Loading...
HomeMy WebLinkAbout921081015 Geotech Assessment~o~ d~--t~oo~j STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 Apri127, 2005 Jim and Ellin Larimer P.O. Box 65253 Port Ludlow, WA 98365 Re: Geology Evaluation, Shoreline Bluff Parce1921081015 Flagler Road, Marrowstone Island Jefferson County, Washington fir, _ a { ~' J U ~. 1 5 2(305 i Dear Mr. and Ms. Larimer: Stratum Group conducted a geology evaluation of the above referenced property on a shoreline bluff. The 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 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. Based on my evaluation, a home can be located on the upland portion of the property such that it will not be at risk from landsliding or erosion within the life of the structure.. The home should be set back at least 50 feet from the top of the steep shoreline slope and site development recommendations presented in the conclusions of this report should be followed. 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. April 28, 2005 Parce1921081015, Flagler Road, Jefferson County, WA Geology Evaluation, Shoreline Bluff ~' ~1 !i ?005 ~ ~u~ ~ 5 I 3 j .... ...__... _~._,.~.. .. .....J 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 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 Geolotuc Map of the Port Townsend 30 by 60 Quad_*A~le Puget Sound Region, Washington (Pessl, Dethier, Booth and Minard, 1989) and the Geolo~c Map of Northeastern Jefferson County, Washin tg_on (Gayer, 1976) indicate that the surface unit on the upland on the subject property is Vashon glacial till and the steep bluff slope is glacial advance outwash deposits. Observations on the upland area and on the bluff face on the subject property are generally consistent with the mapping.described above. The upland area and portions of the upper part of the bluff are underlain by glacial till. The till consists of very compact silt and clay with sand, gravel and cobbles as well as occasional boulders firmly embedded within the silt and clay matrix. The till was deposited duectly by glacial ice during the last glacial period approximately 15,000 years ago. Underlying the till and exposed on most of the shoreline bluff are layers of sand, gravel, and silt consistent with a river depositional setting. The lack of organic material within the deposit would indicate rapid deposition consistent with advance glacial deposits. The unit is very compact, however, sand units with little clay lack cohesion and are subject to erosion. At the very base of the bluff at the subject property an older glacial drift or till deposit is present. The till is very hard and resistant to erosion. It is exposed at the base of the bluff only at the subject property and on the property immediately south of the subject property. 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 is located on the southwest shore of Marrowstone Island. The property consists of a gently sloping upland area bounded on the east by Flagler Road and on the west by the steep west facing shoreline blur The upland portion slopes very gently toward the west to the top edge of the bluff. The upland area is covered with a wide variety of second growth trees and a thick understory of brush. One exception is an area near the top of the bluff that is covered by a thick Stratum Group Fik:3.22.056 2 Apri128, 2005 Parce1921081015, Flagler Road, Jefferson County, WA Geology Evaluation, Shorelinc Bldtf stand of small Douglas fir with little undergrowth. _ __ .~ _......-"ter - ~ ~ _~ ` ~r .. '_j-Z ~ JllL 1 5 2(~5 Approximately 180 feet west of Flagler Road, the slope steepens and slopes steeply down to the shoreline. The height of the steep portion of the shoreline bluff is approximately 55 feet. At the northern property line the bluff is steeper and slopes down at an overall angle of approximately 65 degrees and at the southern end of the property the bluff is less steep with an overall angle of approximately 40 degrees. Portions of the bluff slopes are steeper, especially neaz the base of the bluff and along portions underlain by glacial till or compact silts. The base of the bluff is eroded by wave action that undermines the base of the bluff. The shoreline at the site is relatively protected from large waves due to its location within Kilisut Hazbor. Generally the distance across open water at the site is very small. Northwest winds may generate waves during high tide that could erode the base of the bluff. Because of the very compact nature of the bluff soils, the bluff is generally fairly stable except where over-steepened by erosion at the base of the