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HomeMy WebLinkAbout701205012 Geotech Assessment (2002)06/25/2002 14:48 2067060808 CREATIVEACCOMPLICE ~~ s~rRATU~ ~~a>~P 1451 Grant Stz~t, BeIlinghaxn, WA, 982.25 Phone (36{?) 724-9409 May I o, 2002 Totta Atkinson 6704 23`~ Avenue NW Seattle, WA 98117 Re: Geology Hazard Evaluation, Shoreiw~e ~3iaff Tax parcel 701245012, Lot 4, Last Camp Quilcene, WA Dear Mr. Atkinson: PAGE 02 Stratum Group is pleased to present the results of our geology hazard evaluation ofthe above referenced property on a shoreline bluff, The purpose of this geology evaluation was to qualitatively evaluate the risk of slope failures impacting the proposed residence on the subject property and provide general site development and maiantenance recommendations. This evaluation was limited to a visual inspection of the property and vicinity, a visual inspection of the slopes on the proper. ty and vicinity, review of available geologic mapping ira th.e area, and inspection of the shoreline at the base of the bluff. Tl~e property is located on an approximately SOOwfoot high bluff on the east side of the Dolton Peninsula. The location of the property is indicated on )i~igure 1. The property is approximately 5 acres in size and is located in the southeast 114 of Section 20 and the southwest 114 of ~ection 21., Township 2'7, Range 1 West W.M. sco>P~ o~ sERV><cl~s The scope of our services included the following: 1) Conducted a site visit to visually inspect the subject property including t slope conditions, shoreline conditions, and relevant conditions in the vicinity o~'the property. 2) Observed surface soil conditions on the slope at~d on the uplands above t~e slope. 3) Prepared this report summarising our findings, including a qualitative evaluation of the slope stability in regards to the proposed residence on the property, recommendations. for site development, and recommendations for furtha .investigation, if necessary. 06/25/2002 14:48 2067060808 CREATIVEACCOMPLICE PAGE 03 May ~o, zoo2 Tax Pance1701„2A5012, Lot 4, Last Camp Quilccne, WA, C~ala~ic ~valusition of Sho>"eline Bl~ GENERAL GEQLOGX Northwestern Washington has been occupied by continental glaciers at least four times during the Pleistocene Epoch (1.6 mili,ion to 10,000 years ago). During these. glacial and accompariyixtg 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 Geolo 'c a of S isl_Deposits in the Seattle 30' by 6U' Q~a~ran~l_e~'UVashington (Yount, NIinard, and Dembroff, 1.993) indicates the bluff i.s underlain by undifferentiated Pleistocene deposits. These deposits are described as consisting of poorly exposed gravel, sand, sih, and clay of uncertain. provenance. The Geolo~cM.~n of~ast~Central Jefferson COQ nu~,_,yt w ton (Birdseye, i 976) indicates the shoreline and bluff' e,~posed on the subject property and vicinity is underlain Pre-Vashon ,glaciation stratified deposits. Birdseye indicated that there are three glacial till deposits on the bluff in the vicinity of the property, the Possession Till, the Double >31uff Ti11 and the Vashon Till. The pre-Vashon strarified deposits are described as consisting of sand, gravel, sift and clay deposited prior to the last ice age. Birdseye also indicates that Vashon advance outwash deposits are present on the east slope north of the subject property. Observations on the subject property, slopes in the vicinity of the property and upland areas above. the subject property are consistent with the mapping described above.. The proposed building site is located on a ridge that is underlain by very compacted pre-Vashon glacial deposits consisting of very hard silt. Compact silt deposits are also present at the base of the bluff: Relatively loose sand and gravel deposits i~tre present along t>te access road above the property and on the slope below the proposed building site to very nearly the shoreline. These sand layers are either pre~Vashon outwash deposits that have not be compacted because oftbe lack of silt and clay within th.e urnit or they may be a mantle of recessional outwash that was deposited along the ice nrtargins between the compact soils of the bluff and the ice within the present location of Dabob