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HomeMy WebLinkAbout601061013 Geotech AssessmentDecember 6, 2002 Joon Song 2626 NW 204t~ Street Seattle, WA 98177 STRATUM GROUP 1451 Grant Street, B¢llingham, WA 98225 Phone (360) 714-9409 ~.2~:pT. JEFFERSON COUNT'( OF C()i,~r.,':~i4~Ty DEr'ELOPe'lENT Re: Geology Evaluation, Shoreline Bluff Gustavson Road Lot Section 6, Township 25 N, Range 1 W Jefferson County, Washington Dear Mr. Song: We are pleased to present the results of our geology evaluation of the above referenced property on a shoreline bluff. The purpose of this geology evaluation was to l) 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 bluff. This evaluation was limited to a visual inspection of the property and vicinity, a visual inspection of the bluff face, review of available geologic mapping in the area, and inspection of the shoreline at the base of the bluff. SCOPE OF SERVICES The scope of our services included the following: 1) Conducted a site visit to visually inspect the subject property including the bluff face slope conditions, shoreline conditions, and relevant conditions in the vicinity of the property. 2) Observed surface soil conditions on the bluffface and on the uplands above the bluffby excavating shallow hand dug test pits. 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 shoreline bluff stability, recommendations for site development, and recommendations for further investigation, if necessary. 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 December 6, 2002 Song Property, Gustavson Road, Jefferson Cotmty, WA Geology Evaluation, Shoreline Bluff interglacial periods, the underlying bedrock was eroded and glacial related and interglacial fluvial sediments were deposited over the underlying bedrock in the vicinity of the subject property. The Geologic Map of Surficial Deposits in the Seattle 30' by 60' Quadrangle, Washington (Yount, Minard, and Dembroff 1993) indicate the upland portion of the property and vicinity is underlain by glacial till and that the steep shoreline bluff exposed on the subject property is underlain undifferentiated Pleistocene deposits. The Geologic Map of East-Central Jefferson County (Birdseye, 1976) indicates that the steep bluffis underlain by Double BluffTill at the basel of the bluff, Pre-Vashon Stratified sediments on most of the bluff, and that the top of the bluffis underlain by Vashon Till. Observations on the bluff face on the subject property and on the bluffface in the vicinity of the property are consistent with the mapping described above. The steep bluff face is underlain by very compact glacial till and very compact sand, silt and gravel layers. SPECIFIC SITE OBSERVATIONS A site location map is provided on Figure 1 and a general site plan sketch is provided on Figure 2. The property consists of a very gently sloping upland area bounded on the south by a steep south facing shoreline bluff The upland portion slopes gently towards the south, and is covered primarily with second growth Douglas fir with an understory of salal and bracken fern as well as other brush. The property is accessed from Gustavson Road via a gravel access driveway. A cleared potential building site is located on the upland portion of the property 105 feet from the top of the steep shoreline bluff slope. The overall slope of the steep shoreline bluff averages approximately 55 degrees; however, portions of the slope are steeper. The total height of the bluff is approximately 200 feet. The bluff slope is covered with a mix of mature Douglas fir, relatively young trees, brush covered areas and! a few areas with very little vegetation. The uppermost 10 feet of the bluffis underlain by very compact glacial till. The slope below the upper 10 feet is primarily underlain by very compact silty, sandy gravel. Glacial till is present at the base of the bluff The bluff appears to be eroded primarily by wave action that undermines the base of the bluff The shoreline at the site is exposed to a fairly long fetch of open water to the south such that south winds will drive waves against the base of the bluff. Hence south winds coinciding with high tides will generate large waves that erode the base of the bluff Because the bluffis underlain by very compacted soils the apparent erosion rate is low. Evidence of past landslides are readily apparent throughout the bluff slope. Based on the age of trees and vegetation, it appears that slides have taken place on portions of the bluff within the last few years. However, older slide scarps are Stratum Group File: 10.:28.02 December 6, 2002 Song Property, Gustavson Road, Jefferson County, WA Geology Evaluation, Shoreline Bluff apparent as well. All slope failures observed appeared to be shallow slab-type failures or very small circular failures none of which exceeded 10 feet of depth. The cause of the slides is oversteepening of the slope by the undermining of the base of the slope by wave action. Once the base of the slope fails, the upper slopes loose support and the slides progresses through a series of shallow soil failures to the top of the bluff. No beach armoring is present in the vicinity of the property and the beach appears to be in good condition. That is, the level of the beach appears to be such that downward erosion is minimized by the erosion of the entire bluff along this section of shoreline. We did not observe any evidence (tension cracks or trees rotated inward away from the top of the bluff or indications of past uplift of the beach area) indicating an incipient large-scale global-type or deep-seated failure on the subject property. Slope failures appear to be limited to fairly shallow failures of upper soil as the base of the bluffis eroded by wave action. CONCLUSIONS AND RECOMMENDATIONS Based on our visual inspection of the subject property and vicinity, we conclude that a residence can be located on the upland portion of the property such that bluff erosion or landsliding would not threaten the structure within the expected life of the structure (75 years) as long as our recommendations are followed. The residence should be located at least 75 feet from the top of the shoreline bluff'as indicated in the Site Sketch Map (Figure 2). The existing cleared area on the property is 105 feet from the top of the bluff A 50-foot buffer of native vegetation should be maintained on the upland side of the steep bluff We do not anticipate that the development of the subject property will cause any negative impacts on the stability of the slopes or the shoreline bluff as long as our recommendations are followed. Because the bluffis an eroding bluff, slope failures should be expected to occur on a periodic basis. The only way to prevent continued erosion of the base of the bluff is to construct hard armoring at the shoreline at the base of the bluff. However, the eroding bluff acts as a feeder bluff (erosion of the bluff provides sediment) for the beaches east and west of the subject property, and any shoreline armoring will have a negative impact on properties down drift from the bluff The construction of shoreline armoring on other properties may cause an increase of erosion of the subject property. The owner of the subject property should contact Jefferson County officials regarding any proposed shoreline protection projects along the bluff or any shoreline protection construction. There are currently no shoreline-armored areas along this stretch of coastline. Stratum Group File: 10.28.0:2 December 6, 2002 Song Property, Gustavson Road, Jefferson County, WA Geology Evaluation, Shoreline Bluff Soils on the upland portion of the property appear to be relatively well drained, and therefore we do not anticipate discharging of footing drains will be necessary. Roof drainage and any storm water catch basins must not be introduced into the perimeter footing drain. We recommend that roof drains and any other drainage be discharged to a water infiltration system that will distribute water into the ground at the site. The access road should be graded such that storm water run off is directed to the forest floor instead of all being channeled to the same discharge point. Site grading soils or debris, landscape debris, or any other material should not be disposed of over the bluff face or placed at the top of steep bluff. Trees on the bluffface and within the setback buffer may be limbed or trimmed for view purposes as long as limbs and debris generated is removed from the bluff face and the top of the bluff. No trees greater than 1 O-inches in diameter at chest height should be removed from the native vegetation buffer. The septic drain field should not cause problems for slope stability as long as it is not located with within 50 feet of the top of the bluff. It is possible to construct a trail down the bluff slope to the beach. However the trail alignment should follow the contours of the slope such that cutting into the slope is minimized. A few deer paths are already present on the slope and can be partially used to access the beach. 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 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, Dan McShane, M.S., P.E.G. Professional Engineering Geologist Stratum Group File: 10.28.02 -- , 122~5"21 ·' W NAD2'? 122~'4~'51 = t~