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HomeMy WebLinkAbout990900016 Geotech Assessment (2003) ! ~• ~ ~,~ ~ ~ GEOT`ECMNICAL REPORT ~ ,~ ~ Q ~ ,~ r~,L.~! ®~ ®1G~ Prepared For Mark Hinkle • 1 May 14, 2003 _ • Fore PropertyLo~ted d®s~ibed as ~~~~ ~~~,,. ~~~ 1~~° ~~,°~1~~ . Lat 16 of Port Ludlow #5 Sub Division, Montgome ' h` ' ° ` 9ectlon 9, Township 28 Nortl~, Renge 1 East. W.M. Jefferson Coin, Washington • Prepared by NQRT~IWESTERN 'TERRITORIES, Iti1C. \ 717 S. P®abody Street Park Anseies, Washington 98362 Phone 3@0-4x2-8491 Fax ~6p-4526498 Web S(te www.ntiAlu.com E-masl info~ntJ4u.com Z8 3J~'d df10t~J S'it' I1N 86b8Z91y99E b8~9T E00Z/DT190 ~v rvrV !r VV•lli VV~.V t1LLV Vllll Il L1\ IVI\1\LVI 11 IL IlV VVIr ! ~ ~. .( • • ! regriv• tvvv ic•vv+ie vv~vn~~~ un+u+a.n ivnn+..v+ ++• i~. ++vvv+r. + v May 14, 2003 ~ I~~ ~~~~1~1~ Mark Hinkle ~ ~` ~`~~~ ~~ ~ ~~:~ C/o Coldwell Banker, Bill Roesler Port TownsendRWA sr33s5 ° ~~~~~~~~~~~ ~~~~~~ Sub~eot; Geotachnloal report for Lot 16 of Port Ludiow #5 Sub Division located In Secdan g, Township 28 North,l2ange 1 East, W.M., Jefferson County, WA Dear Mr. Mlnkle: Background At your request, Bill Peyton, Engineering Geologist with Northwestern Territories Inc. cxtinducted a bluff stability inspection at the above mentioned property on Apn'I 30, 2003. The purpose of this Inspection was to examine the marine bluff at the subject alts by visual means in order to determine the relative stability of the bluff and make recommendations in regards to th® proposed construc~on of a single family residence with possible basement and beach acxess trail. Site p®scrlptlon The subject watertront property Is located on Montgomery lane overiooklrig Admiralty Inlet to the east (Figure, 1, Photo 1 and 2). Th® property Is currently undeveloped except for the possibility of a municipal sewer line running across the property. The upland .portion of the property slopes eastwarcl at about 4 degrees and is vegetated prlman7y with grass end weeds with a few young to mature trees. No evidence of surface erosion nor swampy areas were visible on the upland. The bluff at the property 1s roughly 65 feat high with'an overell slaps of about Z7 to 30 degre®s. The upper X35 feet of bluff Is about 3~ to 40 degrees and the lower section is about 16 degrees then steepens to around 34 degrees near the toe (Figure Z). These slopes are gen®rally below or within the wangle of repose" which Is defined ass the maximum angle at which Iaose, coh®slonless material r®means stable (assuming no destabUlzing forces), This angle commonay ranges between 33 and 37 degrees. About half of the toe of the bluff is protected from wave erosion by a rock bulkhead extending from the neighbors property. The bluff face is vegetated predominantly wkh brush' and young trees with 'some mature trees (Photo 3). A spring exits the bluff roughly half way dawn the bluff. 'There is little or no vegetation in the spring channel, A large slide Geotechnical Depart Lot 18 of Port Ludlow #5 Sub Division E0 3~tld d(1~2,~J 51C T1N 86b8Z5b89£ b0 ~9T ~09~rtiS190 muriv cvvv ~~•vv~m May 5, 2003 vv~vrr~~~ viumi..n ~ vn~~~.v ~ ~i • i ~ Geotechnica! Repoit Lot 16 of Port Ludlow #5 Sub Division Mark Hankie Clo Coldwell Banker, Bill Roesler ~ ~~~~~~~~~y~~~, ~~~~`~,, ~~~C 9522 Oak Say Road Sts.1 QO Port Townsend, WA 98366 SubJect: Geotechnlcal report for Lot 16 of Port Ludlow #5 Sub Division [ocated in Section 9. Township 26 North, Range 1 East, W.M., Jefferson County, WA Dear Mr. Hankie: . Background At your request. Bail Pay(On,~Engineering Geologist with Northwestern Territories Inc. conducted a bluff stability inspectiori at the above mentioned property on April 30.2003. The purpose of this insp®ctlon was to examine the marine bluff at the subject site by visual means in order to determine the relative stability of the bluff and make recommendations in regards to the proposed construction of a single family residence with possible basement and beach access trail Site Description The subject waterfront prop®rty !s located Qn M©ntgomery Lane overlooking Admiralty Inlet to the east (Figure 1, Photo 1 and 2}. The property is currently undeveloped exc+®pt for the posslblllty of a municipal sewer line running across the property. The upland portion of the property slopes eastward at about 4 degrees and is veg®tsted primarily with grass and weeds with a few young to mature trees. No evidence of surface erosion nor swampy~areas were visible on the upland. "The bluff at th® property is roughly 66 feet high with an overall slop® of about 27 to 30 degrees. The upper ~35 feet of bluff is about 34 to 40 degrees and the lower section Is about 16 degrees then steepens to around 34 degrees.near the toe (F`lgure 2). These slopes are generally below or within the "angle of repose" which is defined as the maximum angle at which loose, cohesfonless material remains stable (assuming no destabilizing forces). This angle commonly ranges between 33 and 37 