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Home My WebLink About502153052 Geotech Assessment I I I I I I I I I I I I I I I I I I I GEOTECHNICAL REPORT Prepared For Roger Myhre September 27, 2006 RECE1[\\~D OCT-S. JEffERSON COUNlY OCD For the Property Described as Tax # 502153052 Section 15, Township 25 North, Range 2 West, W.M. Jefferson County, Washington Prepared by NTI Engineering and Surveying 717 S. Peabody Street Port Angeles, Washington 98362 Phone 360-452-8491 Fax 360-452-8498 Web Site www.nti4u.com E-mail info@nti4u.com I I I I I I I I I I I I I I I I I I cl ". NT' NORTHWESTi:RN .TERRITORIES, INC. AJLSGROUP COMPANY 717 SOUTliPEABOOY STREET,PORT ANGELE~, WA 98361 Engineers Land Surveyors Geologists c Constl1,Jction Inspection. , , Materials Testing (360) 45f~8491 FAX452-8498www,nli4u.c;omE-Mail: i1}fo@nti4u.c6m JLS GROUP, INC. , September 27, 2006 Roger Myhre 4135 Ashworth Ave. North Seattle"WA98103 Suj)ject: Dear Mr. Myhre: Bac.kground At yourreque~t, NTIEngineering, and Surveying (NT!) condllctedabluffstability . c inspection at the above referenced property. The purpose of this inspectiQnwasto examine the bluff attnesubjectsite,byvisual means in orderto determine the relative stability of thebluftand to make recommendations in regards to the proposed construction ota single f(;tmilyresidence ingeneralaccordancewithSecti,on 3.6.10.of the Jetterson County,WA Unified Development Code (UDC). The, inspection co.nsisted of office ,research ofavaHa,bleliteratureandgecilogicmapsofthe area and asitevisitJor visual observations madeonSeptember13,2ootl . . , Site Description , , The subject "noba~k" waterfront property is located at 248 Robinson, Road off of Highway 101 south of Hri n non (Figure 1). Theproperty overlooks HoodCanal tethe south, and is bounded on the east and west by residential property,and orrthenorth by a -150' high ascending slope (Figures 2csnd 3). , " , . Existing development on the property, includes a mobHehorneand Shed and concrete . bulkhead (Photo 1).ltisunderstood that plans for the property include the dernolitionof the existing home and shedand>the construction of a new sing Ie family residence, detached garage and septic system (Figure 4). >Thehollse wilLbeapproxi/Tlately30~ north of the bulkhead and 55' south ofthe toe oftbeslop~.Thegarage \Hill b~ approximately 50' north of the bulkheadand55'southofthe toe oHheslope~ The new septicdrainfieldwill be located near the base,oftheslope.ltisouf understanding that there will benoslope disturbance in C()njunct~onwiththis.project ' I I I I I I I I I I I I I I I I I I I The cleared d,eveloped portiol'1oftheproperty isrelativelyflatandlevel. and vegetated ' with grass with a few shrubs. The property extends partway up the~lope, which is vegetated in young to mature trees and brush. Someoftnetreeson the slope have curved 'trunJ<sil'ldicating, downslope creep of the ,surface' soil is ,occurring. ' , ," --', .-- , The slope is'approxirnately 150 foot high with an average slope angle of about 38 toAO \ de, ,g, rea.s. S, o,m" ,eshallow, slide, ", s, c, a, rswe,r"e, not, ,i,ce,d on ',the,s,I"o, pe" fCitee,' Re"porte, "d, Iy"" SO, ' ,m, e,o, f these slides were the result ,Qf ,cutting into the toe of the slope. No springs or seeps " Were noticed on the slope face. There is an old jeep trailthatreadsuptheslope to the top (Photo 3). ' . Site Geology The Washington State Department of Ecology's CoastatZone Atlas, 1978 maps the soils in the area of the subject property as Vashon tHI(Qyt) atthejop of the slope and underlain by Vashonadvance outwash (Qva)., The tin is described asavery comPact mix of poorly sorted,nonstratifiedgravel, sand,sJlt andclaywith occasional boulders. This sOilis,exceUenUbrfoundation stability, good fOf.seismicstability, and stands in s~eep naturaJand.cutslopesfor long Periods. It may ravel and spall by wetting'.and drying, and freezing and thawing. ' ' " . The advance outwash is described as wett stratified, well