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Home My WebLink About701324004 Geotech AssessmentPO Box 44890 Tacoma, WA 9&448 253-537-9400 253-537-9401 Fax ERA `1~1 t.~4`~--3 tv3 June 19, 2007 T07192 Patsy and Vern Hoisted 940 McDonald Road Quilcene, Washington 98376 (360) 765-499b Subject: Geotechnical Report Parcel # 701324004 940 McDonald Road Quilcene, Washington ~ ~~ j° ~~ t u~ _ JUN 2 7 2007 _:,~~d1Y L._._ ....-...... Cr.~tELOPP~EN7 Dear Mr. and Mrs. Halsted: E3RA is pleased to submit this Geological Assessment-Landslide Hazard Geotechnical Report for the above-referenced project site. The purpose of our assessment is to address Title 18 of Jefferson County's Critical Areas Ordinances as they reia~te to landslide and erosion hazards on the site. This report is in response to some brush clearing that has already occurred on the site. Jefferson County has requested this report as paint of the permitting process. We understand that no fiuther development is planned. Our scope of services is limited to surface observations, geotechnical research, and report .preparation. This report has been prepared for the exclusive use of Patsy and Vern Halsted and their consultants, for specific application to this project, in accordance with generally accepted geotechnical practice. PROJECT DESCRIPTION The project site is a long, narrow, irregularly-shaped parcel located on the Dabob Peninsula between Red Bluff and Lindsays Beach. The site measures about 1,200 feet east to west, reaches a maximum width of somewhat more than 300 feet, and encompasses slightly more than 5 acres. The site is located at the end of McDonald Road ,in an area of timberlands and large residential parcels. The east boundary of the site is formed by the shoreline of Dabob Bay. Understory brush has been cleared from a lightly forested area, measuring about 150 feet by 200 feet, on the east half of the site. We understand that the area where brush clearing has occurred has recently been seeded with clover. No other development is planned for the site. SITE CONDITIONS During our reconnaissance of the site and nearby areas on June 14, 2007, we observed much of the site is an upland that is bordered to the north by the valley of an east west trending creek that flows into Dabob Bay at Lindsays Beach. The upland part of the site descends down to the east and Dabob Bay. i :; ~~. {~;~,~ ~ , u~ JUN 2 7 2007 ~` June 19, 2007 - ERA, Mc. .~ T07192J Hoisted Geotechnic~al Report ~ ,~~~, i~ i7E1tELOPMENT Grades on the north edge of the site measure about 60 percent over an elevation change of 100~feet on the northeast portion of the site and 250 feet on the northwest portion of the site. Grades on the remaining upland portion of the site vary. The westernmost three quarters of the upland area slope down to the east at grades that measure 10 percent or less. Grades then steepen, sloping down to the east at about 30 to 35 percent for a horizontal distance of about 150 feet (this is the area in which brush has been cleared}; grades then increase again farther to the east to about 45 percent over a horizontal distance of about 100 feet. Farther to the east, near the shoreline at the east boundary of the site, grades increase to more than 100 percent at a steep bluff, descending 150 to 200 feet to Dabob Bay. Site grading, in order to provide relatively level building and lawn areas, was conducted on the gently-sloping upland during original site development almost two decades ago. A house, shop, and septic system occupy the gently sloping, west three quarters of the upland. Vegetation on the gently sloped portion of the upland consists mostly of grass lawn, ornamental shrubbery, and a few scattered trees. Vegetation west of the gently-sloped portion of the upland, where slopes measure 30 to 35 percent and understory brush has been cleared, consists mostly of madrona and fir trees. Vegetation east of this area, where slopes measure 45 percent, and on the steep bluff, consist of large firs and madronas with a dense understory of salal, evergreen hucklebemes, and other brush. Vegetation on the steeply-sloped north edge of the site consists of large firs, cedars, alders and hemlocks with an understory that is predominantly sword ferns. The Geologic Map of Eastern Jefferson County identifies on the upland portion of the site as site soils as Vashon Lodgement Till, while soils at lower elevations, on the steep bluff and in the steep creek valley wall on the north edge of the site, are identified as Pre-Vashon Stratified Sediments. We observed Lodgement Till in small grading cuts, conducted during original site development, on the gently-sloping upland portion of the site. The Natural Resources Corr~servation Service maps the upland, gently-sloping portion of the site as AiC, Alderwood Gravelly, Sandy Loam, 0 to 15 percent slopes and the east portion of the site, where clearing has occurred, as a QuE, Quicene Silt