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Home My WebLink About992600001 Geotech AssessmentGEOTECHNICAL REPORT Prepared For SeaHome Services, Inc. March 28, 2006 For the Property Described as 247 Franks Lane Tax # 9926000019 Section 36, Township 27 North, Range 1, W Jefferson County, Washington ~i~~ ~~~ ~; APR 2 1 2006 J Jr~"=i"~~GN COUNTY C~f'l. OF COiv1MUNITY GEVEL4PA4ENT Prepared by NTI Engineering and Surveying 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 f~~;;. ~~ NORTHWESTERN TERRITORIES, INC. /~ A JLS GROUP COMPANY ~ -l. - ~, r i~' 717 SOUTH PEABOdY STREET, PORT ANGELES, WA 98382 -1 Enginoors Land Surveyors Geologists N~-l Construction inspocfion Materials Testing (3G0) 452-8491 FAX 452-6498 www.nli4u.coin E-Mail: into c@nti4u c°m JLS GROUP, INC. Geotechnical Report Tax # 992600001 March 28, 2006 SeaHome Services, Inc. Attn: Greg Barron 7425 Thorndyke Rd. Quilcene, WA 98376 Subject: Geotechnical Report for Tax # 992600001 located in Section 36, Township 27 North, Range 1, W.M., Jefferson County, WA Dear Mr. Barron: Background At your request, Bill Payton, Engineering Geologist with NTI Engineering and Surveying (NTI) conducted a bluff stability inspection at the above referenced location on January 24, 2006. The purpose of this inspection was to examine the marine bluff at the subject site by visual means and provide an opinion regarding the relative impact that a proposed repair/reconstruction of the existing beach access stairway would have on the stability of the bluff. Concurrent with this inspection is an engineered drainage, sediment and erosion control plan that is being performed by David Hanna, P.E., also of NTI. It is our understanding that you desire to repairlreeonstruct the existing stairway (Photo 1) due to it being unsafe and very steep (48° to 50° 1111 % to 119%}, which includes constructing a landing (in the area of the bottom of the existing stairs}, with a change in direction in order to lessen the steepness of the stairway (Figure 1). We understand that the newlrebuilt stairway will be in approximately the same location as the existing stairway and be above the ordinary high water mark. We further understand that the stair system will be founded on piers and that the toe of the bluff will not be excavated out in order to construct the stairs. And further, that bluff disturbance will be limited to the immediate vicinity of the stair location and will not encroach onto adjacent properties, and that disturbed areas on the bluff will be restored upon completion of the stairs. Site Description The subject waterfront property is located at 247 Franks Lane, off of Thorndyke Road on the Toandos Peninsula. The property overlooks Hood Canal to the east and is bounded on the north and south by residential property, and on the west by Thorndyke Rd. (Figures 2 and 3) Existing developments-to the property include the main house, guesthouse, garage and other buildings (Photos 2 and 3). There are existing tightline drains (Photo 4) on the bluff north of the stairs that are causing erosion/sloughing of the bluff and it is understood that these drains will be repaired as part of the proposal. The upland portion of the property, in the area of development, is predominantly open and vegetated with grass, with some trees and shrubs. The rest of the property towards the west is wooded. The topography of the upland slopes gently eastward toward the bluff (Figure 4). The property south of the garage was wet and had runoff flowing into a culvert under the driveway and then into another culvert on the bluff edge south of the property. The bluff at the property (Photo 5} is about 35 feet high with an average slope angle of about 55° to 60° with some short steeper sections at the top of the bluff that are 70°+. The lower bluff angle is about 20° to 24°. The bluff is mostly vegetated with English ivy and berry bushes with some young alders, cedars and horsetail ferns. There has been some recent sloughing north of the stairs as mentioned above. The existing house is about 45' to 55' from the top edge of the bluff. young alders and brush. Groundwater seeps were noticed on the bluff face. Site Geology The Washington State Department of Ecology's Coastal Zone Atlas maps the soils in the area of the subject property as Vashon lodgment till (Qvt1) over Vashon advance outwash {Qva) over Undifferentiated stratified sediments older than Vashon lodgement till (Qpf) over Possession(?) Drift {Qpd} (Figure 5). The till soil is described as consisting mostly of a compact mixture of boulder to sand size particles with some silt and clay. The Atlas lists this soil as excellent for foundation stability, good for seismic stability, and says that that it stands in steep natural andlor cut slopes for long periods. The advance outwash typically consists of well-sorted, well-stratified sandy pebble to cobble size gravel. The undifferentiated soil is described as consisting mainly of sand and gravel, but in some areas contains silt, clay, peat, and possibly till. Environments