The URL can be used to link to this page

Home My WebLink About001302022 Geotech Assessment 11 Li - lb -a. 1 STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone(360)714-9409 January 3, 2001 Colum Tinley 4225 56th Street Sacramento, CA 95820 Re: Geotechnical Report, Shoreline Bluff Tax Parcel#s 001302012 and 001302022 Township 30N, Range 1W, Sections 19 and 30 z: ° 4".J Jefferson County, Washington Dear Mr. Tinley: We are pleased to present the results of our geology evaluation of the above referenced property located on a shoreline blur The purpose of this geology evaluation was to 1) determine the suitability of the property for the siting of a residence, 2) qualitatively evaluate the risk of slope failures, and 3) provide general site development and maintenance recommendations for development of the property adjacent to a potentially unstable steep slope. This evaluation was limited to a visual inspection of the property, the steep slope on the property, other steep slopes i the vicinity of the property, available geologic mapping in the area, and the shoreline at the base of the bluff. SCOPE OF SERVICES The scope of our services included the following: 1) Conducted a site visit to visually inspect the subject property including the steep slope conditions and shoreline conditions in the vicinity of the property. 2) Observed surface soil conditions on the steep slope and at the top of the slope. 3) Prepared this report summarizing our findings, including an evaluation of the feasibility of building a residence on the subject property, a qualitative evaluation of the slope stability, recommendations for site development, and recommendations for further investigation, if necessary. January 8,2001 Tax Parcel#001302012 and 00130202022, Jefferson County, WA Geotechnical Report 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 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 Sulfa] Geologic Map the Port Townsend 30- by 60-Minute Quadrangle, Puget Sound Region, Washington (Pessl, Dethier, Booth and Minard, 1989) and the Geologic Map of Northeastern Jefferson County,Washington (Gayer, 1976) indicate the subject property and the steep slopes in the vicinity of the subject property are underlain by undifferentiated pre-Fraser glaciation deposits and Vashon advance outwash deposits. The pre-Fraser sediments are described as consisting of interbedded oxidized brown, red-brown, and gray gravel, sand, silt, and clay. The layers are moderately to well bedded and the unit contains minor amounts of ice-contact deposits and outwash gravel and sand. Generally the unit is nonglacial and may contain peat and woody debris. These sediments were deposited sometime prior to the last glacial event and include deposits of the Whidbey and Olympia nonglacial periods. The sediments have been overridden and consolidated by glacial ice. The Vashon advance outwash deposits consist contain sand, gravel, silt, and clay deposited by the meltwater flowing from the advancing ice margin of the Puget lobe of Vashon age. The advance outwash deposits have been overridden and consolidated by glacial ice. Observations on the steep slope of the subject property and in hand dug test pits on the upland portion of the property are consistent with the mapping described above. The uppermost 30 to 40 feet of the very steep shoreline bluff is underlain by very compact gravel with sand, silt and cobbles that we interpret to be advance outwash deposits. Because of the silt and clay content the unit is very dense and compact, it is