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HomeMy WebLinkAbout937800120 Geotech Assessment~~ - --. ~® ~wru~.nriaa.can NORTHWESTERN TERRITORIES, INC. 717 SOUTH PEABODY STREET, PORT ANGELES, WA 98302 Englnaers ~ Land Survoyors t Geologist Consfrucflon Inspacflon ~ Maferlais Tesfing (3G0) 452-8491 1-800-854-5545 FAX 452-8498 E-Mail: info(~nli4u.c ~ .: - ~ ~,_ APR 1 0 2001 GEOTECHNICAL REPORT for Lots 20 and 21, Victoria Loop Cape George Colony ~n Section 12, Township 30 North, Range 2 West Jefferson County, Washington Prepared for Dr. and Mrs. Barton J: Blinder Prepared by NORTHWESTERN TERRITORIES, INC. 717 S. Peabody Street Port Angeles, Washington 98362 March 13, 2001 -.,. NORTHWESTERN TERRITORIES, INC. 71T SOUTH PEABODY STREET, PORT ANGELES, WA 9862 ~ Englnoors ~ Land Surveyors ^ Geologlsl Conslnrc(ion lnspecfion !• Mafcrlals TosUng (a80) 452-8491 1.800-054.5545 FAX 452-8499 E-Mail: Info~nll4u.com ~I rvirrv.n lion. com March 28, 2001 Dr. and Mrs. Barton J. Blinder 2008 Yacht Resolute Newport Beach, CA 92660 Subject: Geotechnical report for the property described as Lots 20 and 21, Victoria Loop, Cape George Colony in Section 12, Township 30N, Range 2W, Jefferson County, Washington. Dear Dr. and Mrs. Blinder: [ntroduction At your request, Bill Payton of Northwestern Territories, Inc. conducted a geotechnical inspection of the above mentioned property on March 6, 2001. The purpose of this inspection was to examine the marine bluff at the subject property by visual means and make recommendations regarding the development of the property in accordance with the Jefferson County Unified Development Code. It is our understanding that you plan to construct a small, temporary vacation home on the property, Site Conditions The subject property consists of two adjacent lots located near the northern end of Victoria Loop in Cape George Colony. The property is bounded on the north by an existing home and on the south by an existing home that has been condemned due to its close proximity to the high marine bluff to the west. The property is bounded on the east by Victoria Loop and on the vvest.by a high marine bluff. . , .. The upland portion of the property is relatively flat and level, and is covered in grass. It is reported that the bluff varies in height from 130' to 145'. The upper portion of the bluff face slopes at an angle of about 80 to 90 degrees from horizontal. There is a mid-bluff bench approximately two-thirds of the way down the bluff. This bench is roughly 50' wide. The lower portion of the bluff slopes at an angle of about 40 degrees to the beach. There is recent slide debris at the base of the upper portion of the bluff and the bench is littered with slide debris. There is also a large slide mass at the toe of the bluff that is covered with young alder trees (Photo 1). Recent sliding has also taken place at the toe of the. bluff (Photo 2) and cracks were noticed on the bench close to the edge of the lower bluff. Except for sparse, isolated patches of grass, there is no vegetation on the upper portion of the bluff. At the time of the site visit, the base of the upper portion of the bluff, the bench and the lower portion of the bluff were saturated with water. It appears that groundwater exits the bluff at bench level. The presence of this groundwater contributes to the erosion and sliding of the upper bluff material. The presence of excessive groundwater is also a factor in the recent sliding at the toe of the bluff. Site Geolo A visual inspection of the bluff face revealed the upper portion of the bluff to consist of stratified sandy and gravelly deposits. The lower portion of the bluff consisted of laminated silty deposits. The Coastal Zone Atlas of Washington classifies the soils in the area of the subject property as "Undifferentiated stratified sediments older than Vashon lodgement till" (Qpf). The Atlas describes these soils as consisting mainly of sand and gravel, but in some areas contain silt, clay, peat, and possibly till. The Atlas also labels the slope stability at the property as "Unstable-old slide" (Uos). The Atlas was printed in 1978. The marine bluff at the subject property is very typical of the bluffs found throughout northwestern Washington, where a layer of pervious sandy material overlies an