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HomeMy WebLinkAboutBLD2006-00572 Geotechnical Report • ,,, NTI L!AGINEERING & SURVEYING, C. 70 SOUTH PEABODY STREET,PORT ANGELES,WA 98362 Engineers III Land Surveyors i Geologists Construction Inspection■Materials Testing 4141,11!, (360)452-8491 1-800-654-5545 FAX 452-8498 E-Mail:info@nti4u.com n ir.nlr4u.cnm July 27, 2006 Dean Rosenthal 341 South Keel llv iy Pt. Ludlow, WA '! ::t65 Re: Addendum to Geotechnical Report dated July 8, 2003 for Parcel# 821103012 Dear Mr. Rosenthal: At your request, NTI Engineering & Surveying, Inc. (NTI) performed an updated geotechnical review of your property described as Lot 8 in Tala Point Estates, Parcel#821103012. The purpose of this review was to determine if the geologic conditions at the site were the same as was reported in the July 2003 report completed by NTI for the property, and to address the proposed location of the septic/drainfield location. This review was requested by the Jefferson County Department of Community Development in a letter dated June 27, 2006. The review consisted of a review of the July 2003 geotechnical report and the septic system site plan completed by Creative Design Solutions, and a site visit made on July 13, 2006 for visual observations of the property and bluff.. The current conditions at the site are essentially the same as was reported in the July 2003 report, and the proposed septic/drainfield location behind the recommended 150' bluff setback is acceptable from a geotechnical perspective. If you have any questions regarding this matter, or need further assistance, please feel free to contact us. Sincerely, NTI Engineering and Surveying, Inc — isa 3111 Payton, L.E.G `* :ngineering Geolc .. .. L"' ��•� ( is cop.-. 191 /• ��/ c:Torn Mitchell i' i Sao;csCP- Rick Lander William C. Payton Jr., Expires 11/06/06 G\Gen\Bill\Reports\POSD0601.10(28-1 E).septic location letter.Tala Pt.doc O • 4." • . _ GEOTECHNICAL REPORT Prepared For Dean Rosenthal July 8, 2003 For the Property Located described as Lot 8 in Tala Point Estates, Tax# 821103012 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 NORTHWESTERN TERRITORIES, INC. A JLS GROUP COMPANY 717 SOUTH PEABODY STREET,PORT ANGELES,WA 98382 ❑ Engineers ❑Land Surveyors o Geologists Nil0 Construction Inspection ❑Materials Testing (360)452-8491 FAX 452-8498 www.nti4u.com E-Mail:info@nti4u.com JLS GROUP, INC. Geotechnical Report Lot 8 in Tala Point Estates, Tax # 821103012 July 8, 2003 Dean Rosenthal 45 McCoord Woods Dr. Fairport, NY 14450 Subject: Geotechnical report for Lot 8 in Tala Point Estates, Tax # 821103012 located in Section 10, Township 28 North, Range 1 East, W.M., Jefferson County, WA Dear Mr. Rosenthal: Background At your request, Bill Payton, Engineering Geologist with Northwestern Territories Inc. conducted a bluff stability inspection at the above mentioned property on July 2, 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. All measurements were made with hand held instruments and should be considered approximations. The site is heavily vegetated especially towards the north end of the property and thus the topography of the property and the location of the top of the bluff is not well defined. For increased accuracy and assistance in development planning, it is recommended that a topographic survey be completed for the property including mapping the top of the bluff. Site Description The subject waterfront property is located on East Ludlow Ridge Road overlooking Port Ludlow Bay to the west and Admiralty'Inlet to the north (Figure 1). The property is currently undeveloped. There are residential homes on each side of the property. 1 • • The upland portion of the property is heavily vegetated in young to mature native trees, brush, ferns and grass. There is a gully that runs northward through the property and drains out at the top of the bluff (Figure 2). In the gully, dense brush predominates. No springs or seeps were noticed and the gully appeared dry. Because of the gully, the only potential building site is located at the southeast corner of the property near the cul-de-sac (Figure 3). The limited buildable area may hamper development of the property with respect to the size of house and suitable area for a septic drainfield, From the cul-de-sac, the property is roughly level for about 112 to 126 feet. At this point, the gully becomes the dominant feature and the property begins to slope northward at about 18 degrees (33%) then increases to about 25 to 30 degrees (47%-58%). As the gully develops, the sides of the gully also steepen to around 25-30 degrees. There is a strip of relatively flat land between the gully and the east property line. The top of the bluff appears to be roughly 345 feet northward from the cul-de-sac or south property line. Beach access was gained via a neighbor's property several lots south of the subject property. The bluff at the property is roughly 120 to 150 feet high and mostly devoid of vegetation due to recent slide activity (Photo 1). The upper bluff is near vertical, while the lower bluff has a slope of about 36 to 38 degrees. The sloping soils that comprise the lower bluff are predominantly slide debris derived from the upper bluff. Some young alders have started to grow on this slide debris. The bluff is composed predominantly of sand with lenses of gravelly sand and silt. No springs or seeps were noticed on the bluff face. 