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Home My WebLink About921191019 Geotech AssessmentH i NORTHWESTERN TERRITORIES, INC. A JLS GROUP COMPANY 717 SOUTH PEABODY STREET, PORT ANGELES, WA 98382 Engineers Land Surveyors Geologists /VTR (360) 452-849D1n FAX 452-8498 ewww nti4u. matE-Mas n~otQ ~i4u com JLS GROUP, INC. Geotechnical Report Tax # 921191019 May 8,-2006 "~ ... ~Y,~r. t ...,rwa d. SeaHome Services, Inc. Attn: Greg Barron 7425 Thorndyke Rd. Quilcene, WA 98376 Subject: Geotechnical Report for Tax # 921191019 located in Section "19, Township 29 North, Range 1 East, W.M., Jefferson County, WA Dear Mr. Barron: Background At your request, Bill Payton, Engineering Geologist with NTI .Engineering. and Surveying (NTI) conducted a bluff stability inspection at the above referenced location on April 19,2006. The purpose of this inspection was to examine the marine bluff at the subject site by visual means and provide an opinion regarding the relative impact that a proposed beach access stairway would have on the stability of the bluff. It is our understanding that you desire to construct a stair tower on the beach with a bridge leading from. the top of the bluff to the tower as approximately shown in the preliminary drawing (Figure 1). We further understand that the stair system will be founded on piers and that the toe of the bluff will not be excavated out in order to construct the stairs. And further, that bluff disturbance will be limited to the immediate vicinity of the stair location and will not encroach onto adjacent properties, and that disturbed areas on the bluff will be restored upon completion of the stairs. Site Description The subject waterfront property is located at 3133 Oak Bay Road, south of Port HadlocK. The property overlooks Puget Sound to the east and is bounded on the north and south by residential property (Figures 2, 3 and 4). Existing improvements to the property include a gravel driveway, garage and log home which sits approximately 85' (75' for the deck) back from the top of the bluff (Photo 1). Drainage for the existing homesite arrears to be controlled with catch basins in the driveway and roef~~uf~rs--with - . ~._- ~~- downspouts conveyed to tightline drains over the bluff. The upland portion of the property slopes gradually to the east and is mixed wooded and open. A ditch cuts across the middle of the property in a north-south direction and directs water from the south .through the property and eventually to the beach via tightline drain on the property north of the subject property. Water was flowing in the ditch at the time of the site visit. The bluff at the property is approximately 60 feet high. Slope angles are variable but generally range from about 40° to 55°, with the lower third of the bluff at about 70°. The upper and mid bluff areas are predominantly vegetated with young alders and brush, with a few larger trees (Photo 2). Some of the trees on the bluff have curved trunks, indicating that shallow down slope creep of the surface soils is occurring. The steeper lower sections of the bluff are predominantly void of vegetation due to wave erosion during the recent winter storms (Photo 3). Some areas of the bluff were moist from groundwater seeps. Site Geology The Washington State Department of Ecology's Coastal Zone Atlas maps the upland and upper bluff soils as Vashon lodgment till (Qvt1), and the lower bluff as Tertiary shale and mudstone (Tg) (See Appendix). The till is described as consisting mostly a compact mixture of boulder to sand size particles with some silt and clay. The Atlas lists this soil as excellent for foundation stability, good for seismic stability, and says that that it stands in steep natural and/or cut slopes for long periods. The Tertiary shale and mudstone is described as interbedded siltstone and sandstone. The Atlas maps the stability of the upland as Stable and the bluff as Intermediate. The Department of Ecology's "Geology and Ground-Water Resources of Eastern Jefferson County, Washington" maps the soils in the area of the subject property essentially the same as the Coastal Zone Atlas. This publication further states that these units are impermeable. According