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HomeMy WebLinkAbout601342022 Geotech Assessment GEOTECHNICAL REPORT Prepared For Brian Vanloon October 5, 2007 For the Property Described as Tax # 6013422022 Johnson Road, Community of Coyle, Quilcene, WA 98376 Section 34, Township 26 North, Range 1 West, W.M. Jefferson County, Washington Prepared by NTI Engineering and Surveying l17 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 ~~~cC1E,d~%1(~,iC lip 15 2007 EFER30N COUNTY err ~~ f~ www.nti4u.com NTI ENGINEERING & SURVEYING 717 SOUTH PEABODY STREET, PORT ANGELES, WA 98362 Engineers ~ Land Surveyors ~_~ geologists Construction Inspection ~~ Materials Testing (360) 452-8491 1-800.654-5545 FAX 452-8498 E-Mail: info ,~nti4u.com October 5, 2007 Brian Van Loon 54 Goliath Lane Port Ludlow, WA 98365 Subject: Geotechnical Report for Tax # 6013422022, located in Section 34, Township 26 North, Range 1 West, W.M., Jefferson County, WA Dear Mr. Van Loon: Background 0 At your request, NTI Engineering and Surveying (NTI) conducted a geotechnical inspection at the above referenced property. The purpose of this inspection was to visually examine the property in orderto provide an opinion regarding the presence of landslide hazard areas and make recommendations in regards to the proposed development of the property. The inspection consisted of office research of available literature, previous reports completed by NTI, and geologic maps of the area and a site visit made on September 24, 2007. This study was performed in general accordance with the Jefferson County Unified Development Code. It is our understanding that the property will be developed for seasonal recreational use with the possibility of a cabin in the future. Potential building sites include locations at the east and west ends, and in the center of the property. Site Description The subject property is located on Johnson Rd at the southern end of Toandos Peninsula near the community of Coyle (Figure 1). Access to the propertywas via 505 Johnson Rd. The property is surrounded by rural residential, vacant timbered, and forest designated lands (Figure 2). The property is partially cleared and existing improvements include a driveway and septic system (Cover Photo). The property slopes down towards the east at an average slope angle of approximately 9° (~16%) (Figure 3). There is a shallow seasonal drainage swale along the south side of the property, another swale west of the property, and a ravine near the north side of the property that has a spring in it. The head of the ravine is near the northwest property corner. The steepest part of the ravine has a slope angle of approximately 30° and is 0 about 40' in elevation. Just a short distance from the head, the ravine widens and becomes less steep. Vegetation on the property includes young to mature fir, cedar, and alder with a brushy understory (Cover Photo). Site Geology The Washington State Department of Ecology's Coastal Zone Atlas maps the majority of the property as Vashon lodgment till (Qvt1), and the eastern side, approximately below the 200' contour line as Undifferentiated stratified sediments older than Vashon lodgement till (Qpf) (See Appendix). The till 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 forfoundation stability, good for seismic stability, and says that that it stands in steep natural andlor cut slopes for long periods. The Undifferentiated stratified sediments mainly consists of sand and gravel, but in some areas contains silt, clay, peat, and possibly till. The Atlas maps the stability of the property as Unstable (See Appendix). The Department of Ecology's "Geology and Ground-Water Resources of Eastern Jefferson County, Washington" maps the soils in the area of the subject property essentially the same as the Coastal Zone Atlas. According to the Soil Survey of Jefferson County Area, Washington (United States Department of Agriculture, 1975), the majority of the property is in an area mapped as Dabob very gravelly sandy loam (DaC). This unit consists of moderately well-drained soil that formed in very gravelly glacial till. This soil is listed as having moderately rapid permeability, but becomes very slowly permeable at ~20" to 36". Runoff is listed as slow to medium and the hazard of erosion as slight to moderate. Other minor soils mapped on the property include Alderwood gravelly loam (AmC) and Kitsap silt loam (KtE). The Alderwood soil also formed in glacial till and is similarto the Dabob soil. The Kitsap soil, mapped in the ravine to the north, is a silty, clayey soil that formed in glacial lacustrine or marine sediments on canyon sides and ocean bluffs. The Soil Survey descriptions pertain to the upper 5' of soil only. Visual observations of the surface exposures of soil are generally consistent with the above descriptions. Figure 2 ,: . ~~ ~~ '~ ~r t ~ ~a Figure 3 Conclusions and Recommendations The subject property appears grossly stable and the project seems feasible from a geotechnical perspective. We flagged one area in the northwest corner of the property (Figure 3) as a landslide hazard area with white flagging tape. We recommend a minimum 30' building setback from the top of this slope. We further recommend a reduction of the landslide hazard area vegetative bufferwithin the building setback to 15' for this area. Other moderately steep slopes exist on the property. For slopes greater than 18.5° (33.3°/a), we recommend that building foundations comply with Section 1805.3 of the International Building Code (IBC) which deals with footings on or adjacent to slopes. That text and related illustration can be found in the appendix of this report. Several dead trees were noticed in the vicinity of potential building sites, including the site at the west end of the property near the landslide hazard area. These trees could pose a danger to structures or people if they fell. They also could cause significant soil disturbance if their root balls were ripped out of the ground. We recommend that dead and diseased trees be removed from the vicinity of structures, even within the landslide hazard area vegetative buffer. The following recommendations should also be considered with regards to the proposal: 1. Itwill be necessary to maintain ground coveron the slopes and on the flatter areas in order 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 enclosed publications forfurther information. 