The URL can be used to link to this page

Home My WebLink About702273014 Geotech AssessmentSTRATUM GRGUP 1451 Grant Street, Bellingham, WA 98225 -Phone (360) 714-9409 March 22, 2007 Mr. Vanderweghe 345 Hidendale Drive Quilcene, WA 98376 Re: Geology Hazard Assessment Tax Parcel 702273014 Quilcene, Washington Dear Mr. Vanderweghe: Stratum Group is pleased to present the results of our geology evaluation of Tax Parcel 702273014 in Jefferson County, Washington. The purpose of this geology evaluation was to evaluate the risk of slope failures impacting the subject property and determine an appropriate building area that would not be at risk from slope failures. 'The property is located west of Highway 101 on a terrace area above the Big Quilcene River near the base of the northwest flank of Walker Mountain and approximately 2.8 miles southwest of Quilcene. The property is located in the southwest quarter of Section 27, Township- 27 North, Range 2 West. The location of the subject property vicinity is indicated on Figure 1. SCOPE OF SERVICES The scope of our services included the fallowing: 1) Conducted a site visit to visually inspect the subject property, including slope conditions and relevant conditions in the vicinity of the property. 2) Observed surface soil conditions on the slopes of the subject property and vicinity. 3) Reviewed geologic and slope stability maps for the subject property and vicinity. 4) Prepared this report summarizing our findings, including a qualitative evaluation of the stability in regards to the proposed residence on the property, flagging of the potential building site envelope, and recommendations for site development. ~.I~~~g~ ~~c 2 s zoos JEEfE~~ COSY ~~~ lft]E~'~d~g~ March 22, 2007 Tax Parcel 702273014, Jefferson County, Washington DEC ,~ ~ ~° Geology Hazard Assessment J~EFF~flS~~ QTY D~' GENERAL GEOLOGY Northwestern Washington has been occupied by continental glaciers at least four times during the Pleistocene Epoch (1.6 million to 10,000 years ago). During these glacial and accompanying interglacial periods, the underlying bedrock was eroded and glacial related deposits and interglacial deposits were deposited over the bedrock in the vicinity of the subject property. The Geolo ig c Map of Surficial Deposits in the Seattle 30' x 60' Quadrangle, Washington (Yount, Minard and Dembrof, (1993) indicates that the subject property is underlain bedrock. The Bedrock Geolo is Map of the Seattle 30' x 60' Quadrangle, Washin on (Yount. and Gower, 1991) indicates the bedrock is the Crescent Formation. The Geologic Map ofEast-Central Jefferson County, Washington (Birdseye, 1976) indicates the subject property is underlain by thin drift over basalt. Observations on the subject property are not consistent with the mapping described above. The subject property is underlain by layered silts, sand and gravel with cobbles. The cobbles are well rounded and the sediment is poorly sorted and not heavily compacted. I interpret this unit to be outwash sand and gravel likely deposited