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Home My WebLink AboutBLD2006-00134 Geotechnical Report • • STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone(360)714-9409 February 27, 2006 John Ewald P.O. Box 207 Lilliwaup, WA 98555 Re: Geotechnical Report Tax Parcel 502313004 Forest Drive Brinnon, Washington Dear Mr. Ewald: Stratum Group is pleased to present the results of our geology evaluation of Tax Parcel 502313004 on Forest Drive south of Brinnon, Washington. The purpose of this geology evaluation was to qualitatively evaluate the risk of slope failures impacting proposed development on the subject property and to provide general site maintenance recommendations for the property. This evaluation included a visual inspection of the property and vicinity and review of available geologic mapping in the area. The property is located on the east side of the Olympic Peninsula on a slope above Hood Canal. The location of the property is indicated on Figure 1. The property is located in the southwest quarter of Section 31, Township 25N, Range 2 West W.M. SCOPE OF SERVICES The scope of our services included the following: 1) Conducted a site visit to visually inspect the subject property including slope conditions and relevant conditions in the vicinity of the property. 2) Observed surface soil conditions on the slopes of the subject property and vicinity. 3) Prepared this report summarizing our findings, including a qualitative evaluation of the stability in regards to the proposed garage and residence on the property, recommendations for site development, and recommendations for further investigation, if necessary. GENERAL GEOLOGY 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 • • February 27,2006 Tax Parcel 502313004,Forest Drive,Brinon, WA Geology Hazard Assessment interglacial periods, the underlying bedrock was deeply eroded. Glacial related deposits were deposited over the bedrock in various places on and in the vicinity of the subject property. The Geologic Map of Surficial Deposits in the Seattle 30' by 60' Quadrangle, Washington (Yount, Menard and Dembroff, 1993) indicates that the subject property and vicinity is underlain by Vashon recessional outwash deposits. The recessional deposits were deposited by melt water streams from the retreating glacial ice near the end of the last glacial period approximately 13,000 years ago. Observations on the subject property are generally consistent with this mapping. However, the deposits on the property are more consistent with the site being underlain by ice-wasting deposits. As the glacier melted it left behind sand and gravel and boulders in a poorly sorted manner. SPECIFIC SITE OBSERVATIONS The subject property is located on a slope above Hood Canal and U.S. Highway 101 on the west side of Hood Canal. The location of the subject property is indicated on Figure 1, Site Vicinity Map. A site plan sketch showing pertinent features is provided on Figure 2. The property is accessed via Forest Drive. Forest Drive bounds both the upper and lower parts of the lot. However, the proposed building area is on the upper, western side of the lot and is accessed via a driveway from the upper side of the Forest Drive. A level building pad area has been graded on the southwest portion of the property. Otherwise the property slopes down to the east at an angle of 26 degrees or less. Vegetation on the lot consists of a mix of brush and cut back blackberry vines. The lower east side of the lot is covered by thick growth of blackberries with a few stands of alders. At the time of my site visit in late January 2006 there were a few area of minor water seepage approximately half way down the slope, but otherwise the site was free of standing water and test pits for septic drain fields were free of water. It should be noted that my site visit coincided with a period of extremely wet weather. No areas of past slope movement or landsliding are present on the subject property or on properties in the immediate vicinity of the subject property. CONCLUSIONS AND RECOMMENDATIONS It is my opinion that the subject property is stable and that there are no geologically hazardous slopes on the site. Furthermore, development of the property will not increase the risk of landslides or slope movement on the property or elsewhere. Stratum Group File: I.16.05B 2 • 0 February 27,2006 Tax Parcel 502313004,Forest Drive,Brinon,WA Geology Hazard Assessment It is my understanding that the proposed home may be constructed with a daylight basement. A daylight basement will be feasible on this site as long as adequate drainage is provided around the daylight basement walls and floor. At the time of our site visit the building area appeared to be well drained as was no standing water in the proposed building area despite very high rain fall the morning of my site visit and over the previous month prior to my site visit in late January 2006. I recommend that all roof drains and paved surfaces be directed into a level dispersion trench located on the lower slope below the home site as indicated on the attached site plan sketch map. Foundation drainage and access road drainage should be constructed to prevent soil saturation beneath the foundation. Where possible, the land adjacent to buildings should be sloped away from them at a minimum 2 percent slope. Drain systems are recommended around the perimeter of the proposed house, which should consist of 4-inch perforated pipe surrounded by washed (coarse sand). Structural fill should consist of clean sandy gravel with less than 5%passing the#200 sieve based on a wet sieve analysis of that portion passing the 3/4-inch sieve(GW by USCS Classification). Stratum Group appreciates the opportunity to be of service to you. Should you have any questions regarding please contact our office at(360) 714-9409. Sincerely yours, Stratum Group 7,„-------- QMCS& ., M.Sc. �"` r\\ ��diT.,:t 4Y5 Licensed Engineering Geologist v=1 . [ , 0eoloee."430 f \ 0 13I9 _�4 s ,]cSK 03 i _ Stratum Group File. 1.16.05B 3 • r Figure 1. Sit\Adnity Map 123°00.000' W 122°59.000' W WGS84 122°58.000' W Obi JJ 3,t / ;•: t i, `. r`1 - �' `�_�,, _- l Yam' o o _ js ao : _ ' if �,r. / - .ems. f " "�+ ,' ' ' ,--— - ,, o caR. "' i if \ / t\ 'IN \--' -' s.---/ /, .- -----.3 ,' `'','''-'- . cr-r-7.,,---Y____,I47.,-- ,/ "Nk,.\-,_„-P- - .,,.., ---- gC,:tet,--':-. 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