Loading...
HomeMy WebLinkAbout001193011 Geotech Assessment '. GEOTECHNICAL REPORT 1 l' Prepared For Rick Seaborne March 31, 2006 , i..,." For the Property Described as Tax Parcel # 001193011 ( . : ~ -~~-~--- Section 19, Township 30 North, Range 1 West, W.M. Jefferson County, Washington Prepared by NORTHWESTERN TERRITORIES, INC. 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 0..... - ., , NTI NORTHWESTERN TERRITORIES, INC. AJLSGROUPCOMPANY JLS GROUP, INC. 717 SOUTH PEABODY STREET, PORT ANGELES, WA 98362 Engineers Land Surveyors Geologists Construclfon Inspection Materials Teslfng (360) 452-8491 FAX 452-8498 www.ntl4u.com E-Mail: Info@ntl4u.com Geotechnical Report Tax Parcel # 001193011 :..,._..~.,,# March 31, 2006 Rick Seaborne 9670 Battle Point Dr NE Bainbridge Island, WA 98110-1485 Subject: Geotechnical Report for Tax Parcel # 001193011 located in Section 19, Township 30 North, Range 1 West, W.M" Jefferson County, WA Dear Mr, Seaborne: Background At your request, Bill Payton, Engineering Geologist with Northwestern Territories Inc, conducted a geotechnical inspection at the above-mentioned propertyon February 9, 2006, The purpose of this inspection was to observe the bluff at the subject site 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 own the subject property and plan to build a single-family residence, detached garage and an alternate dwelling unit (ADU) on the property (Figure 1). Site Description The subject property is located on Cape George Road across from driveway # 5663, which is south of Kruse Ave. overlooking Discovery Bay (Figure 2), The property is bounded on the north by Cape George Road, and on the south, east and west by wooded residential property (Figure 3). The north "upland" end ofthe property, where the proposed development is planned, is flat to gently sloping towards the south. The east side of the property is a spur that slopes southward at an average slope of about 9 degrees. There is a ravine along the east property line and another ravine that bisects the middle of the property (Figure 4). The east ravine is approximately 60' to 70' deep with an average slope of -24 degrees, with -30 degree slopes at the top. The central ravine varies from approximately 30' deep at the head to approximately 50' farther down, with slopes of - 30 degrees. These 1 slopes are below the typical "angle of repose" of the soil. The "angle of repose" is defined as the maximum angle at which loose, cohesion/ess material remains stable (assuming no destabilizing forces). This angle commonly ranges between 33 and 37 degrees. There is a "Type 5 Stream" located in the bottom of the east ravine according to Jefferson County. However, at the time of the site visit, the soil was damp but there was no water in the ravine. The bottom of the ravine was thickly vegetated with predominantly salal and sword fern and there was no defined channel. The central ravine was dry. The upland is mostly cleared and vegetated with grass, and there is a small shed in the clearing (Photos 1 and 2). The slopes and ravines are well vegetated in young to mature trees, shrubs and brush except for a small cleared part of the slope west of the central ravine which has been replanted with trees (Photo 3). No slide activity was observed in the ravines and no springs or seeps were observed. Site Geology The Washington State Department of Ecology's Coastal Zone Atlas maps the soil in the area of the subject property as Vashon lodgement till (Ovt1) over Vashon advance outwash (Ova) (Figure 5). These soils are generally composed of sand and gravel and are glacially derived. The upper till has more silt and is generally more compact than the lower sandy soil. This upper till is also capable of standing at steeper angles than the lower sand, The Atlas also maps the