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HomeMy WebLinkAboutBLD2007-00077 Geotechnical Report • 1 • • -y.- .' I. a1% w1 A GEOTECHNICAL REPORT 1) O(o - 1q Prepared For Mar Hinkle I '"' V E May 14, 2003 C(r)C=25-4 (,_______ rL3 2 7 For the Property Located described as liFNMA rump irp Lot 16 of Port Ludlow#5 Sub Division, Montgome Section 9, 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 ZO 3Wd drtoNo Sir IUN 86ti8Z9b09E 00:9t E00L/DT/90 • • Geotechnical Report Lot 16 of Port Ludlow #5 Sub Division May 14, 2003 ' l A'LnriF f) • Mark Hinkle f E) 2 7 2717 C/o Coldwell Banker, Bill Roesler 9522 Oak Bay Road Ste. 100 1�1��!l U'���� `�w��lY�l Port Townsend, WA 98365 �U l' Subject: Geotechnical report for Lot 16 of Port Ludlow#5 Sub Division located In Section 9, Township 28 North, Range 1 East, W.M., Jefferson County, WA Dear Mr. Hinkle: Background At your request, Bill Payton, Engineering Geologist with Northwestern Territories Inc. conducted a bluff stability inspection at the above mentioned property on April 30, 2003. The purpose of this Inspection 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 property Is located on Montgomery Lane overlooking Admiralty inlet to the east (Figure 1, Photo 1 and 2). The property Is currently undeveloped except for the possibility of a municipal sewer line running across the property. The upland portion of the property slopes eastward at about 4 degrees and is vegetated . 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 the property is roughly 65 feet high with.an overall slope 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 (Figure 2). These slopes are generally below or within the 'angle of repose"which Is defined as the maximum angle at which loose, cohesionless material remains stable (assuming no destabilizing forces). This angle commonly 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 with 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 in the spring channel, A large slide 1 E0 39Vd dnod9 Sir I1N 8608Z9009E 00:91 E00Z/01/90 i • • Geotechnical Report Lot 16 of Port Ludlow#5 Sub Division R liVED May 5, 2003 r 2 ,., � T 2777 Mark Hinkle C/o Coldwell Banker, Bill Roesler .lff i1iNi CUM 9522 Oak Bay Road Ste. 100 Port Townsend, WA 98365 • • Subject: Geotechnical report for Lot 16 of Port Ludlow#5 Sub Division located in Section 9, Township 28 North, Range 1 East, W.M., Jefferson County, WA Dear Mr. Hinkle: • Background At your request, Bill Payton, Engineering Geologist with Northwestern Territories Inc. conducted a bluff stability Inspection at the above mentioned property on April 30, 2003. "The purpose of this inspection 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 property Is located on Montgomery Lane overlooking Admiralty Inlet to the east (Figure 1, Photo 1 and 2). The property is currently undeveloped except for the possibility of a municipal sewer line running across the property. The upland portion of the property slopes eastward at about 4 degrees and is vegetated 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 the property is roughly 65 feet high with an overall slope 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 (Figure 2). These slopes are generally below or within the "angle of repose"which is defined as the maximum angle at which loose, cohesionless material remains stable (assuming no destabilizing forces). This angle commonly 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 with 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 In the spring channel. A large slide • ve 39 d dn089 S f UN 8608Z5009C b0:9t Ee9Z/vTI98 • • 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 slide scars were noticed on the bluff face, which suggests that the bluff has been relatively stable since the slide. Commonly, after a slide, the slope angle of the bluff is reduced thereby increasing the stability of the bluff. • Site Geology 0 The Washington State Department of Ecology's Coastal Zone Atlas maps the area of the subject property as Undifferentiated stratified sediments older than Vashon lodgement till (Qpf) and as Possession Drift (Qpd). The Qpf soils consist mainly of sand and 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 boulders, 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 Department of Agriculture, 1975), the subject site Is in an area mapped as soil type Swantown gravelly loam (Sub). 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 descriptions. 