resistant to erosion and in has formed essentially vertical slopes at several locations along the bluff. Layered silts and sands ere exposed on the lower portions of the bluff and glacial drift deposits are exposed on the mid portion of the steep bluff. We interpret these deposits to be the pre-Fraser sedimentary deposits. The Slope Stability Map of Northeastern Jefferson County, Washington (Gayer-,406) indicates the subject property is located in an area of recent landslides, former landslides and areas of poor Stratum Group File: 12.12.00 January 8, 2001 Tax Parcel#001302012 and 00130202022, Jefferson County, WA Geotechnical Report natural stability. The unstable slopes and active landsliding in the area is a result of the steepness of the shoreline bluff and the eroding shoreline. SPECIFIC SI'Z'E OBSERVATIONS The subject property is approximately 5 acres in size and consists of two parcels. The property extends from Lane de Chantal approximately 880 feet to the shore of Discovery Bay. The property width varies between approximately 360 and 240 feet wide. The location of the property is indicated on the attached Vicinity Map. A gravel-covered access road switch backs down the slope from Lane de Chantal to the proposed building area on the property. A water line and septic line hookup are located near the end of the switch back road. These connections are located approximately 180 feet from the edge of the very steep shoreline bluff The upland portion of the property slopes to the southwest at an average angle of approximately 10 degrees from the top of the very steep shoreline bluff to Lane de Chantal. The upland portion of the property is partially tree covered with mature Douglas fir and madrone. Cleared areas include the access road and portions of the proposed building area. The steep shoreline bluff on the property is approximately 200 feet high. The overall slope of the bluff is approximately 65 degrees; however, significant portions of the bluff are vertical. Most of the vertical portions of the bluff are unvegetated. The non-vertical portions are grass and tree covered. Trees consisted of Douglas fir and madrone. Landslide debris was present along the base of the bluff at several locations. The exposed soils on the bluff consist of compact gravel with sand, silt, clay and cobbles, silt layers, and glacial drift consisting of very compact silt and clay with gravel. The lower bluff is eroded by a combination of wave action that undermines the base of the bluff and raveling of material off the exposed soils on bluff face. The material eroded from the bluff face is removed from the base of the bluff and transported to the northwest along the shore by wave action. Landslide debris covers the base of the bluff at most locations along the shore including the subject property. At other locations the base of the bluff is exposed to wave action during very hide tides and/or storm events. Where present, the landslide debris provides erosion protection for the base of the bluff. The presence of numerous large trees within the landslide debris will provide natural shoreline armoring. No ground water seeps were observed during our site visit on December 15 2909,anywhere on Stratum Group File: 12.12.00 January 8,2001 Tax Parcel#001302012 and 00130202022,Jefferson County, WA Geotechnical Report the bluff face and no wetland type vegetation was observed on the steep bluff either. We did not observe any evidence of deep-seated slope failures anywhere on the subject property. All trees on the upland portion