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HomeMy WebLinkAbout021333008 Geotech AssessmentSTRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 September 22, 2005 Mark Peters 8551 Mossford Drive Huntington Beach, CA 92648 Re: Geology Evaluation, Shoreline Bluff Parce1021-333-008 East Marrowstone Road, Marrowstone Island Jefferson County, Washington Dear Mr. Peters: Stratum Group is pleased to present the results of our geology evaluation of the above referenced property on a shoreline bluff. T'he purpose of this geology evaluation was to 1) 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 included 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 bluff face and on the uplands above the bluff by 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. September 22, 2005 Parcel Number 021-333-008, East Matmwstone Road Jefferson County, WA Geology Evaluation, Shoreline Bluff GENERAL GEOLOGY C _. .. __ __ _ __... I ... ...__ _ _. --- --~ _. . _ _.... I OCT - 5 200 L~'_. 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 Surficial Geolo ig_c Map of the.Port Townsend30 by 60 Quadran>?le, Puget Sound_Re~ion, _ _ _ _. Washington (Pessl, Dethier, Booth and Minard, 1989) indicates the steep shoreline bluff exposed on the subject property is underlain glacial till and advance glacial deposits. Observations on the bluff face on the subject property and on the bluff face in the vicinity of the property are consistent with the mapping described above. The upper 25 feet of the bluff face and the entire upland area on the property are underlain by very compact glacial till. The lower 75 feet of the bluff consists of compact advance glacial outwash sand and silt. SPECIFIC SITE OBSERVATIONS The site location map is provided on Figure 1 and a general site plan sketch is provided on Figure 2. The property consists of a neally level to very gently sloping upland area bounded on the east by a very steep east facing shoreline bluff. The upland portion slopes very gently towards the northwest, north and northeast. The upland area is covered primarily with Douglas fir with a few madrone trees and a thick understory of a variety of vegetation. The overall slope of the steep shoreline bluff averages approximately b0 degrees; however, the upper.25 feet of the bluff and portions of the lower bluff are vertical or very nearly vertical. The total height of the bluff is approximately 100 feet. On vertical portions of the bluff soils on the bluff face are exposed. The upper 25 feet of the bluff consist of very compact silty clay and clayey silt with pebbles, sand and cobbles. I interpret this unit to be glacial till. The exposed soils on the lower 75 feet of the bluff consist of medium to fine sand and silt layers in a very dense condition that I interpret to be advance outwash sand and silt. Where the slope is not vertical or nearly vertical the slope is covered with grass and brush with a few small trees and slopes at an angle of approximately 30 degrees. Slope failures of the bluff have occurred within the past few years on portions of the bluff face on the subject property. Routine slope failures are apparent all along the bluffto the north and south of the property. There is very little bluff vegetation and the bluff is generally very steep all along Stratum Group File: 8.30.05 2 September 22, 2005 Parcel Number 021-333-008, East Marrowstone Road Jefferson County, WA Geology Evaluation, Shoreline Bluff this section of shoreline. Very little landslide debris is present at the base of the bluff. The bluff is eroded primarily by wave action that undermines the base of the bluff. The shoreline at the site is exposed to relatively open waters of the Puget Sound. Wave action during high tide from south, east or north winds will erode the base of the bluff. The cohesiveless sand unit at the base of the bluff is readily erodable. Because of the very compact nature of the bluff soils, the slope failures appear to consist of __ _ .relatively narrow slab type_falures. Ravelling of material off the exposed soils on the _ _ _ _ _ unvegetated portions of the bluff, and periodic shallow topsoil failures also play a minor role. The material eroded from the bluff face is removed from the base of the bluff and transported along the shore towards the north by wave action. We did not observe any evidence (tension cracks or trees rotated inwazd away from the top of the bluff or indications of past uplift of the beach area) indicating an incipient global-type or deep- seated failure on the subject property. No seeps or azeas of seasonal wet areas were observed on the bluff face. .~, ~ , ~^-- ~` l5 ~E ~y('~ ,. -- ` _ -; _~---~ `; -~,.. _ t_ Stratum Group File: 8.30.05 3 September 22, 2005 Parcel Number021-333-008, East Marrowstone Road Jefferson County, WA Geology Evaluation, Shoreline Bluft 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 aze followed. The residence- should be located at least 50 feet from the top ofthe-shoreline-bluff as indicated in the Site Sketch Map (Figure 2). This distance will be adequate to provide protection for a residence for at least 75 years even if an erosion rate of 6 inches per year is used. It is our opinion that this rate of erosion is very conservative and is likely less rapid. Please note that this setback distance may be precluded by shoreline setback regulations which. may require a greater setback. 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 bluff is 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 north 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 bluffor any shoreline protection construction. There are currently no shoreline-armored areas along this stretch of coastline. Soils on the upland portion of the property appear to drain poorly due to the .very compact glacial till underlying the site. However, the upland area of the property has very gentle slopes and drainage is generally towazds the north and not towazd the top of the bluff. All storm water drainage should be directed away from the top of the bluff and should allowed to infiltrate into the ground surface via dispersion trenches. 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. Strawm Group File: 8.30.05 4 September 22, 2005 Parcel Number 021-333-008, East Matrowstone Road Jefferson County, WA Geology Evaluation, Shoreline Bluff ~', I ~~ OCT - 5 2005 , Bluff erosion and stability is overwhelming dominated by shoreline process. If not for the erosion at the base of the bluff, the bluff would be relatively stable. The rate of shoreline erosion may be reduced if trees are left within the established setback from the top of the steep slope. Large trees within landslide debris can effectively protect the base of the slope from erosion for long periods of time. At least one tree per 30-foot center should be left within the geology setback area. Trees left within the setback area can be limbed for view purposes. The septic drain field should be located at least 50 feet back from the top of the steep slope. The minor_volume of water from a household drain field-will not cause slope stability problems at this site. The septic drain field should not cause problems for slope stability, as the general slope direction of the upland portion of the property will not direct drainage towards the top of the bluff. Please note that there aze 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 appeaz different than those described in this report, or other concerns azise, 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 please contact our office at (3b0) 714-9409. Sincerely yours, Stratum Group Dan McShane, M.Sc., L.E.G. Licensed Engineering Geologist Stratum Group File: 8.30.05 5 Figure 1. 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