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Home My WebLink About821103010 Geotech Assessment I Pro~~,,,.~~ Subject ~/- -~~ ~ ~'~'"' m ~-- ~.-~ Prepared For David Reid July 8, 2003 For the Property Located described as Lot 11 in Tala Point Estates, Tax # 821103010 Section 10, Township 28 North, Range 1 East, 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 NT/' JLS GROUP~ INC. NORTHWESTERN TERRITORIES, INC. A JLS GROUP-COMPANY 717 SOUTH PEABODY STREET, PORT ANGELES, WA 98362 . []' Engineers r~ Land su~eyors [] Geo/ogists [] Construction Inspection [] Materia/s Testing (360) 452-8491 FAX 452-8498 www.nti4u.com E-Mail: inf0@nti4u.com Geotechnical Report· Lot 11 in Tala Point Estates, Tax # 8211030'!0 July 8, 2003 David .Reid 46 Village Way, PMB 131 Port LudloW, WA 98365 SubjeCt: Dear Mr. Reid: GeotechniCa/rePort' ~or Lot 11 in Tala. Point EStates, Tax # 821103010 located in Section 10,-TownshiP 28 North, Range 1 East, W.M.. Jefferson County, WA · . Background At your request,.Bill'Payton,.Engineering Geologist with Northwestern Territories INC.. conducted a bluff stability inspeCtion at the above referenced property on March 12, 2003. The purpose of this inspection was to examine the madne 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. Site Description The sUbject waterfrOnt property is loCated on East Ludlow-Ridge Road overlooking Port Ludlow Bay to the west, Existing deVelopments at the-property include a.driveWay and a septic system. It is our understanding that the septic system is being redesigned and that the proposed location of.the house'will be where the old existing drainfield is- located (Figure 1). The upland portion of the PrOperty is well vegetated in young to mature native trees, brush, ferns and grass. There is a Iow-hill. on the property and the ground slopes eastward and westward from the hill (Figure 2). In the area Of the' proposed home site the property slopes westward toWard the bluff at.about 15 degrees. Between the.hilt and East Ludlow Ridge Road, there is a depression on the north side of:the driveway that- appears to collect runoff from the road and neighboring property, whiCh appears, to then move off site towards the north' or northweSt. This depression area is only partially located on the Subject property and the rest is designated as common area. The . presence of the hill on the property helps to isolate the home site from this runoff. The section of driveway that flows past the depression may need improving 'in order to make it passable in wet conditions. The bluff at the property is roughly 100 feet high with an overall slope of about 42 ' degrees, with some steeper sections. The bluff is mostly'well vegetated with young to mature trees and brush. A slide scar was noticed on the bluff face' that now has young alders and brush growing on it. SOme of the trees on the bluff app'ear t° be in the 60 to 80 year old. range, which sUggests that the bluff has been generally stable except for local sloughing. NTI surveyed the property in 1990 and :measured a distance of 20 feet from a marker on the nOrth property line to the edge of the bluff. The marker was found during this investigation and was measured to be the'same distance as in 1990.-This ' further attests to the relative stability of the bluff' over this time frame. ~ Beach access was not gained during this investigation, but from previous investigations in the area, it has been observed that the toe of the bluff does get. eroded by wave action causing oversteepening of the bluff and periodic-sliding of the upper bluff soils. Mechanics of Bluff Recession There are many fOrms of bluff recession that occur in the cOastal regions of northwest Washington. Two common processes are the erosion of the toe of the bluff by wave action, and the sloUghing of upper bluff s°ils due to saturation of the soil during the rainy season. · When waves attack the toe-of an unprotected bluff, the lower bluff soils are eroded away. Eventually, 'this erosion will oversteepen the bluff to a point where the soil can no longer support itself at-such a steep angle. Then the bluff soils will slough'off, depositing . material at the toe of the bluff. This will have the effect of temporarily redUcing the angle of the .bluff to a more stable angle, and then the whole process will start over again. Many of.the landslides that oCcur in-our, region happen in the winter or spring When th'e ground is saturated with water, and espeCially after heavy rainfall events. When the soil. becomes saturated, there is a decrease in the cohesion between the soil grains and an . increase in the pore-water pressure. This condition can trigger landslides and debris ' --flows on slopes. Often, there-will be an impermeable soil layer part way down the bluff,~ which prevents the downward migration o.f groUndwater and causes the water to migrate laterally, exiting the blUff. above the impermeable laYer. Surface -runoff flowing down the face of the bluff can also cause erosion and damage vegetation on.the bluff. For this reason, it is important to control on-site drainage and runoff in order to minimize negative impacts to the bluff. It .is also important to maintain vegetation on the bluff face 'in order to reduce erosion of the bluff soils. · Site Geology. The Washington State Department of EcologY% CoaStal Zone Atlas maps the bluff area of the subject property as Undifferentiated stratified sediments older than Vashon lodgement till (Qpf). This soil consist mainly of sand and gravel, but in some areas contain silt, clay, peat and posSibly till. The Atlas .maps.the upland area of the property as the Vashon lodgement till (Qvtl); a compact mixture of boulders, cobbles, pebbles, sand, silt and