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HomeMy WebLinkAbout701191016 Geotech AssessmentSTRATUM GROUP ~~~ ,. 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 ~ ~ _._ _ ~ - March 8, 2004 APR 3 0 2004 , ~ ~ `: Kristen Kennell - P.O. Box 598 ; ` ~ _ . _. Quilcene, WA 98376 Re: Geology Hazard Evaluation, Shoreline Bluff. 1520 East Quilcene Road, Tax Parce1701191016 Quilcene, WA Dear Ms Kennell: Stratum Group herein presents the results of our geology hazard evaluation of the above referenced properly on a shoreline bluff. The purpose of this geology evaluation was to qualitatively evaluate the risk of slope failures impacting the building located on the subject property. It is my understanding that you are considering remodeling the existing structure, and this evaluation should be considered in your remodeling plans and provides general site maintenance recommendations. This evaluation was limited to a visual inspection of the property. and vicinity, a visual inspection of the slopes on the property and vicinity, review of available geologic mapping in the area, and inspection of the shoreline at the base of the bluff. The property is located on a 20-foot high bluff on the west side of the Bolton Peninsula on the northeast shore of Quilcene Bay. 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, shoreline 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 site development, and recommendations for further investigation, if necessary. Mazch 5, 2004 1520 East Quilcene Road, Quilcene, WA G~logic Evaluation of Shoreline Bluff GENERAL GEOLOGY Northwestern Washington has been occupied by continental glaciers at least four times during the Pleistocene Epoch (l.b 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 Geolo~c Map of Surficial Deposits in the Seattle 34' by b0' Ouadran~le, Washington (Yount, Minard, and Dembroff, 1993) indicates the bluffis underlain by undifferentiated Pleistocene deposits. These deposits are described as consisting of poorly exposed gravel, sand, silt, and clay of uncertain provenance. The Geologic Map of East-Central Jefferson County, Washington (Birdseye, 197b) indicates the shoreline and bluff exposed on the subject property and vicinity is underlain Pre-Vashon glaciation stratified deposits. Observations on the subject property, slopes in the vicinity of the property and upland areas above the subject property are consistent with the mapping described above. The subject property and the steep shoreline slopes on the subject property area underlain by compact sand and gravel. The Relative Sloe Stability in East-Central Jefferson County, Washington (Birdseye, 1976) map indicates the subject properly and slopes in the vicinity of the subject property are unstable. However, Birdseye did not identify any areas of former or recent landslides on or in the vicinity -- ofthe subject property at the time the map was produced. Observations confirm that there are potential unstable slopes on the subject property and the vicinity. The shoreline at the base of the bluff is erosional and thus the slopes above the shore are potentially unstable. Unstable slopes are located on the steep slopes east (inland) of the subject property, particularly in areas of convergent topography near the upper slopes of the Bolton Peninsula. However, no convergent topography is present near the proposed building site area. Subsequent to Birdseye's slope stability mapping several slides have occurred within the pre- Vashon sediments in the vicinity of the subject property as well as elsewhere on the Bolton Peninsula such as on the Old Lindsay Hill Road and the new Lindsay Hill Road. Two shallow surface soil failures are apparent on the shoreline slopes to the south of the subject property. SPECIFIC SITE OBSERVATIONS The subject property is located at 1520 East Quilcene Road. The property is located between East Quilcene Road on the east side of the .property and Quilcene Bay located on the west side of the Stratum Group File: 2.25.04B 2 March 5, 2004 1520 East Quilcene Road, Quilcene, wA Geologic Evaluation of Shoreline Sluff ~ 1 .~~F t' S~ 1 # ~ ; i~: y 1 ' ~ - " • i i ; ' ~ ` ~ F ~ ~~ `~ ¢ - ~ cg f t~' y: '~h" 'rii"R'~~R~f property. A sketch map of the property with pertinent features indicated is provided in Figure 2. The property slopes down from East Quilcene Road to the top of the steep shoreline slope at an overall angle of approximately 20 degrees. Except for the driveway and pazking area and home location the property is tree covered primarily with mature second growth Douglas fir. The steep shoreline bluff is approximately 20 feet high and slopes down to the beach below at an overall angle of approximately 65 degrees. However, an approximately 7-foot high vertical section is located on a section of the slope approximately 5 feet below the top of the slope. The slope is underlain by compact sand and gravel. Except for a vertical section of the slope, the slope is well vegetated with brush and trees including several mature trees. No clay layers, impermeable zones, springs or indications of seasonal wet areas aze present anywhere on the steep shoreline slope. The shoreline at the base of the bluff consists of a sand and gravel beach with silty azeas. The beach extends out onto extensive tide flats that underlie the upper part of Quilcene Bay. The base of the bluff is lined with lazge drift wood logs and azeas of heavily vegetated debris from very old past slope failures. A series of logs have been chained together along the base of the slope for several hundred feet both north and south of the subject property and along the subject properly itself. This passive protection appears to have been in place for many yeazs. Otherwise much of the large wood debris at the toe of the slope. along the subject property and north and south of the property appeazs to be natural. The shoreline along the base of the slope is in an overall fairly natural condition. The steep shoreline slope becomes progressively less high to the north of the property. A slope failure is located