HomeMy WebLinkAboutAL Pettygrove Block 17 - Geotechnical Investigation - 1998.06.00t
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Geoloclrrfcal lnve*iigafi on
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1634 Jackson StreetFort Townson d, Wash ington
prepared for
The Amanda Herrtrom Trust,
Eltzabeflr A- gkirvin, TrustegOvmeraf lfte prropcrty t-Sfi*in H"use,?
Prepared by
Northwestem Territori€s, lnc-_ 717 South peanooy
Port Angetes, Wsshington SaSOZ
June, 1998.
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Geotcchnical lnnesti ga$on
al1634 Jackson Street, port Townsend, Wanhlngton
ouring May of lggs,.$teve s. Luxton, M$c.. p.E. of Nrr Engineerrng andSurvr:ying carried out a reconnaissarce geotectrnical slLrdy r€lgted to the eingtetamitv reddence al 1684 Jr"d;biilr In p",r i;;;io, w"rt ingron. rheproperty lies within Brock 1? of the pettygroveseooiti-oi'io *re city of portTqrnsend. within Je'ge,'"on gornty. wiJnington.- rt" aitracti"" brick house etthe Jrckson str'et address is $iru"i"-a n"€r'-,-l9.-*p oi " o""*ndhg erope andrnarine bluff at an slevation "r.u"u-tnigs,fcet'.td*;: LeL The hor.rse sffor'a spectacr.dar view over Admira*y tnret ano the pugeibornd hat incrudEs Mt.Eaker and the v€rious peaks of tire Gascade Mounlains, At the tima of ftereoonnaissance, ne re-srdentiarproperty was for =ar*- n,* orA,nerrequested an,irnrestigation into the stabirity "rirt* "r.p"s beyoncr tn* p.p*rty on the norg.'ea$tand long-term oulrook ror the q*p".rti";ilffiH;iile pr*pecrive buyer.The infonnation berorr su*r"iir*lr,u fi ndings of rhe reconneissancegeotechnhat study of the FroFsrty.
Geologic *td Structunl $etripg
The skirvin house is located near the riT of a descending srope qs illustrqted inthe attacfied Figure l. The house is undertatn,b!:a,20 tqot tt{ict<ness of e\lougrnent titt thst t€s deposited ar the site ar ttra'ciosi J f,e rce Age some -15'000 to 17,000 rya.F ?so. T*ilF;lithcepep.r,il;.;"-;;;ilr*rd*rure,of bourders.
"srfl{d'e1,.'s-qFd,,Fnd silt thatfr,ere iip#il8;iih;;;#'fiaciar ice. rhE stactatice is thought to have bd6n two * rtri** thousand r"*iin-na"kness in the puget,$ound region. As temper"tur** *"r,ri*d rrigrrtry, the pressure of the ica caueedmelting to oocr'rr al the bottom o'f tne ioe maaa. Tha antire body or ice rnae fllledwith rock sand andsr'rt, t-ru'. "* *"uir,g. occ.gr"q, a d€posit de.rar<rped at thebase of the ica- over. aJong peri; ;iiirne the erl"i";;A the tilr depo€r-rs seentoday beneath the skirvin tir=u, ine ufl.,cepoite,rr"rrl][*rrr,wmpe.tc.*andsliglrtlp,w.nemt.d by the weight or il,Jou"rtying ie and the presence of frne-grain soilbinders. Gene,tgjliJhe till is stronb e;c durabtJ
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Yil"Ihl';;Jl:f ffi ffi ls""'#?fl-ll*:,H""meltvmter that was detained ,"*ty i"*ri"g p;G?; r;k;.. si's setued fsmthese ponds forming ayers oi ;;;'";rr; sirts ttrat were rater ovr-ricrcren and
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Geotechnlcat and Gcometric Arul1nrls
Field eranrination and meesursnrenr of ths brqfl berory tfre skirvin house
frL*'$eff tlFffi tff fi#-6p:ffi ffi ;ffiWea5:g:iffi *HH#,i#,.#1ffi"^qffiE'tjso''at'ilhe'beeroh,teto,,p-rG g"irt""hd"., I*#on* of interest havebeen dravm acnoss urJii"t n'Iiin" iln u,o^, of the site.