stability of the upland as Stable and the slopes as Unstable (U) (Figure 6). The Washington State Department of Ecology's "Geology and Ground-Water Resources of Eastern Jefferson County, Washington", April, 1981 gives essentially the same description of the soils in the area of the subject property. The Soil Survey of Jefferson County Area, Washington (United States Departmentof Agriculture, 1975), classifies the soil. in the area of the property as Tukey gravelly loam (TuG), found in the upper north end of the property, and the Tukey gravelly loam (TuD) found on the rest of the property. This gravelly soil formed onterraces in glacial till and has a very slowly permeable cemented layer at a depth of 20 to 40 inches, which can cause a perched water table during the rainy season. Runoff is listed as slow to medium and the hazard of erosion as slight to moderate. Visual observations made of surface exposures at the subject site were generally consistent with the above soil descriptions. ~-'--~;- ~- --ij~'--'-- .----- ~, .. ----i \ i ( i ',~' ~- 2 Conclusions and Recommendations No ongoing or recent sliding was observed on the slopes below the proposed development area. The measured slope angles are below the angle of repose, and no springs or seeps were observed on the slopes. Based upon our investigation, we recommend that the landslide hazard area buffer be reduced to 15' from the top of the slope bordering the flat area where the proposed development is planned. We also recommend that the International Building Code (IBC) guidelines for footings adjacent to slopes (Figure 7) be followed. Figure 1 shows the approximate location of the IBC footing setback, which varies from 15' to 23' based upon the slope height. The IBC allows for structures to be closer to the slope if the footings are deepened to maintain the required separation from the footing to the face of the slope. The necessary depth can be calculated by using the IBe Figu're (Figure 7) and the slope height, which, can be determined by measurement or by using the topographic map (Figure 4). However, the IBC setback should not be reduced to less than the 15' landslide hazard buffer. Further, we recommend that septic drainfields not be constructed less than 30' from the edge of the slope. The proposed location of the drainfield for the primary house as shown on Figure 1 is acceptable from a geotechnical perspective. We recommend that the drainfield for the ADU not be located east of the ADU as shown on Figure 1, A better location from a geotechnical perspective would be north or west of the ADU, thus avoiding the ravine on the east side of the property, The following recommendations should also be considered with regards to the proposal: 1. It will be necessary to maintain ground cover to reduce erosion from surface runoff. Any bare areas that develop should be revegetated. Native deep-rooted vegetation that requires little or no irrigation would be the most beneficial. 2, Vegetation on the slope face and within the 15' landslide hazard area buffer at the top of the slope provides stabilization to the slope soils. Existing established vegetation should be left in as natural a state as possible. If a better vieW is desired, minor thinning and pruning should be done in such a way that minimizes disturbance to the soil and rootzone and that insures the continued health of thevegetation. Damaged trees or trees thar are .in dangerof falling, due to undermining of the roots for example, should be evaluated for removal becauseof the damage potential to the slope caused by the root mass of a large tree being pulled out of the ground. If trees are removed, th!3 trunks should be left in place so that the root mass can continue to provide stabilization to the soil. A tree expert could provide valuable consultation in this matter should the need arise. . 