clay, generally overlain by-l-5 feet of ablation till. The Atlas also describes this soil as being exCellent for foundation stability and goOd for seismic .stability. The Atlas maps the slOpe stability of the bluff in the area of the prOperty as Unstable old slide (UOs) and Unstable recent slide (Urs) (Figure 3), Soil descriptions from the Department of EcOlogy's "Geology and Ground-Water . . Resources of Eastern Jefferson County; Washington" were consistent with the Coastal Zone Atlas soil descriptions. According to the Soil Survey of Jefferson County Area, Washington (United States Department of Agriculture, 1975), the subject site is in an.area mapped as the Kitsap silt loam along the bluff and as the Cassolary sandy loam on.'the upland. The Kitsap sOil consists of moderately well drained soils that formed in glacial lacustrine or marine sediments. The Survey state's that the" runoff of this soil is rapid andthe hazard of eroSion and slippage are severe, The Cassolary soil Consists of Somewhat Poorly drained soils that formed in glacial lacustrine or marine sediments. Runoff of this soil is classified as slow to rapid depending on slope; and-the hazard of erosion is classified as slight-to Severe again' depending On slope. - . Visual observations made in the area were.generally m°re consistent with the C°astal Zone Atlas s°il deScriptions Conclusions and Recommendations The bluff at the subject property appears grossly stable at present and the proposal seems feaSible from a geotechnical perspective, however, this report cannot guarantee that a Slidewill not occUr at the.property: There have been slides along the bluff in the past and fUture occasional sliding should be expected. .. Based upon Our investigation, w.e-rec°rnmend~that;a 15ofocjt..minimum setbaCk bb established :f~om the top.~edge of.the bluffto,the foundati°n of:the-proposed house and that a.100.foot, minimum s-etback be-eStablished from.the top.edge of the-bluff to the . septic system. "if The follOwing reco. mmendations should alSo` be considered with regards to the proposal: , 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. Please consUlt the online publications mentioned below for'further information. 2. Vegetation on the bluff face. provides stabilization to the blUff face soils and 'helps remove water from the soil. The vegetatiOn on the bluff face should be left- in as natural state as pOssible. If an enhanced view is desired, trees should be 3 pruned such that they are not damaged. It may be worthwhile to consult ia tree expert in this matter. The proposed 'house should' not be founded on fill material. Any fill that is encountered should be remoVed and the house foUnded in. compact undisturbed native 'soil or proPerly compacted engineered fill'. . . . . . Heavy irrigation or other activities that would contribute large qUantities of water to the soil should be avoided. · · SUrface runoff should not be allowed to fi'ow Over the face' of the bluff and cause erosi°n of ,the bluff face soils. one way to 'mitigate this if necessary would be to construct a swale Or interceptor drain on the upland and direct the water to a catchment area and then to the beach via tightline. PleaSe~see the Online publications recommended below for more-information on this .subjeCt. . Surface runoff from hard SurfaCes sUch .as roofs, drivewayS, Walkways and patios should-be controlled and roUted to the beach via.tightline such.that surface water 'discharge to adjacent properties does not significantly exceed predevelopment conditions. : Silt~fences or other sediment control devices may be needed during construCtion such that 'sedimentatiOn to adjacent properties does not significantly exceed predevelopment conditions. · Drainage control devices shoUld be maintained in good working order and inSpected at least onCe a year. An engineered drainage and erosion control plan should be-developed for this. property to address items 5, 6, 7 and 8 above. Based on the findings, recommendations and limitations of this repOrt: .. 1. There has been a landslide 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. observations of slope stability indicate that the proPosal should not be subject to risk of landslide under the current conditions that exist at the site. .." ,. . , The proposal should not increase surface water discharge or sedimentation to adjacent properties beyond, predevelopment conditions. ~ The proposal should nOt decrease slope.stability on' adjacent properties. The proposal should be stable under normal geologic conditions.. For further information please review the three online publications published by the Washington State Department of Ecology (DOE) entitled: "Slope StabilizatiOn and Erosion Control Using Vegetation", "Vegetation.Management: A Guide for Puget Sound Bluff Property OWners" and "Surface Water and Groundwater on Coastal Bluffs", TheSe publications are now Out of print but can be obtained from the DOE website at: http://www..ecy.wa..qov/biblio/sea.htmllunder the 1993 and 1994 year'heading. The DOE website also contains much mOre useful information regarding slope stabilitY and site develOpment; this reference is highly recommended, ' Limitations This report has been prePared f0'r 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 by others 0niy with the expressed written permission of the . Engineer. Within the-limits of scope, schedUle-and.budget, this reportlwas 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 appeardifferent 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 Bill Payton, L.E.G. Engineering Geologist G:\Gen\Bill\Reports~:~EDA0301 .bluff stability. 10(28-1E).Tala Point.doc 5 Expires 11,.06-03 .,: - . . APPENDIX Figure 2 i _ U~t IFigure 3 J