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Home My WebLink About501032010 Geotech Assessment ~p/15/06 10:01 FAX 1 360 647 3744 STRATUM GROUP 14101 STRATUM GROUP 1451 Grant Street.. Bellingham, W A 9g225 Phone (360) 714-9409 sJsf(JfJtJ s,~k () p~V~ f JftfV .to Re: GeololY Assessment Tax Parcel 501032010, Smith Lane Jefferson County. Washington 11 IL I . I1.r Laf/ll ~ If11 ~ ---r-t O^ r r,~ II. ()UV\"-WA Y S' e ( e.. 9 t;. ,..(/1 /\ r; ~ ~-,:; ~ ~'f73 (]bO) October 21, 2005 Janet !..egg 2505 South 320lh Suite 110 Federal Way, WA 98003 Dear Ms Legg: Stratum Group is "leased to present the results of our geology assessment of the above referenced ~roperty adjacent to a shoreline bluff. The purpose of this geology assessment was to 1) detennine the guitability of the l'fOperty for the siting of a residence. 2) qualitatively evaluate the risk of slope failures. and 3) provide general site development and maintenance recommendations for develo~ment of the property adjacent to a potentially Wlstable bluff. Based on my investigation of the site and vicinity. it is my opinion that the site is sujtable for development as a re.qidential property Drovided the recommendations presented in this report are followed. This asse~~ment included a visual inspection of the property, slopes on and in the vicinity of the property. available geologic mapping in the area. and 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 and vicinity , including the bluff slope conditions. slope conditions on and in the vicinity of the property and shoreline con4itions- 2) Observed surface soil conditions on the shoreline slope and the slopes on the west side of the property and at the top of the bluffby excavating shallow hand dug test pits, 3) Prel'ared this report summarizing our findings, including an evaluation of the feasibility ofbuildlne 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. . . .0$/15/06 10:01 FAX 1 360 647 3744 STRATUM GROUP 141 02 <ktober 21, 2005 Tax Parcel 501032010, Smith Lane, JetTerson County, WA GeoloO Assessment GENERAL GEOLOGY Northwestern Washington bas been occupied by continental glaciers at least four times during the Plei~tocene 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 Geologic Man of Surficial Deposits in the Seattle 30' x 60' Ouadrangle. Washington (yount, Minard and Dembrof. 1993) and the Geologic Man of West-Central Jefferson County. Washington (Birdseye. 1976) indicate the subject property and the bluff exposed on the subject pro:t)erty are underlain undifferentiated J)1'e-Frager glaciation deposits. These sediments are described a.1l consisting of interbedded oxidized brown. red-brown. and gray gravel. sand.. silt. and clay. The layers are moderately to well bedded and the unit contains minor amounts of ice- contact deposits and outwash gravel and sand. Generally the unit is nonglacial and hag abundant peat and woody debris. These sediments were denosited sometime prior to the last glacial event and include depOsjl~ of the Whidbey and Olympia nonglacial periods. The sediments have been overridden and consolidated by glacial ice. My observations on the bluff face and within road cuts and test pits on the upper portion of the property and on the bluff face in the vicinity of the property is consistent with the mapping described above. Most of the property and bluff face is underlain by coarse, well graded, rounded gravel with sand and silt Several discontinuous one- to three-foot thick sand lay~ and silt layers on the bluff face. Very hard. varved silts are located near the base of the bluff at several locations. All of the silts and gravels were very com.1)act and dense. No woody layers were present on the blutIface on the subject J)rooertY~ however. woody layers are present at nearby areas on the blutIface. I interpret the deposlb to have been deposited in a fluvial environment prior to the last glacial advance as they are in a highly coml'act and dense condition and some organic material is present. SPECIFIC SITE OBSERVATIONS The location of the subject property is indicated on the attached site vicinity map. A schematic site plan for illustrative purposes is provided