4.16.01B F~ze I. vimmty Map :::-'.: .:~~: :.. NAD27 122°54'~0'' W - ,. -. -j '. 9