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Home My WebLink About976200606 Geotech AssessmentS'I'RATT7M GROUP 1451 Grant Sweet, Beltingfiam, W A 98225 _. Plwrn1360)714-9409 ' .~,:. ~y{};i 41...;:x. larch 24, 2003 George N~or~' 6023 166 Ave SE Bellevue, WA 98066 Re: C,~eobgy Ev~uation, Shoreline $~ Parce197b200b06 ~~ ~~ Fast Matxowstone Road, Maim ]effersonCo~nty, Wa~~°A Dear Mr. NorbY: ve referenced prop~Y ]eased to present the results of our geology evaluation of the abo evaluation was to l) determinefailures, and 3) W e are p -~ purpose of this g~lO~! the risk of slope on a shoreline bhtff. of a residence 4 rece a moons for development of the the property for the siting was lunited to a visual provide general site development amble bluff. This evaluation property adjacent to a potenuallY rion of the bluff face' review of available insl~ and vicinity, a vo~ thmse~shoreline at the base of the bluff. coon of the properh' geologic maPP~ in the area, and a SCOPE OF SERVICES The scope of our services included the following including the bluff l) Conducted a site visit to visually llzspect the subject PiOP~' . . face slope conditions, shoreline conditions, ~ relevant condtuons in the vicinity of the property • ~ on the uplands above the 2) Observes surface soil eonditi~ du~ st pitsfaCa bluff by excavat°'-8 X10 ~ ev~uation of the report suemna~-n8 ottr findings. tncltrdtng a qualitative evaluation ent, ~ 3) f of building a restdetx'z on the s~o fo~° ~'~dcvel°p'n of the shoreline bluff stabilttY. ~.~ on, if necessary. mendatioas for fiuther G~'RAL GEOLOGY continetrtal glaciers at least four times dw'ing ears ago). During these S~'al and accorr'Far'y'n8 Northwestern Washington has been occupied by the Pleistocene EP°Ch (1.6 million to 10,000 y 5t+E658£09£ 5b~GT yggZ~LO/40 ld 3B3YAt3m'IIM £0 ~d April 10, 2003 Parcel Number 976200606, l.ot 1 Gtp~ge Norby Shott Plot, Jefferson County, WA Geobgy Evdaatiow, Skaretine Blatt 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 Geologic Mau of the Port Townsend 30 660 Quadrangle 1'uaet Sound Reeion. Washineton (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 25 feet of the buff consists of compact advance glacial outwash sand. 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 nearly level upland area bounded on the east by a very steep east facing shoreline bluff. The upland portion slopes very gently towards the west, away from the top edge of the bhtff. The upland area is covered primarily with grass and low growing brush. The overall slope of the steep shoreline bluff averages approximately 60 degrees; however, the upper 25 feet of the bluff and bwer 1 S feet are vertical or very nearly vertical. The total height of the bluff is approximately 55 feek On vertical portions of the bluff soils on the bluff face are exposed. The upper portions consist of very compact silty clay and clayey sih with pebbles, sand and cobbles. I interpret this unit to be glacial till. The exposed soils at the base of the bluff consist of medium to fine sand in a very dense condition that i interpret to be advance outwash sand. The middle portion of the bluff above the sand unit slopes at an angle of approximately 30 degrees and is underlain by silt and sand. This slope is grass-covered with a few bushes. 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 bluff to the north ~delo g of the property. There is very little bluff vegetation and the bluff is generally very steep 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 widermutes the base of the bhtff. 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. Svatwn Crtoup 2 Fi1e226.a3C b0 3JGd ld 3'J3W2130NIM 54£658£09£ 5Z~Lt 400Z/L0/40 April ID,?003 Panel Number 976200606, Lot I George Norby Short Plan, Jefferson County, WA Geology Evaluation, Shoreline 131aR Because of the very compact nature of the bluff soils, the slope failures appear to consist of relatively narrow slab type failures, Ravelling of material ot]• the exposed soils on the unvegetatcd 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 evidet~e (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 global-type or deep- seated faiture on the subject property, CpNCLUSIpNS AND 12LCOMMENDATIONS 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 tbe structure (75 years) as long as our recommendations are followed 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 for at least 75 years even if an erosion gate of 6 inches per year is used. tt is our opinion that this rate of erosion is very conservative and is likely less rapid. W e 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 recommendationsere followed. Becaase the bluff is an eroding bhr~ slope failures should be expected to occur on a periodic basis. The only way to prevent continued erosion of the base of the bhrff 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 btuffprovides 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 ~Y proposed shoreline Protection projects along the bl uff or any shoreline protection construction There are cutraldy ~ shoreline-azmored areas along this stretch of coastline- Soils on the upland portion of the property appear to drain poorly due to the very compact glacial dll underlying the site. However, the upland area of the property slopes very gently away from the bluff. All drainage shouk be directed away from the top of the bluff. s~ ~ F~~aze.osc 3 50 ~ltld ld 32~NROQNIM 5D£658E09£ Sl~LL ti00b/G0/1'0 ApriI1Q2003 I'rit' Parcel Numlxv 876200606, [.ot 1 George Nortry Short Plaz, Jefferson County, WA Geology Evairstior, 3horrtiue BLft' Site grading soils or debris, landscape debris, or any other material should not be disposed of over the bluff face or placed at the ~p of steep bluff. There currently are no trees on the top of the bluff or on the bluff face. 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. h is possible that the rate of shoreline erosion may be reduced if trees are established within the shoreline setback area, if large trees were growing at the top of the bluff, they would eventually be incorporated in landslide debris when the upper bluff fails. The presence of large trees within landslide debris at the base of the bluffmay act as a natural shoreline armoring at the base of the bluff and slow the rate of erosion. At present the lack of large wood within slope failures on the beach has meant that landslide material does not remain at the beach level for very long. The septic drain field should not cause problems for slope stability, as the general slope direction of the upland portion of the property will direct drainage away from the top of the bluff. 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 n~ogniu and be willing to accept If conditions appear diE'erent than those described in this report, or other concerns arise, we request that we be notified so we can review those areas and modify ow~ 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) 7149409, Sincerely yours, Stratum Group ~. /~ %~ Dan McShane, M:Sc., P.E.G. Professional Engineering Geologist Strffium Group 4 FiIc2.26.U3f: 90 39tld ld 3N3V2136NIM 56E658E09E SL~Lt ti00Z/L0/b0 ~rraas•cv z t ~1 ti'~i0. \1~ ~..InN. 1 ` `~. ,I, t { :w `a~S ~ ~ ~ t' I' ; 1' t~, .. 4; ` 1 ,'fTT<.. ;..~„$x ~ ~~~ _ayy ~ '~ _ .~ t~1 ~' ~~'\~ ~~) to .u i'.i'K'" e ~. i ~:~-~ t~J ~. Y l1 4 ~tjJ j ..I I ~"j~, ,r,r it ..'IUlY"r~~.`.~~ a ~ ~,~ { Il,l'~ ~'~'. E I]7'4Y6' W tN'}/A9i F~ 1. vimtl~~OE T ' 1 -_ . ~\` a f~ s ~ ~ , ~~ ~ { ~zJ N: 1 "~ r a ~ ~, ~ . ~ > . \ , s r. E. "-q i .<~ ..~` ~ . ~~~ 4 n ~ ~ > 1 ,,. ~ ~ (( ii ~ ®w + .j ~ . N ~....,a ~~ 1 ' ~: i S ': ~'''t~a ~^ J ' ~ .:. d I ~y~e,5 '~, ~. 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