Bay_ The lielative Slope S~bi.lity in East-Central. Jefferson CountyWashin ton {Birdseye, 1376) map indicates slopes in the vicinity of the subject property are unstable and identifies an area of recent landslidin,g in the immediate vicirty of the subject property. Observations confirm that there are potential unstable slopes on the subject property and the vicinity. The shoreline at the base o£the bluffis erosional and thus the slopes above the shore are potentially unstable and in some places are unstable. Unstable slopes are located on the steep slopes west of the subject property, particularly in areas o£eonvergerrt topography near the upper slopes of the Bolton Peninsula. However, no corrvergent topography is present near the proposed building site area. straw eiroup Pile; 4,23.02 0612512002 14:48 2067060808 CREATIVEACCOMPLICE PAGE 04 May lo, 2002 Tax Parce1701.205012, Lac 4, Last Camp Quilcenc, WAS Gepl~gic Evuluatwon~ of Sh~retirle Bl~ SPEC~F'IC S11TE OBSERV'AT'IONS The subject property is approximately 5 acres in size. A sketch map and cross section of the property with. pertiner-t features indicated are provided. in l~igures 2 and 3. The building site is accessed via a spur road off of Last Camp Road. The building site essentially xs located on the end of a ridge that extends out from the upland areas of the Bolton Feninsula_ The relatively flat building area is 145 feet long by 'l0 feet wide. This building area is located. approximately 220 feet above the shoreline to the east. The area east of the building area slopes down towards the shore at an average angle of approximately 40 degrees. The upper portion of the slope and the base of the slope at the shoreline are underlain by very compact silt. These portions of the slope are steeper with slope angles up to 55 degrees. The entire building area i,s underlain by very hard silt that is most likely pre glacial fluvial or lake sediments that have been compacted by at least one glaciation. Most of the rest of the slope is underline by loose sand and gravel. These deposits appear to be recessional glacial outwash deposits or ice margin deposits as they are in a loose condition. The slope istree-covered with a mix of mature Douglas fir, big leaf maple, and madrone trees with an understony of salal, Oregon grape and other brush. Soil creep is evident throughout most of the slope. The property also slopes towards the west down into a stream drainage. This slope is approximately 40 degrees and is cover. ed with a forest of Douglas fir., weste~cia red cedar, big leaf maple and red alder with an understory of salal, elderberry and other brush. Several. of the Douglas firs on this slope appear to be greaten than 100 years old. The property is accessed via a spur road o~F of .Last Caanp Road. The spur road and Last Camp Road follow the same ridge down from the upland area. of the Bolton Peninsula. A portion of Last Camp Road above the subject property has been paved and sltored up with sheet .piling. This portion of the road is steep and traverses a narrow stretch of the ridge between very steep scopes. No immediate probl.exns were observed on this section of the road. Along the beach a few slope failures are apparent along the shareli.ne of the property and in the vicinity of the property. All of these the slope failures appear to be relatively shallow soil failures due to the steepness of the slope. The primary cause of slope failures is from undermining of the slope by wave action at the beach. As the slope is undermined the lower portions of the slope fail first anal the slope failures progress up ttae slope. iJltimately the slope failures will progress up to the top of the bluff However, zzo slides that involved the entire blufl" slope were absezved anywhere on the bluff in. the vicinity of the subject property. The landslide debris from these slides acts as a natural protection for thebase ofthe bluffuntil oven time the debris is washed away and the process of progressive slope failures up the shoreline bluff repeats itself Approximately half of saw,,, o~,~ 3 ~~ie: a.as.oa 06/25/2002 14:48 2067060808 CREATIVEACCOMPLICE PAGE 05 May 10, 2002 Tax 1'arce1701205012, Lot 4, I.