of the property were relatively straight and no incipient headwall scarp slope breaks were observed. Block and deep-seated slope failures were observed on the properties to the south of the subject property. An approximately 350-foot wide block of the bluff south of the subject property has dropped approximately 30 feet. Several mature second growth Douglas firs are growing on this block indicating that significant slope movement has not occurred on the order of 100 years or more. However, a large slope failure has taken place further to the south within the past year. This large slide involves several blocks with a head wall scarp that extends all the way to the top of the bluff The uppermost block appears to have dropped approximately 10 feet and a middle block appears to have dropped 10 to 15 feet. The toe of the middle block has collapsed and appears to have mingled with an even lower block that has completely collapsed. The soil and debris including large trees has slid down the lowermost 50 feet of the bluff onto the tideland below. No springs or seeps were observed on the slope and no recent active movement appears to have occurred on the uppermost block. However, the collapsed soil appears to be still creeping down the lower slope. Soil in the uppermost headwall scarp consisted of massive fine sand and silt. This slide appears to be the result of lateral stresses built up into the compacted fine sand. The lack of confining pressure to the west has caused the slope to fail in a deep-seated failure. Based on features in the area, this slide was a pre existing slide made of several blocks that recently reactivated. According to a resident in the area the slide took place in late April and no movement has taken place since that time. CONCLUSIONS AND RECOMMENDATIONS Based on our visual inspection of the subject property and vicinity, and our assessment of the landslides in the area, we conclude that a residence can be located on the property such that slope failures will not threaten the proposed residence within the expected life of the structure (75 years) as long as our recommendations are followed. We recommend a set back distance of 150 feet from the top of the steep slope. It is our opinion from an engineering geology standpoint that this distance will be adequate to allow for expected bluff failures. The presence of mature Douglas fir trees on the steep slope and on the failed blocks south of the subject property indicates that slope failures in the vicinity are episodic. Although we did not observe any slide blocks on the subject property, we anticipate that becarAd`of the st lepness and Stratum Group File: 12.12.00 January 8, 2001 Tax Parcel#001302012 and 00130202022, Jefferson County, WA Geotechnical Report height of the steep shoreline bluff block failures potentially could take place on the subject property in the future. Hence, a setback of 150 feet is warranted for any residential structures built on the property. This distance will allow for up to three 50-foot wide block failures to take place without threatening the proposed residence. However, as noted earlier the soils on the steep bluff at the subject property consist of very compact silty sand gravel and glacial till. The soils observed in the large landslide consisted of less compacted massive fine sand and silt. Soils on the property are very well drained, and therefore we do not anticipate discharging of footing drains will be necessary. Roof drainage and any storm water should be discharged into the ground near the