to the Soil Survey of Jefferson County Area, Washington (United States Department of Agriculture, 1975), the subject property is in an area mapped as Alderwood gravelly loam (AmD) and Cassolary sandy loam (CfD). The Alderwood soil formed in glacial till and is composed of silty sand or silty gravel. The Cassolary soil formed in reworked glacial and marine sediments and is composed of stratified silty sand with a layer of clay: Visual. observations of surface soils on the bluff face are generally- consistent with the above descriptions, including siltstone and sandstone (Photo 4) and very compact silt. and glacial till (Photo 5). The bluff soil and rock at the site is very dense. As the bluff surface is weathered and softened, vegetation can begin to take root and grow, except in areas where waves frequently contact the toe of the bluff. As the vegetation grows, its roots further loosen the surface layer of soil, typically a foot or so thick depending on the size of vegetation. When a slide occurs, triggered by winter storm waves eroding the lower bluff in this case, and compounded by the saturation of the soil from winter rains, this thin surface layer of soil that has been loosened by roots is typically what slides down taking the vegetation with it. The resulting bare slope has a visually dramatic appearance but the now exposed underlying soil is usually left relatively undisturbed and the process begins all over again, resulting in a typically slow landward retreat of the bluff. 2 Conclusions and Recommendations Based upon our understanding of the project and our site investigation, we conclude that the beach access stairs will have minimal impact on the stability of the bluff. However, the natural processes of bluff retreat are occurring at the subject property and periodic sloughing of the bluff should be expected to continue which may necessitate future maintenance/repair of the stair system. As mentioned earlier, it appears that drainage for the existing homesite has been addressed previously. The following recommendations should be considered with regards to the proposal: 1. It will be necessary to maintain ground cover in order to reduce erosion from surface runoff. Any bare areas that develop, on the bluff or on the upland, should be revegetated if possible. Native deep-rooted low growing vegetation that requires little or no irrigation would be the most beneficial. Please consult the enclosed publications for further information. It should be noted however that vegetation establishment in the very compact soil and rock on the bluff may not be possible. 2. Vegetation on the bluff face provides stabilization to the bluff face soils and helps remove water from the soil. Existing established vegetation should be left in as natural state as possible. Once construction of the stair system is completed, any areas that were disturbed should be revegetated as needed. As far as pruning for a better view, minor thinning and pruning should be done in such a way that minimizes disturbance to the soil and root zone and that insures the. continued health of the vegetation. 3. Tree limbs, lawn clippings, etc. should not be thrown over the bluff because these piles have a tendency to damage/kill the underlying vegetation which in turn increases the erosion potential of the bluff soils. 4. Heavy irrigation or other activities that would contribute large quantities of water to the soil should be avoided. 5. Surface runoff from hard surfaces such as roofs, driveways, walkways and patios appears to have been controlled already. The existing, and any future drainage control devices need to function such that surface water discharge to adjacent properties does not significantly exceed predevelopment conditions. We recommend that on site infiltration of this water not be considered for this site due to the impervious nature of the soil 6. Silt fences or other sediment control devices may be needed on the top of the bluff during construction such that sedimentation to adjacent properties does not significantly exceed predevelopment conditions. 