sorted sandy pebble to cobble sized~ravel. In some places, sand predominates.J'hissoi.1 is good for seismic stability and good to excellent for foundation stability, but may ,be poor on, slopes that approach , the angle of repose ,of the material. This soil is generally stable in slopes up to the angle , of repose"and may stand in steeper slopesfof$hortperiods. The "angle of repose" is defined as the maximum slope or angle at which ,Ioose,cohesionlessmaterial,remains stable, and commonly ranges between 33 and 37 degrees onnatural sl()pes.The Atlas maps the stability oftt)ebluffas Unstable old slide and recentslide (Figure 5). The DepartrnentofEcology's "G.e6Iogyahd Ground-VVaterResources of Eastern Jefferson County, Washington" map of the, soils in the area of the subject property is generally consistent with the Coastal ZoneAtlasdescripfions. · ' According to the Soil Survey,ofJefferson.CotJntyArea,VVashingtQn(United States DePartment of Agriculture, 1975), surface soitsin th~areaofthe subject property, include Grove very graveUyloamysancl (GoE,.GoD) and,'Cof;lstalbeacnes(Co).The' sandy gravelly Grove soil formedinmaterialcJepositedbyglaciers. The.Sl.lrveystates that water moves rapidly through this soil. lheCoastal beach deposits occur af the base of coastal bluffs or lowlands and consistsof stratified gravel and sand. Water also moves rapidly through tl'1issoiL ' I I I I I I I I I I I- I I I I I I ;1 I , Conclusions and Recommendations . There havebeeh periodic shallow slides on the slope, some of which may have been triggered by cutting into the toe ofthe'slope:rheaverage~ngleof,the slope is approximately 38to40 degrees,whichis steeper than the iypical"angleof repo~e". The typical "angle of repose" is defined as the maximum slope or angle at which/oose" cohesion less m(iterial remains stable commonly ranges between 33 and 37 degrees on natural slopes. However, the soil that the slope is composed ofwascoll)pactedby , glaciers and exhibits a steeper angle of reposeuntU disturbed. 'This js evidenced by the amount and size, of the trees growing on the slope face. , , The proposed house and garage willbelocatedapproxill)ately55feefaway fromthe toe of th,e slope, which complies with Section 1805 of the International Buit,d"ing'Code (IBC)~ Section 1805 recom me rids 'buildings; bea distanceofat,Ieast'H/2awayfrom the' toe oUhe slopewhereH ::: height of slope, but need norexceed 15Jeet. , Based upon au rinvestigati(:>nandouru nderstand i ng. of the ,project,itisou f()pinion that the proposal will not contribute, to slope destabilization and We recommend approval of the project, from a geotechnical perspective, as snownonFigure4. ' , , ,-I The following recommendations shoufdalso be considered with regards t0111e proposal: , " 1. Vegetation on~theslopeface providesstabilizationtothe slope face soils and should be .left in as natural state as possible. ,Large tr~s thatareindangerof fallin~.tdue to disease, undermining of thero()ts, etc. should be evah.Jated'for possibleremoval because large trees that faUover on aslqpecan cause considerabledamage.IUrees are removed" the trunk should' b,~ leftin place so that the. roots can continuefo providestabilizationto the ,soil. 2. It will be necessary to l1laintaingroundcoverontheslopeinordertored~ce erosion from. surface runoff. Anybare areasfhat develop should be ',' revegetated. Nativasmall, deep-rooted vegetation that requires little or no irrigation would be the mostbeneficiaL Please consulttheencl~ed publications, forfu rt~er information. 3: Excavation into thetoeofthe slope should be avoIded. 4. Surfacerunoff from hard surfaces. such as roofs I driveways, walkways and patios shol:Jldbe,controlledandrouted to a drainage;contr()1 device such that c . surface waterdischal'geto adjacentpropertie;sdoesnot significantly exceed . predevelopment' conditions. The Departll)entof Ecology guidelines, as well as. any other local codes should be followed . 