building, and should not be concentrated in any given spot. We recommend a 50-foot native vegetation buffer consisting of tress and low brush at the top of the bluff. The top of the bluff should be disturbed as little as possible, and we recommend that no grading be performed on the steep slopes of the bluff face. Trimming of tree branches for view purposes will not have any negative impacts to the steep slope. However, no trimming debris should be left on slopes steeper than 30 degrees as this material may become saturated and slide down the slope. We do not anticipate that the development of the subject property will cause any negative impacts on the stability of the slopes in the vicinity of the subject property as long as our recommendations are followed. Site grading soils or debris, landscape debris, or any other material should not be disposed of over the steep slope on the property or placed at the top of the slope. The septic drainfield is proposed to be located on a property across Lane de Chantal well back from the bluff. We do not anticipate that the septic drainfield will have any adverse affects on the stability of the steep slopes on the subject property or elsewhere. Because the shoreline bluff is an eroding bluff, slope failures on the bluff should be expected to occur on a periodic basis. The only way to prevent continued erosion of the base of the bluff is to construct hard armoring at the shoreline at the base of the bluff However, the eroding bluff acts as a feeder bluff(erosion of the bluff provides sediment) for the beaches and the sand and gravel spit north of the subject property, and any shoreline armoring will have a negative impact on properties down drift from the bluff The construction of shoreline armoring on properties along this bluff may cause an increase of erosion in the vicinity of the subject property. The owner of the subject property should contact Jefferson County officials regarding any proposed shoreline protection projects along the bluff or any shoreline protection construction on the beaches to the 2 8 LA Stratum Group File: 12.12.00 • January 8,2001 Tax Parcel#001302012 and 00130202022,Jefferson County, WA Geotechnical Report south of the subject property. There are currently no shoreline-armored areas along this stretch of coastline. CLOSURE Please note that there are inherent risks associated with building on lots near or adjacent to steep slopes. In addition, we emphasize that the steep upper slope of the property is potentially very unstable and may be subject to large slope failures. Although the proposed residence will be setback far enough from these potential failures that the residence will not be threatened, large failures should be expected. 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. We appreciate the opportunity to be of service to you. Should you have any questions regarding our reconnaissance please contact our office at (360) 714-9409. Sincerely yours, Stratum Group \e nil a Dan McShane, M.S. Geologist L 8 231 Stratum Group File: 12.12.00 !tine I Vicinity Map NAD27 122'49'57"W , ....,. 123°2-27"W , ... , .1 f. , .,... ,. ., k,• t ', ... ; 0 ‘ ' .Z... \ ......\,,:c.,c, • ., , k 1 \ \ , zj : : 1',A t ' t \\: \ \ ,.. \ %..,1 N, s.• . `,. 4,• • .„ , /- ' '' ••\\\ • \ ti \r‘ . ' 5 •4, ‘'5,. .- s \t\---.) : .. . ''. •:. .. . .: **• 4 , ,/ ..„,...,,,./„.,,,,, ,. — . ,.., v , .... v: ,...„..,,,.... -,..„\\,,,. ; • ,, • ••‘; \ .• , e ,:c. 7.- . ,:.. ..iJ 1 . ,, i i 1.. .... ..i.. ......7.,,,,\ \ !,-',.t .. . \. •:::. ... ' ... • . ,• .,.../ /..,;.,;,.1•i. .