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Home My WebLink About502112036 Geotech AssessmentSTRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 December 22, 2004 Jan Baker 920 Mountain Trail Brinnon, WA 98320 Re: Geotechnical Report Tax Parce1502112036 323 Talons Roost Road Brinnon, Washington Dear Ms Baker: Stratum Group is pleased to present the results of our geology evaluation of Tax Parcel 502112036 within the Eagle View subdivision at 323 Talons Roost Road 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 was limited to 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 properly is indicated on Figure 1. The property is located in the northwest quarter of Section 11, 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 durrng~~ , the Pleistocene Epoch {1.6 million to 10,000 years ago}. During these glacial and~~^++accompf~anying ~. J b j CSiJ.;' December 22, 2004 Tax Parcel 502112036, Brinon, WA Geology 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 Geolo ig c Mai 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 glacial till and bedrock. Observations on the subject property are consistent with this mapping. The property is underlain by a veneer of glacial till very compact ice-contact deposits over bedrock that consists of basalt. The Bedrock Geolo i~Lc Ma~of the Seattle 30' by 60' Quadrangle, Washington (Yount and Gower, 1991) indicates that mapped bedrock in the vicinity is basalt of the lower Eocene Crescent Formation. The proposed building sites on the lots are underlain by glacial till. The till deposits consist of cobbles and boulders within a matrix of sand silt and clay. The till is very compact and hard. The till on the site is well exposed in the cut slopes on the property. 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 within the Eagle View subdivision. The property is access via a gravel road that is part of the access to several other properties north and west of the subject property. The proposed building area is located on the upper northwest portion of the property at an elevation of approximately 360 feet. At the time of my site visit a cleared and level building area had already been completed on the site for both the home site and the garage building site. The graded area is underlain by glacial till and ice contact deposits. The west side of the leveled area has been cut into the above slope. The home site has a cut slope of approximately 8 feet with an upper approximately 15-foot wide bench cut into the slope above. The cut slope above the bench is approximately 10 feet at its highest but averages approximately 5 feet. The cut slopes are cut into very compact till. The slope above the cut slope slopes up at an angle of less than 10 degrees and is covered by thick brush and young trees. The cut slope and the slope above are stable. Minor seeps are present at a few locations within coarser grained areas of the till. In addition it is likely that perched gr8und water within the more weathered till and topsoil above the very compact till is intercepted by the cut slope. A drainage line has been constructed around the base of the cut slope and routes water to an area south of the garage building site. The water is discharged onto a gentle 10-degree slope that gradually steepens to approximately 30 degrees 60 feet further down slope. The slope below the level building area consists of side cast soils from portions of the cut slopes. The side cast slopes down from the building site at an angle- of approximately 30 degrees. The Stratum Group 'File: 12.13.04[3 2 December 22, 2004 Tax Parce1502112036, Brinon, WA Geology Assessment side cast covers a slope approximately 25 feet high. The original natural slope in the area is only a little less steep; hence, the side cast is only a few feet thick at most. Based on the natural slope angles south of the building site, the thickness of the side cast soils at the edge of the level area is on the order of no more than three feet thick. Some minor erosion riling has taken place on the exposed side cast soils from surface run off from the very compact till underlying the level building area. CONCLUSIONS AND RECOMMENDATIONS It is my opinion that proposed building area on the level area of the property is stable and that the proposed construction will not be at risk from landslides or other slope stability problems nor will the proposed development on the property cause any instability on other slopes. All proposed buildings should be set back from the down slope side of the level building area at least 5 feet in order to minimize differential settlement that may occur along the outer edge of the level building area. Buildings should be set back from the base of the cut slope on the west side of the building area a distance equal to the height of the cut slope in any area where the cut slope exceeds 6 feet. Storm water run off from the roof gutters and road access as well as drainage should be routed in such a manner as to avoid excessive concentration of water onto the side cast soil on the slope below the level area. The current discharge of drainage south of the proposed garage site is an appropriate location as long as that area remains undisturbed. As the building sites are underlain by very compact till, the impact of structures on storm water will be minimal. Please note that there are inherent risks associated with lots near or adjacent to steep slopes. These are risks that the building owner should recognize and be willing to accept. If conditions appear different than those described in this report, or other concerns arise, we request that we be notified so we can review those areas and modify our recommendations asrequired. 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, Strum Group 1 1 -. t~„'a.. ~/ H Dan McShane, L.E.G., M.Sc. Licensed Engineering Geologist Stratum Group 3 V , v Xj IC S~. 3 Figme 1. 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