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Home My WebLink AboutBLD2009-00071 Geotechnical Report • • RECEIVED October 23, 2008 MAR - 5 2009 Mr. and Mrs. Richard Bollinger u J fY r[f r � PIUIRI D�n l 1220 Schwartz Road it U Nordland, Washington 98358 Re: Geotechnical Slope and Staircase Reconnaissance Bollinger Residence 1220 Schwartz Road (�oKacAAID-Eigoir, Washington Project No. 080193-001-01 Dear Mr. and Mrs. Bollinger: Thank you for the opportunity to perform a geotechnical slope reconnaissance for a proposed beach stairway project. We performed the reconnaissance at your property located at 1220 Schwartz Road in Nordland, Washington. Our services were provided in accordance with our Contract Agreement form dated September 12, 2008. This letter summarizes our field observations and conclusions regarding the nature of the site slope, our conclusions regarding the risk of damage to the slope and the proposed stairway to the beach, our recommendations for minimizing the risk of future erosion and slope damage, and reducing the risk of damage to the stairs. Our observations are the result of a reconnaissance-level investigation only and are intended only for developing general design criteria.No subsurface explorations were conducted. We understand that you wish to access the beach by building a staircase near the southern property line, from the top of the ramp cut into the top of the bluff, to the shoreline. The stairs will be built across the steep slopes that fall within the Jefferson County's steep slope ordinances. The purpose of this investigation and report is to specifically address the slope conditions with respect to the feasibility of construction of the stairs. Observations Site Conditions and Topography Our field reconnaissance was conducted on September 15, 2008. The site is located on the waterfront of Puget Sound, about 1 miles north of the town of Nordland, on Marrowstone Island. The site is several hundred feet long east to west and about 175 feet wide north-to - south. The eastern property line is oriented northwest-to-southeast and occupies about 190 feet of medium-to high-bluff waterfront. Site developments include a single-family residence located on the eastern portion of the uplands. The western and southern portions of the parcel are primarily woodlands. Grass lawn is present immediately surrounding the residence. The southeastern corner of the parcel is • C E NE 1� • Mr. and Mrs. Richard Bollinger October 23,2008 MAR - 5 It'14 Project No. 080193-001-01 sediments. Like the diamict sediments above,these lower soils have also been deformed by movement of the sediment prior to the end of the last glaciation. All of the sediments described above are typically very dense or hard due to compression by the weight of the Vashon glacial ice sheet. The very dense to hard in-situ soils that form the core of the steep slopes are locally covered with a thin layer of loose to very loose colluvium and landslide debris comprised mostly of silty, gravelly sand. A wedge of loose colluvium/landslide debris is present in places at the base of the steep slope where it accumulated after sliding from the slopes above. In many places,the colluvium/landslide debris has been removed from the toe of the bluff by wave erosion. The beach slopes seaward about 7 degrees. The upper 35-foot-wide horizontal band of the beach is composed primarily of slightly gravelly sand with scattered cobbles. The lower 10- foot wide band of exposed beach consisted of sandy gravel with cobbles. At the time of our investigation,there were moderate amounts of transient logs up to 2-foot- diameter and woody debris on the beach. The logs are concentrated in a band about 10 feet wide on the upper beach, and about 2/3 of the beach front had some log cover. Very little of the logs and drift wood appears to have originated on the nearby steep slopes. Drainage Soils on the bluff were generally dry. We did not observe evidence of concentrated water flowing from the uplands toward the bluff, and no well developed drainage channels or seeps were noted on the bluff. Corrugated plastic drain pipes were noted crossing the steep slope near the north end of the property, near the middle of the property, and near the stairway site at the south end. The pipes at the north and south end extended from the top of the bluff to the beach. The middle line was visible at the top of the slope but could not be seen at the beach, and appeared to be stretched tight and buried in sediment on the mid slope. Vegetation Vegetation on the uplands near the residence consists of grass lawn and landscape shrubs and trees. In the northern half of the site, grass lawn generally extends to the about 5 to 10 feet from top of the bluff. Salal, wild