Loading...
HomeMy WebLinkAbout001063020 Geotech Assessment STRATUM GROUP 1451 Grant Street, Bellingham, W A 98225 Phone (360) 714-9409 __._c_-:~'" ..~-;:~~',". {J;:;.,.c, \,,,.j .... r \ .,. I.... ! r' . ,r , . :." r,' r, \,~} '~;<\' ":' '-, \ . \ r'\~ .,\f'! t,,,, '.. ... \,;... \ '\. d...' \ . ,., :----l" .' ' t':~ ,~, r--- t.~ I ' i. :.' . \, JUN 1 2 2006 I;., . i t.f.........' May 22, 2006 Tim Haney 2023 East Simms Way Port Townsend, W A 98368 Re: Geology Hazard Assessment Tax Parcel 001063020, Porter Lane Jefferson County, Washington Dear Mr. Haney: I visited Jefferson County Tax Parcel 001063020 on Porter Lane in January 2006 to evaluate the slope stability of the steep shoreline bluff on the northwest side of the subject property. The purpose of the site visit was to assess the slope and shoreline and determine an appropriate minimum setback distance for the construction of a home on the site as well as other development recommendations. Based on my assessment of the geology and geologic processes at the subject property a home sited on the property should be set backat least 100 feet from the top of the steep bluff slope. SCOPE OF SERVICES The scope of our services included the following: 1) Conducted a site visit to visually inspect the subject property including the bluff face slope conditions, shoreline conditions, and relevant conditions in the vicinity ofthe property. 2) Observed surface soil conditions on the bluff face and on the uplands above the bluff by excavating shallow hand dug test pits and observing test pits excavated for septic drain field evaluation. 3) Prepared this report summarizing our findings, including an evaluation of the feasibility of building a residence on the subject property, an evaluation of the shoreline bluff stability, recommendations for site development, and recommendations for further investigation, if necessary. May 22, 2006 Tax Parcel 001063020, Porter Lane Geology Assessment 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 deeply eroded. The Surficial Geologic Map of the Port Townsend 30- by 60-Minute Quadrangle, Puget Sound Region, Washington (Pessl, Dethier, Booth and Minard, 1989) indicates the upland portion ofthe subject property is underlain by glacial till from the Vashon-age ice sheet. The mapping indicates that the shoreline bluff slope on the subject property is underlain by undifferentiated non-glacial and glacial sediments. The Geologic Map of Northeastern Jefferson County (Gayer, 1976) indicates that the top of the shoreline bluff is underlain Vashon ablation till. The shoreline slopes are mapped as consisting of pre- V ashon stratified deposits. In addition, Gayer (1976) provides a profile of the stratigraphy of the bluffto the southeast of the subject property. The stratigraphic profile from the top of the bluff to the shoreline is Vashon till, Kitsap Formation, Possession Till, Whidbey Formation and Double Bluff Formation. Site observations on the subject property, bluff face, and in the vicinity are generally consistent with the above-described mapping. However, the upland portion ofthe property is underlain by compact sand and gravel stratified sediments not the glacial till indicated by the mapping. The shoreline bluff slope on the subject property is primarily underlain stratified sand and gravel with very minor silt. My site observations are consistent with these units being advance outwash near the upper part of the bluff and interglacial alluvial deposits in a river environment on the rest of the slope. A glacial till unit is present approximately half way down the slope to the southwest of the subject property, but was not observed on the subject property itself or on the shoreline slope to the northeast of the subject property. Glacial till is present at the base ofthe slope at the subject property and along portions ofthe shoreline southwest and northeast of the subject property. SPECIFIC SITE OBSERVATIONS The subject property consists of a gently sloping upland area bounded on the northwest by a very steep shoreline bluff that is approximately 145 feet high. The upland area of the