5. Silt fences or other sedime.nt control devices maybe l1eededduring constr~ction such that sedimentationoffsite does not significantly exceed predevelopment conditions. ' I 1 I I 'I,' , " I I I 01 I I I I I I I I I I 6. Drainage control devices should be maintained ,in good working order and inspected at least once a year. For further information please review the three publications (included with theoriginaL of this report) pub~ishedby the Washington State Department of Ecology (DOE)entilled: "Slope Stabilization and Erosion GontroLUsing Vegetation", "Vegetation ManagemeotA Guide forPliget Sound Sluff Property Owners" and '~Surfac~ Waterand Groundwater On Coastal Bluffs". These publications can also be viewedontheDQE websiteat http://www.ecy,wa.qov/bibliolsea,htmlunderlhe1993and 1994 year heading:Tne DOE website also contains additional useful information regarding slope stability ~:md site development; this referehceis highlyrEmommended. li.mitations This report has been preparedforyourexclusive use in conjunction with the above referenced project ,The <report has not.been prepared for usebyothersorfor,other locations. It may be used for other purposes only with the expressedV\lritt~n permission of the Engineer. . , . Within the limits of scoP~rscheduleand budget, this report was prepare-q in general accordance with accepted professional engineering and geoJ(jgical principles and practices i~ this or similar'localities atthetime the. report. was prepared. Nopther warranty, expressed or implied, is made as to theconclusionsa,nd professio!1aladvice included in this report. Theobservations,conclusionsand recommendations presentedin'this report were based on our visual (jbservationsofthe subject property at the time Of our site visit; no laboratory tests were perforJlled~SoiLandgeologic conditions can vary significantly . between test holes and/orsurface, outcrops. If there is a substantial lapse of tim~i conditionsatthe site have changed or appear differenffhan thos,e described in.this report, we should be contacted and retained toevaluCltethe changed conditions and make modifications to our report if necessary. . Sincerely, NORTHWESTERN TERRITORIES,. INC. ~., ",'",',., '",'.,""..","""','",,..'. I,."" . . . " .' /, " Robert A. Leach,P.E., MBA Principal Engineer &~~? I . I I ' I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I ,- - - - - - - - - - - - - - - - - - - ~""1 ~ - ! G' ~ G D )) ~ (il E.p. li\ Z -\ \,/ ____...f..ROPERTY UNE ---------------~ ~T----- , , I , , I , , I ~ ~ a ...- ...- ...- ./' /' / / -------- \ "- " '-.. " --... (/) - --I m -u r- )> Z .. l!l ~ gJ l5 lXlO co ~Q ~~ , , ~<:I~ ~~! ~~~ --... --... , ~<:I\I)~~ ~~~~i ~ ~ C) 1 o . ," .,. '-l I" ...- -.........--..------------ , , ~ l5 ~ 1= ffiIDG1 ~ ~ ~ ~ I I !'~i~~ O!l ~ II lJ' --0 ~ii II; SITE PLAN --- ...- -, ,/ /' / / / / I I \ \ \ \ "- " '-.. , I ~ -~ J Myhre Residence 248 Robinson Road Brinnon. Washington 98320 /360) 643-0598 1IIIIW1UHlli [[ill]~ -Northwest Design iOQ-5 ;;I;~h ' CAD. SeNIoeeL.L.C. -- IhlaQII~QIIJ "~8.~-o.o- R ~H UJut h -...-- I I I ~nle l :24.000 to:.:. .~ I ~Hk!: I ~oo . 11'1(1 . l~n~ Ml1ro:\ I .' l'!!.l- 1i!I-.-~ I ~"",............__....... G1- iiJ'~- c.1'U'...... -:::';;;1."~'~~" ~t:.G;i "i;:~ =~~':~;";.;d:;~~':'::'l:~'-. n.'~"""'.._-:.I"'''''''.l""""",,d'''''l''''''' ~..._. "'_... ............-..--~.-.,." J.. ...'l;Vf....."'_IllW"._....,..u_.y~,~ "I............ .....~_.....~l:(-j, I I I I Approximate location of subject property, I I ,.;.; I I Figure 5. Coastal Zone Atlas Slope Stability Map I I I I I I I I I I I I I I I I I I I I I I I I I Photo 1. View of property from beach "<.1.~<<";iA~'~t'$*:-:C ',~:fj;'~--.:"\~"\;L~yt')ih":":';;:,;,-~~:~,ft~j'"-- j~;\"'>~(,:V~~"J~;~!Z:~,'~~"':>'::::~_"'iI~\t.~'~'lT" I I I I I I I I I I I I I I I I I I I Photo 2. View of vegetated slope behind proposed home site I I I I I I I I I I I I I I I I I I I '''^<', ""; ,... t~~>~' .'i(;;':~. 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