bluff. Except for the very. steep till and clay/silt slopes, the bluff slope is tree-covered with a fairly large areas of the bluff supporting. mature trees indicating that slope failures are very infrequent. Past slope failures of various ages are evident on the steep slope. Where verry compact till, silts and clay underlie the. slope the failures are shallow slab failures that involve a few feet of soil. A very old circular type failure covered with mature trees is located on the southern portion of the property and other circular failures are present on neazby properties. The circular failures have taken place in low cohesion soils and are a result of over-steepening of the low cohesion soil units. The circular failures are limited to the low cohesion unit. The lazgest failure observed involved a slump into the top of the slope of approximately 20 feet. Ravelling of material off the exposed soils on the unvegetated portions of the bluff, and periodic - shallow topsoil failures also play a minor role on the stability of the slope. The material eroded from the bluff face is removed from the base of the bluff and transported along the shore towards the south by wave :action. We did not observe any evidence (tension cracks, 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. SUr~um Group Fiie:3.22.O5B 3 Apri128, 2005 Parcel 921081015, Flagier Road, Jefferson County, WA Geology Evaluation, Shoreline Bluff i~ 7 1 i J U L 1 ~ 2005 r _~a CONCLUSIONS AND RECOMMENDATIOr1S ~ __ __.. _~ _:... _...; 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. Setback Distance The residence should be .located at least 50 feet from the top of the shoreline bluff as indicated in the Site Sketch Map (Figure 2). This distance will be adequate to provide protection for a residence even if two circulaz failures were to occur. The distance will also allow for adequate room to disperse storm water from the development of the site such that slope stability will not be impacted. 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 axe followed. Drainage Drainage such as roof drains, foundation drains and driveway run off should be dispersed as much as practical. Driveway drainage should be allowed to sheet flow to the sides of the driveway or the driveway should be constructed with materials that will allow infiltration of storm water. Roof run off should be either allowed to drip directly onto the ground or if gutters aze used routed to a dispersion trench. The dispersion trench should be located at least 30 feet from the top of the steep slope. The septic drain field should not cause problems for slope stability as long as the septic drain is limited to asingle-family residential use. Vegetation 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. The properly is currently mostly tree covered. Tree roots provide cohesive strength to slope soils, Strawm Group Fik:3.22.056 4 ~ ~ . a~ ~ ~~ ,~~ April 28,181015 FI ler Road, Jefferson County, WA J U L 1 5 1005 Parce192 ag Evaluation, Shoreline BMfl' Geology I ~ .. _ .. ~.. _ . _ a reduce frequency of saturated soil conditions by intercepting rain w2-ter~anFl-snew-1°( primarily), and in the event of a slope failure on the bluff slow erosion rates by providing natural shoreline armoring. Trees on the steep slope and within 30 feet of the top of the slope should be left. Some thinning and limbing is acceptable as long as all debris is removed from the slope. Mature trees greater than 8-inches in diameter should be left on at least 30-foot centers within 30 feet of the top of the bluff. Shoreline Armoring 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 south and 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 bluff or any shoreline protection construction. There aze currently very few shoreline-armored areas along this stretch of coastline and the beach processes aze still functioning in a fairly natural condition. Please note that there are inherent risks associated with building on lots neaz or adjacent to steep slopes. These aze 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 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 our reconnaissance please contact our office at (360) 714-9409. Sincerely yours, St taro Group McShane, M.Sc., L.E.G. Licensed Engineering Geologist SUatum Group D~ni~! l~C~~' ~; t:: Fik:3.22.056 5 i ~ i 0.5 r! ~ / ^ ~ j -~ ~~'~~a~- ~_;~,. r ~~ ~ ~ ~ -\ . .~ ~.. ~ ~~ l''S t v ~ e 0! ~ '.~ \? ~~ ~ i .``1 E 1 _ ,' <,c3~ ~ i .; .. -~ ~= .~, ~r N v ~ ~ ~ ~ ~ ~ ~~ ~ ~, s~ ~' ~ ~ ~^ U1 C'3 ~~~ v o -~- .e Q O p ,~ ~\ ~ ~ t ~ . ~ n ;` K~ X11 _~ ~ '~ J s J U L i 1 5 2005 . _ _..~ ~ ~, ~ ~----