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 soils 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. This . process has been partially suspended at the subject property due to the presence of the bulkhead at the toe of the bluff. If the bulkhead 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 cohesion between the soil grains and an Increase In the pore-water pressure. This condition can trigger la nds(ides � r�s flows on slopes. Often, there will be an impermeable soil lay _ nr i • n2 FEB 2'7 L.Ji ,1FF� 1O 2 tIII' 1 11.1 GO 39Vd doom AC S llN 0608Z9009E 00:9I E00Z/OT/50 • • which prevents the downward migration of groundwater and causes the•water to migrate laterally, exiting the bluff above the Impermeable layer. This mechanism may have been the trigger for the slide mentioned above. For this.reason, it is Important to control on-site drainage and runoff In order to minimize the infiltration into and resulting erosion of the soil. It Is also important to maintain vegetation on the bluff face in order to reduce erosion of the bluff soils. 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 slide will not occur at the property. The toe of the bluff is partially protected by a rock bulkhead. The bluff is well vegetated and the slope angle is generally at or below • the angle of repose. The bluff soils are generally coarse grained and compact. Accordingly, we recommend that a 50 foot minimum setback be established from the top edge of the bluff to the foundation of the proposed house, The presence and location of the sewer line should be verified prior to construction and easements maintained. 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 on the upland or on the bluff 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. The vegetation on the bluff face should be left in as natural state as possible. If an enhanced view is desired, trees should be pruned rather than topped. It may be 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 the lower bluff, rock could be placed 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 properly constructed trail down the face of the bluff would • not significantly decrease slope 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 irrigation 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 d od • condition, , [ �M k ET) • • 3 FE3 2 t� • 98 39ad • d1021J S"1t IUN 8608Z9009E 00:9t E00L/bt/90 • • 7. Surface runoff from hard surfaces such as roofs, driveways, walkways and patios should be controlled and routed to the beach via tightllne such that surface water discharge to adjacent properties does not significantly exceed predevelopment conditions. 8. Drainage control devices should be maintained in good working order and Inspected at least once a year. 9. The Soil Survey mentions the possibility of a shallow perched water table during the wet season. Thus It would be beneficial to allow for wetness under the home In the building design. One measure would be the use of footing drains. 10.SIIt fences or other sediment control devices may be needed during construction such that sedimentation to adjacent properties does not significantly exceed predevelopment conditions. 11.An engineered drainage and erosion control plan should be developed for this property to address items 7, 8, 9 and 10 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. 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 properties beyond predevelopment conditions. 4. , The proposal would not decrease elope stability on adjacent properties. 5. The proposal would 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: rttpJlwww.ecy.wa.gov/biblio/sea.html under 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. • RFkEIVF1 ) 1D2, 72 • 4 • 1414.14 L0 39Vd domm s1r IlN 8608L8609E VO:9t E00L/bt/90 • • 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 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, �,�,,' q, ien' qr of WIS,y4 '7�' NORTHWESTERN TERRITORIES, INC. h�� �� 4 k�0/BTBa.4e. Fsa/atiAt etc EXIVE6 1 zr3onoa Robert A. Leach, P.E., MBA Principal Engineer vi o //.2e),24. tgtZZ - % 1.4 ../ Bill Payton, L.E.G. ��191 ,..k0,� Engineering Geologist �d �'�° lyytiliam C. Payton Jr. • Expires 11-08-43 Gt10en181111Reporta\HINK0301.bIufl stebillty.9(29-1 E).Port Ludlow.dno 5 80 30bd drION9 Sit llN 86v8Z5t709E 00:91. 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LI 1 - 1 s a T B e R i E. DATE: S-7-0 3 NORTHWESTERN TERRITORIES, INC. 11 DESIGNED BY: G. c? . PROJECT: H I N K G�O I Enemas 0 find Surveyors■Planners `/ .. construction Coordination.liiprlale 4orin9 S FOR: Lo-r 1 6, PoIAr LocLew -so s SNT: I QF NT' 717 SOUTH PEABOOY■PORT ANOSL60,WASHINGTON OM■(360)4624101 F-I G;URC •, 0T 3Dt1d df1OH5 SirIlN 66b9Z9b09E. b0:91 EOM/PT/SO