S€ction BB
the house-The Skirvin house has substantial setbackIro{n the rirn of the bluff in the direction of Section BB as shown on theilfustration.
$ection BB depicts thenortheasterly Jine firrm
re.w9!..p,qp[, a,lqag,Bgnsprlyi A portion
rnarine-erwion and
apprqdmate shapeand compssition of lhe bluff on a
sectisn of"$le,bluff abeut,450 feet noi'rffl'df,the.$kir,uinof the slide debrts from that event fias been rermved by
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littoral transport,but a 1O-foot high rnound cf slide clebriscontinues lo form an obstacle to those watking the beactr at hagh tide-
Secfion AA
T'heiliirib'trf8d(sd''d'aari.:lT npbablesllde1 on,{lection Eg is ttre,largest sildetbat le likety to ocqrr "n til;d;l"Jig rd;;;#cinl orgre stdrvihhils6.such a sride uourd be rikery dil *;;riced by the resldenrs of rha house as rheslide scarp wourd be weil GvofrJ tL" iour* on the east
$ection AA presenf .ry_lpproTlnate geometry_gf the descending €lope atong aline that runs roughty nortn-nortt"asi.fior m* skirvin houge. The seqtrondepicts the geometry andapproxi,tn"ie'georogy sf a line from the northeast
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't'hg*fuor4f"thst€doh"ts,doifi'pol*ad of'rlide'd6brs, A 20-foot thickness of stidedeposits, overrving the or^rwictr ;;rdn *;;ffiTabove rhe berch. The'dfs['6hiffa'.6dira]€n'y'been'the site-or ,fairiy G,g;,d-rffitriggered,by rhc;n ::,:!.'';s' tt'lGlitiuri'Eit(l'mbbllitlttid,n'6f the unde'lyipg uJna-*ilpLits,b ercisqsivo rdirator{. :i';
@mer of fl16
line
W.terls a Key Factorin Erosiorr at the Gulch
point to the $kirvin hor.se.
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ffio" Hill Reservoir Drainage and Str€ot $riainage Arc Llkely Sourcoe ofWater
Beacon Hill reservoir.tuui abandonod, tiers at the tcp of Root street about go0feet southusst of tre Sktrvin house. ti"proie";ffi itre Citv of porl Townsenoreported that a *lcg3 drainage pipe dbei "o*" io*'Ro4 streer from thereservoir. There is evidenoe rrai rre urai"A#;;;;iied direcily to ths sutchat on€ rime' At pl"TtF? drqinage pipe is routed ctown Root stre€t toJacksonstreet and , re'.c6, ro cnazem"iGhir. ano trren to *,. u"""n. Mrs. Bew$kirvin recafls that durirq ner oiunood. hsr father, the rormerownsr of thehouse, becanre Envare sfine erusiorit"rng geate! by drainage from thereeeryoirs arrd strEetssbove' nccorurq 6 M*.-6ki"ii,i, in" drainage pipeswgrer thersan€f. routd dovun Jackson Street.
Although the events of that time are be.vond^the rgmory d .he nmxetired vrarerdepartment cfrief.
,ti :ff.*"_,ilU Mrs. Skirvint u-c^ilrnt of the historicdrainage problern is aocurate. Root btreet ard the un#o*y*d were within cityproperty and the routeforms the shortest line from tf* io*ruoir to the bsach.Thus Root Street vrould haw tonneJ a loginal drainage corridor in times whenlittle artention was paid b stop..'t uiritv and water erosion.
Procpects for the Long Ternr Strbrlity af the gits
depicted in Figure 2 Eliders-SndFmrddelidtr SuCh as that
house in have gn affect m the sefety or vqlue crf theesufd.form.a to:th4.
