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Home My WebLink About502082004 Geotech Assessment NT! JLS GROUP~ INC. NORTHWESTERN TERRITORIES, INC. A JLS GROUP COMPANY 7t7 SOUTH PEABODY STREET,.PORT ANGELES, WA 98362 r~ Engineers [] Land Surveyors [] Geologists [] Construction Inspection [] Materials Testing · (360) 452-8491 FAX 452-8498 www.nti4u.com E-Mail: info@nti4u.com GEOTECHNICAL REPORT For Proposed Remodel of Existing Deck At 2134 Duckabush Road~ Tax ID 502082004 Located in Section 8, Township25 North, Range 2 West, W.M. Jefferson County, WashingtOn. Prepared for 'JUDITH ASHLEY Prepared by 2003' JEFFERS0~ COU~.TY DEPT. OF ~'~,,t ~ '~' ng~l r-~.-- NORTHWESTERN TERRITORIES, INC: : 7'17 S, Peabody Street Port Angeles, Washington 98362 Phone 360-452-8491 Fax 360:452-8498 E-mail info@nti4u~com December 6, 2002 GEOTECHNICAL REPORT for Proposed Remodel of Existing Deck at 2134 Duckabush Road, Tax ID 502082004~.---~,~ December 6, 2002- !L!- ~i ~'~{ - 6 2003 Judith Ashley . 2134 Duckabush Road Brinnon, WA 98320 Subject: i ,~E~ERSO~ c0~J DEPT. OF COMMUNIT¥ DEVELOPi'¢EN.,f Geotechnical report for the prOposed remodel of an existing deck at 2134 Duckabush Rd., Tax ID 502082004 located in SectiOn 8, T25N, R2W, Jefferson County, WA. Dear Ms. Ashley: IntrodUction At your request Bill Payton; Geologist With NorthWestern Territories, Inc. (NTI.) met with you and Gary' Streid On site on November 19, 2002 to perform 'a geotechr)ical inspection for the above referenced project. It is our understanding -that you plan to remodel'your home and the deck .located on the west side of the house. This deck-is currently supported on pier blocks at ground, level and is settling. The Purpose of the-inspection wasto examine the hillside below the existing deck by visual means and provide foundation recommendations for.the new deck. Please see the enclosed photos. Site Conditions The subject property is located off of Duckabush Road on the north side of'the DuckabUsh 'River valley. The property consists of 25+ acres of mostly wooded land,surrounded by foreSt land..The proPerty is developed, with a home, garage and other improvements. The hillside'in question, below the existing-deck, is about 18 to 20 feet high' and slopes down towards an intermittent creek at ab0ut'40 degrees.from horizontal. The hillside is well vegetated with trees, brush and grass. Some of the tr. ees on the hillside have curved .trunks, Which suggests that shallow downslope creep of -the surface soils is occurring. There is a large old growth stump at the 'base of 2 the slope, which suggests that the hillside has been grossly stable for close to a century or more. The presence of this stump also suggests that the flow of the creek has not been great enough to undermine the stump. The creek-bank is lined with large cobbles and boulders, which helps reduce the effects of erosion. Geologic Conditions The hillside at the subject prOperty is mapped as Hoodsport-Grove very gravelly sandy loam (HrD) by the. USDA Soil Survey of Jefferson County. This soil formed in glacial till and glacial outwash. The Survey states that there is a cemented layer at 20" to 36" with a seasonal high water table at 2' to 3'. This. soil has a Iow shrink:swell potential and is subject to slight frost-action 'during very cold periods. The Survey classifies this soil as GM or GP based on the Unified Soil Classification System. The State of Washington Department of Ecology Geologic Map of Eastern Jefferson County, Washington maps the area of the subject property as Lodgement till (Qvt) of the Fraser Glaciation. This material is described as boulders, cobbles and pebbles in a matrix of sand, silt and clay;.a compact and unsorted mixture. Generally covered by a few feet of recessional drift or Holocene sediments. Visual obServations made at the site revealed compact gravelly material consistent with the above descriptions. Conclusions and Recommendations Visual evidence from the' site investigation suggests that the hillside is grossly stable with only surficial sl°Pe creep 'occurring. This usually only affects the upper few feet of the hillside. Accordingly, we recommend that the new deck be founded on piles embedded about 5 feet deep.- Due to the access-limitations of the site and the .architects desire to not use concrete, we suggest the-Use'of steel pin piles' or screw anchors: Pin piles are. typically driven with a pneumatic hammer and screw anchors are screwed in.With a power head mounted on'a bobcat or similar equipment. Both of'these systems have' limited accessTCapability'and are generally less disruptive to the surrounding environment than concrete pile construction.-' However, concrete' auger cast piles Would .be suitable as w.ell. _ 3 The following recommendations should also 'be considered with regards to the project: 1. It will be necessary to maintain ground cover on the hillside 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. -2.' Heavy irrigation or other activitieS that would contribute large quantities of water.to the soil should be avoided. One causeoflandslides is the presence of excessive groundwater in the soil. 3, Surface runoff should be directed away from the hillSide or shunted past it in order to prevent surface erosion from this source. Silt fences Or other sediment control devices may be needed during constrUction so that sedimentation,o.nto adjacent properties does not exceed predevelopment conditions. 5. Any drainage control devices should be maintained in good working order and inspected at least twice a year, sayin SePtember and February. Based on the findings, recommendations and limitations of this report: 1. There is minimal landslide hazard as proven by the lack of evidence of landslide activity'at the subject property in the recent past. 2. An analysis 'of slope stability indicates that' the proposal would not be' subject to risk of landslide under normal conditionS. 3. T.h'e proposal would not significantly increase Surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions. °. The proposal would not decrease slope Stability on adjacent properties. The proposal would be stable under normal geologic'conditions. For further information please, review the three pamphlets, which are enclosed with the original of this report, published by the Washington State Department of Ecology entitled: "Slope Stabilization and Erosion Control Using Vegetation", "Surface Water and GroundWater on Coastal BlUffs'' and "Vegetation Management: A Guide for Puget Sound Bluff Property Owners". - Limitations This report is based on a visual inspection of the existing site conditions. No · .' scientific measurements, tests or calculations were performed. The findings: and . report are limited to the normal standard of care of the industry for an investigation without benefit of scientific data and calculations. The sole objective of this investigation has been to diagnose the geotech~ical conditions at the site. The report has been prepared for the exclusive use of our client in conjunction with the above mentioned project. It may be used by others only with expressed written permission of the Engineer. The observations, interpretations, and.conclusions herein are based on generally accepted professional engineering and geologicprinciples'and practice. This warranty is in lieu of all others, either expressed or implied. Sincerely, NORTHWESTERN TERRITORIES, 'INC. Robert A. Leach, P.E., MBA Principal Engineer EXPIR ES 12130/2004 Bill Payton, L.E.G. Engineering Geologist G:\Gen\Bill\Repo!ls~ASHL0201.geotech report.8(25-2).Duckabusb Rd.doc 5 Expires 11-06-03 Photo 1. View of existing deck-north side. Photo 2. View of existing deck-south side. Photo 3. View of creek. Photo 4. View of hillside and deck from creek.