near the end of the last glacial period along the valley. walls of the Big Quilcene River. The river has likely incised down through these deposits since the end of the last glacial period. The sediments generally become finer grained with depth further down the hill slope; however, the units consist of lenses of materials such that depth to silt rich zones appears to vary. The Relative Slime Stability in West-Central Jefferson County, Washin on (Birdseye, T976) indicates the subject property is mapped as a normally stable area-that may become unstable if modified- by -man. The stability mapping is broadly based on slope aspects and soil types underlying the area. SPECIFIC SITE OBSERVATIONS A site plan sketch in Figure 2 illustrates the building envelope and other pertinent information at the proposed building site area. The property consists of a relatively level bench area between Highway 101 and a steep northwest facing slope that slopes down to the floor of the Big Quilcene River with Riverdale Drive at the base of the slope. The upper part of the slope is fairly steep with slope angles on the order of 35 degrees. The slope has been logged in the past and an old logging skid road cuts down across the slope. Old slope Stratum Group File: 1.5.07C 2 March 22, 2007 Tax Parcel 702273014, Jefferson County, Washington Geology Hazard Assessment failures are evident on the slope and the headwall of two of the slope failures has slopes as steep as 60 degrees. Ravelling and shallow sloughing of soil is evident at these headwall areas. The slope is mostly tree covered with a mix of trees and a mixed understory. Springs and seeps of water are evident on the lower slopes where. ground water is perched above silt/clay layers. With the exception of the southernmost portion of the property, the old slides on the property appear to be inactive with the exception of minor shallow soil movement where the slopes are steep and where some more recent movement has taken place due to the cut slopes along the abandoned logging road. The area with the most soil movement is the result of the old logging road cut slope. An old growth tree stump is located within the central portion of this old slide area indicating that no large scale motion has taken place at the site for many years. More recent slope movement associated with seeps. and very wet soils above a compact, layered silt is present on the steep slope on the southernmost portion of the property. The-upper portions of the slope have the potential to become saturated in this area due to the impermeable layered silt immediately below the permeable soils above: The, upland area of the property is part of a fairly level bench area extending from base of the northwest side of the slope up Mount Walker east of Highway 101 to the steep slope on-the property that slopes down to the floor of the Big Quilcene River. The bench area on the subject property is underlain by well drained sandy gravel with cobbles. A seasonal stream is located on the eastern portion of