3. Heavy irrigation or other activities that would contribute large quantities of water_,__ ... to the soil should be avoided. .. :.:,.'."-- \ . ~ -~.~ ---.-1 3 4. Surface water should not be allowed to flow over the face of the slope as an uncontrolled or concentrated flow and cause erosion of the slope face. This can be controlled with vegetation and using berms or swales to direct runoff to a drainage system. Please see the attached DOE publications for more information on this subject. 5. Surface runoff from hard surfaces such as roofs, driveways, walkways and patios should be controlled and routed to a drainage control system such that surface water discharge to adjacent pro'perties does not exceed predevelopment conditions. 6. Silt fences or other sediment control devices may be needed during construction such that sedimentation to adjacent properties does not significantly exceed predevelopment conditions. 7. All drainage control devices should be maintained in good working order and inspected at least once a year. 8. An engineered drainage, sediment and erosion control plan should be developed for this property to address items 4 through 7 above, NTI can accomplish this task if requested, A contractor knowledgeable in the construction of these types of features should be retained for this work. 9. We recommend that footing drains be incorporated in the design of the stFuctlHes. Feoting drains should be kept separate from roof drains. Given the findings, recommendations and limitations of this report: 1. There appears to be minimal landslide hazard to the proposal as suggested by the lack of evidence of landslide activity on the slope at the subject property in the recent past, and by the recommended setback distance to the proposed . structures. 2, Observations of slope stability indicate that the proposal should not be subject to risk oflandslide under the current conditions that exist at the site. 3; The proposal would not increase surface water discharge rates or sedimentation to adjacent properties beyond predevelopmentconditions. 4, The proposal would not decrease slope stability on adjacent properties. 5. The proposal would be stable under normal geologic conditions. . '''') For further information please review the three attached copies of booklets published qy the Washington State Department of Ecology (DOE) entitled: "Slope Sta~i1ization and: ., Erosion Control Using Vegetation", "Vegetation Management: A Guide for Puget Sound Bluff Property Owners" and "Surface Water and Groundwater on Coastal' Bluffs". These \ 4 -"'~_....-- '--<~"'; I I I publications are now out of print but can be reviewed at the DOE website at: http://www.ecy.wa.gov/biblio/sea.htmlunder the 1993 and 1994 year heading. The DOE website also contains more useful information regarding slope stability and site development and 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. Others may use it 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 obseNations, conclusions and recommendations presented in this report were based on our visual obseNations 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 ther~ 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. &d Robert A. Leach, P.E., MBA Principal Engineer /EXPIRES 1213012006 4LL.~~ Bill Payton, LE.G. Engineering Geologist G:\Gen\BiU\Reports\SEAB0601.bluff Slability, 19(30-1 ).Discqvery Bay.doc ,~ ("'~ ----, I Expires 11/06/06 't- , ...:...,J \ ._- , 5 ,:V;:','UWMENT ...J \" , '"' ~ . . " m ~" + 't;,~ t1 ~ ~ ~ ~ ~ :z:, "< ~ ~ r- ~ '> c. ~ '" ~T'T\ ~z.. <: ~ ,., ~ ~ ~ c ~..~ }\~ f"- ~ ~ hi '1~ ~%. '<. \r. ~ '> ..... ~ ~ ).. " ~ ~ I ~ ti ~ D V, ~ ~ ~ ~ .9'~ ~ '\ ~ OJ · I :b~! ! ~ ~ I ~ 1=;J ' ~ ~ ~,~ .