in Fi2lU'e 2. The subject property consists of a sloping upland area bounded on the south by a steep shoreline bluff. J ~ . tel ."d=~~ . . <, .,,", . and another small ravme 18 a ong the top of the bluff near the east side of e property. e upland portion slopes gently towards the south, and is covered with mix of second growth Douglas fir) western red StralIIm Group Fi1e:L10.P7.05 2 05/15/06 10:01 FAX 1 360 647 3744 . , STRATUM GROUP 14l 03 October 2 I, 2005 Tax Parcel 501032010, Smith Lane, Jefferson County, W A Geology Assessment cedar. red alder, big Jeafmaple and madrone trees. The understory consists of a wide mix of brush, grass and blackberries. An area cleared of trees is located in the central DQrtion of the site_ An access road from Smith Lane enters the northwest portion of the property. The slopes on the upland area are less than 10 percent. The overall slope of the steep shoreline bluff averages approximately 70 degrees; however, portions of the bluff are verticaL The total height of the steep portion of the bluff on the subject property is approximately 45 feet, Except for the nearly vertical to vertical sections of the steep shoreline bluff, the steep shoreline sloJ'e is well vegetated with a mix of brush and trees. The brush consists of a mix of evergreen huckleberry, sala!, fems. salmon berry. and poison oak. A number of mature madrone, Douglas fir and cedar trees are growing on the steep bluff face. The gteep bluff slope above the shoreline on the subject properly consists primarily of very compact sand and gravel with silt and clay that I interpret to be pre-Fraser fluvial deposits. A few discontinuous silt/clay layers less than one--foot thick are present on the bluff face. A siltlclay layer extends across a portion of the bottom two to six feet of the bluff. The silt/clay layers are very hard and compact. \l\:JlRu.l1J:rd' ~tlinl. ..&.,~.~ "U.;II.IIIdIil~.___n"ll.n"_~.... 'lir.,Mi.,tIIJ t 1~,~..t!I\'.iln!t*~__*IIl~.Jt~:IfJ.J._~_. j~I!lJ ~~8bJ1.i~Jtk ."tt)R..Wl.~.\l<1 r .....'..ag-me'~Jidl'~fl_i-i.L~~iR'II;~kn AU .AI"- ,UMt mrt--=rg"_iMI.JlJ~8.1tiM.~~'~fmmr.~~Nte'~ _~~ti:u!'M~.~ ,_If illr"". .JBftlt ~mJJJ!1f.t'lrQWlfMl ~IV"''' ttE wry ti'fiin ~m.~.. ami 1[;..~1d lJIdl_lJh~"~.-Jliitll.I_..dlIle, :ft.lnnme 1tM"';d.., ]ffi~!JM!~~tJllMIh!1y'M _ul.md tq&.rwr .I'JIB ..lJftm~y ~e"trtt<~~~'witttft'lR ....~.. _i_ A similar sinal I valley is located on the east side of the property near the tOI) of the bluff.. However. no water is present in this valley and there is no indication that water tyl)ically is present. The bluff is eroded by a combination of wave action that undennines the base of the bluff. ravelling of material off the exposed soils on the unvegetated ~rtions of the bluff. and Deriodic shallow topsoi I failures. The material eroded from the bluff face is removed from the base of the bluff and transported along the shore towards the ea.qt by wave action. Landslide debris has accumulated along ~rtions of the base of the bluff and at other locations there is no landslide debris at the base of the bluff I did not observe any evidence (tension cracks or trees rotated inward away from the top of the bluff) indicating an incipient global-type or deep-seated failure on the subject property. The presence of straight mature Douglas firs on the bluff face indicates that the erosion rate on the Stratum Group File: 1.1 0.07.0S 3 .05/15/06 10:01 FAX 1 360 647 3744 STRATUM GROUP 141 04 October 21. 2005 Tax Parcel SO 10320 I 0, Smith Lane, Jefferson County, W A GeolO(O' Assess....! steep slope must be relatively slow. The areas of seeping water did not appear to be eroding at a faster rate than the layers above or below the seeps. Except for shallow minor soil creep, there was no evidence of slope instability in either of the shallow ravines near the top of the bluff. The easterly ravine may have been a location where spring water formerly flowed from the upland area down and across the steep bluff face. A slight change of water flow in the upland area has subsequently sent water down along the west side of the property. CONCLUSIONS AND RECOMMENDATIONS Based on my visual inspection of the subject