approximately 300 feet south of the home site. This slope failure appears to have been the result of sidecast soil or topsoil failure along the steep slope below East Quilcene Road. The existing home is located neaz the top of the steep shoreline slope. The southwest corner of the house is-located approximately 9 feet from edge of the slope. There is no evidence of potential lazge-scale slope failures anywhere on the upland portion of the property or in the vicinity of the property. CONCLUSIONS AND RECOIVIMENDATIONS Because of the compact nature of the pre-Vashon silt sand gravel underlying the steep slope, the slope is not inherently unstable except for the potential shallow failures that should be expected because of the steepness of the slope. The steepness of the slope has been caused by wave action undermining the toe of the slope. Erosion at the toe of the slope appears to be fairly slow because Stratum Group File: 2.25.04B 3 March 5, 2004 1520 East Quilcene Road, Quilcene, WA Geologic Evaluation of Shoreline Bluff of the protected location of the property and the lazge logs both natural and chained together at the toe of the slope. In addition, the well-vegetated nature of the slope, if maintained, adds slope stability and in the event of a slide the tree branches and root wads will act as natural armor to stop erosion at the toe of the slope for many years. Observed slope failures and slope conditions indicate that slope failures that have occurred in the past have been fairly shallow and have involved only a few feet of the bluff face at a given time. Based on my visual inspection of the subject property and vicinity, I conclude that the existing residence is at a moderate risk of being impacted by a slope failure within the expected life of the structure (75 years). This risk is the result of portions of the foundation being as close as 9 feet from the top of the slope. It is my opinion that there is a high risk that a slope failure could occur that would move the top of the steep slope back as much as 4 or 5 feet. If a second similar failure should occur, the structure would be damaged. I recommend that a minimal setback for foundation support of the home be 14 feet from the top of the steep slope. This distance will approximately achieve a 1:1 slope from the foundation support to the base of the slope as well as achieving a 1: i slope from the foundation support to the base of the neaz vertical section approximately half way down the slope. In addition to the 14-foot setback vegetation on the steep bluff should be disturbed as little as possible except for the management of blackberries which should be prevented from becoming well established on the slope. Roof run off and any other storm water run off should be directed away from the steep slope in front of the house. Run off should be directed via a tight line to the ~-- base of the bluff above the ordinary high water mazk north of the home. A deck may be constructed within the setback from the top of the steep slope as long as it is not an integral part of the structure of the residence. That is if a slope failure occurs, the deck will detach from the house. No debris should be placed on the top of the bluff. Thinning and trimming of trees on the bluff slope should be limited to maintaining view corridors. Any debris from thinning on the bluff slope itself should be removed from the steep slope. Stratum Group File: 2.25:04B 4 ~., - ~ ~ ~' ~ - ~ t; ~ ~ ~ ~.. APR 3 0 2004 ~; , March 5, 2004 ' 1520 East Quilcene Road, Quilcene, WA ( k _ Geologic Evaluation of Shoreline Bluff ~, ~ _. c }= ~ ~ ~ k ~ ~~ CLOSURE Please note that there aze inherent risks associated with lots near or adjacent to steep slopes. These aze 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 azeas and modify our recommendations as required. We appreciate the opportunity to be of service to you. Should you have any questions regazding please contact our office at (360) 714-9409. Sincerely yours, St tum Group Dan McShane, M.Sc., L.E.G. Licensed Engineering Geologist Stratum Group File: 2.25.048 5 Fiigane 1. Vicinity Mnp 122°5737' W NAD27122°~'09 W v. _ r V i J '1 < <' .. i ~ ~ ~: ri 1 ~ ` l'~.. ~ '' x ~ I ~ _ ~ •. r \ V 'S E (F ~i ~~ '1 1 ~ X14 t~` ~\, ~~. -5-. ~_. \ ~ 1 ~~~ t ~ ~i ~ -o. 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Box 598 Quilcene, WA 98376 Re: Drainage 1520 East Quilcene Road, Tax Parcel 701191016 Quilcene, WA Dear Ms Kennell: In my Geology Hazard evaluation dated March 8, 2004 I recommended that roof run off and any other storm water run off should be directed away from the steep slope in front of the house. I further recommended that the run off should be directed via a tight line to the base of the bluff above the ordinary high water mark north of the home. Infiltrating the storm water run off using a horizontal perforated pipe located north of the house would also be acceptable. My primary purpose of recommending a tight line to the shore north of the house was because of concerns I had about the existing system that cause a risk of surface water flowing on the bluff face immediately in front of the home. As noted in my previous report there are no impervious layers on the bluff slope. Hence, there is no risk of infiltrated water becoming perched on an impermeable layer and causing slope problems on the bluff face. Best regards on your project. Should you have any questions regarding please contact our office at (360) 714-9409. Sincerely yours, Stratum Group ,~ Dan McShane, M.Sc., L.E.G. Licensed Engineering Geologist ~~ ,., ~ k .~k k f~ Y' ~ ~-~{ ~~~ ~~f'~ ~~ ~~ ~~ ~ \ .4~ l ~ r ~ j' ! 4 ~` s`~' \~ ~,` ~f= e5 .~.' `, ~~ ~ . `~ :, MAY 2 0 LOu4 ~i,~;~~ 6w~; ~~ ~~~ ~~ ~~`; -~'~, ~ ~~'" Zoca'~~ o~,g Tq~?V' .5~~ d{ Ci~p~ C3X ,;1.4 ?@ (~ ~.~;, ~~ Stratu M~ Group e,~,.r: ~ .~, ~,. ~ e~ n: py Figure 2 Site Sketch Plan 1 B~gn4ham. MIA ai:19/~0~ tcw~ NA ~ NA • -- -- -- 3~) 714-8404 l''~~ ~ M:AY 2 0 ~~~~ ~r -~o ~~ -~ t a k~~' r °'~- ~- ti ~~~ ;~~ ~ `~ .~ -~- -i Y VVV C~ ~ ~ `_ JJ T n 1 T `~ a_ • \r ~" TL! *' ~~RY ~~f__{ ~`