• ,..., ,....‘ .. :• ....1, s.„,.._ ....„„ ...,, ,.„ :0 f .,v -,...--......„„,s,,,,,,,,,,4=__, , ...„,, t .: , '."'• ...• '-'-'• ''' i ;•• ?:.;`) f •• •1 • • •'..\\s\''''\' ;! '! : .. \ • • *.`"k5,:......?‘" --,',..•- : '''•:".f. ...;4'...:-./-'i I.. i .• it 1. . .• -''...."‘-r.: \\\ ,• \ ii• "N, ., '.'• - 1 — ''. ' .•. . —."''.-— ...), ,,,,,1 . .........,...,...-,.....„.:.:." , ,., .i ti,••:•:. .. '2, # ".....,,,„ r-%,,,,x, \1,..,,.. .,, ••,, 1. - ''' ' - : ,•/''WI r''''• •-•"''' ', / / .'' ...1. .- ..7.'' . . • \.•\‘" 'XL \ \A • . • . ••••!l•.,:.2....'."//':./...",L..'..,.••1••.,.,,,,,•,,..4./...-.;„52,.;.•7•D•,•,„.„..,.„.!."•.0.•'.4';7.,.4,-,-i•-..i. ...;.1...:' .el•.%!,..:e'4'„,3,,/.,.,.,-•.• .,•,/-,-.i.,,,-t';f't1?''.'s.'e.'--'),7,....1:.-.0•.•4,.4.,,.."7:';..!','\t,',-‘.,5'-'"\'4 k N I 1'..,‘_\\.).,,;'.•.4 1„.,••!.-,•,4:5-:'\1'i:1i•5',;.5..5\ti'5••\,,•y1.'.\'-••5 v,4.). ,,• ,,.4•..r.. • -','I1„,X.1•.F1.i., '.i't' 1;i I •-- •! ••• 1„:: . ,,,,,,, „..- :•..›.. ••”, fr,' .5.44 1 ,,,r • '5,,,. 5....-.....,,,-:‘, 'Ali i 't,:..ZYN).: ,k,,•,----••• '". .,:•.:•,.. :I.. i ,t,.. ; . i; . , r. .... •• . / • '' \'''', ' ';.• • x. . s N''-';•.. 0.11 0. . , i 7 . z. './. :, j ...,, ‘ . 0 . , vA Vt.*\ 1,i..,. .., ...v:.•...!. 1 ,v., ,,, ,., I . 1 .. .7 //• • - •;/,' . '•,: .1 r.--'-...--N4".....‘,t,‘.?,...,\I, i i\ V,.• ,... A. ''r. ..., .L .•. .1, ..//le/ - ' ,.' . ' , •' / ,'1,„ .. • ' k.5,,r, '5„.v,A ,,i, !, ...3,. . -. .:\..t, .. i.; • ' ,,'.../!........-..:44-..r,.'•-'A ''4'''`• •-'••••' L\ '"ti ' A' • 'X, ; . 's .4, • •-•'"•% 1 ' 7 1.'•-../ ' -.. /It . /" ! . i \4'.--\\'A‘ti 4,"•i 1.i.141.1:\ 1 ':734,,';'' •N.: , (, ,... . , .. , • . . , , v A •, tt,111',t1t . ..1 . .N. • A•t •'.\' \ ;,41...)\ I ... 1:•.,.. ii .• .:.?..,.y...4,•i; , ... . .„.. "..... ....,.,.4 li 1 A.,1 I,....,:t..1,,\k,\41,1 .L.,,,-•,..,\--•\;, T`), ,;:' ) i ,,._:■.;),\ j ' ...1..-..• /.... • -.: i; ,t. • .I. -• .. •-• . i ., -..--ti.•,,. • `‘,,-.. , ., 1. \ , ---,-- ;,. - ...,.." .'•• . 4.••••"':' 4 :4' 4',1 i t,:'Ilk 1.5,-....„,Z..4: '',5 V s•-• •'• ' ' t ,.. • •' • f - • '. ' :I. . • "-!*' . ,-,' ,.,..."—a i • 4. 'i..: . '. ..1;‘, (, . . 1 - - ..' -4"-- \ ! ' !'!:,.. t ?Si/P ; /-; • 1 ; ' Pl 1 ; r', . • , ..j,, .1, . , - --,,?..".Y. "''I- f, , ' i , „, ,' ........",i 4 , "."14 •. ',. Ei"5 t.„...,\ ,.4' • .I. ,. •...- •, - A i• ••:- ' ) 4 • ' ''''', 5 5 C .." r• • (., , . . _ .... 5, .. ,. , .,. .1 i "\1,'..Y.‘"•.‘5.111 r. .. ' •': ' ••••\ r I.. 1 r•!„,..." '\ • ..., \• .., r ! Si. ! ,0• ‘ , . ( r---.7.- ,..1 '1 / ..„.... i , 1 I I 0.',. . N;r.. ""..•• t.' ',' ....rr 1 , 0 I;,;.i i!.■?i 1,i , '........... ' ..,....:. t.., t.; •• . 'IA.. ". 4.,........,. . ."0,,,..." ., // 1';" 0; ."'N,..,..1 .7'. / / l''01. .1 1.:0 f., '• '0. . .,\, "..) : --4----=.-- . ..,7,:„....,-..-,-,,i ,,tx.:1 e : .y.., .i.,•,, ..,.,.,, ; ? .• ' !•.:Z4.:.-x.....4„:„:,.....riP11,,,„,;_ ;,,,„..,4.,,,, ,,; , ' •...'''..)(.,,T..."-. - • ' 2 \ i, '•\ti liti■ti li - '.. ' A' • \•' i •• ' ' 1 ■•t •• •• • ! 't , • •••...,7:-..;.,,..PC•., . :','''•.;,1., 1 ., : ''.',.-t•tt,#.- • . 4 .\ 1- '11 t'I I t ••' '' •..' 1 t' \''S '- • ,-,,,,,.-,..;;-----; ..?"•:. A: -,—,,,.:,.. - 4. ...t...*:': •.'.•-. • -''',.-. — • ?..,.V 1.• r ., , ,,,,,t,..,,.;,::,,I.,.i. . .. 1.\ No ....,:z.,-....„.. -_,.. . ,.‘„,•,-;:,.........lt , ....._..%.• •, -...,.._,...... i •,.. „LI i ,, ,r., , : ; •. , . . :. , . • ... c ••••,, .... „.,..„...... ..... . _.... .......„....„: .. •., .,. . .„,...,; \.., k•i•, i 7 ! • '' ' It .‘,.. :,,44;;`,.,,,,.;,•■■,",: ::\\'‘..,`,.:•"-!"