loam, 30 to 50 percent slopes or BdE,l3eausite Alderood Complex, 30 to 50 percent slopes. AiC soils are not considered a severe erosion hazard, while the other soil types are considered severe erosion hazards. The Seismic Hazards Map of East Jefferson County does not depict the project site as a seismic hazard and the Erosion Hazards .Map of East Je,~ferson County depicts the site as an Erosion Hazard. No indications of flowing water or surface hydrology were observed on the upland portion of the site. A spring, which is .used as a water source for the parcel, is located on the c#~eek valley wall on the north-central portion of the site. No other seeps, springs, or other surface expressions of groundwater flow were observed on site or immediately off site. Springs or seeps were not observed within 1,000 feet of the site, but, due to dense vegetated cover and rough terrain, could exist without our notice. 2 June 19, 2007 .. T07192/ Halsted Geotechnical Report ice. '.. .t~ ~i ~~, ~ - f11 Y i rt ~ u~ 2 ~ 2ao~ ~ ~, ~~ ~? ~ .,~~n i_:-._ . ..,~_.w.__,. ,. DEVELOPMENT E3RA, Inc. We did not observe Landslide Hazard Indicators, such as hummocky terrain, scarps, slump blocks, etc., on the site or nearby the site. The north edge of the site and the steep bluff at the shoreline of Dabob Bay are mapped by the Coastal Zone Atlas as U, Unstable. The moderate slopes west of the steep blu.~ which include the area cleared. of under brush, are mapped as I, Intermediate. The upland portion of tha site is mapped as S, Stable. The Coastal Zone Atlas also indicates that recent land sliding has occurred in the steep bluff several hundred yards south of the site and just north of the site. Stable and Intermediate slopes have no known landslides while Unstable slopes have small slides or talus piles to small to map or are potentially unstable due to steepness or geologic conditions. CONCLUSIONS AND RECOMiV~NDATIONS Slopes over most of the site are stable and slopes in the area where brush was cleared, and just east of the cleared area, are classified as intermediate. In our opinion, the brush clearing will not affect slope stability on the site. If future development is to occur, we recommend that development be limited to areas where slopes are less than 45 percent and that no development occur on the steep bluff on the east edge of the site or on the area just west of the steep bluff, where slopes measure about 45 percent. The area where brush has been cleared is not in either of the areas in which we do not recommend development. ' According to the Natural Resources Conservation Service, the area, cleared of brush presets a severe erosion hazard. This area has already been seeded with cover. We recommend that the area be monitored to assure that germination occurs; if germination does not occur within one mouth of seeding, the area of brush-clearing should be re-seeded. Because the site is considered an erosion hazard, we recommend that basic erosion control system be implemented. This system should collect and filter all surface runoff through either silt fencing or a secured barrier of straw bales. The silt fencing/bale barrier should be erected on the down slope side of the area cleared of brush. if silt fencing is selected as a filter, this fencing fabric should meet the requvrements of Washington State Department of Transportation (WSDOT) Standard Specification 9-33.2 Table 3. In addition, silt fencing should embed a minimum of 6 inches below existing grade. If straw baling is used as a filter, bales should be secured to the ground so that they will not shift under the weight of retained water. Regardless of the silt filter selected, an erosion control system requires occasional observation and maintenance. Specifically, holes in the filter and areas where the filter has shifted above ground surface should be replaced or repaired as soon as they are identified. If noticeable erosion in the form of rill marks or small gullies occurs on the surface of the cleared area,. we recommend that a cover of straw be placed until vegetation has been re-established. 3 June 19, 2007 TO7192/ Hols#ed Geotechnical Report E3RA, Inc. CLOSURE The conclusions and recommendations presented in this report are based, in part, on our interpretations and assumptions regarding subsurface conditions; therefore, if variations in the site conditions are observed at a later time, we may need to modify this report to reflect those changes. We appreciate the opportunity to be of service on this project. if you have any questions regarding this report or any aspects of the project, please feel free to contact our office. Respectfully submitted, E3RA, Inc. ~u~ ,~u~ 2 ~ 2007 ~~n DEVELOpMEN7 Fred Rennebaum, L.E.G. Senior Geologist P.O l3ox 44890 Tacoma, Washington 98448 (253) 537-9400 r-Ex:~m TACO\\Tacoma-serverUOB FILES\2007 JOB FILES1T07192 PATSY HALSTED\T07192Landslide EvaluationReport.doc Four copies submitted 4