~St Camp Quilcctte, WA Geologic Evalnation o~ Shoreline B1urtE the soil exposed at ttze base of the bluff along the shoreline in the vicinity of the subject property appeared to consist of landslide failures. other portions of exposed soil consisted of very compact silt that is relatively tesistarn to erosion. Ax the locations where the silt is exposed, the slope immediately above the beach is nearly vertical. The time period betweezt the slope failures has been. generally sufficient to allow for the growth of mature trees on. the landslide soils. Hence, most of the slopes above the beach on ttte subject property axtd vicinity are heavily waod~ed. Based on the presence of 100 year old or older Douglas firs and madroaes on most of the bluff slope oz~. this stretch. of coast line, and the consistent geology. along. this portion of the coast, it appears that the frequency of slope failures that remove all the trees has been less frequent than 100 years. CUbTCLUSIONS A1V11 RECOMMI~NDATIt)N$ Based on our visual .inspection of the subject property and vicinity, we conclude that a residence can be located on the relatively level building area as indicated in the site plan. sketch.. A, residence located at this site should not be at risk from bluff erosion or la»dsl.iding that would threaten the structure within the expected life of the structure (75 years) as long as our recommendations ate followed. The building area is approximately l45 feet long and 74 feet wide. The building should be setback from the top of the st east t from t e top p the steep slope to the west. A deck may be constructed. within the setback from the top of the steep slope as long as it is not an integral part of the structure of the residence. That is if a slope failure occurs, the deck will detach from the house. No debris should be placed on the top of the blu#1' Thinning and trimming oftrees on the bluff slope should be lirrtited to topping of alders and maples attd limping for view corridors of Douglas fir. Any debris from thinning on the bluff slope itself should be x~moved from the slope- Storm water run off from the roof gutters should not be routed tvwatds the east bluff and should be infiltrated into the ground in a dispersed maruter either north or soutl- of the building. 'a"li.e septic drain field is located within a communal drain5eld adjacent to the access road south of the subject property. The septic tanks maybe located south of tlZe proposed building site near the building. 'The wastewater will be pumped from the tanks up the slope to fire drainfi.eld. The drair~eld is adequately sized and located such that the addition of water at this location does not pose a risk of causi.rt$ slope stability problems. ststum P F+le: 4.23.Q2 06/25/2002 14:48 2067060808 CREATIVEACCOMPLICE PAGE 06 May 10, 2002 Tax Parce170120541Z, Lot 4, Last Cannp Quilccnc, WA Ccolo~ic Bvehwation of Sharelinue Bluff (~OSUi~ Please xxote that there are inherent risks associated with lots tear or adjacent to steep slopes. These are risks that the building owner should retxagnize and be willing to accept, If camditions appear. different than those described in this report, or other ca~ncerns arise, we request that we be notified sa we can review those areas and modify our recornmend~tions as ;rewired. We appreciate the opportunity to be of service to you. Should you have any questions regarding please contact our office at (360) 714-9409. Sincerely yours, Stratum Group Daft McShane, M.S. Geologist scrnt-m, orate 5 Fitt 4.23.02 06/25/2002 14:48 2067060808 CREATIVEACCOMPLICE ::...., u:.~.~..w..,. PAGE 07 r~ 19° oa r~tr ,„ ~Mdllo1h7C1~O1 bk99& W~Inw~eFxo~lldrae (m'-a'~tup°-~°m} 06/25/2002 14:48 Id orth G 0 ~~~ p -~ ~~~ ~ fi ~~~ ~ c~ ~ ~. ~ ~ ~ ~ cu N r+ 2067060808 CREATIVEACCOMPLICE x~~-- PAGE 08 .~ -- h b '~a~ ~a o 06/25/2002 14:48 ~. 2067060808 CREATIUEACCOMPLICE t_t ~ ~7 4 ~ o ~~~`' ~ ,fie I it .I I I~ ~ I~ ~~~ o ~ ~~~ ~ ~~ ~ ~ ~ rn ~ ~ cn c~ ~ ~ rn ~ ~ Cb ~~ CJ.i rn ~+ c~ ' + ~ .~ ~\ . ` ~ ~- `~. s ~. e,-~ .. t' ~ .. ~ . '~ ~, o ,. ,~ t ~- ~~~ ~; ~ 6 ~p '~ ~ ~ ^ ~ ~~ ~ ~ ~ ~ ~ ~ r'~ N Q ~ "- ~ - ~~~ ; , ~ a r .~` a- , ' ` ~ ) o ;~~ ~ C .~~ ~ ~- '~ ~ ~,l ,' r- ~~ ~ ~ ~ ~, ~.. ~. , ,-- ~ ~ ~ ' ~ ° / .,~: ~~ r PAGE 09