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Home My WebLink About902133004 Geotech Assessment~~r~oc~ so3 STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 December 13, 2005 ~ ~. ~ , .,~ David Lu ' r F ~: a 3447 Avocado Hill Way Hacienda Heights, CA 91754 ~ ,~ , J~~~~~~~ ~~~~~~ ~ Re: Geotechnical Report Parcel # 902133004 Port Townsend, WA Dear Mr. Lu: ..~ 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 that is adjacent to a steep slope and shoreline. This evaluation included a visual inspection of the property and vicinity, slopes on the property and vicinity, 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 is located on the southwest shore of Discovery Bay. The location of the property is indicated on Figure 1. SCOPE OF SERVICES The scope of our services included the following: I) 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 stability and slope stability in regards to siting a residence on the property, recommendations for site maintenance, and recommendations for further investigation, if necessary. December 13, 2005 `'- `~ Tax Parce1902133004,Defferson County, WA ,: Geologic Evaluation of Shoreline Site r - _ , GENERAL GEOLOGY tl~~~ri~~`~ ~~~~~~~~ t~~ 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 Geolo i~ c Map of Surficial Deposits ~n the Seattle 30' x 60' Ouadran~le, Washington (Yount, Minazd and Dembroff, 1993) indicates that the subject property is underlain by beach deposits and glacial advance outwash. The Surficial Map of the Port Townsend Quadrangle, Puget Sound Region, Washin on (Pessl, Deither, Booth and Minard, 1989) indicates the azea immediately north of the subject property is underlain by glacial recessional outwash deposits. Both maps reference the Geologic Map of Northeastern Jefferson County, Washington Gayer (1976). (Gayer, 1976) indicates both advance and recessional outwash is present in the vicinity of the property. The subject property and slopes in the vicinity aze 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. This interpretation is consistent with the mapping of Gayer (1976). SPECIFIC SITE OBSERVATIONS The subject property is bisected by Old Gardiner Road. The proposed building site is located on the lower portion of the lot between Old Gardiner Road and Discovery Bay. The proposed building site is located on a neazly flat bench of land created sometime in the past by the grading of the property for a rail line that has been long abandoned. The bench is approximately 60 feet wide at the proposed building site. The bench is bounded on the southeast by a shoreline bank that is approximately 8 feet high and slopes down to the beach at an angle of 50 degrees. 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 aze located on the slope and at the base of the slope. There appeazs to have been little if any erosion of the shoreline slope by wave action. The presence of mature firs on the slope down to the beach indicates that the slope erosion rate is very slow. Given the southeast facing aspect of the shore at this location and the very small fetch across this far end of Discovery Bay, lazge waves do not impact the site under any storm Stratum Group File: 11.29.OS 2 December 13, 2005 Tax Parcel 902133004, Jefferson County, WA Geologic Evaluation of Shoreline Site 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 appeaz to be effecting shoreline transport of sediment. The number of bulkheads and their locations do not appeaz to be having a significant effect on the sediment available to the beach from erosion. The level building area is bounded on the northwest by a steep slope sloping up to Old Gardiner Road. The steep slope above the building site is approximately 15 feet high at the south end and approximately 20 feet high at the north end. A rough access road has previously been graded down across this slope to the building site from Old Gardiner Road. The access road begins at the north side of the slope and angles down slope to the base of the slope. The access road is approximately i05-feet long. Approximately 40 feet of the road has been cut into the slope such that the slope immediately above the road is vertical. The slope below the road down to the level area slopes at an angle of approximately 40 to 45 degrees. Except for minor ravelling of soil on oversteepened portions of the slope the slope appears to be stable. Above the steep portion of the slope, the slope up to and across Old Gardiner Road is very gentle. The gentle slope is covered with mix of brush and a few mature fir and cedaz trees. 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 ditch built to transmit storm water run off from the uphill side of Old Gazdiner Road is located just to the north of the subject property. The ditch along the road is underlain by sandy soils. There is no evidence that water ever flows across the lower portion of the property or on the adjoining properties between Old Gazdiner Road and Port Discovery. Only under very unusual circumstances would storm water flow over land to the bay given the very porous nature of soils at the site. ~~ ~- r I ~~~ ~ ~~~ii~ ~:~~~~~ u~D Stratum Group File: 11.29.OS 3 -~~ ~ ~- '' ~ !~ December 13, 2005 ~ ' ` ~ " Tax Parcel 902133004, Jefferson County, WA Geologic Evaluation of Shoreline Site ~ ~ ~ ~. ~ ~ ~ ~ [~ ~ ~ i ~ ~ ~ 1 CONCLUSIONS AND RECOMMENDATIONS Based on my inspection of the subject property and vicinity, I conclude that shore erosion, landsliding, or soil movement will not threaten the proposed building site as long as my recommendations aze followed. The proposed residence should be located at least 10 feet back from the top of the slope above the beach. As noted above, the erosion at the base of the slope is very low as evidenced by the presence of mature fir trees growing at the ordinary high water mark (OHWM). This distance is greater than the shoreline setback of 1 foot for each foot of bank height as measured from the OHWM for bank heights of less than 10 feet (Jefferson County Shoreline Management Master Program). The Ordinary High Water mark is located at the base of the slope. The slope above the level area between the level azea and Old Gardiner Road is stable except for minor ravelling of soil on oversteepened portions of the slope. The only over steep sections are where the road was graded down from Old Gazdiner Road to the level building area. Development of the building site may require cutting into the slope either for purposes of siting the home or for purposes of accessing the home site from Old Gazdiner Road. A cut into the slope is feasible and will not pose a risk to Old Gazdiner 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 cut slope during final home design and or access drive 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 shoreline slope. Storm water from the roof and driveway should be evenly distributed on or within the ground at least 20 feet from the top of the shoreline slope. In addition the following general recommendations should be followed: Native vegetation, particularly trees 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 and erosion resistance. The present native vegetation on the shoreline slope of the site and on the slope between the level building area and Old Gazdiner Road is very effective and therefore should be disturbed as little as possible. Blackberry vines should be removed from Stratum Group File: 11.29.05 4 December 13, 2005 Tax Parcel 902133004, Jefferson County, WA Geologic Evaluation of Shoreline Site 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 slope, is a very effective erosion control plant. English ivy is present on portions of the site and should not be allowed to spread as it crowds out more desirable plants and can grow up into trees killing the trees. Yard waste and other deleterious material should not be placed on or over the steep slopes. 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 office at (360) 714-9409. Sincerely yours, Str tum Group i ~. Dan McShane, L.E.G., M.Sc. Licensed Engineering Geologist ~j~/o~r- ....o...u.~ ....:Y ~~ ,J ~ V, ~ ' J File: 11.29.05 Stratum Group Figure 1. Sit Viditity Map L ~S' D D Z 0 o ; 0 0 O D c '~ Z D O n 7 122°54.000' W 222°53.000' W 122°52.000' W WGS84 122°51.000' W woo .ji ~~~ a~1~!~ ~ a~ , ~ 2 ~~ ~ e {"`~ ~ ~, ~ ~~e i ~ ,•~ ~ ~ - -~ ~ ~ ~ ~ + ~~`` ` ~ , , ~ ~;~~!! 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