property slopes gently towards the top of the steep shoreline bluff. The top edge of the bluff is very abrupt and very steep. The overall bluff slope on the subject property is approximately 45 degrees with steeper areas particularly on the upper part and lower part with some slopes of approximately 35 degrees on the mid sections of the slope. The upland area is underlain by sand and gravel. Most of the upland is tree-covered with Douglas fir and an understory of fern and salal. The steep shoreline slope is covered with a mix of alders and Douglas fir with grasses and other brush within the understory. A few areas of bare soil are Stratum Group File: 5.3.06F 2 May 22, 2006 Tax Parcel 001063020, Porter Lane Geology Assessment n~'~;\\"""'()~-'" r -..--, "'") f ~ ~ i ,J ___~~,., ~ : ,J r --""-'''''.""__.''1 . .: , !: I JUN 1 2 2006 ! !_- ~ i, present on steeper slopes on the uppermost and lowermost portions of the slope. The beach at the base of the bluff consists of a cobble beach with areas of sand and a few scattered boulders. At the time of my site visit, which coincided with a fairly high tide, waves were reaching the base of the bluff at a few places. It has been my observations that waves reach the toe of the slope along this section of shoreline on a fairly regular basis during periods of higher tides. Wave action on the lower part of the bluff has been causing the bluff to become over steep triggering periodic landslides. Shallow soil landslides are evident all along this section of shoreline. Where the bluff is underlain by more cohesive sediments to the northeast, the bluff is very nearly vertical. The predominance of sand and gravel units with little silt and clay and hence lower overall cohesion has led to the shoreline near the subject property sloping less steeply than the vertical shore to the northeast. The geologic units underlying the bluff at the subject property consist of very compact sand and gravel from the top of the bluff to the approximately 10 feet above the shoreline. The lower most 10 feet of the bluff is underlain by glacial till consisting of sandy silt with cobbles and boulders. A till unit may be present along the mid section of the bluff as till was observed on the midsection of the bluff slope to the northeast and southwest; however, the till was not directly observed on the subject property. No seeps or springs were observed anywhere on the bluff slope on the subject property. A rotational failure has taken place in the past on the shoreline bluff southwest of the subject property. Although this slide is not on the subject property, I spent some time evaluating the slide because of its proximity to the subject property. The lower third of the shoreline southwest of the subject property is underlain by a uniform sand unit. Immediately above this sand unit is a layer of glacial till. It appears that the sand unit was oversteepened due to shoreline erosion and subsequently failed in a rotational collapse. This rotational collapse caused the overlying units to collapse as cohesive blocks onto the failed slope below. Hence, there is a fairly distinct bench at the top of the slump. As noted above, the same conditions are not present on the subject property as the soils on the subject property consist of alternating sand and gravel units instead of a massive sand unit. There are minor seeps of water in a few places on the slope southwest of the subject property, but otherwise no springs or seeps are evident on the slope despite the fairly heavy rainfall that had impacted the area the over the previous month. Heavy rainfall may playa role in slope failures but the main driving force of the rotational slide area appears to be the shoreline erosion. Stratum Group File: 5,3.06F 3 May 22, 2006 Tax Parcel 001063020, Porter Lane Geology Assessment j'~"''''r:;'';~-' rcc.~.-,..~.