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HomeMy WebLinkAbout701253005 Geotech Assessment STRATUM GROU~ 1451 Grant Street, Bellingham, W A 98225 Phone (360) 714-9409 June 19, 2006 Kim V ogley and Jon Smaker P.O. Box 971 Reno, NV 89504 Re: Geology Hazard Evaluation, Shoreline Bluff Tax Parcel 701253005 Jefferson County Dear Ms Vogley and Mr. Smaker: Stratum Group is pleased to present the results of our geology hazard evaluation of the above referenced property on a shoreline bluff. The purpose of this geology evaluation was to qualitatively evaluate the risk of slope failures impacting development on the subject property and provide general site development and maintenance recommendations. This evaluation was limited to a visual inspection of the property and vicinity, a visual inspection of the slopes on the property and vicinity, review of available geologic mapping in the area, and inspection of the shoreline at the base of the bluff. The property is located east of Thorndyke Road, between the road and the shoreline of Hood Canal on the east side of the property. The location ofthe property is indicated on Figure 1. SCOPE OF SERVICES The scope of our services included the following: 1) Conducted a site visit to visually inspect the subject property including the slope conditions, shoreline conditions, and relevant conditions in the vicinity of the property. 2) Observed surface soil conditions on the slope and on the uplands above the slopes. 3) Prepared this report summarizing our findings, including a qualitative evaluation of the slope stability in regards to the proposed residence on the property, recommendations for site development, and recommendations for further investigation, if necessary. JUN 2 7 2006 June 19, 2006 Parcel 701253005, Jefferson County, W A Geologic Evaluation of Shoreline Bluff GENERAL GEOLOGY Northwestern Washington has been occupied by continent~l 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 eroded and a'relatively thick layer of glacial related and interglacial fluvial sediments were deposited over the underlying bedrock in the vicinity of the subject property. The Geologic Map of Surficial Deposits in the Seattle 30' by 60' Quadrangle, Washington (Yount, Minard, and Dembroff, 1993) and the Geologic MaD ofthe East-Central, Jefferson County, Washington (Birdseye, 1976) indicate the property is underlain by Vashon advance outwash deposits. Birdseye (1976) indicates Pre- V ashon stratified sediments are present on the east end of the property while Yount, Minard, and Dembroff (1993) refer to this unit as Pre- Frasser sediments. The advance outwash deposits consist of clay to gravel stratified deposits deposited by meltwater from the advancing glaciers at the start of the ice age. The pre- V ashon stratified sediments or Pre-Frasser depsoits consist of stratified silt, sand and gravel with peat layers. Observations on the subject property, slopes in the vicinity of the property and upland areas above the subject property are consistent with the mapping described above. However, I observed a glacial till unit approximately in the mid-slope of the shoreline bluff. This till is likely an older pre Fraser till. The Slope Stability Map of East-Central Jefferson County, Washington (Birdseye, 1976) identifies the subject property area as having unstable areas. Due to the steepness of the shoreline bluff and erosion at the toe of the bluff the steep shoreline bluff is unstable. The upland area away from the bluff is stable. SPECIFIC SITE OBSERVATIONS A sketch map of the property with pertinent features indicated is provided in Figure 2. The property is bounded on the west by Thorndyke Road. The property along Thorndyke Road is approximately 200 feet above sea level. The property slopes down to the east very gently to the top of the shoreline bluff above Hood Canal. Most of the subject property is tree-covered with a mix of trees and a thick understory of brush. The overall slope of the steep shoreline bluff averages approximately 45 degrees; however, portions of the bluff are much steeper. The total height of the steep portion of the bluff on the subject property is approximately 60 feet. Most of the steep shoreline bluff slope is well vegetated with a mix of brush and trees. Trees consist of a ffi,ixofmatlJ.re,alder and fir. Shallow r: ... Stratum Group File: 5.29.06B 2 2 7 June 19, 2006 Parcel701253005"Jefferson County, WA Geologic Evaluation of Shoreline Bluff surface soil failures have taken place in the past and the toe area of the slope immediately above the shoreline has wedges of landslide debris with trees growing on the surface. Soils on the shoreline bluff consist primarily of layered very compact sand and gravel and silt layers including a few organic rich silt layers. A glacial till deposit is also present on the mid- bluff slope. This till is likely the Possession till, which is mapped on the slopes