the property. The stream flows parallel to Highway 101 towards the northeast then turns to the west and passes through a culvert under the access road onto the property. From there the stream flows down the slope. The source of water for the stream is a seasonal stream that comes off of a small area of the slope of Walker Mountain combined with drainage ditch water from the east side of Highway 101. The water passes through a culvert under Highway 101 and is discharged onto the subject property: At the time of my site visit in early February water was flowing through the Highway 101 culvert and flowing along the stream route on the property; however, the stream fully infiltrated into the ground well before reaching the culvert that passes under the access drive to the property. It appears- that the stream does not reach the slope on the north side of the property except during very heavy rainfall events. The construction of Highway 101 and the rerouting of the stream has likely led to erosion at the top of the slope and down the slope due to the concentrated water where it had previously not been located before. 1l~lEC~I~~ stratum croup DEC 2 g 207 File: 1.5.07C ~~ ~~~ ~~ March 22, 2007 Tax Parcel 702273014, Jefferson County, Washington Geology Hazard Assessment EVALUATION OF GEOLOGIC RISK AND RECOMMENDATIONS Based on my visual inspection of the subject property, it is my opinion that the steep slope on the western side of the property is potentially unstable: However, the stability of most of the upper slope adjacent to the bench area is not at risk from large scale failures due to the slope saturation. Slope movement on this portion of the slope is subject to minor soil creep and slope raveling due to the over steep nature of the slope in this area from past very old slides. The southern portion of the slope does appear to be at risk from saturation due to the presence a compact. impermeable silt unit. Deeper slumps involving as much as 10 to 15 feet of soil could fail in this area under unusual climatic conditions. Home setback It is my opinion that a home can be located on the site such that the risk posed by the steep slope will have no impact on the structure and that development of the bench area can be accomplished without causing an increase of frequency or magnitude of slope failures on the slope. The home should be setback at least 30 feet from the top of the steep slope on the west side of the bench area. The home should also be set back. at least 50 feet back from. the steep slope on the west side of the bench area where impermeable compact silt is locate on the steep slope. The setbacks are indicated on the attached site plan sketch map. Septic Drain Field The septic drain field should be setback from the top of the steep slope at least 15 feet and should not be placed on the southern portion of the property where it potentially could add to water on the impermeable silt layer that underlies a portion of the steep slope. Locating the drain field to the north will not cause