<----':j--'~'--I \., . I', ' . ' , 1 JUN 1 2 2006 \ i 'I ASSESSMENT OF GEOLOGIC RISK Observations along the shore and the top of the bluff indicate that the primary cause of slope instability is the undermining of the base of the bluff by wave action. As.wave action erodes the base ofthe bluffthe lower slopes fail and the failed material withthe exception of boulders is readily washed away by the large waves that periodically hit the shoreline. As the base of the bluff fails and the bluff becomes over steep the slope failures progressively move up the slope until the uppermost part of the slope fails as well and the process repeats itself. This stretch of shoreline bluff is in various stages of this process. Most sections of the shoreline in this area have very little or no landslide debris along the base of the bluff. Overall the bluff is very steep and erosion at the toe of the bluff should be expected to continue, As the very steep slope weathers shallow slab type failures are likely to occur. Based on my observations along the top of the bluff at the subject property and all along the top of the bluff northeast and southwest of the bluff, it is my opinion that these slab type failures will be on the order of a 5 feet. Erosion rates along the eastern end of the Strait of Juan de Fuca are estimated to be on the order of 6 inches per year. Because of the protection afforded by Protection Island and the height of the bluff (the higher bluff height means a greater volume of soil would need to be eroded), I estimate that erosion rates along this section of coast to be somewhat less - on the order of 4 inches per year. The top ofthe bluff will not systematically retreat 4 inches per year, but the bluff will progressively get steeper and mid slope failures will take place and eventually another failure will take place at the top of the bluff. As noted above the size of top of bluff failures should be expected to be on the order of 5 feet. The recurrence interval of top of bluff failures that send material to the base of the bluff appears to be on the order of25 to 30 years based on the age of tree stands along the steep shoreline slopes. This estimate is complicated by a several factors: 1) the bluff height varies, 2) clearing of trees on the slope and at the top ofthe slope varies, 3) drainage and slope management on the developed properties and undeveloped properties likely has varied, and 4) variability in the underlying geologic units causes some slopes to have more frequent small slides and other slopes to have larger less frequent failures. Stratum Group File: 5.3,06F 4 r'--~'._-~"'<-' May 22, 2006 Tax Parcel 001063020, Porter Lane Geology Assessment JUN 1 2 2006 CONCLUSION AND RECOMMENDATIONS I recommend that all occupancy structures sited on the property should be set back at least 100 feet from the top of the steep shoreline bluff. The septic tanks and drain field should be set back at least 100 feet from the top of the steep shoreline slope. Decks may be closer as long as they are not structurally attached to the home. The setback of100 feet would allow for an erosion rate of 1 foot per year. This rate is substantially greater than the estimated rate of 4 inches per year. The setback of 100 feet will also provide protection of the building site in the remote event a sand unit on the bluff face rotationally fails in a similar manner to the failure observed on the bluff slope southwest of the subject property. I am of the opinion that due to the differences in the soil units on the slope this risk is very small. Although this setback distance is very conservative, it should be kept in mind that stabilizing erosion at the base of the bluff is not practical given the size of the waves, frequency of toe erosion and the need to coordinate shoreline works with multiple property owners. As describe above most of the factors impacting the bluff stability over time are beyond the control of a property owner. However, some actions can be taken to reduce the frequency of slope failures. Based on site observations and my assessment of the geologic risk, I make the following recommendations: . Native vegetation, particularly tress and low native brush on the slope should be disturbed as little as possible. No grading should be performed on the bluff face. The existing trail on the bluff face is not causing any slope problems. If a failure