impervious silty layer. Groundwater percolates through the sandy strata and then is restricted by the impervious strata. This causes the overlying material to become saturated and increases its pore water pressure. The groundwater also migrates towards the bluff face and exits the bluff above the confining layer. These processes weaken the bluff and contribute to landslide activity. These processes also contribute to the formation of a mid-bluff bench such as is found at the subject property. Another mechanism for erosion of the bluff is the relentless attack of the toe of the bluff by wave action. When slides occur, the slide debris that forms at the base of the bluff acts as a temporary stabilizing factor. The debris temporarily protects the toe of the bluff from wave action. The debris also acts as a buttress, 2 and sometimes, trees will grow on the debris. The trees help reduce the erosion of the debris (Photo 1). Historical Data The subject property is in an area that has received much attention over the past several years due to continuing slide activity. The home just south of the subject property has been vacated due to recent slides that have [eft it within a few feet of the bluff edge. Several reports have been written on this and other nearby properties dating back to at least 1976. These reports can provide insight into the recent activity that the bluff has undergone. A 1982 report for a property on Victoria Loop mentions two landslides that occurred in 1979 and in 1982. In each of these slides, the top edge of the bluff was reported to have receded approximately 8 feet. The report further states that the average rate of recession is approximately one foot per year. Another report from 1983, and also for a property on Victoria Loop, mentions activity within the two years prior to 1983 that had resulted in the loss of approximately 15 feet. This report states that prior to the past two years, residents had reported little if any bluff regression. This report further states that residents report observing cracks in their yards prior to the material between the cracks and bluff sloughing off and cascading down the slope. Ina 1985 follow up letter to the above report, it is estimated that since 1982, the maximum average rate of regression of the top of the bluff is approximately 8 to 10 feet per year (for the time period from 1982-1985). From the above mentioned reports, it appears the bluff is receding in a manner typical of many bluffs in the region. An event will remove severa( feet of material at once, then become relatively stable for a time until the next event. Please review the enclosed Figure 21, which describes this cyclical process of bluff recession in more detail. Conclusions and Recommendations It is quite evident that the bluff at the subject property is undergoing bluff . recession, sometimes at a rapid rate. There is no way to accurately predict rates of bluff recession or when a landslide will occur. There is also no way to know how far the bluff will recede in any one event. From the above mentioned reports, it appears the bluff has receded 16 to 20 feet or more in the last 20 years or so. Thus, the average rate of recession over the last 20 years would be 3 around one foot or more per year, even though several feet are lost in a single event. Preliminary plans for the property call for the home to be situated on Lot 21 and for the septic drainfield to be situated on Lot 20. Based on preliminary septic design plans and the size of the property, the proposed home would sit about 67 feet from the edge of the bluff. It must be understood that this is a marginal situation and that any development of this property should be considered temporary and short term. Based on the recent history of the area outlined above, it may take several large slide events for the edge of the bluff to recede to the proposed home if constructed 67 feet back from the bluff. At some point, it would be necessary to determine when the home is no longer habitable. This report will not make that determination. Consideration should be given to a type of home that could be relocated when the bluff encroached too close to the home. As noted above, residents have noticed ground cracks prior to slide events. It would be prudent to develop a monitoring program in order to be aware of the possible development of such cracks. It would also be helpful to monitor the rate of bluff recession by establishing a benchmark and taking regular measurements. The following recommendations .should also be considered with regards to development of the subject property: 1. It will be necessary to maintain ground cover to reduce erosion from surface runoff. Any bare areas that develop should be revegetated. Native vegetation that requires little or no irrigation would be the most beneficial. 