deers®s. About half of th® toe of the bluff is protected from wave erosion by a rock bulkhead extending from the n®ighbors property. The bluff f~co i® vegetated predominantly witfi brush and young trees with some mature trees (Photo 3). A spring exits the bluff roughly half way down the bluff. There is little or no vegetation Ira the spring channel. A large slide 1 b8 3~dd df1DaJJ S'it' g1N 86ti6ZS6096 b0 ~9T E00Z/bT/B0 occurred on the bluff In the past, as evidenced by the bowl•shaped upper bluff. The age of the slide could not be determined. However, the slide scar is now revegetated and no bare slidescars were noticed on ifie.bluff }ace, which suggests that the bluff has been relatively stable since the slide. Commonly, after a slide, the slop® angle of the bluff is reduced thereby incr®asing the stability of the bluff. site Geology The Washington State .Department of Ecology's Coastal Zone Atlas maps th® area of the subject property as UndifFerendated stratlf~ed sediments alder than Vashon lodgement till (Qpf) and as Possession Drift (Qpd~. The Qpf soils consist mainly of sand snd gravel, but in some areas contain silt, clay, peat and possibly till. The Qpd soils derived from glacial till and consist of a compact mixture of bouidsrs, cobbles, pebbles, sand, silt, and clay. The Atlas also maps the stability of the bluff in the area as Unstable old slide (Uos) and Unstable recent slide (Urs). According to the Soil Survey of Jefferson County Area, Washington (United States Departm®nt of Ag~lculture,1975), the subject site Is In an area mapped as soil type Swantown grav®Ily loam (Subj. This unit consists of somewhat poorly drained gravelly soils that have a very slowly permeable cemented layer'at a depth of 18 to 24 inches. These soils formed in glacial till. Visual observations made on the bluff face at the subject site were generally consistent with the above soil d®scriptions. M®chanlcs of Bluff Recession There are many forms of bluff recession that otxur in•the coastal regions of northwest Washington. 'Theo common processes are the erosion of the toe of the bluff by wave action, and the sloughing of upper bluff soils due to saturation of the soil during the rainy .season. When waves attack the toe of an unprotected bluff, the low®r bluff soils are eroded away. Eventually, this erosion w(II oversteepen the bluff to a point wher® the soli can no longer support itself at such a steep angle. Then the bluff soils will slough off, d®posking • material at the toe of the bluff. This will have the effect of temporarily reducing the angle of the bluff to a mars stable angle, and then the whole process will start ov®r again. This . process has been partially suspended at the subject property due to th®presence of the ..bulkhead at the toe of the bluff, if the buikh®ad were ever damaged or removed, this form of erosion would begin again, thus destabilizing the slope and triggering the typical recession with landslides. Many of the landslides that occur in our region happen in the winter or spring when the ground is saturated with water, and especially after heavy rainfall events. When the soil becomes saturated, there is a decrease in the c~heslon b®tween the loll grains and an increase In .the pore-water pressure. This condition can trigger land ides ~ s flows on slopes. Often, there will be an impermeabl® soil lay ~r~~p~ b~ifF, 2 ~. u ~ .,,;, ! ~.. 50 mad ~ ~moza sir r1N ~~~~~~~~~~ ~~~~~~~~ ~~lo~. 86b8ba~b0~B b0~9T E08ZfbT/58 ^Iq/•IV' LVVV IL'YV~Ifl VVLUIILLL U111t1\LI1 IVI111LV1 rl' fL I t V' V V / L I' V which prevents the downward migration of groundwater end causes the~water to migrate laterally, eiciting the bluff above the impermeable layer. This mechanism may have been the trigger for the slido mentioned above. For this, reason, it is important to c~ntro(on-sfte drainage and runoff !n ord®r to minimize the infiltration into and resulting erosion of the soil. It Is also important to matntaln v®getation on the bluff face in order to reduoe erosion of the bluff soils. Gonclualons and Rscommond~atlons The bluff at the subj®ct property appears grossly stable at present and the proposal seems feasible from a geotechnical perspective, however/ this report cannot guarantee that a slide wll! not occur at the property. The toe of the bluff is partially protected by a rock bulkh®ad. The bluff is well vegetated and the slope angle is generally at or below the angle of repos®. The bluff soils are gen®rally coarse grained and compact. Accordingly, we recommend tha# a 50 foot minimum'setback be established from the top edge ofthe bluff to the foundation of the proposed house) The presence and location of the sewer line should be verltied prior to construction and easements maintained. The following recommendations should also be considered with regards to the proposal: t. it will be necessary t0 maintain ground cover to reduce erosion from surface runoff.