of deposition of this unit include glacial outwash, glacial lake deposits, and interglacial alluvium and lake deposits. The Possession Drift is described as a till deposit. The Atlas also maps the stability of the bluff as Unstable (U) with small areas of Unstable recent slide (Urs} (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 as mentioned above. According to the Soil Survey of Jefferson County Area, Washington (United States Department of Agriculture, 1975), the subject property is in an area mapped as Cassolary sandy loam (CfC). This soil formed in reworked glacial and marine sediments and is typically composed of stratified silty sand with a layer of clay from 23" to 38". Visual observations of the bluff face soils are consistent with the above soil descriptions. Conclusions and Recommendations Based upon our understanding of the project and our site investigation, we conclude that the beach access stairs will have minimal impact on the stability of the bluff. However, the natural processes of bluff retreat are occurring at the subject property and periodic sloughing of the bluff should be expected to continue which may necessitate future maintenancelrepair of the stair system. 2 Drainage is an issue at the property and is likely contributing to the bluff retreat process. Thus, anything that can be done to reduce the amount of water reaching the bluff should be beneficial to bluff stability. Controlling surface water and directing it away from the bluff as well as curtain drains to intercept shallow groundwater are two of possibly many ways to achieve this goal. A more involved approach that could deal with deeper groundwater would be to install a series of wells across the yard at some distance back from the edge of the bluff at a depth that would intercept groundwater and to continuously pump the water out, Vile recommend that the existing tightline drain located north of the stairs be repaired and that the tightline be continued to the base of the bluff. Specific drainage recommendations will be in the drainage, sediment and erosion control plan being completed for this property, also by NTI. The following recommendations should also be considered with regards to the proposal: 1. It will be necessary to maintain ground cover in order to reduce erosion from surface runoff. Any bare areas that develop, on the bluff or on the upland, should be revegetated. Native deep-rooted vegetation that requires little or no irrigation would be the most beneficial. Please consult the enclosed publications for further information. 2. Vegetation on the bluff face provides stabilization to the bluff face soils and helps remove water from the soil. Existing established vegetation should be left in as natural state as possible. Once construction of the stair system is completed, any areas that were disturbed should be revegetated as needed. As far as pruning for a better view, minor thinning and pruning should be done in such a way that minimizes disturbance to the soil and root zone and that insures the continued health of the vegetation. 3. Tree limbs, lawn clippings, etc. should not be thrown over the bluff because these piles have a tendency to damage/kill the underlying vegetation which in.turn increases the erosion potential of the bluff soils. 4. Heavy irrigation or other activities that would contribute large quantities of water to the soil should be avoided. 5. If new surface runoff from hard surfaces such as roofs, driveways, walkways and patios will be created as part of the proposal, it should be controlled and routed to a drainage control device such that surface water discharge to adjacent properties does not significantly exceed predevelopment conditions. We recommend that on site infiltration of this water not be considered for this site. This issue, as well as on site surface and groundwater derived from off site sources (i.e. not from roof's, etc.} will be addressed in the above mentioned drainage, sediment and erosion control plan being completed for this property. fi. 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. Drainage control devices should be maintained in good working order and inspected at least once a year. For further information please review the three publications {included with the original of this report) published by the Washington State Department of Ecology {DOE} entitled: "Slope Stabilization and Erosion Control Using Vegetation", "Vegetation Management: A Guide for 3 Puget Sound Bluff Property Owners" and "Surface Water and Groundwater on Coastal Bluffs". These publications can also be viewed on the DOE website at: http://www.ecy.wa.gov/biblio/sea.html under the 1993 and 1994 year heading. The DOE website also contains additional 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 for other purposes 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. Expires 11/06/06 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, NORTHWESTERN TERRITORIES, INC. Robert A. 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