2. Surface water should not be allowed to flow over the face of the bluff and cause erosion of the bluff face. Likewise, it would be beneficial to reduce the amount of infiltration near the bluff. A formal drainage control plan should be developed for this property to address the issues of surface water runoff and infiltration. 3. Heavy irrigation or other activities that would contribute large quantities of water to the soil should be avoided. One cause of landslides is the presence of excessive groundwater in the soil near the bluff. 4. To help reduce the level of groundwater, it may be possible to drill one or more wells on the property and pump out the groundwater. 5. Surface runoff from hard surfaces such as roofs, driveways, walkways and patios should be controlled and routed to the beach via tightline. Surface 4 water discharge to adjacent properties should not exceed predevelopment conditions. 6. Silt fences or other sediment control devices may be needed during construction such that sedimentation to adjacent properties does riot exceed predevelopment conditions. 7. All drainage control devices should be maintained in good working order and inspected at least once a year. 8. Septic systems and/or detention basins should not be constructed between the house and the bluff. Based on the findings, recommendations and limitations of this report: a. The landslide hazard to the proposed home would be reduced due to the mitigation measures proposed in this report. b. An analysis of slope stability indicates that the proposal could be modified such that hazards are reduced. c. The proposal would not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions. d. The proposal would not decrease slope stability on adjacent properties. e. The proposed home would be stable under the normal geologic conditions that have occurred over the last 20 years at the property. For further information please review the three pamphlets, which are enclosed with the original of this report, published by the Washington State Department of Ecology entitled: "Slope Stabilization and Erosion Control Using Vegetation", "Surface Water and Groundwater on Coastal Bluffs" and "Vegetation Management: A Guide for Puget Sound Bluff Property Owners". Information online can be found at: www.ecy.wa.go~/programs/sea/shorelan.hfm! and at: www. wa.goy/dnr/hfdocs/qer/index.htm! . Limitations This report is based on a visual inspection of the existing site conditions, barring major catastrophic events such as earthquakes or abnormal weather conditions. No scientific measurements, tests or calculations were performed. The findings 5 and report are limited to the normal standard of care of the industry for an investigation without benefit of scientific data and calculations. The sole objective of this investigation has been to diagnose the geotechnical conditions at the site. The report has been prepared for the exclusive use of Dr. and Mrs. Barton J. Blinder. It may be used by others only with expressed written permission of the Engineer. The report has not been prepared for use by others or other uses. The observations, interpretations, and conclusions herein are based on generally accepted professional engineering and geologic principles and practice. This warranty is in lieu of all others, either expressed or implied. Sincerely, SORRY N~ NORTHWESTERN TERRITORIES, INC. ~ oe wnsy l ~ ~ 5 ~ V J.R. Jerry Newlin A t+ ~' ~ ~~AS~CISTtiR~~fi~ ~sf ~~` Principal Engineer 0'v~` E41'IRES Bill Payton Geologist 6 ~i /' // /~ '~~'.,f ^M,,~~•/, ~ drain and local runo~f•~ ~_, ~ - -- ,~„ ' d ` ~ " ,rr 'r / soak Into ground% ~ /~ AI tiro boglnning of an Idoallzod cyclo, rho buff r . ,. • ti ~ / /'-~ has a unllorm slopo. Walor Inflltratos trio sur- ~(/ ~ t ' ~.