2. Heavy irrigation or other activities that would contribute large quantities of water to the soil in the vicinity of the steep slopes should be avoided. 3. Surface runoff from hard surfaces such as roofs, driveways, walkways and patios should be controlled and routed to a drainage control device such that surface water discharge to adjacent properties does not significantly exceed predevelopment conditions. It may be feasible to control release this waterto the existing natural drainage corridors with quarry spalls or perforated pipe. 4. Surface water should not be allowed to flow freely down the face of the slopes and cause erosion of the slopes. If this occurs, the water should be directed away from the slopes and/or conveyed to a safe discharge area, possibly into the natural drainage corridors mentioned above. 5. Footing and/or curtain drains maybe beneficial if the ground around the selected home site is excessively wet. 6. Silt fences or other sediment control devices maybe needed during construction so 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. 8. All drainage control features should conform to the current regulations that apply to the development of this property and be constructed in accordance with standard engineering practice in orderto insure that surface waterdischarge or sedimentation to adjacent properties does not exceed predevelopment conditions. Forfurther 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 Puget Sound Bluff Property Owners" and "Surface Waterand Groundwater on Coastal Bluffs". These publications can also be viewed on the DOE website at: http:Ilwww.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, and 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 orfor 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. An analysis of the potential for large scale earth movements was not performed. If you desire a more detailed analysis of you site, you are encouraged to obtain such services. 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 subsurface exploration or laboratory tests were performed. Soil and geologic conditions can vary significantly between test holes andlor 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, NTI ENGINEERING &SURVEYING r Robert A. Leach, P.E., MBA Principal Engineer Y Bill Payton, L.E.G. Engineering Geologist B:IReportslVANL0701.geotech.34(26-1).Coyle.doc !o -5-07 ~ EXPIRES 12130/2008 vvaa~a~ ~~~~a n~~a~ v~ ~~a~~~~~~y~v~~ - v~v~vyy LJI 1 0 0 1805.3 Footings on or adjacent to slopes. The placement of buildings and structures on or adjacent to slopes steeper than one unit vertical in three units horizontal (33.3-percent slope} shall conform to Sections 1805.3.1 through 1805.3.5. 1805.3.1 Building clearance from ascending slopes. In general, buildings below slopes shall be set a sufficient dis- tancefromthe slope to provide protection from slope drain- age, erosion and shallow failures. Except as provided for in Section 1805.3.5 andFigure 1805.3.1, the following criteria will be assumed to provide this protection. Where the exist- ing slope is steeper than one unit vertical in one unit hori- zontal (100-percent slope}, the toe of the slope shall be assumed to be at the intersection of a horizontal plane drawn from the top of the foundation and a plane drawn tangent to the slope at an angle of 45 degrees (0.79 rod) to thk° horizon- tal. Where a retaining wall is constructed at the roe of the slope, the height of the slope shall be measured frc ~m the tap of the wall to the top of the slope. 1805.3.2 Footing setback from descending slope sur~ face, Footings on or adjacent to slope surfaces shall be~ founded. in firm material with an embedment and set back from. the slope surface sufficient to provide vertic al and lat- eralsupport for the footing without detrimental settlement. Except as provided for in Section 1805.3.5 and Figure 1805.3.1, the following setback is deemed adequate to meet the criteria. Where the slope is steeper than 1 unit ver- tical in 1 unit horizontal (100-percent slope), the required setback shall be measured from an imaginary plane 45 degrees (0.79 rod) to the horizontal, projected upward from the toe of the slope. 1805.3.3 Pools. The setback between pools regulated by this code and slopes shall be equal to one•half the building footing setback distance required by this section. That por- tion of the pool wall within a horizontal distance of 7 feet (21.34 mm}from the top of the slope shall be capable of sup- porting the water in the pool without soil support, 1$05.3.41'oundationelevotion. ~n graded sites, the top of any exterior foundation shall extend above the elevation of the street gutter at point of discharge or the inlet of an approved drainage device a minimum of 12 inches (305 mm) plus 2 percent, Alternate elevations are permitted sub- ject to the approval of the building official, provided it can be demonstrated. that required drainage to the point of dis- charge and away from the structure is provided at all loca- tions on the site. 1805.3.5 Alternate setback and clearance. Alternate set- backsand clearances are permitted, subject to the approval of the building official. The building official is permitted to require an investigation and recommendation of a registered design professional. to demonstrate that the intent of this sec- tion has been satisfied. Such an investigation shall include consideration of material, height of slope, slope gradienC, load intensity and erosion characteristics of slope material. FACE OF FODTJNG TOP OF SLOPE ........ .,. FACE OF ~E--~-, .,...,,... STRUCTURE TOE OF H/3 BUT NEED NOT N SLOPE EXCEED d0 FT. MAX. For SI: l foot = 3(k~.& mm, N/2 BUT NEED N07 EXCEED 15 FT MAX. FIGURE 1805.3.1 FOUNDATION CLEARANCES FROM SLOPES 2006 INTERNATIONAL BUILDING CODE SOILS AND FOUNDATIONS 347 i~