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HomeMy WebLinkAboutBLD2007-00377 Geotechnical Report • • ` D.0--7-37 7 STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone(360)714-9409 May 15, 2007 444 3 TI / Mike and Sue Joachims kit) 50 Mockingbird Lane JUL 13 2001 Port Ludlow, WA 98365 rttKllli �i,i,u,YI Re: Geology Hazard Assessment Tax Parcel 802274006, Elkhorn Lane Jefferson County, Washington Dear Mr. and Ms Joachims: I visited the Tax Parcel 802274006 on Elkhorn Lane on March 1, 2007 to evaluate the slope stability of the site. The purpose of the site visit was to assess the slopes on the site and vicinity and other relevant geologic conditions at the site. Based on my assessment of geologic conditions,the lot is not at risk of being impacted by slope failures. Because of potential perched shallow ground water within the soils at the site, attention to foundation drainage will be particularly important. The foundation drainage system be carefully designed to allow for complete drainage from behind the back (upslope) foundation wall. This can be accomplished in a variety of ways including a well sealed foundation and well draining gravel and drainage lines placed along the outside of the walls and beneath the foundation. If a daylight basement foundation is used, it is my opinion that some over design of the drainage system would be well worth the extra costs that may be entailed to ensure that the foundation remains dry. The extra design could include additional drain rock and drain lines, and I suggest that clean out risers be provided for the drain lines so that they can be readily maintained in case of clogging. GEOLOGY This geology description of the area is to provide some background regarding the conditions at the site. Northwestern Washington has been occupied by continental glaciers at least four times during the Pleistocene Epoch (1.6 million to 10,000 years ago). During these glacial and accompanying interglacial periods, the underlying bedrock was deeply eroded. The Geologic Map of Surficial Deposits in the Seattle 30' x 60' Quadrangle, Washington (Yount, Minard, and Dembroff, 1993) indicates that the subject property is underlain by glacial till. Site observations on the subject property and in the vicinity are consistent with the above-described mapping. The till on the site consists of very compact silt with a poorly mixed assortment of sand, cobbles and boulders embedded in the silt matrix. Fragments of local basalt bedrock are embedded within • • May 15,2007 Tax Parcel 802274006,Elkhorn Lane Geology Assessment the till at the subject property. The till was deposited directly by glacial ice associated with the Vashon ice sheet that covered the Puget Sound lowland between 20,000 and 13,000 years before the present. Because the till is very compact and contains a significant percentage of silt and clay, the till soils on the site are not well drained and shallow perched ground water is likely present on the site on a seasonal basis. Site Observations Slopes on the site are relatively gentle with slopes generally less than 15 degrees throughout the site. The property generally slopes towards the east. Elkhorn Road angles across the eastern portion of the property. An access road enters the central portion of the property from Elkhorn Lane. A grass-covered clearing is located in the central portion of the property. Most of the remaining areas of the property are tree covered with alders being the predominant tree. A former road or possible rail line grade presumably associated with past timber harvest cuts across the property west of the clearing area. Conclusions No potentially hazardous slopes are present on the subject property. As noted above attention should be paid to foundation drainage as soils underlying the site will generally have low permeability. Stratum Group appreciates the opportunity to be of service to you. Should you have any questions regarding our reconnaissance please contact our office at (360) 714-9409. Sincerely yours, iii S um Group �- 0 �_ "-_ 10; opip ,--/-c- / D an McShane, L.E.G., M.Sc. a ....... 04o100e '!t», Licensed Engineering Geologistfr, ' 378 l . Stratum Group File:3.1.07 Figure.ite Location 47°52,987' N, 122°54.62 WGS84 122°56.000' W 122°55;000' W 122°5 0' W WGS84 122°53.000' W V 1 s, 'I '1 • --,-- ,_I e'-N) (SN 1 i it: r„..N ii ji V. I I 1 I \; •y, '2 2 �i i t I )/r44 0 / 0 O 1 ) �0 9 • 11�f f 1 V,. 6 . 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