an increase in slope instability as there are no perching layers on this portion of the slope. Storm water Storm water from the roof and drive way areas should be directed into dispersion/infiltration trenches or allowed to simply sheet flow onto the ground at least 30 feet from the top of the steep slope. No storm water should. be directed towards the south property line in order to avoid adding to ground water recharge above the portion of the slope that has impermeable silt underlying the steep upper portion of the slope. Stratum Group ~E4 2 8 Z0~7 File: 1.5.070 4 ~EFFEt1aI~~ C(l~~ t t ~t,~ March 22, 2007 Tax Parce1702273014, Jefferson County, Washington Geology Hazard Assessment Tree Management No trees or vegetation should be removed from the southern portion of the slope where periodic saturated conditions take place. Thinning and limbing of trees for view purposes is acceptable on the steep slope outside the area where the upper slope has the potential of becoming saturated. At least one tree per 30-foot center should be left in place. Please note that there are inherent risks associated with 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 please contact our office at (360) 714-9409. Sincerely yours, Stratum Group ~~ Dan McShane, L.E.G., M.Sc. Licensed Engineering Geologist ~' .ea fig. 3 ~.,~T _~ k ~, S ~ ~ ~ t; ~~. ~, '_- r ~{ ~_~ ~_ 3 ~rt~~rr Cc~ ~ , -~: ; . :.- ~ --~ ~~ ~]ECEI~ DEC E 81U07 JfF~Efl~'N COI(NiYUCD Stratum Group File: 1.5.07C 5 TOPS! map printed on O2J20J07 from "Untitled.tpo" 122°57.GOG' W 122°56,000' 1N 122°55.000' W WGS84 122°54,000' W i ~ (t r 1 p ~ F l ~( ~~ ~~~~1 ' a ~ _ ~ ~ `'~ j T~ ~1\ `~~ r°` °o. - ~. ~ « J .11111 J ~~4 , t , J ~~ f ~i~. ~ ~ .~~ -..> Jas ;-~ ~ {j!~JJ tr t ~~ ~~~~~ ~ #`~z~ 1'~ ~ ~ ~`~(f ~ ~._~ '--. ~iv~~ r' #~' ~ ~~`• ~.- ~. (~ ~ ~ r r ~ . d,_. ~` ~ ~.~ s~'~ ~j~t i p 1t ). ~ ~'~: '..it _~ { ~ ~ZL ~ 'a t- ~ tti ~~'t~ti ,, ~t`~'~' '-~~V ~ ~C ~~ ~,`°# yam` ~F ~~ ~i ~ ~~. ~1~~ ~`~t°, TifL~B1 ~lC ~t~'C6 ,. I7~d ~ ~~~ ~~ .'. ' II ~~,#~`vt+t4 ...~'* \ --:~..k. `-~.,"~ ~J~r ~4~J ~' N _ r~''~ 1J~: ~ ~}r~ ~tft'~`~ ~ ' ti '~, 4 +~ 3~ 1111 ~°~ I'ti `, l t~~.l f ~Y, ` I~~~^ h L , ~ ( ~~ ti~ ; 3c y !!,(I' 'r•''^,~~' ~~ w l ( ~ ~J- \ =~ f1 ~ ~ t~ ~/ir ' ~ ~~,C;~r.. ~. L 1 ~' ~•t_~. ~J ,~ \ tlli f~~ 4~` ~ ,;~ ~ I~ ~ I j , ` ` \~. ~ ', T - , Q' :,I # ~ IJg F~Sie-k+9~ ~ "/ ` . / ~~-,., 4.,,`~k _ :.~-1's~!` ~-~ c~... ~ ~- ~ »~l ~ ~Y a - { ,:.i },~,,AV.-r''~.._. --"-r-i- ~._......- '. .... ~,~,1.. ~~' _i 1. ~ \l r,;=,Jx~, ~ .~ ._~s..v.. ~C~ ~ ~I ~ ~, ~ s ~' /~ y~ i ` 1~~,~~ 1 :4t ~ f ~ ~ I ~ ~~ ~q i f~;6 ~ ~... f.~~,/ /.s° Y ~r;~ \, ~ 'Z ~+ IJ(lJ~r~#~ ~'# t ~~ 1~~ i~ '# f i~~J , ~~~¢ fl~j~ \ ~`ti,,,~ ,r/~,f'„-~-' pc - 1 s, ,~ ~ , ~~~><'~~ ~ i fr~j ~~ '~ ~~~~~ s~ fit./ ~ r~ ~~ ~ ~~~~ ~// ij ~>'""~---~ \ ~~:,t~/r/,'~ ~ ~_ ~ f li ~i -~ ( ~ ~ 1 ~ JI I ~ # t i 1 i~ ~' ~ 1 ~1 ~. i ~` t~ ~ ~i~ f ~.1 ~i, /~ N~ p '~ ~ !fJ I L } ~ ~~ / J t ~ q ~r ~ A~~. ~,~~~1,7 [ r,, ~~~ ~~~ )'~ 'd~~i ~~! r~~~o i ~ .