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Home My WebLink About977100905 Geotech Assessment'Ja~~21~ 2005 2~15PM 1 X60 ~~7 X744 STRATUnf GROUP Ido~7655 P~ 2 r~oi ~N ~~ ~ ~ U~ ~a~ s~r~mun~ ~~ouP i 4~ ~ v~f Street, aellmgham, wA Phone (360) 71~.94~}9/ January 24; 20 5 ~ ' ~~~~-a^ Pete Varnien r 1478 West $r' wood Avenue /~~/,'~ ~.~/~~ Littleton, Cp 0120 r Re: Geolo Hazard Evaluation, Shoreline Blutr 1 ax P ei 977100905, Beckon Faint ~effer~~in County, WA ~~~ ~ 7 ?005 dear IV1r. V~ ~tx~stum t3rou is pleased to present the results al' our geology hazard evaluation of-tha above xeferenccd property on a shoreline bluff. The purpose ofthis geology evaluation was to quAli,katively a aluate the ri9k of slope failures impeotln,~ the proposed residence on the subject pr~operLy as~d p ide general site develaptnent and maintenance recanunandations. This evaluation w limited to a visual inspection of the property and viclzuty, a visual inspection of the slopes on prvper#.~r an,d vicinity. view of available geologic mapping in the area. and inspection of shoreline at the base oi't~e bluff'. The prop4rLy i~ located an 8ixt approximately 200-foot high blufF on the east side of L7iscove.ry Bay narthea3t f Beckett ~Uint. The location of the property is indicated on Figure 1. The property is located in the northwest quarter 4f Section Z~, Township SUN, lZange 21Ncst W.M. SC4PE QF SERVICES The scope pf ~ur services included the fallowin(~: i) 2) 3) Conducted a site visit to visually inspect the subacot property including the slope conditions, shot~eline cnrtditions~ and zelevant conditions in the vicinity of Cho properly. observed surface sail conditions on the sloAC and on the uplands move the slope, Prepared this report surnrn~si~ing our ftndirlgs, including a qualitative evaluation of the slope stalsility in regards to the proposed a~sidence on the property. recomtnondations for sit® development, and recommendations for father investigation, if naaessary. GENI±,RAU ~EULQt~Y Northwestern Washington has been occupied by continental glaciers at least four times during the Pleistocen Epoch jl .b million to IO,ppO years ago). During Chase glacial and acoompanying 'Jan~27. 2005 2~15PM l sae e~~ sza~ J4nuery Z5, 2005 Tax Pareek 97710 Geologic l;valua ~5TRa7'U3i CROUP i, Beckett Point, aeffarson County, VYA of Shorelir~~ glint' No•1655 P~ 3 ~~ ~ ~ ~ zoos interglacial pe?j iods, #l~e underlying bedrock was eroded and .a. relatively thick layer of glacial related gnd int rlaacial filuvietl sediments ware deposited over the ur-derlyir~ bedrvtWk in the vicinity of the ~SUbjcct property. X10 2 The tt ~u~ o ru et aouna see vn ~~ ( esa1, iJetlxier, Booth and MinFtrd, 1989} and the Ciealogjc Map of 1Vorth4s~tcmt Jefi'ergon Qu_nty Waghinato~ (Grayer, 197b) indicates the bluff ~ subject property are underlain by ashen advance outwash depasi'~ and glaaiel till. The advanec outwash dspoaits consist Qf clay to $retvel stratified depasil~ deposited by meltwatez~ from the advancing glaciers at the start of the last glacial period. Thy glacial till oorlsists of a poorly sorted mixture ofrock fragments dap sited directly by 8lacial ice. Tho larger fragments of cobbles and occasional boulders and are embedded in the very ceznpact silt and clay with sand. Observations n the subject property, slopes in the vicinity o:F~e prapcrty and upland areas above the subj et property axe evnsistcnt with the mapping described above. The upper stapes of the property underlain glacial till. Except fbr the uppemxast sieges that are underlain by till, the steep she line bluff is underlain by sand and gxavel in a compact condition, which is consistent wit the slope being underlain by advance outwash deposits. _ ~~ of N'orthesstem Je er n Coon Was ' (Gayer, 1976} indiva~tas that the stECp harcline bluff slopes ii1 the vicinity of ~e sub}act property is an area of active and former landsli es and is an area of poor natural stability. The glacial till area is mapped by Gayer as an area of s b1G slopce. Observations ~onfirm that there are unstn,blc slopes an the steep shoxelinc buff on the subject property and tie vicinity. Tho shoreline at the base of the bluff is erosional and thus the slopes above the she c sure areas where old slides and recent slides have taken place. The elides along the shoreline onsist of relative]y shallow Landslides associated with shoreline erosion at thaw base of the bluff p er stecpcning the slopes above. The areas in outwash dap that allow sa la.Ck Of silt a~ Co1;iC83oA1e88 lnittlBdlBte V; The glacial gcncrally st, the till_ The Stratum Umup y vicinity mapped by Gayer as being of poor natural stability cozrsist of advaztce t"s. Those sediments are potentially tttnstable if underlain by impermeable zones ~ation of sand and gravel units to occur or are instable or, stccp slopes if there is a slay cohesion in the unit. Conditions that would lead to saturation and areas of awash sediments do not appear to ba present on the subject property and due to its compact coherent nature has been mapgad by Gayer as stable. The till is unless it i$ on very step slopes or is undermined by weaker material utttdet'lying lea present on the steep shoreline bluff slopes consist of sk~ailow sail failures and Fite:1213.04 3 ~ 2 I1 ~ 2~ . 