above the shoreline south of the subject property. Soil at the top ofthe bluff is very compact with very low permeability. Perched water flows down the slope at one location. A small trickle of water was flowing at this site at the time of my site visit in June 2006. Very little erosion is taking place at this location. However, the top of the bluff is indented inward slightly at this location due to the failures on the saturated slope below. The shoreline at the base of the bluff consists of a sand and gravel beach. The bluff is eroded by a combination of wave action that undermines the base of the bluff, ravelling of material off the exposed soils on the unvegetated portions of the bluff, and periodic shallow topsoil failures. The material eroded from the bluff face is removed from the base of the bluff and transported along the shore towards the north by wave action. Landslide debris has accumulated along portions of the base of the bluff and at other locations there is no landslide debris at the base of the bluff. I did not observe any evidence (tension cracks or trees rotated inward away from the top of the bluff) indicating an incipient global-type or deep-seated failure on the subject property. There is no evidence of deep-seated movement indicated on the UDAR imagery of the area either. The presence of mature trees on the bluff slope indicates that the erosion rate on the steep slope must be relatively slow. Stratum Group File: S.29.06B 3 ,<", = L I June 19,2006 Parcel 701253005, Jefferson County, W A Geologic Evaluation of Shoreline Bluff CONCLUSIONS AND RECOMMENDATIONS - Based on my visual inspection ofthe subject property and vicinity, I conclude that a residence located at least 40 feet back from the top edge of the steep upland slope will not be at risk from bluff erosion or landsliding within the expected life of the structure (100 years). It is my opinion from an engineering geology standpoint that this distance will be adequate to allow for expected bluff face failures that will result from the continued erosion of the base of the bluff by wave action and shallow soil failures on the bluff slope. I do not anticipate that the development of the subject property will cause any negative impacts on the stability of the slope as long as the following recommendations are followed. Roof drains and any drainage around the home should be discharged into a perforated dispersion pipe and infiltrated into the ground at least 30 feet from the top of the steep shoreline bluff. The dispersion pipe should be placed in a level infiltration trench excavated perpendicular to the slope. The access road to the house should be designed such that water can sheet flow off the sides of the road and not be concentrated at any given location. Site grading soils or debris, landscape debris, or any other material should not be disposed of over the bluff face or placed at the top of bluff or placed on the steep ravine slopes. Over time debris will break down and form an unstable mass that will slide damaging the slopes below. Native vegetation, particularly tress and low native brush within 30 feet of the top ofthe bluff should be disturbed as little as possible. If trees are thinned or limbed for view purposes on the bluff face, all debris should be removed from the bluff face. Limbing and topping of trees should be limited to maintaining the view corridor only and the native brush understory should be left in place. A septic drain field should not have any have any adverse affects on the stability of the bluff as long as the drain field is located at least 50 feet from the top of the bluff. Because the bluff is an eroding bluff, slope failures should be expected to occur on a periodic basis. The only way to prevent continued erosion of the base of the bluff is to construct hard armoring at the shoreline at the base ofthe bluff. However, the eroding bluff acts as a feeder bluff (erosion of the bluff provides sediment) for the beaches north and south of the subject property, and any shoreline armoring will have a negative impact on properties down drift from the bluff. The construction of shoreline armoring on other properties may cause an increase of erosion ofthe subject property. The owner of the subject property should contact Jefferson County officials regarding any proposed shoreline protection projects along the bluff or any File: 5.29.06B Stratum Group 4 June 19,2006 Parcel 701253005, Jefferson County, W A Geologic Evaluation of Shoreline Bluff shoreline protection construction. There are currently no shoreline-armored areas along this stretch of coastline. - Please note that there are inherent risks associated with building on 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 as required. 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, 111r, , mGro~up /' // .~ Dan McShane, M.Sc., L.E.G. 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