There is now about 4O feet horizontally from the $kirvin house to ttt€ rim of the€sctrprnent along the tine shown in $eclion
; Tl.e prosp€ct or continued headwand er^olpn of lhe gurch caused by the fairurei tc diverr drainage waters from the area is r,, -oet-tiJIi"=o,n* pot€ntiqr- prob{ern whidr murd affecr the varua anJ;;g'#;;;rir, *rn" house.#Al[loudh it is clear that the "r"*ion iac aavaiceJ "iiiuu"tantial rate in earier''times' the preponderance or evidencs, ouilined above, suggests that the sourcesI or water o*ving the prosion t'"u* nuliilluerr eil;i;;€d. Thus. the rapid'eroeion thal might threaten the value or tt e nbu=e *:rfgn in" foreseeable futureFppears to now bo absslt
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sumrnary and RecornmGndauons
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Attrougl"' ther€ has been-historic gulch+brming erosion nsar ths skirvin hor,*e,$ie rrater that dmw th" "t*ion-rtlllpparentty ,,qr b"*n diverted. Large scareand potentiailv disastrou"3riqg.r roe"*, to oe iuite uniireu at t1re srte due ro the$trengrh and endJrarrce of . *,1"[iiii o"po*ir-uii;ffi;\" srtirvin house.
$lab debminations of .tjll deposits and slour creep of debris slideg wi' mntinrain the gurch noilh-northe""t iin" sr,i*in r,orrs, itl Jltu"ry brv rate so ror
fr-,,F, tj*,iff #"Fnrj*ffi:"**u:;nffiif#tr ffi:i: Iirre skirvin horns "Li"rg iil;il,';;tne economi;iif;; rhe house. rt is quitl Ir'rnlikaly' however. that ths
"ro"-lon"i mecnanisrnsl;i,J,iwirr remove more Ithan 1o fuet of the blutrin tnt "in,"ar zone north-norrn."& of rhe house duringtFenensovearsp'o$@.1hst;;;+;;lt#iJrii,i.Il"ivsf,.i€d.rrqngqs;psss,4,
The follouring actions are suggeSted:
' Hire an experlenced clitnber to carefully explore the face of lhe escarpment atthe easterly e)dension or nooi st=ilL o6rccf po*ru[ ni"t*ic drainage pipesof sources of water. Exarnination of the escarpment during or direcry afterheavy rain tyould Ue loear. pfu;-;fu;, refiiov€ eny Fipes that might be fq.rnctand divert water elsgwhere.
' The interseclim of Jackson Street €dirched.sJui"i;;";u.i;;il;;i;il#Hr[tr.'iffi1,-rmxt"^B1fr fgff l,would be wise to monitor street ai.ln"g" arong tne easrerry elt€nsion of Rootf,'ffi,:il':tt*:?H,:1t' Fiil;il Jii.n Jrr'Itil;ffi;# tn"t m.y o;;;t*i;s
;"Hffi*ffiil,,*?.Toot streer sasterty extension over time to proteel*
Limitations
This report is basect on a.visuat inspedion sf the existing aite conditions. Noscientific mcasurernent$, tests or cjicuiation= Gre p*#dr,r*o. The findrnge and
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report are limited tothe normal standard of care dthe incfustry for aninvestiga hon wirho'r berref ir oJ $cieiifr " aat, un J *r'irr'*]on *.
The sore objective ?i_rl,:.,f*st(Tation ha.s been -to diqgnose the geotectrrulcarend/or other rcndrtions at rhe sG, ftr. oossrvationi,Eielpretatrons, andcorrctusions herein Te basgd on gen.iatry
"m"ffibniiir"i*arenginem.ng:lfrggfl:fr'ffi S:"-'Jp'Iiii"!.'lniiGdn-il-iJ;li""srauorhers.eiriler
Steve S. Lurtm, MSc. p.E.
Profeesional Engineer
J.R. Jerry Ne$rfh, p.E.
Princip€l Engineer-
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