~ fir,, J/+ctl ~l ~ % ~ ~~I,r ~ ~ ~ _ u ., ~ rjl 7 /J! / ~ 1 , ~ ~ !, ~y ~ i /r / _ ,f !tf 4 i _' t~ ~1 ~/~J r! t~1 , i y ~ r f i Ir t~` ~ ,,,.,. ~ f 1 3J J ! a'~1 ~'}~ I'~/.t :!~j ~ 1,~,~ l ~~~. } ~~~~. ~~ ~:~ A~~ ,..~~ ~~iJ ~ J.~r;• !. r' II ~ - r- ~ ~"-'~.,~ ~ ,~~ /:1~1 ~~, i ~,~~ . J , ~ j tt ~~~ ~ ' ~~! yff ~ (JIB. ~ aJJ r i t ~.,.'~",..~ ~ ~~~;°~~` ( r jl >~, , `~ tiv~ ~ '1 ~ t ! ~5/J~ ~'J`~I ;~ {!! 7 } 7 ~ ~ ! ~ if ~ r~i ~ t ~{~~ w`'~J~-~~.~,.-" ,` ~ ~ ]~~~ ti 3i'tl ~i 1J~1~'~ff l#~ !' r~ ~'' l ~~ ~ ii} ` ~~ ~~~J / /' /,~ ~/ r~'`~J` r1"~.~-~ _~ f ~ i +jt `tj l ~~ ~~ I 1, ~'~j f f { ,~ ill_ ~ ! ,. cN ! ~ f j° . _ 11 ~~r._s` "'^-+`'- ~-',~ +, n . ~ , ~,, f/ ; ~ ~ a~jl ~I i#`~f,~r ia,~~~ i~ ~ ~. ~ ~~ }~~ ~` r~' ~ SIIBJECT PROPERTY - ~ ?> ~„r-'--y`~~i~ ~.~~,~. ~ ~~~~ 1~ ~ v i I ~ e I q ~~ sij~~ j J tiS ~/~J ~ ~~ #+ ~ ~ ` JI , ~~` ~,~/~'` ,,-....-.. ~ f ~ {°~~ ti~ . --k ~\. , ~~ _/~ ~~0 % 1f ~ i . ir~J i ,~ l zi # (! ~ r • ~' ~~ J e i ~~~ ~ !, ~ t~ t r ' ~I ~~~j' f~ r%~--,...-. ~"ll f 11~ ~!/~ 4~~`~~ --.' ~~>~ t/i' r, of - t~~~, ~ ~ .,q~~ t 5 ~ ~. ~~ ~~ ~ i ~(~r.. ~ e-t2° : r-^+~k.._• tJf ~ ~ P~bt/""'r ~'~:-„ :~ ,~' ~ ~•~ ~ ~ i ~ ,~"'~ '`f~~i ~~- ~ ./~~ f~~ 1 ~ ~. ~t ~ i 1~ }~ t j ~ ' FJI'I ~l ~ ~-r~~,,~ ~~ '''" ~ ..~ ~ ~ ~l ~ ~ ~ ~~~ .., r t~~;,,~i; . /~ ' ,.~ - f~~~,,~%r~ _ r , ~ ~+f f f/`'~ 1 ~ `~~,~\ `.~;~.}~ ~',~-'-''~ ~~~ V~ `~ Yom. t -J'- f. ' . ~ ~ v'~ °'`1~/ / "' a , r ! r^' I i f) t !- (' ' `'r' , A - ~~'" ~-~ '~, ~ `~_.`~A a~ ~;--:. S~ =x.11'.1 1 ~.# >~-~~~~1.~ .S~ ~~~ L4 ~[t.~!~: 1 #`~ ~ -~~/1 ~ i- ~, ~°`- ~d. A~'~~" ~~.'~~- ! j ~J// ~>~51~"#11} ~~ ~' _ ~ t~ , 4,;##'~ -.'-~ ~'!~' i~~# ~5 rr/ - O f ~ :~~ ~ iT ~ ' ~~ Y• ` 11~ ~ I,~~~ ~ ~\,, (~~ j ~. ~ ~ f 41 {}tt ~c~~ t '~i l . r=' ~ `:, ~' ~ i ~ ~ A~ t f ~ ~ ~. .. ,' '; \ t t f ~, x \ \ ~ , , y, r.~ ~ t 1 a j t r•" ~C3 ! +' ~i~; }c ~~ 5:,x,1 {~ 1 ,,~~ `~, ,v\ ~ e i ~~# ,/ r ~ P/, ~ ~~l ~.----~d., ' \;,N~,~~ C u ~~ - _~ y_-_ ~ ~ ~ ~ ~~~\~t'? ~,,~~• ~ ~~ tt'~ .~ ~¢ I ; ~ ~ j~ ~~'?' >ri~{ ~ i ii ~ ~ ~ ' 11,1 " ~ o ~; j i ~~ ~ ~.;~`. ~~ ~''~ o; 4'x4 *o i~ 51 ,1,#1'.s lti ~.~ .r.~,~~~,,,~~, h ..r.+'l rte: ~~J $ ik~ {,;~ ` \ l t _1"„"..^. ~~~~ \, Safi ~I t`t 3 ` ~N1\` ~ ~~ ~ 1~, ~ "w-.,,;..w,,;.,.,,._ I ~ r~~l. ~. , t ,. ~ ~ ~ ~ ~ ~ 1 dA ~1 i t' ~~ 1, .ti-, """-+.,...."+, -.- y I ~. -.~•r"t ~ ~ # '-=,'~~~ il~ ~~ \~ < t~"'`~~°fl\ ~~ ,~ ~\`~' # ~ ~`'~t~~ ~a`i ~ ~ t ~ . 1~ `~ ~~ `~-.:may- 1 + 1 # ~~ ~~ _t :~ ---,_ _ _ + ~ ,~ r : ~, '~ :~ _~, -._.- _ 1GG-7I.VVV YV TN MN l~° t~~"56•~~O' W 122°55.000' W iNGS84 122°54.000' W a 5 1 ;41iE ~a:a =Eki ~~ - Sao -- - ,a:,? rsr.rrRs IvIap created anth TOPO!~5 02003 National Geographic (urww.nationslgeogaphic.corrJtopo) O O O n d' ~d~-. ~' r r \ ` d1 ~' ` _ °` ~ ~~ e U ` ~ .`~ N -+J ~ ,- ~ ~- ~ ~" ~~ ~. d ~,_ ~ ~ ~ ~ ~" ' ~ ~~ ~ ~ ` j ~ ~ v `\ "3 ~ ~ c 7 ~ ~ i -,~~ ~ .. ~ ' ~ vx~~y Sd ....~ -'~ i \ ` C1 ~~ J ~s LmCI O ~a9,+p\ ~ ~ J ~ yy ~ ~, ~~ ~ ti..J ~m~ ~ f~~ ` ~~4 ,,, ~, ~.. , ~ ~' y _ ., \ ~ ti ~ ,, ~~ (yam. ~ ~ ~-~_~ ~ ,~~,., ~ ,~ v ~ ~ ~ ti• 1 l ~ ~, ~ " J ti `ti. ~ , 4 ~ \.. j = ~ `Tar (5 ti ,i ~•~"~ i --F: `~ \ u ~ ~~A'~ i ~ e ~. ~ ~ .~ 3 ~' A. ,, ~ a .~ ~