removes this trail and a new trail is constructed, the trail layout and support structures should be approved by a qualified engineering geologist before construction. . Waste and other deleterious material should not be placed on or over the slope. Such material smothers vegetation, retains water and acts to destabilize the slope. . Storm water from roof run-off and driveway run-off should be dispersed as much as possible through allowing sheet run off and lateral dispersion spreaders such that storm water is not concentrated anywhere on the property and onto the bluff face. File: 5.3.06F Stratum Group 5 May 22, 2006 Tax Parcel 001063020, Porter Lane Geology Assessment I' ' I JUN 1 2 2006 As noted above the stopping erosion and slope failures at the site is not feasible. Any shoreline works on this stretch of shoreline should be reported to Jefferson County and Ecology officials. Any reduction of erosion on properties to the southwest of the subject property would have a negative impact on the shoreline of the subject property. Except for a short section of rock rip rap at the low bluff section of Cape George, there are no armored sections along this section of shoreline. Stratum Group appreciates 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 fJovciltlL Dan McShane, L.E.G., M.Sc. Licensed Engineering Geologist Stratum Group File: 5.3.06F 6 .' . ~ j . 122.883330 W Agure 1. Site Vidntty Map 122.866670 W 122.850000 W WGS84 122.833330 W z z ~ . ffi .. ~~JC ; ~~~'1....2: v ~~7~~ '..i.~~/('N'~ i 'k~: ~'=~fVA1f.;"...".. :) ,{.t ~ M. .......\,.. /... .' '.,1 ...:.".~ /' ... .' '.. .... ,~.....~ " / ..'1/'-""- ... ...~;;;"..~.~..... / ,/ , ",...... ..j" ~lf'; 1''' "/( +a ~~/ /~ ~.,....,. .." ~97. I 1 IrJ!:b, ' ~:':R)f z ,// /~"" SUBJECT PROPERTY V" '7~~~ ..,.p:~: k ~ ,,/' \.{ / 4 /~l ....- I L.I. "'.....~ '..\\ ~ ~ //~/~ //, ~t.&i I -1, ;/ x, \f " ):t\ ..~ ; / I ~'-- ,6 ,... · ii. .....;~... / 91' /./ II ~... ...... ~~ t '.'J'. I '..:. .'. . / ~ or /",.iJ! ,........., ~ J ~ / /."--'//~:>'::-'~ .r---~ rr9" i"~~ ' ' .,.... re..... ...... ;' ,,;> ,/ ...---' /'/. '.:': ~".... ".. t / //./ /"""~,:,:'::;";; ~. *~ .' I " .., .."-<-;.. ! ". / /..'.'..,.... '. I '. I I / /-' / ,;;(:~/:', ~ ... '" I ... ! .,../. .'./.....f.......,.....'(.'..;..'.. .. \ .l'. . ...... .._......1If I :..... I ,I (./ ;'#:.,~ / ,~'\: r r-. -:""- J...... ..., ..': I i"I?)f llJ/I'.t,. ~. ~I,~I. \~ :, .:...........\.j ')1 ..".!.:"',. 'Yl ~.'..I. ~~~ 0 --.Q.<ilr.... " , \.\ .i........::;......'..."j:;..;.r....""..'.... .... :--!t...:. .' ~. q.,J.' .....l. ......._! l .,.. \.\ ~.\ /:i(~' :,: ~\f?- ',' .>'. ~" .;- ~ · '." ..V ~.2z"':"'),\'1' 1") .,..~\.t>) ~........'... Geo~" \,~, .}:;)". "~ ,Mfl>~N "! >A ~ri"'~' :,..J F" '':',~C ....."'.. . ....~ ,,,,-~' .'c".. ji., " Z ../....\\.;,~"'i ~~'".:;:t~-'5', . . " ~~ GrMIf'It . . u.' 'l "..~--:,~ z g \<'-'\~"'~\ I." .. b.~ ~ -'- .... ~,oo~ . u ..~~~t.. \~. .~~. \.'l~ g g \\' . . ~~\ '. ~ ~1~\.~.~ ' ..::- -=:- ^L~~~~~~~~:'l\.<' - g .... \,.\. :'. >"'~.' <, ~ \ "'\ 'u .... LaMina, :~, ~'"'-""'".''\. .>---:.c. ........;r .... CD \ \ ~,,: ~;. \' ... S; StftP::" -:. '" '. ..... - CD V \\....\...,.:~~~1).~;.....:...'5~.~..)..'....\..'~.~ ~:'\\ \ 0\..1..~.' ::< 'J .',;1,,,;.. r;;: v \\ ~. ".~:.I/ tr.) ~,..<~,~\\~I\ . i). "\ l \ . ":,;~t .. J..)t ,\\\,\ 11 - " - i ~ ...,.f!. /' ~ 1 r Iii. ~ff.j{fJ r~ 122.883330 W 122.866670 W 122.850000 W j ~+~8s~W, I o ('t') ('t') ('t') ('t') .... CD V z o I"- W W .... .... CD V TNj I.MN V 18%0 o .5 "'AIlE L _ lID) fEET 9 _ ~ _ ---1000 MmllS Map CJe8led with TOPOI(il@2003NationalGeographic(www.nationalg8ogmphic.comltopo) i I ! r JUN 1 2 2006 '< ,'----..,...--.-.- ~---- .. . N '^ It--'/ ,,07 Y . /, / ;/ ,/' /' / ,/''' ./ /' /' /" Daniel McSh'anelj -~:'~!,'l!oii 5co,)t CA",J lo~oA';o"j Ot1f,"Oi"iYh-rle. I ,,,e,L:: loa ..{.~e+ ... NO.: -lIY:lJII r.....'1'..r..rrrr.~....tSt t '::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::.; ra u "".",...,.,.,....."....,",. G ':';':':';':':':':':':':-:':;':':';':':':':':':':':':':':':;:':" roup Figure 2. Site PI an Sketch ,/ \~~~ ,'.--5 II1.i.....'~~.! - "~I ~ z: I~ ~ \ I~~.-~ I (C:\' L~~ N