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HomeMy WebLinkAbout001061006 Geotech Assessment (2002)'~ 10/28/2002 16:16 3603854196 JOHN L SCOTT PTT PAGE 02 ~TRA.'~'UIYI ~RC1UP . la~sl ~ ,stmt, s~wngnam, w~ gszzs Phone (3fo) 71A~9409 18, 2402 Bill Bvans CIO Terry M. ,C~ugh John L. Scott Real Bstate 2219 West Simms Way Port Townsend, WA 983b8 Re: Geology Es*aluation, Slro~-eline Blatt' Parcel 001061.006 Section S, Township 30 N, Range l W Jefferson County, Washittgtan Dear Mr. Bvans: ~We are pleased to present the results of our geology evaluation of the above referenced. property on a shoreline bluff, Thee purpose of this geology evaluation wars 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 tnairttenance recommendations for development of tre property adjacent to a potentially unstable bluff: This evaluation was limited to a visual inspection of the property and vicinity, a visual itspection of the biutl'face, review of available geologic miapping in the area, and inspection of the shoreline at the base of the b1u#~ SCGPE UX' SERVICES The scope of our services included the following: 1) Conducted a site visit tt~ visually inspect the subject property includitg the bluff face slope conditions, shoreline conditions, and relevattt conditions itt the vianity of the property. 2) Qbserved surface soil conditions on the bluffface and oa the uplands above the blutFby excavating shallow hand dug test pits. 3) Prepared this reporC summarizing our bnditigs, including ari evaluation ofthe feasibility of building a residence on the subject lsroperty, a qualitative evaluation of the shoreline bhaff stability, recommendationsfnr site development, and reeotnmendations for further investigations if necessary. 10/28/2002 16:16 3603854196 JOHN L SCOTT PTT PAGE 03 October 18, 2002 Parcel Number OO1o61006, Middle Poix-t, Jefferson County, WA, Geology Evaluation, Shoreline Blufi' GENERAL GEOLOGY Northwestern Washington has been occupied by continental gliders at least four times during the Pleistocene Epoch (1.6 million to 14,ddd years ago). hiring these glacial and accompanying irttergla~cial periods, the underlying bedrock was eroded and a relatively thick layer ofglacial related and interglacial .fluvial sediments were deposited over the underlying bedrock in the vicinity of the subject property. The Surllcial Geologic Man oi`the >'oxt ~Qw,ttsetld 30 by 60_Quadraoale_ Puget Souttd Real n, Wgs: ' ton (Pessl, I7etl»er, Booth and Minard, 1989} and Geologic Map of Northeastern Jefferson County, Washington (Gayer, 1976} indicate the steep shore]ne bluff exposed on the subject property is underlain pre-Fraser glaciation deposits. Gayer (1976) provides several sdctoon interpretations of the bluff in the vicinity. Near the subject property Grayer describes the base of the bluff as being underline by Possession Fill overlain by Kitsap 1~ormation, overlain by Vachon advance Dulwich, overlain by Vachon Till. Observations on the bluffface on the subject property and on the blufffaoe in the vicinity of the property are co~aslstent with the mapping described above. The steep bluffface is underlain by very compact silts and gravel. The base of the slope is underlain. by very compact silty sandy gravel and appears to be a sediment deposit deposited directly by glacial ice. Most of the bluffis underlain by very compact silts and sa,ads with increasing gravel"content higher up the bluff. A definite ice contact zone and glacial till deposit is present near the top of the b1u:E£ The upland portion of the- property is underlain by compact sandy gravel. This unit appears to be a thin layer of recessivz~al glacial sand and gravel or reworked glacial till. s>P>~C~o~C sliT>~ o>as>~a~.vA~><o~vs The site location r~nap is provided on Tigure 1 and a general site plait sketch is provided on Figure 2. The property is approximately 5.5 acres in size and consists of an upland area bounded on the west by a• very steep northwest facing shoreline bluff The upland portion slopes very gently towards the west, and is covered primarily with second growth Douglas fir, western red cedar., grass and brush. The overall slope of the steep shoreline bluff averages approximately 6d degrees; however, portions oi'tlle uppena~tost and lowermost sections of the bluff are vertical ar nearly vertical. The total height of the steep portion of the bluff is approximately 150 feet. Qn vertical portions of the SlrMwn Group rate,zo.6.oz 2 10/28/2002 16:16 3603854196 JOHN L SCOTT PTT PAGE 04 aw>xr ls, zooz pa~i Nuunber p01.