~ / taco soils and ponchos abovo rho rolatlvoly Im• ~ Qj ~ { ~ ; ~ ~^^~ l ~ l pormoablo matorlals al rho baso of this sandy soil/ ~ ,_• ; .. ~` ~ ~ erosion of ground surface soquonca. Saturation croates poro-wator till ~ -~ ~' i ~_ 1 ~•t ~ ~ -~_ '~' ~ during overland flow prossuros that roduco Iho offoctlve strongth of _ _~ thoso matorlals. Sand -- ~ T~,, \.. 4~';~~, perched ground-water and - ~ .~~ 5pring '~~.c1+: gravcl S~zn'ou~ra ~~ ~~ , / .,'..~ --_~~L-~ ;~~ wave cro5ion '` ~ ~ rcmovc5 Ic55 I ~~~~ I t l + slide dcbri5 permeable ~~~. potential failurc y matcrial5 ~~` ~t surface potentaa( '-'-----" dccp failurc 5urfacc ~iV ~~ - ~~ - r 't / / ~ ~ ~ Runoff and precipitation introduced by the • ~~ d' i / ~- B sources shown in A have infiltrated and weak- t ® ~ , ,~~ / ened the sediments, causing lailure of the un- ~ i ' ~ ~••~••••••••. l prcviou5 51opc profile consolidated upper sand unit. Once mobilized, Soil/ t the sand moves (sometimes episodically, till ~-= t ~ ~'~ ••''•~ mid-slope sometimes continuously} along the contact -`i` bench dcbri5 from upper with the underlying less permoable unit on the Sand ~~,~' ~~ , ~ , ~-.. 51opc contributc5 to mid-slope bench, often cascading as a secon- and `~~~~`.~ ~~'= 5ccondary land5liding dory landslide oft the bluff formed by the lower ~ •. L-` ~ '••., unit. This migration of material across the gravcl ~~ •~ ' ~~. •• ~ ~ bench decreases the buttressing of the upper '~ `•• ~~==_`~ ~:..°• •-4 :~ `Y'~~ • ~ ~ bluff. Failure surfaces can be deep (those that ---- ~-l'~`` I ~-- ~ -~ , , ~ lower project into the lower, less permeable materi- Ic55 ~~ I I l + ^' •~"' -~ bluff als} as well as shallow. permeable ~~, :~ - matcriais `~~, ,~ patefTtial failurc - ___ ______- surfaces If ground ~ potential not stabilized dccpp failurc 5urfacc ~~ , i t / / ~ ~ Benched biuff retreat continues. Movement of ~ ~ ~~ ^~ ~'I / ~- C slide debris toward the lower bluff further de- ~, ® ~~. •~ stabilizes the upper bluff, causing continued . -. ~ ~ ~;~ ~ ~ l sloughing onto the bench. Either failure of the 5O1(/~ -~ , , tapper bluff onto the bench or failure of the till -_ t E-Z~ 51opc continuc5 to retreat, slide debris off the lower bluff can trigger a cy- ' .~~yt ,5~~ ~ cvcntuallycncroaching on the house cla of movement. Movement along adeep- sand ~ . ~ seated surface can reset this sequence of ~ }~' ~' events. and ~ - - ~t ~ _ \ _ .._ ~. gravcl \ _ _. _ ..._.. `~` \ _ ~ . ~~ I ` IG55 ` I ~`. permeable ~, matcrial5 ~` potential ~\' - -----___--_ dccp failurc surface Figure z1. This sequence of sketches shows the idealized, potentially cyclical process by which bluffs in the northern Puget Sound area are forming and retreating. 26 Washington Geology, vol. 25, no. 1, March 1997 PHQTQ 1 View ofi Bluff at Subject Property PHOTO 2 View ofi Recent Slide Activity at Toe of Bluff - - _ 7. ~+. _ NORTHWESTERN TERRITORIES, INC. ~ - ~ A JLS GROUP COMPANY ~ ~ ~ 2~"`~---_ 717 SOUTH PEABODY STREET; PORT ANGELES, wA 88362 ~ v ^ Engineers ^ Land Surveyors ^ Geoiogisfs . ~~I ~ ^ Consfructlon Inspection ^ Materials Testirig FED 4 (36~) 452-8491 'FAX 452-8498 www.nti4u.com- E-Mail: info@nti4u.cgm. f ' JLS GROI~P, INC. : DEPL'i3F COMMUN TY DEVE February 5, 2003 LOPMENT Dr. and Mrs. Barton J. Blinder- - c/o George W. Andrews, Design and Construction`Consultant. ' ' P.O. Box 480 . Chehalis, WA 98532 Subject: Geatechnicai report.adderidum.forthe Blinder properly described as Lots 20 .and 21,. Victoria Loop, Cape: George Colony in Section 1.2, Township - • 30I`J, mange. 2W, Jaffersan .County, Wushir:gton. Dr. and Mrs. Barton J, Blinder: - ~ You have. requested. that we clarify- our geotechriical -report for. the -above referenced ..:location dated March 28,..2001, .to reword the setback language from '`house" to "foundation" since the house is a cantilever design. , We hereby amend our -report to recommend a setback distance of. not less than 59'=6`': . from the top edge of the bluff to the foundation of th.~ proposed home. If you have any questions. or need further assistance, please do not hesitate to contact - this effice. _ _ Sincerely, ~ . - ~~R~ A ~F9 O~~ ~~ \` ~ of wasy, cy - - NORTHWE$TERN TERRITORIES, INC. ?