2 0 0 5~ 2~ 15 P M t~ e o e 4 7 3 7 4 4 STItA~'iJbi G1i0LiP i January 25, 2005 Tax N"arcel 977 i 0 Gbologic Evalua the only till are the erosion of No.1655 P, 4 ~ os ~, t3eckett Palni; Jefferson county, wlt, J~ N 2 7 2005 aP&hataline lilllfP that are potentially unstable are those areas that are cvctatually undermined by steep sh,orcline bluff slope, ~PEC)lF`IC SITE QBSERVATIONB The subject ~perty is located an a high shoreline bluff on the northeast store of Discovery day. The location o~the properly is shown on Figure 1. A sketch map of the property wiih pertinent features indices ed ie provided in Figure ~ The steep she line slope on the northwest erul of the subject property is appraxirnately 200 feet high. except r portions ofthe upper slope end the base slope, the shoreline slope is plainer and slopes at aaa gle of approximately 40 dcgrccs. Except foe the uppermost slops, the shoreline slope is primarily underlain by sand and gravel advance Outwa.5h deposits. The uppermost park of the slops is s because it is underlain by very corr~paet glacial till.'~'hc base of the s1apG is steeper than sl pas above in planes because ii is being underrtuned by erosion, and in other lor~tions the lia.9e of the bluff is less steep because of landslide material at ~e bass of the bluff: "1'hc slope has ~ mix 4f vegetation including stands of tall Douglas firs, stands of°aldr~rs and grass- and bru~h covered areas. Evidence of past shstllow soil failures is partially expressed by the variety of Vegetation on the slope and topagraphio features, `The thickness of the shallow failures appeal's to he on the order of s few feet. The uppermost slope on the bluff is underlain by silty glacial tit for approximately 10 to 1 S feet. This very hard aozzapact material stands morn steeply than c stapes below and is in plar,~q vertic~l.lvidcnce ofpestslab failures o#'the outer portion ofthe ill is apparent by the presence of br4lten pieces of till on the slope below. All the slides ' ppear to be primarily t1~d result of erosion at the base ol'the bluff from wave action. Asa the lull becomes oversteepened the lower slopes fail first and overtime Ehe slope failure progre see up the slops to the top o£the bluff. All the landslides observed appear to be shallow slab type failures na more than, five foot thick. Tha Bhorcline ~at the base of the bluff'is subject to some ocean swell. However, tltt shoreline orientation vii. n Discovery Bay and tlae presence of Protection Isl~d approximately 1.S miles aaorthwcst oft a subject property limits the size of waves generated that can impact the base of the slope, Ero ion rates appear to be 1`airly slow xalativc to areas to the north at' Cape George approxirnatel one mile north. of the subject property. Zandslide debris at the bass of the slope along this scc~ion of shoreline supports mature trees indicating the slow rate of erosion Most of the pr~opGrty extends across an upland area above the shoreline slops. Slopes on the upland area. are unrelated to the steep shoreline bluff slope located to the northwest Qf the property, '1`lre ~tproperty slopES downward into essentially a dry valley. That is a valley chat has rte Fflo: 12.13.04 Swum O~cup Jan • 27 ~ 2065' 2 ~ 16PM i 38U 847 9744 ST12A~LThi GROLiP January 25, 2pp5 Tax P$res1977J0 GeologlC lbv~lu~ evidence of w compact odes the southeast ; portion of the property is tre Douglas l~ir fi 5, Beckett F'oint.l8ffierson counq, wl# of gl~orCltne )duff No~7655 P• 5 ~ oa flow taking place. lvlost ofthe slopes of this valley-are underlain by till ar autwash sand and ,gravel. The slopes of the valley range from very gentle on ion of the lot and gradually steepen to as much as 30 degrees toward the central before decreasing agairt at the bottom oi'tho valley. The upland area of the ~vored primarily with mature second gr4wlh western rod c+rdar and a few CUNGLUSIONS AND Y~COINMENDATIpNS Buiddirrg .Sr1Q Based on my isual inspection of the subject property and vicinity, T conctud,e that a residertcQ can be located on the upland area of the property. The building should bs setback from the top of the steep northwest slops a minimum of 30 feet. A deel,; xn,ay be constructed within the setback from the top othe stecg slope as long se it is riot sa integral part of the structure of the residence, That is iF a slo fazlure occurs, the deck will detach from the house. ,A-,11 the slopes ~n the upland portion. of the site arc stable. I~owevcr, if a home is built on airy of the slopes gre~tef' than ~5 do$rees, specific engineering may be required if sail greater than 4 Poet thick will be r tsincd by foundation, walls. No debris eho ld be placed on, the top of or on the steep shoreline slope. Thitulizt~ and trimming of trees art the bluff slope and within the setback urea should ba limited to topping and limbing for view corn vrs. Thirn',~; should leave at learnt one tree on a 30-#'oot center within the 30-foot setback area ~ m the top edge of the steep shoretirte bluff Any debris from thinning on the steed slaps itself sh uld be rerna~+ed from the slope. Storm water ran oiT from the roof gutters should not be routed towards the bluff and should be infiltrated' into the ground in a dispersed manancr. T recommend that any storm water drainage be directed into level spreader towards the dry valley and should not be discharged within 30 feet of the top of too steep shor®lin~ bluff slope. The septic dra'n ficlcl :should be located at least 3Q feet back from the top edge of the bluff: The addition of w~~er at this lacatian should not pose a risk of causing slope stability prob]ems. f Conclusions' l~egaTdin~ S,lc~pe Features err the Upland Portia,? of the Site It is my opini n based an my site inspection and geology interpretation that the slope features on the upland po tor! Srxl?ject propelrty are very old features. There era numerous dray valltrys end ravines much~ikc the one on rye subject properly located on the slopes southeast of the subject Siuvluto Croup I Filn: 12.13.04 4 ,, Jan~27~ 2005 2~16PM 1 ~(i0 647 6744 STRATUM GROUP N0,]655 P, 6 f~06 . ' JAN 2 7 2005 January Z5, a0os Tax Parcel 9'1710 905, $ntkott Paint, Jefferson County, WA Geologic ~volua Ina orSi;orellne Muff property. I hav~ over the past few yeaza inspected several of these dry valleys and all of them have no evidc~ce of water flow. All the ravines inspected ag well as others al,ruptly' and along the top ofthe steep slope break that runs fOr soma distance to the southeast of the sul5jcct prvporiy. I have intCrprat~d these valleys us being a remnant load featuro vreated at th® and of the last glacial period. ' i The abrupt slope break is not unusual far high bluff slopos in the P'ttget Sound rc~ion. HawavCr, the presence ofi tho dry han,gin~ valleys is rather unique. i have two theories as to rho how those now dry valleys may lavo formed. C7ne is that the steep bluff may hQVm beers ClCated by glacial ice near the enii of ~e last glacial period. 'Che steep slope forrrted by a remnant glacial lobo within what is ~novv D1'sGQvbry BEty. 8trcams flowing from melt water from remnant glacial ice C» the uplands above carved the ravines doom to the tap of tho ice and deposited the vutwash from the strealxl9 onto khe glacior, Another possil5le explazxation is that relative sea-level at the end of sloe lust glacie~t porind~ was briefly highe~han it etnrcantly is today because the land in the area had been depressed by the Puget and Ju de Fuca ice sheEts. In northern Puget Sound relative sea level was ae rnuckt as ESQ feet higher than current sea levels at the end ofthe last glacial period, Tho steep slope already existed from glacial erosiQtl. The ravines wore created by glacial melt vvrtttrr from the retreating ice shoot e~rtd the ravines flowed into an early Discovery Bay that was higltaY or! the bluff elope than it is tQC1ay. As the land rapidly rebounded the source of glacial meltwa~tar fxom the retreating ice ended because the glacial shut refireated from the aroa and the valleys became dry, Regardless of the concoct interpretation, there is no ovidenee of slope instabllit~r nn the subject p>•opo['ly that v~ri11 pose a risk to structures on the upland portion of the property ~ long as the above site dQveiapment zecgil~klcAClations are followed. i Stratum Group Pia: r2.is.o4 S Ja~~27~ 200,;, 2~16PM X 3130 047 3744 STRATUM GRQUP N0,]655-~, 1 :~ boa U -r-~ N ~ 4~ ~ C7~ ~ tL v~ ~ ~', ~~~ ~°~F ~~~~ i _I. i~- ~' +~ .~ ~ ~ ~ ~ . a ~ ~ ~~ '~ x 0 ~~ ~ ~- W ,, a o ~ ~~ ~ u m "'~ t `. ~_ -~ ~ y J an ~ 27 ~ 2005 2 :11PM 1 3so e47 3744 s'r17a~rtrni G>zou~ No ~ 1655 P ~ 8 ~ °"' . ~' January z5, zoos ~ ~A ~ 2 ~ ?OQS fax Parcel 977100905. $eokett point Jefferson Counl~, WA Ceolo~ic Fv®luatio[~ of Shoreline bluff - ~~QS~J~ Please n~-te that there ®re inherent risks associated with late ncsu' ox adjacent to steep slopes. These ere risks that the building awncr should reeogni7e and be willing to accept. If ao>«ditions appear different than those descrihBd ~ this report, or other concerns arise, we request that we be nofii~iad ao wQ Cann review those areas and modify our recommendations as x®quire-d. We appreciate the opportunity to be o~service to you. Should you have any questions regarding please oontact our office at (360) 714-9409. Sincerely yours, ' 6trratum Group Dan McShane, M.Sc., L.~.C`r. Licensed Engir~earit>~ f~eologist SOsNIRI C}Rtup Pilo: iz.i3.04