rose, and grasses grow at the edge of the bluff.Vegetation on the uplands in the southern half of the property consisted of third-growth conifers to 18 inches, and groundcover vegetation. Steep slope vegetation consisted of sparse alder trees, wild rose,berry vines, grasses and shrubs, sword fern. Large conifers are present on the uplands near the top of the bluff to the north and south of the subject slope. Most of the bluff slope is either bare or covered with small maple trees, blackberries, sword fern,and other shrubs and groundcover. Some trees on adjacent slopes show indications of rotation and soil creep during growth.Vegetation patterns generally suggest moderately well drained soil conditions on the slope. Page 3 • REC IVED• Mr. and Mrs. Richard Bollinger October 23, 2008 MAR - 5 2009 Project No. 080193-001-01 Conclusions f � COUNTY DCD Based on conditions present at the time of our site observations, we consider the steep slope in the southern portion of the bluff(the proposed location of the beach stairs)to possess a moderate risk of colluvial debris landslides during the anticipated life of the structure. The site appears to be at low risk for deep rotational landslides. The rate of retreat of the bluff(the rate at which the bluff's face is moving landward) is estimated at about several inches per year, averaged over many years. The actual retreat rate will be episodic with no observable movement for a number of years punctuated by loss of several feet or more in one season. In our opinion,the area of lowest landslide risk is the southern portion of the slope,adjacent to the re-graded ramp. The risk is lower here due to removal of the upper steep slope which has reduced the hazard from slides originating on the upper half of the slope. Groundwater and surface water are key factors in the stability of the bluff. Slope failures are generally most active above the seepage horizons and below areas of concentrated surface water.No natural seepage was observed discharging from the bluff,which is favorable from a slope stability perspective. If good surface water and bluff management practices are not followed,the rate of bluff retreat will increase. If runoff from the top of the bluff were to increase significantly and discharge onto the slope, erosion of the bluff and the chance landslides would increase. The presence of logs at the toe of the slope will tend to provide some protection from shoreline erosion and undermining of the toe the slope. If the logs and the debris at the toe of the slope can be maintained, they will help stabilize the lower portion of the bluff which will provide some long-term buttressing stability to the slope. Shoreline stabilization and lower slope protection will provide relatively little near-term reduction of risk of landslides from the middle and upper portion of the bluff north and south of the ramp to the stairway site. Beneficial vegetation on the slope will help reduce the year-to-year erosion of the slope soils, and will ultimately create fewer landslides. Based on our observations of conditions at the time of our site visit,we believe that construction of the staircase is feasible.A properly constructed stairway will not increase the rate of bluff retreat or result in a decrease of the stability of the bluff, or have adverse impacts to adjacent properties.A properly constructed and maintained staircase will not impact sediment transport to the shoreline. The owners must understand that the bluff slopes will move again and toe erosion will continue. Structures on the steep slopes should not be considered permanent.Future repair or replacement of the staircase should be anticipated. We believe that the risk to life posed by landslide hazards is very low during normal use of a properly constructed stairway that has been well maintained. Jefferson County may have other shoreline regulations that must be addressed other than the geotechnical slope stability issues. We suggest that these be investigated to determine the full Page 5 • • Mr.and Mrs.Richard Bollinger October 23,2008 Project No. 080193-001-01 Thank you for the opportunity to provide these services to you.If you have any questions, please do not hesitate to call. Sincerely, • • Aspect consulting, LLC i, .$eirififf&ii k ,14,1.PEP.4) ,...All 4,?%./., sla 1T Xv./. • ,44119 r Adr,.-7,,*, 0- 417 9..) ' %4114, 18802 ii§)dp 0/1 y", ,T. \ T / David H. McCormack ZONAL �1yCs David H.McCormack,LEG John L.Peterson,PE Associate Engineering Geologist Senior Associate Geotechnical Engineer dmccormack@aspectconsulting.com jpeterson@aspectconsulting.com cc: Pete Brockman,Brockman Builders,Inc. W:1 GEOTECH1080193 BollingenDeliverables\Stairs Reportdoc • RECEV41D MAR — 5 2009 Jfff[ 7f p COEJNTY OCO • Page 8