7. Existing, and any new drainage control devices should be maintained in good working order and inspected at least once a year. For further information please review the three publications (included with the original of this report) published by the Washington State Department of Ecology (DOE) entitled: °'~lope Stabilization and Erosion Control Using Vegetation", "Vegetation Management:, A`Guide for Puget Sound Bluff Property Owners" and "Surface Water and Groundwater on Coasfat Bluffs". 3 These publications can also be viewed on the DOE website at: http://www.ecY.wa.gov/biblio/sea.html under the 1993 and 1994 year heading. The DOE website also contains additional useful information regarding slope stability and site development; this reference is highly recommended. Limitations This report has been prepared for your exclusive use in conjunction with. the above referenced project. The report has not been prepared for use by others or for other locations. It may be used for other purposes only with the. expressed written permission of the Engineer. Within the limits of scope, schedule and budget, this report was prepared in general accordance with accepted professional engineering and geological principles and practices in this or similar localities at the time the report was prepared. No other warranty, expressed or implied, is made as to the conclusions and professional advice included in this report. The observations, conclusions and recommendations presented in this report were based on our visual observations of the subject property at the time of our site visit; no laboratory tests were performed. Soil. and geologic conditions can vary significantly between test holes and/or surface outcrops. If there is a substantial lapse of time, conditions at the site have changed or appear different than. those described in this report, we should be contacted and retained to evaluate the changed conditions and make modifications to our report if necessary. Sincerely, NORTHWESTERN TERRITORIES, INC. Robert A. Leach, P.E., MBA Principal Engineer t, Bill Payton, L.E.G. Engineering Geologist G:1Gen\Bill\Reports\SEAH0603.b1uff stability.l9(29-1 E).Port Hadlock.doc S-(l-D(o ~,,~~' ~a ~~ ~• , K" ~ ln4heark-g, Q ~ „5~1 i ~ dE, ''c~~, 191 ~~ William C Payton Jr, 4 EXPIRES 12/30/2006 27'-7» " BANK 8' RISE 12' RUN 4, _ ~„ TOP EDGE ^F LANDING O I 0 I O O C7 [V to 35'-0" ORDINARY HIGH WATER 6' - 0" c T -~-, t _ ~ ~. SCALE I/8 ifv = 1 r ^GT PRELIMINARY Figure 1 Provided by SeaHome Services n: e ~_... ~_ _ ,... _. -+ ,. ~,, r ~~; ~~ ~ ; ~r: ~_' ~ ~ _ ~ ~~ ~ ~ ~r ~g ~r ~~ ~±~; ~ ~ i~: 3 0~ ~~~~ ~~~ ~ _ .. l~" _ _ .. ~ s 1 ~ J P- ~ `a '' «r LL, Subject : ~ ~ ~~i ~, Property .~*y ~'~ ~ Figure 3 AAxr_n_ xandnd Ur .c-~_•~i~urc: Cn'~: a .3rs•.~c+~s GtS 0 - 262fA ~;, ~.,. ry„ ~ ,:_ .tit .. ~., - ~,~ ~~ - '} ~,x'r ~+ ~ . . ~ i - - ~_~,~~ s Y b' _ ,~~ p.ci ~l i~4 f ~ ... t »~ - _ _ - ~... 1 s--^ ~ ~ n ~. a o--- •_----~ R.`.,. . ~ ~ _ Photo ~3 -. - . t ,~;, ~k";1~ 3 ~ 2~(~6 `I V A r1 ~t _ ~'/Qva ,; ~_ i( .+ .; t =~ ~~ ,\ ~.~ ~ Approximate Location ' ~ of Subject Property Tg Qvt7 45 Qvt1 f 35' Qva ~~ '~ t i_~Qva c, .~ Tg `Coastal Zone Atlas of W Qvti Qva Qpf 40' ~s~iing~on ~ 'r N S~ ~,, ~., w. , f M ~ t':: y _. ~ ~ ~ ~ F6' p~f ~ MAY 3 0 2006 t ,a... ~.~ _ .. .} ^ r, i , Protection ~ Island ~ Port To send (~ ` --- ' wort ~,I 1 Towns~~ld ..... i'J4i~c~lr8f48d4r I~Q ~ . ~~ Q a J v _ tat~~fona Forest _ ,TO) ~' ~ ~ Port Hadloc .1~~ ~, ~1tDU i WASHMGI~O ? I Olscovery Bay- ~, , J E; F F E R S O:~N ~~~ ~Q7 ~ _--tcr- ~G...^, ~...... .. . ~. Calnall4 ~ ~ i c}~ Island ~» ~ -- -oiGreenbank 1 -~yx ~ fort Flagler ~ „~ ~~ State Park ~ I _ ~ ~i _ ~ °s . ~ s6 ~ Indian ~ ~ ~' . Island ~ d Nordlan ~ ° 1; Whidbey = ~ ~~ uy 525i Island iy ~ ISLAND ~ , Print Rd l- ~? 'd Free~ ., --- and _..- ' ~~ I m Property Vicinity -l '~ Port Ludlouh° = .. J ~.Hansville _ ~ :_ ~ > ~+ I r ~ ) MTSAA ,' w I f , u~ it -- ---- , GEOTECHNICAL REPORT Prepared For SeaHome Services, Inc. May 8, 2006 For the Property Described as 3133 Oak Bay Road Tax # 921191019 Section 19, Township 29 North, Range 1 East, W.M. Jefferson County, Washington Prepared by NTI Engineering and Surveying 717 S. Peabody Street Port Angeles, Washington 98362 Phone 360-452-8491 Fax 360-452-8498 Web Site www.nti4u.com E-mail info@nti4u.com