~~ ...... -r ~ i V'b .,t ~D: '\ \.4 _ . ..... ~. \);l L-l'l"~ ~ -j ~ ~ ~ ~ "'II '):... ft\ ~ '- ~ ~ ~ 'i ~ Cl ~ ~ ~ l 'I~ ~~ ~ - I~ mlJ\ ~ .~ C7~ ~~ ~p <:) ~;g ~O V ~rn 03 ~, \J~ - ~ ~~ I o C> ~ 0 -D \J'I o ..... I ~ , .)'>.1: --I ------ ,/ _/ ------ \ ~ ~ \ ~ ~ \ \ ------ .-/ / / \ \ ./ ------ ./ ./ \\ \ ~~~~ .----.-/ /./--1, \~ L_;. .-/ ./ . \ ~ ---- 'r- _ ---- ,-- '\ \ ~ ~\ I ~ ~ ~/./ \ > '-'--- Q /.----/./ \ - ----- ----r ~ \~ ~~~~ ~'-"--~~~~~ , \ --, ,,-. \ !l "----1---1 ( I I I I 1 I ----11 0_. 0l(Q ~. C :J -, COco "0 o~ < a: CD 0. 0- '< Q.. (ii' ~ - O-l I !l~ ON O~ 6{,~~ ~ J !~ ------ '" i: ~"" \ ~t , -13" :::r0 CD ..., W ~=O'g- '-" 9. ..., "T1 OJ (J) (J) S' 6" o :J 6i 6ico Vl Oa.:J:JVl;:;: ::!:O 0.0.:E CD :J :J Ol Ol 0 en ce -0. .., a. c: q- ~, ::r - c :ECDO'O'o.Q. O:E09.~c c CD ::!: _. < ..., 0: !e. c5 c5 CD g: ~ !!? 0. g- 0 ~ ~g-{g~g:T '-+0 :T::T" CD O.....enen ~ 0-:T0l0l ~ CDCD@@ or ~)>CllCll :J 0.00.0. 0. CD CCD CD !!!.. CD Cll-g-g Ci ~,@ Cll ~ CD :JOlCOCD ~ :T s' S' N CD __ Q) N ::r::r a. W CD CD 0- ~ -'--'- C o W-...j 4: 9. ..........0 CD 00"" ~.-+ tl tl :J :J CD CD ~~ :T=r cp CD 0."0 CD ::!. en '-+3 a: ~Cll CD ~..:2 a o..::r :T coO CD Cll C ..., en )> OlCD o COOl C CD..., Cll CD ..., Cll CD Ol N o :J CD ~ s: CD CD Cll !e. UJQ ~ A rN .~ ~~ .~ "11, ~ d~ ~ if' I I ~ * 1 J I I I '/ I I ~I ~ "1 I I I I I I I I I I I ~ ~ "-" -':-j "- ~ ~ o ~ .....e:::. ~ ';J \; ~ - " ~t ~ m?< - ~ ~ ~l: \( rr\ 0 '-J c- d":,- _~~ a ~ ~':::--(; M. '~r * :"'\ _ :0;1- ~ ~ i~/ 1f"~ ~ g. ~ \~ i \~ ) ~, ""~ , ~ -.t ",.. J <>~ '" /...\ 1~. " //~ ....<:..; r .---_.!i ~, w : ~ ~ ~/ )> ~ .t" /( g '> ..~ "rt1 \ .." r- ;! --- " I), ~ .r---- a- ", l::: i;t ~~))o I'. --rc. .... r:~ ~ ' . : -.2,,\~U' ...,...~ , '11- ~ "',. ~ ^ - !" y~ /~ G ~ . , ~ / 1 .; .~ )L g -I (J) CD .-+ e- OJ (') '" c: ::J CD , ~ g .J i GB, I p. ~ ~ ~ o .lfl ~ ~ 2 ~ t 1 ). ~ f .(/) -f ::7:l 5 $ (I) )!( ~: i I I I I I I I I I I I I / J I I I I I ! I I I i I 1 ! i I I I I ~. Ij) ~ I I , _.;I- , ~c- 1 '"':.---------~--__D___ ~------- ---t .'l _ /, POCKET LN- I A~c~:r . . . 1== ,j .' '. '.- ))"12.'1 I .,~~l:"c" _ _~~:J" j_", .~-j '-~l- . . :~=~I ~ig~re 2 t " .\C~--------jJ t~ ,... -:;,,;: < ~1LL-..-::t-- ,.-'--:; ! \i.-c".". ;n:r,,-do:i :t,/,' ..;.;fe',:'Q~1 C(J;J'-:~' ':~fTr... s.~~,".~a!:. G~S o ,_.., .e...._ ,. ,.',. 1J.... .; ..-......-"!'... ,..,~..---..."... ,. .c,____ 19 ~---:t~~_=:::' "-'~_-,._"...ii~~--'--T ---- ~-~.-'.,.-----., l C [. r.~. ---t-----.:~ ~' (j) ~_ ,-'i.,[ ))....1'0'1 ~: --c' [ >( -,~'-:' " i..~.~, ~~._-_L..-J'i: . '~::; 'I -F-.i ""-' -- '-J" ",' l~., / ----,: - -"-, ~.. ( \.,,-,# 40' / ..A '1 , ..~ ,4' '-.J 'J/.. .~~.- -~-~,,,.; 0-. ... .. ' . ,.~ /' ~",~f.':,:, n.,_ f_ ~ :6 /(' R3~:/ /:'. ~",='.,J ,: / ;I' ',i i ,/ t , .: - "'i , -l r. ".. :/ J P~1~;;,~=~-I-')17 ... .. -' .""'-'174 '~. ./" \ / . ,) - ,:'}\ / (',.' . r...! (t, l,"% .r.l', .,' , TAltY ~,\ ~\... -. /. ~ ERVA1r10N.4" -t~d.~~~:'~ I , ! i j..,.. ,:.;>: .:.: '- .p ..... .6(/" /' .... / / / "':~"''''', ......,.> '....Qp.f ~............. ...OO~ "~~: . ,~o".,......,- " /"-" ,'--' "' 'r--' '-.'/ /-'.. ( '../ "0 Figure 5 ',. ~ '. ; '-. \\: Cape Geo~;\ ..' \,\ .~ .' ~ , / ~"~':,,'<;~i~~i~/' i . si~p;~~~:.~~ty , I /" ,'\J' ,Urs, ..,:,' / '~ ",.." 1 tfos/ '/.,...: ,,/ t'\ 'r' ,/ ."'. .. , " .~,\l. " Vt:~ 0' .;?i<r:~~"t;::,/ ~r .J:lf.:>,:,"~ " ~ .' ./"" ~r~l /" " .00- / ./ \ I I ~/ / ....-.... '1- ! ,(J) i I { i I :0....... ~f;~'~~~:;>-:.. ~ U~s; / ;' ,: f / / I : ./ ,/ ,u~ ;'.' :",/ .' ,: Or$' Uo' l;in~" ~~' :' /. // /~/ :;../ _,A-""""'-O ~_~ ~ A:proXimate loc~on of subject prope .tt Beckett ..... J, \ ..~ ''''.-. 'co: " /',~ ',,-j-' /"" '\/ J ..'....1 ' . . ~ , ' <:? I ....\ r-- ~-: . 2003 International Building Code ~FACEOF ST1IUCTIJRE TOE OF SLOPE For SI: 1 fOOl ~ 3M.S DUD. AGURE 1805.3..1 FOUNDATION CLEARANCES FROM SLOPES I Figure7 ~.-J .. ~~ .~:> : ~~~~I. -,....:;.;..