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Home My WebLink About902133009 Geotech Assessment STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 September 23, 2002 Henry Souza Design by Souza, Inc. 164 Swanson Avenue Port Townsend, WA 98368 Re: Geotechnical Report, Steep Slope Parcel # 902133009 Port Townsend, WA Dear Mr. Souza: Stratum Group is pleased to present the results of our geology investigation of the above referenced property located on the southwest shore of Discovery Bay, Jefferson County, Washington. The purpose of this geology evaluation was to qualitatively evaluate the risk of slope failures impacting the proposed building site and provide general site development and maintenance recommendations for the property on a shoreline slope. This evaluation was limited to a visual inspection of the property and vicinity, a visual inspection of the slopes, shallow hand dug test pits, review of available geologic mapping in the area, and inspection of the shoreline at the property and vicinity. The property consists of two lots Lot 3 and Lot 4 and is on the southwest shore of Discovery Bay. The location of the property is indicated on Figure 1. The property is located in the southeast 1/4 of Section 13, Township 29N, Range 2 W 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 the slope conditions, shoreline conditions, and relevant conditions in the vicinity of the property. 2) Observed surface soil conditions on the slopes and on the uplands above the slope. 3) Prepared this report summarizing our findings, including a qualitative evaluation of the shoreline bluff stability in regards to the residence on the property, recommendations for site maintenance, and recommendations for further investigation, if necessary. September 25, 2002 Tax Parcels 902133009 and 902133008, Jefferson County, WA Geologic Evaluation of Shoreline Site 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 interglacial periods, the underlying bedrock was eroded. Relatively thick glacial related deposits and interglacial fluvial sediments were deposited over the underlying bedrock in the vicinity of the site. The Geologic Map of Washington - Northwest Quadrant (Dragovich and others, 2002) indicates that the subject property is undifferentiated glacial outwash deposits. Observations on the subject property and vicinity are consistent with the mapping described above. The subject property and slopes in the vicinity are underlain by loose sand and gravel that is poorly sorted. The sand and gravel is most likely a recessional ice margin deposit of sand and · gravel deposited between stagnating glacial ice within what is now Discovery Bay and the uplands to west. SPECI17IC SITE OBSERVATIONS The subject property bisected by Old Gardner Road. The proposed building site is located on Lot 4 between Old Garner Road and Discovery Bay and is accessed via a gravel access road that is also utilized by the property to the north. The proposed building site is located on a nearly flat bench of land created sometime in the past by the grading of the property for a rail line that has since been long abandoned. The bench is approximately 50 feet wide at the proposed building site. The slope down to the beach to the southwest is on the order of 50 degrees and is approximately 8 feet high. The beach is a gravel beach with a few cobbles. The slope down to the beach is underlain by sandy gravel and is covered with grasses, rose and other brush. A few mature fir trees are located on the slope and at the base of the slope. There appears to have been little if any erosion of the shoreline slope by wave action. The presence of mature firs on the slope indicates that the slope erosion rate is very slow. Cdven the southeast facing aspect of the shore at this location and the very small fetch across Discovery Bay, large waves do not impact the site under any storm conditions. Hence erosion is limited to very small waves during very high tide events. The beach appears to be in good condition. Sources of sediment from eroding bluffs are available to the north and the few bulkheads that are present to the north of the property do not appear to be effecting shoreline transport of sediment, and the number of bulkheads and their location does not appear to be having a significant effect Stratum Group File: 9.19.02 2 September 25, 2002 Tax Parcels 902133009 and 902133008, Jefferson County, WA Geologic Evaluation of Shoreline Site on the sediment available to the beach from erosion. The slope above the building site to the northwest slopes up at angle of 45 degrees for the first 7 feet of elevation gain. Above this point the slope is on the order of 20 degrees up to Old Gardner Road. The slope above the proposed building site is underlain by loose sand and gravel. The vegetation consists of blackberries, rose and other brash with maples, madrone, Douglas fir and western red cedar. In addition, at least one hornet nest was located on the slope at the time of my site visit. Except for minor ravelling of soil on oversteepened portions of the slope the slope appears to be stable. No water seeps were observed anywhere On the slope of the subject property at the time of the site visit, and no vegetation is present on the slope that indicates any area of seasonal seeps. No indication of deep-seated landslide activity was observed on or in the vicinity of the subject property. A culvert built to transmit storm water mn off from the uphill side of Old Gardner Road is located just to the north of the subject property. The ditch along the road is underlain by sandy soils and the low area above the culvert is as well. There is no evidence that water flows across the property from the culvert out let and it appears that water rarely ever flows through the culvert and only under very unusual circumstances would storm water generated flow over land to the bay given the very porous nature of soils at the site. CONCLUSIONS AND RECOMMENDATIONS Based on our inspection of the subject property and vicinity, we conclude that shore erosion, landsliding, or soil creep will not threaten the proposed building site as long as our recommendations are followed. The proposed residence should be located at least 10 feet back from the top of the slope above the beach. It is our understanding that constructing the proposed residence may require cutting into the base of the slope above the building site in order to facilitate the construction of the house. A cut into the slope is feasible and will not pose a risk to Old Gardner Road or the proposed residence as long as the cut slope is supported by an engineered retaining structure. Stratum Group can provide engineering specifications for supporting the base of the cut slope during final home design. Drainage of storm water and roof drainage is not anticipated to be a problem as the site is very well drained. However, storm water run off should not be concentrated at points close to the Stratum Group File: 9.19.02 3 September 25, 2002 Tax Parcels 902133009 and 902133008, Jefferson County, WA Geologic Evaluation of Shoreline Site steep shoreline slope. We suggest that storm water from the roof of driveway be evenly distributed on or within the ground at least 20 feet from the top of the shoreline slope. The following general recommendations should be followed: · Native vegetation, particularly tress and low native brush on the slope and at the top of the slope along the shoreline should be disturbed as little as possible. If trees are thinned or limbed for view purposes, any debris from thinning on the slope itself should be removed from the slope. · Maintaining healthy native vegetation cover on the slope is critical to maintain slope stability. The present native vegetation on the slopes of the site is very effective and therefore should be disturbed as little as possible. Blackberry vines should be removed from the slope and not allowed to spread as they can crowd out desirable plants and do not provide effective erosion control. Salal, which is present on portions of the bluff, is a very effective erosion control plant. · Yard waste and other deleterious material should not be placed on or over the slope. Such material smothers vegetation, retains water and acts to destabilize the slope. CLOSURE 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 as required. We appreciate the opportunity to be of service to you. Should you have any questions regarding please contact our orifice at (360) 714-9409. Sincerely yours, Stratum Group D~an McShane, P.E.G., M.S. Professional Engineering Geologist Stratum Group File: 9.19.02 4