• . -off ~F 13772 ~O ~~ O,t~s ~ t SS S~G~~~ S/n , FC1 Robert A. Leach; P.E., MBA ExPiRla ,z,3o;2002 ...Principal Engineer ------------ L ~~' ; *~ L - - _. : `Bill Payton, L.E.G. Engineering Geologist ,. G:\Gen1Bi111Reports\BLIIV02b1.report addendum 2.12(30-2).Cape George.doc ~~~~ ~'~~~ Wifli~t77 L+. ~r~~~GF1 Jf`. . Expires 11-06-D3 NORTHWESTERN TERRITORIES, INC, _ T~ A JLS GROUP COMPANY ` 717 SOUTH PEABODY STREET; PORT:ANGELES, WA 98362 ^ Engineers ^ Land Surveyors ^ Geologists ~~~ ^ .Construction lnspeGtion ^ Materials Testing (360) 452-8491 ,FAX 452-8498 www.nti4u.com E-Mail: info@nti4u.com JL,S GROUP, INC: ~ ~ ~ - _November 27, 2002_ ~ - .- ., . -. ,- .:. .: ~ . , ~' ~ JAN. •2 i .?003 ,. . Dr. anct Mrs. Barton J. Blinder ~. coo George 1(V .Andrews ~~ .. _J I uesigt=.i acid ic~risit~ctior~ Cor~suitant -_ ~ ~~ G ~'~ ,' ~'' - - ~_ ~~ .. P.O.'Box 480 . - _ . _ _. v - . Chehalis, WA 98532 ~ - _ • - Sukiject: Geotechnica! report addendum for the Blinder-property described - _ . _ as Lots 20. and 2.1, Victoria Loop, CapeGeorge Colony in Section 12;_ Township 30N, Range 2V11, Jefferson County; Washington, Dr. and Mrs. Barton J. 13iinder: -You have requested.:.#hat'we amend- ou_r geotechnical report .for tl~e above. ~ , . - •. referenced 16cation dated March 28,.2001; to provide for a setback-d.istance from- the bluff to the •proposed house of 59~-6" rather than 67'. This amendment is ~ y- desired because of°the building area constraints .imposed on the .property by the - ~_ • setback. Lf 'is .understood that the 6.7'setback .distance. shown on the original site • . - . ` plan was preliminary, and riow_ that the. site plan- has. begin finalized, .an additional . 7.5' is necessary''rn order io site the house on;the lot. This report_addendum, as . • well as the previous geoi:echnical; report does not address septic design issues: A seccinci S11'P VISIt t~taS rs,-+arla to thg prCt~a;-ty~nn RITZ/Pmhar ~! R~ ?fly? i:'1 Cruvr +G . - determine if any new sliding. had taken. place. After observing the bluff at •the . - aubject propertx and reviewing photographs from the. previous report, it. was. . :determined that little or no:sliding had fakeri .place at the property since the. last • site visit: We hereby revise our. report and- recommend a setback:distance. of not less.thar ,~ 59 -6 . from the top -edge. of: th.e bluff fo-.the proposed home.: This new setback distance does not alter-the: conclusions,. •limitatien~s or recommendations of the .. previous report except that the. reduction.:in . .setback may ,result :in_ a :. corresponding reduction in the economicaife of the home. ~, Dr. & Nlrs. Barton Blinder Geotech~nical Report Addendum ~ JAN 2 1 2Q03 November 27., 2002 ., .~ . Page 2 of 2 ~ ~ __ - _. . - It Was noticed on -.the site plan- (Farwest Horimes; Sheet 1, ~1/1/20.a2) that a drywell is planed for roof and driveway runoff. 1Ne~recomrriend that surface runoff . from hard surfaces such as-.roofs, driveways, viialkways and patios be controlled • and routed to tfie beach via tightlirre rather than using a drywell due to the .sensitive bluff conditions. Limitations - _ . - This. re~~rt i~~ase~ orr tw~:visuai;inspections of the:eXisting site-conditions. No ~: ~.. ~ _ scientific measurements, tests~s~r calculations were performed. The~findings and ~. ` report are limited to. the normal standard of care ~of the industry for an investigation without benefit of scientific data Arid calculations: The sole objective of.this:investigation has been to diagnose the .geotechnical conditions at the site. The report:-has been prepared for the exclusive use of our. .client~in conjunction with the above mentioned projeet.~.lt may be used by others only with:expressedwritten pe"rmissiori of the Engineer. The observations, , . interpretations; and conclusions herein are based on generally accepted . ~. ~ professional engineering and geologic pri~iciples an.d~ practice: This warranty is in tieu of all others, Either expressed or.~implied: :. _ ERA A. `,~ :Sincerely, ~ O~ fl~ wasy~'9C'y NORTHWESTERN TE.RF2ITORIES, ING. y- ~ . °2 .. ~ ". ' 'O~px ~F~` 377 O. \`~rc.~ ` obert A: Leach,- P,E., MBA ~ ~SS~aNAI. ~ ENG , Principal Engineer ~ - EXPIRES 12/3D/2002 - . ~~, - '~ ,.rj v ~~+4; ~~ - .Bill- Payton, L-.E.G. ~ ~ ~ . Engineering Geologist . G:1Gen1Bi111ReportslBLIN0201.report addendum.12(30-2):Cape George.doc '~'+,,,;. ~~ry~{~ "`• ~ilta~rri C. Payt~n.,fr: Expires 11-DFrd3