property and vicinity, I conclude that a residence can he 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. I reconunend a set back distance of at least 40 feet from the top of the steep portion of the bluff. I recommend a set back distance from the top of the steep slope of the ravines on either side of the steep shoreline bluff of 15 feet. It is my opinion from an engineering geology standpoint that these distances will be adequate to allow for expected bluff face failures that will result from the continued erosion of the base of the bluff by wave action and ravelling of the exposed soils on the bluff face as well as any possible minor settlement along the upper slope of the small ravines east and west of the building area. I do not anticipate that the development of the subject property will cause any negative impacts on the stability of the slope as long as the following recommendations are fonowed. Because the blutIis 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 east of the subject property, and any shoreline armoring will have a negative impact on properties down drift from the bluff. The construction of shoreline annoring 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 are currently no shoreline-annored areas along this stretch of coastline. Soils on the upland portion of the property from the top of the steep shoreline bluff to a distance of approximately 125 feet from the bluff appear to be relatively well drained. Roof drains and any drainage around the home should be discharged into a pe1forated dispersion pipe and infiltrated into the ground at least 30 feet from the top of the steep shoreline bluff. A portion of this water may be directed towards infiltration trenches along the upper slopes oftbe shallow Snlwn Group t'iIe:1.10.07.0S 4 .05/15/06 10:01 FAX 1 360 647 3744 STRATUll GROUP 141 05 . October 21. 200S Tax Parcel 501032010. Smith Lane, Jefferson County, WA Geology AsseaRlent ravines on the east and west sides of the property. The dispersion pipe should be placed in a level infiltration trench excavated ~dicular to the slope. The access road to the house should be designed such that water can sheet flow off the sides of the road and not be concentrated at . any given location. Site grading soils or debris. landscape debris. or any other material should not be disposed of over the bluff face OJ' placed at the top of bluff or placed on the steep ravine slopes. Over time debris will break down and form an unstable mass that will slide damaging the slopes below. Native vegetation. particularly tress and low native brush within 30 feet of the top of the bluff should be disturbed as little as possible. Tftrees are thinned or limbed for view purposes on the bluffface. all debris should be removed from the bluff face. Limbing and topping of trees should be limited to maintaining the view corridor only and the native brush understory should be left in place. A septic drain field should not have any have any adverse affects on the stability of the bluff as long as the drain field is located at least 50 feet 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 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 areas and modify our recommendations as required. Stratum Group appreciates the opportunity to be of service to you. Should you have any questions regarding our reconnaissance please contact our office at (360) 714-9409. - Str:@tqm Group (.~ 1//1 If ,0J~ DaD McShane, M,Sc., L.E.G. Licensed Engineering Geologist ~ u,':1fi!lh! !\'.,......~J-- ..,~..,~ r ,j,k...z,.....J .' i...,...." ',._4.11 -.............-....-__ _..._ ...",-_,--- __......J Sincerely yours, SIJ1llUI1t Group filc:l.l0.1I7.OS 5 .95/15/06 10:01 FAX 1 360 647 3744 STRATUM GROUP 141 06 ~ TOPOl map printed on 10/21/05 from "Untltled.tpo" 122a4S.000' W 122047.000' W z b o o ~ n .... .,. b o C! N 'll' . I"- 'It ::z h o o .... 'll' o " " " " o v o v o l"- v TNF .j ;: 122047.000' W ,,-,.._,. ;i '"'iIii~UlE ~lGlrm CJ_~ ~ __P.lEIDI:l MIP <Jl'Olc:d wilh TOPOI8e:lOO3 NetioI:llllGt~(.........,...~.coJIllItopo) ~ " f) -I) in/; A...U\ 1 V&o.J ) ,S" SGf~Cir~ ; . 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