(161AOb, Middle point, 7eEl'crson Couniy, WA Geology Evaluation, Shorcliuc Btnfl' bluff soils on the bluff face are exposed. Alder and cherry trees with an undergrowth of brush and grasses cover the less steep portions of the bluff No mature trees are present an the bluff slope or on the bluff slopes in the vicinity of the subject property. Slope failures of the bluff have occurred within the peat few years on portions o£the bluffface on the subject property. Routine slope failurres are apparent all along the bluff to the northeast of the property. Northeast of the subject property there is very little bluffvegetation and the bluff is generally very steep. Southwest of the subject property the bluff is more vegetated and, although still very steep, the bluff is not as steep as it is to the northeast. Very little landslide debris is presem at the base of the bluff The bluff is eroded primarily by wave action that undermines fire base of the bluff. The shoreline at the site is e~pased to open waters of the Straight of Juan de Fuca. Large waves are common at the site and durimg storms the shore and the base ofthe bluffis exposed to very large waves. Because of the vexy compact nature ofthe biutf soils, the slope failures appear #o consist of relatively narrow slab type failures. Ravelling of material offthe exposed sails on the unvegetated portions of the bluff, and periodic sl~aUow topsoil failures also play a minor role. ~'he material eroded from the bluff face is removed from the base of the bluffand transported aloz,g the shore towards the northeast by wave action. 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 inoipient global-type or deep- seated failure on the subject property. The presence of straight mature Aouglas firs on the nearby steep bluff face indicates that the erosion rate ort the steep slope must be relatively slow. This is consistent with the very compact nature of the soils on the bluffface, which are resistant to erosion. calvcL>l~sl~olvs ~lvn xE~al~xnATTaxs Based on our visual inspection of the subject property and vicinity, we conclude that a residence can be located an the upland portion of the property such that bluff erosion or landsliding would not threaterl the structure witltln the expected life of the structure (73 years} as long as our recommendations are follgwed. The, residence should be located ~t least 1S© feet fxo:m the top afthe shoreline bhtffas indicated in file Site Sketoh Map (Figure 2). This distance will be adequate to provide protection for a residence for at least 75 years if an erosion rate of 6 inches per year xs used. It is our opirliot~ that this rate of erosion is very conservative and is Iil~ely less rapid. Stratum Qroup F1o:10.6.02 10/28/2002 16:16 3603854196 JOHN L SCOTT PTT PAGE 05 s C+ctobcr 18, 2002 Parcel Number 001061006, lvliddle ~Pai~nt, Jefferson County. WA GCO1ppy EWdiuation, S114re1ine BInR We do riot aniticipate that the development of the subject property will cause any negative impacts on the stability of the slopes or the shoreline bluiFas long as our recommendations nre followed. Because the bluff is an eroding bluff, slope failures should be expected to occur on a periodic basis. The ozily way tv prevent cotrtanued erosion of the base of the bluff. is to construct hard armoring at the shoreline at the base of the bluff: Hpwever, the eroding bluff eats as a Feeder bluff (erosion ofthe blu$provides sediment) for the beaches northeast ofthe subject property, and any shoreline armoring will have a negative impact on properties down dr#t ffom the bluff. The construction of shoreline armoring on other properties nnay cause an increase of erosion of the subject property. '><'he owner of the subject property should contact Jefferson County officials regarding any proposed shoreline protection projects alontg the bluff or any shoreline protection construction. ~'laere are curnetrtly no shoreline-armored areas along this stretch of coastline. Soils an the upland portion of the property appear to be relatively well drained, and therefore we do not anticipate discharging of footuag drains will be necessary. Roof drainage and any storm water catch basins must not be introduced into the perimeter footitag drain. We recommend that roof drains and any other drainage be discharged to a water infiltration system that will distribute water imo the ground at the site. Site grading soils or debris, landscape debris, or any other material should not be disposed of over the bluff face or planed at flee top of steep bluff' Trees and brush within the building setback buffer and on the bluff face maybe removed without posing a risk of increasing bluff erosion or instabi.Gty. i$lufl'ervsign 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, ~n addition, the high-enes~gy wave action at the shore is such that no landslide material remains at the beach level so the role of vegetation providing natural shoreline armoring is not an issue at this site. The septic dxain £~eld should not cause problems For slope stability and may be located within 50 feet ofthe top afthe bluff. Stratum Getup Filot~U.6.fYl 4 10/28/2002 16:16 3603854196 JOHN L SCOTT PTT PAGE 06 October i$, aooz Parcel Number OO1Q610Q6, Middle Point, JetFerson Ctnutty, WA Gtwlo~y Lvaiuation, Shoreline Biatf Ptease note that there are iunberent risks associated with building on lots near ox adjacezrt to steep slopes. These are risks that the building owner should recognize and be willing to accept. If conditions appear differetzt than those described in this report, or other concerns arise, we request that we be notified so we caa review those areas and modify our recommendations as xequired. We appreciate the opportunity to be of service to you. Should you have any questions regarding our reconnaissance please contact our office at {360) '714x9409. Sincerely yours, Stratum. Group ~..~ / ei1~'~~- Dan 1vlcShane, M.3., P.E.Cr. Professional Engineering Creologist Stratum Group Fila10,6.02 10/28/2002 16:16 3603854196 JOHN L SCOTT PTT Fkoa 1. 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