• Any bare areas that develop on ~e upland or on the bluff should be revegetated, Native deeprrooted vegetatiron that requires Itttle or no irrigation would bathe most beneficial. Please consult the online publications mentioned below for further lr~forma~tion. 2. Vegetation on the bluff face provides stabllixativr~ to th® bluff face soils. The vegetation on the bluff face should be left to as natural state as possible. If an enhanced view Is desired, trees should be pruned rather than topped. It maybe worthwhile to consult a tree expert to ensure that the trees are not damaged. 3. In order to reduce the erosion caused by the spring on th® lower bluff, rock •could be pl2~Ced in the spring.channel and the channel could be revegetated. Please consult the online publications mentioned below for further information. 4. We anticipate that a propeNy constructed trail down th® face of the bluff would not sign~icantly decrease stops stability. The trail should be constructed such that erosion of the bluff is not exacerbated. If a trail is desired, someone • knowledgeable in this type of construction should be consulted. 5. 'Heavy irrrigatlon •or other activities that would contribute large quantities of water • to the soil should be avoided. 6. The bulkhead at the toe of the bluff should be,maintain ~~ hod oondition.. - ~ ~ ~~ °-~ ~ ~ I- .9 J 3 90 3~4id ~lOZI~J 51C' I.LN 86b8Z0b09E b0 ~9~ E00~/bt,'S8 Iglu ~ 1 v I~VVV 1 L •'rVi In VVLV!lL LL U111. 1\LII I V111\LV 1 ll 1 L 7. Surface runoff from hard'surfaces such as roofs, driveways, walkways and patios should be controls®d and routed to the beach via tlgh~in®such that surface water discharge to adjacent properties does not signltlcantJy exceed predevelopment conditions. 8. Drainage cAntrol devices should be maintained in good working order and Inspected at least once a year. . 9. The Soii Survey mentions the possibility of a Shallow perched water table during the wet season. Thus k would be bene~dal t4 allow for wetn®ss under the home in the building design. One measure would be~the use of footing drains. 10.Sllt fences or other sediment con#rol devices may be needed during construction such that sedimentation to adjacent properties does not 5lgnificantly exceed predevelopment conditions. 11.An ®ng[neered drainage and erosion cant-vl plan shouid be dev®Ioped for this property to address items 7, 8, 9 and 10 above. Based on the flnd(ngs~ n~commendatbns and Ilmltatlons of this report: 7. There has been a landslid® at the subject property in the past, however there appears to be minimal landslide hazard to the proposal based upon observations of current conditions at the site and the recommended setback distance. 2. Observations of slope stability indicate that the proposal would not be subject to risk of landslide under the current conditions that exist at the site. 3. ~ The proposal would not Increase surface' water discharge or sedimentation to adjacent propertle~ beyond predevelopment conditions. 4, ,The proposal would not decrease slope stability on adjacent properties. 6. The proposal would be stabl® under normal geologic conditions. Ppr further information please review th® three online publications published by th® Washington State Department of Ecology (DOE) entided~ "Slope.Stabilization and Erosion Control Using Vegetations, °Vegetation Management: A Guide for Puget Sound Bluff Property Owners" and pSurface Water and Groundwater on Coasts! Bluffs°. Thee publlaatlans are new out of print but can be obtained from tC-e D0E webslte et: J~ • ec .wa.goWbibllolsea.htmluntier the 1983 and 1994 year heading. The DOE website also contains much more useful information regarding slope stability and site development; this reference is highly recommended. 4 `~ 28 3~d d110~ S'it' IlN 6fib8S9b09B bB:9t E00Zfbt/59 mu~iv~ cvVV IL•711In VvLVtILLL unnn~n tvnii~vi n• i~ i~v•vvir. i ~ v Limitations This report~has been Prepared for the exclusive use of our client 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 by others only with the expressed written permission of the Engineer. . Within the omits of scope, schedule and budget, this report was prepared in general accordance with accepted professional engineering and geological pdnclples and pr'a~Ctices 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 subjeck property at the time of our site vlsh; no laboratory tests were performed. Sail and g~eologlc conditions can vary signlflcantiy 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 thos® described in this report we should be cron'tacted and retained to ®valuate the- changed oondiflons and , make modiilcations to our report tf necessary. Sincerely, NORTHWESTERN TERRITORIES, INC. Robert A. Leach P.E., MBA Pdnoipal Engin®er ~'R'1 ~ l~ '~~,~ op~~M.Siy,~y 4y '°~~, ~~a,~ ago `~~' `~ss~orra~ ~' ~xES ,~onaa ~~- ~°~ Bill Payton, L.E.©. Englr~eering G~logist G;10en181tI1RepaimlHINK0301,b1uf1 seahillry.s(28.1 E).Port Lu~ow.doa 5 88 3Jb~d d(10~J S"l[' IJN 86b8Z5D99E ti0 ~9't . 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