..'7A,:7"-.., . 7-. . .,•-- c..\ , AA ,', t,t'\''-,I -' '-3".., ."%'',;is'.'\!" '---.-...2.'''..,---.'2---''...------- - -. '-- ': k \.. , .\. AA\,'.,t'',,,.........t'...,'.. .'.. • , . .• •.. •-,,,,!..,...,.:::;',...„%,;\4 :::.....:,__,,„:-.2..-.7,-...4... .-,.....::,----,,,-----....,-. , i ,. t..,,,..,t'•:,,'•,, t.,. ,., '... .......„,,,,,,,,, ,....:., :„,.„...,,,,,,.............,.....„..,-.zt•.7.7.\... -., :.,, t. ,,;,,'„,., ,,,, . . • , ..„.,. .! ... ...y....::....,1.......-. --,..,...,,.. ......., -----v,. ‘.. \ ,i 1.....! A 1;,r!' ,r ' •- .• ,........• ,„,.--...,- .....--- - -----...".----, \‘‘ \ .", . . • • - , i :,, 1,•. - • : ! • \ , , •':',";:1::;i:',".;,-,,-,,,,',- :,-1,,.:.---;--,...,•-z-----7:---vi, ,', '•,--,.. \ .. 1, t• r , 5 1.1,r ';,', 1' : • .5 t 5 •It 1 . ''''''.•••••:•••-,••:•!''''!,•;5!...5,4-.,'N '...,_-.---41-:\\5, \:,‘;'‘. -2'5'•••••! •I• i• t' t',t'.t. •1 ,1 ' , . ..." - , "-!•••Aiit,.:.Z t , . • .. • L','•--'t;1::---., - ,\k,g ‘., ••,.7• A' 7 . •' ,'‘" ' '.' ti t ti t. ". • ''‘V'•"'A Ilk • *. ?-.I i;\-','• A ' '' \ - - "T-.• '''''--\-, -\\■"- Subject Prop ''. 4 ■ . ', ;., • ,..4.3::-....„‘,.-, s.„,.,‘ 1. ,,. , ,; 1 , ; :;, ,,, : ,;,,,: •,,,. , \ ;;;:..4-4 '1-e: '-'\''...'• \\'., ' I ' :; t' '. tr'. ; C, \ i \i,'.■; ■ - •. 4..14, •,..--.. .. ,'..,,t,■'s .1 .' i . i: ;., . .1 I. - , 1 :'1'.''''s `• N , \,'\ + .• . . i i,■t' . . ' •t 1 ) • ..,. ..- --„io.a.s ..,..,,,..'.,,, t,...,„ , . . v, , !4 : ' ' ' , '4 . • .'..g.:''''''''-.:". .4.‘V''''' ''••••-, ''''. ti'" I' .* ! .., , ' ' '-,, ', •. •:;,',....,„ '0,,Y7:\V''.',-;‘,- '..,,•""---" • 'RI t!I: , . , • I, i,t •'- . - • . '''',':i.,..,,,,,-.'‘1,-,,'‘:-.-1.,X-,,',,s,\ \ , • 1 111,11'; ' ! : , ', \ , • - - ,•; ' - •''..."....-7,.. --...;',;'.P,--;:.',\V-. '', , ,, ' {is5 )1 I : ', ."... :i . ■? ,. ....',. -....A.,..,:,,..,-- .'.;:\ ■ .--- : • 1. :r , r If t . ,.. .........;,,.,•,,,, ), 0.,.. . .1i1.-• ; : k, ;: : 1 • - ... \\: .., . ,-...r.‘..1 i t•,:. '. ;••••••• , ; I I -• ' ) '',..-. ••• \',', • . ''.;t,”,t, ',111\ '5■ •":“.•,1''. !. .‘ .! 11 •: ,',1 ;, ; ... 5,1' ''..,. ...5, • . ',;:- •'?.V. 's; 5,*(:2•":;: :;4•:,.(III i:i r-•1.• I , . . •••• ..;.:tAlk\\* ‘•5,...JV''T 'I; 1 i‘l. 11.i /' ,.4,4' x• , ...41 :0 , '...'''‘.,,44\k1 t\‘'' *.,,,,..:)i• :,i‘ i'111:'-i. i g; '• e; 1'. :... f i., , \ ,,,,, --, / i i .,,, . .=',k r.• 4, 3 1 /.;/ :of! 1! . .., .:k Ns., ---- .. ! ;, i 1 t... I ■ ''..:, ; , • -t' , ...‘, ,,,. ..., ,,,,,,.. . . • ..,-.....t ;t ; i--; i ...-1 ' '' '' :t • 1 . v:;.`,......- , i ;. , i,. "a ., .. . •. 1 \....‘,„.--, , .0. .;;i:; <Di; • ,. • ....'. , ,,,, . A,,i 10, r , ,e,„.,....:///j' .....'fr - '. .' ..,a ‘;'-‘,..•••'\k..,-k----•‘0,.-, : .._ ..---- i ::.&.. ,. - - . ..N.f.,-:::` ,-, A 60' '/..•••f4,'' ---1.,„t. By E;.•;eTttsktirx, , .,•• ...., scovw y-6), .....,- .:,.,.... .. ...., .. ..." amp Meeting - , " ''••••<:;;:::-......, -0-, •:,.• • < • ;' •• .. .•.., - - ,-- ___.... --- s. - ,.. . . ..__ .- __ ._ .... ,... _.<-"-- ,- _ - ..,....---- - ..---- ,. ., C) 1,,„„1,,, .■ r1CiOntractors ,----- / 8 4.,•,.hu • . 'i..-.---'-' .--:-.Point .,.....--- • • z ?...---- •••••-a 1, -,..2 --- NAD271274757"w :4 &_, ...,..s.,i... 122°5327"W 1/2 1 KEE ? TN*CIN t=can=zsgoo FEET 0 500,51 i000m 19° Printed frnraTOPOI 01998 Wildflower Prodoctions(www.topozom) . I ■ I I 1 P,DPoecl It I Sept; d/a;„ N. : ) a CCPSS 1004 / / q j N l� BC' ` t 1 i „1/4. 1 IP � p i l 1 ■ 0iscover gay st. . ' 0 LJ,J 1 i i a l i ..oara 121100 _:_ :: `. Stratum Figure 2 1 puma)wit°" Group TM ____-._,_ Site Sketch Plan 451 Gant strait I ■L ado='