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. Both of these processes seem to be occurring at the subject property. 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 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 landslides and debris' flows on slopes. Often, there will be an impermeable soil layer part way down the bluff, which prevents the downward migration of 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. 2 • • 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's 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 (Qvt1), a compact mixture of boulders, cobbles, pebbles, sand, silt and clay, generally overlain by 1-5 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 recent slide (Urs) (Figure 4). 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 and as the Cassolary sandy loam. The Kitsap soil consists of moderately well drained soils that formed in glacial lacustrine or marine sediments. The Survey states that the runoff of this soil is rapid and the 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 more consistent with the Coastal Zone Atlas soil descriptions. Conclusions and Recommendations The bluff at the subject property is actively being eroded as evidenced by the recent slide on the bluff face, and future sliding should be expected. Typically, bluff erosion and sliding are more common in the winter and spring due to increased rainfall, higher tides and winter storms. It is impossible to predict when a slide will occur or how much the bluff will recede. Average rates of bluff recession for this area have been suggested to be on the order of 6" to 12" or more per year. However, several feet of bluff could be lost during one event. Based upon our investigation, and the possible average rate of recession of 12" per year, we recommend that a 150 foot minimum setback be established from the top edge of the bluff to the foundation of the proposed house. Also, because of the presence of 3 •• the gully and the steep sides of the gully, we recommend that the proposed house be located in the relatively flat area at the southeastern end of the property mentioned above. In order to minimize the footprint of the house, it may be beneficial to build a two story house, with a basement daylighting at the head of the gully. If this area does not provide enough room for a house and septic drainfield, it may be possible to locate the drainfield in the relatively flat area between the gully and the east property line. The 150 foot bluff setback should be maintained for the drainfield as well. 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. If vegetation cannot be established, say in the gully, then erosion control mats or rock may be useful in order to reduce erosion. 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. Vegetation growth on the bluff face should be enhanced where possible. Any existing vegetation should be left in as natural state as possible. If a better view is desired, trees should be pruned such that they are not damaged. It may be worthwhile to consult a tree expert in this matter. 3. Depending on the location of the house with respect to the gully, deepened footings may be needed in order to comply with the requirements of Section 1806.5 of the Uniform Building Code (See Appendix). 4. Heavy irrigation or other activities that would contribute large quantities of water to the soil should be avoided. 5. Surface runoff should not be allowed to flow over the face of the bluff and cause erosion 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. 6. 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. 7. Silt fences or other sediment control devices may be needed during construction such that sedimentation to adjacent properties does not significantly exceed predevelopment conditions. 4 • • 8. Drainage control devices should be maintained in good working order and inspected at least once a year. 9. An engineered drainage and erosion control plan should be developed for this property to address items 6, 7, 8, and 9 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, the recommended setback distance should provide for minimal landslide hazard to the proposed house. 2. 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. 3. The proposal should not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions. 4. The proposal should not decrease slope stability on adjacent properties. 5. 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.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. 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 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. 5 • • 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, okiA. LF4 NORTHWESTERN TERRITORIES, INC. ��P� ofsH'yccy y.,n ,PE,ss 13772 �p �,, / 4,c LISTER �� ,/d ,oNA I ' Robert A. Leach, P.E., MBA EXPIRES 12/30/2004 Principal Engineer ',` - '- vtq L ".44 BIM Payton, L.E.G. \'`�s 9 4(y, Engineering Geologist Qd Goo [William C. Payton Jrj Expires 11-06-03 G:\Gen\Bill\Reports\REDA0301.bluff stability.10(28-1 E).Tala Point.doc 6 • • • • z— i -? , .-i- . �' - �C +. Subject Property 9 . A610- "` U '071.', +-gam 2 Ip Sill eti t c t31 r i swim a' y � s ( i • f 1 . !L'! 24$! 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