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Home My WebLink About502155008 Geotech Assessment A PO Box 44890 Tacoma, WA 98448 253-537-9400 253-537-9401 Fax E3RA August 28, 2007 T07249 Marvin and Anne Bartelemay c/o Armstrong Homes 2715 Auburn Way North Auburn, Washington 98002-2499 Attention: Kevin Coons ~~~9~1 j~ , ~ 1~~ -_ ~~~ .~ ~ 2pl1? ~E~~E~~~~ C~~~N~i~ Subject: Geotechnical Report Parcel # 502155008 302 Black Point Road Brinnon, Washington Dear Mr. Coons: E3RA is pleased to submit this Geotechnical Report for the above-referenced project site. The purpose of our report ~ is to address Title 18 of Jefferson County's Critical Areas Ordinances as they relate to landslide hazards on the site. Plans call for the construction of a 20 foot by 20 foot two-story bathhouse with attached deck on the north portion of your site. Our scope of services is limited to surface observations, geotechnical research, and letter preparation. This report has been prepared for the exclusive use of Marvin and Anne Bartelemay, Armstrong Homes and their consultants, for specific application to this project, in accordance with generally accepted geotechnical practice. PROJECT DESCRIPTION The project site is a roughly square parcel that measures about 165 by 1,300 feet.. The site is divided by Black Point Road so that approximately 300 feet of the site is north of the roadway and the remainder of the site is south of the roadway. The north site boundary is at the shoreline of Pleasant Harbor. The site is bordered by large residentiaUrecreational properties to the east and west and undeveloped land to the south. Plans call for building a 20 by 20 foot, two-story bathhouse and attached deck on a level area graded into a slope that has grades that measure approximately 40 percent. The building area is located 60 to 70 feet from the top of a steep, shoreline bluff. The deck will be on the north side of the bathhouse, near the edge of the level building area, close to the 40 percent slope described above. August 28, 2007 E3RA, inc. T07242/ Bartiemay-Armstrong Geotechnical Report Because the deck is close to the top of a moderately steep slope, personnel from Jefferson County require that a Geotechnical Report, addressing geologic hazards be prepared for the site. SITE CONDITIONS During our reconnaissance of the site and nearby areas on August 23, 2007, we observed that the north portion of the site, north of Black Point Road, initially slopes down to the north toward the waters of Pleasant Harbor at grades of about 15 to 20 percent. Slopes then steepen about 100 feet north of the road to about 40 percent, descending down to a nearly vertica130 to 40 foot-high shoreline bluff. A level building area was graded into the 40 percent slope approximately 10 years ago. Grading for the building area consists of an 8 to 10 foot high steep cut on the upslope portion of the building area, and a shallow fill, which appears approximately 4 feet deep, on the downhill side of the building area. Currently, foundation forms and rebar for the bathhouse are in place. A graveled driveway winds down to the building site from Black Point Road. Slopes on the site south of Black Point Road vary from 10 to 40 percent in grade and slope up to the south. Soils that underlie the site were visible in isolated areas on the face of the steep bluff at the shoreline, in the grading cut on the upslope part of the building area, and in several small grading cuts created during the construction of the driveway. In all cases, observed soils consist of very dense, cobble and gravel rich, glacial till. The Geologic Map of Eastern Jefferson County identifies onsite soils and soils adjacent to the site as Vlt, Vashon Lodgment Glacial Till. Vashon Lodgment glacial till is deposited at the base of a glacier, in this case, The Vashon Continental Glacier, which withdrew from the area approximately 12,000 years ago. Lodgment till has been compacted to a very dense state by the over-riding weight of the continental glacier. The Black Point Map of Environmentally Sensitive areas maps no erosion or seismic hazards on the site while the shoreline area is mapped a high landslide area. Some of the area south of Black Point Road is mapped as a slight landslide hazard. The shoreline of the site is mapped by the Coastal Zone Atlas as URS, unstable, recent landslide. The remainder of the site is classed as I, Intermediate. No indications of flowing water or surface hydrology were observed on site. No 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 could exist without our notice. Seeps and springs are uncommon in areas overlain by lodgment glacial till. A small landslide deposit was observed along the very steep bluff at the shoreline on the northwest corner of the site. The landslide that created the observed deposit appears to have been shallow and supe~cial; vegetation on the landslide deposit indicates that the landslide occurred approximately 10 to 20 years ago. 2 August 28, 2007 E3RA, Inc. T07242/ Sartlemay-Armstrong Geotechnical Report We did not observe Landslide Hazard Indicators, such as hummocky terrain, scarps, slump blocks, etc., elsewhere on site or nearby off site. Vegetation at the steep bluff consists of a few scattered, large cedars, smaller firs and alders, and dense brush. Vegetation between the shoreline bluff and the building site consists mostly of blackberries and small alders, while the building site itself is covered with grass. Vegetation upslope of the building area consists of smaller fir, alders, madronas, and brush typical of regrowth in logged-off areas. CONCLUSIONS AND RECOD~IlVIENDATIONS Plans call for the construction of a 20 by 20 foot bathhouse with attached deck, on a level area that was graded into a 40 percent slope. The attached deck will be built near the down slope edge of the graded area. The deck and bathhouse will be located more than 50 feet from the top of a very steep, 30 to 40 foot high shoreline bluff, upon which a shallow, surficial landslide occurred within the last few decades. It is recognized that very steep bluffs, comprised of very densely consolidated lodgment till, retreat at an average rate of approximately 1 foot per decade. Generally, bluff retreat occurs in fits and starts, where several feet of the bluff face slides at one time, then no other slides occur for decades. It is-our opinion that the mapped landslide that occurred on the shoreline bluff is of this type, and that the rate of retreat is too slow to affect the planned construction, which is more than 50 feet from the top of the steep bluff. The level area, where the bathhouse and deck will be built, is underlain by very dense glacial till that slopes at about 40 percent. It has been our experienced that 40 percent slopes comprised of very dense glacial till are globally stable and are not landslide hazards. Based on this, it is our opinion that the planned construction will not significantly increase the landslide, erosion, or seismic hazards on or nearby the site. We do recommend additional geotechnical analyses for any construction that occurs within 40 feet of the very steep shoreline bluff. The north edge of the deck will be located on several feet of fill. We recommend that foundations for the north edge of the deck extend down to native, undisturbed soils. We also recommend that roof runoff from the new bathhouse not be allowed to flow freely over the fill slope north of the deck or to flow freely over the top of the steep bluff. Collected roof runoff should be tightlined down an area below the small fill slope, but at least 40 feet from the steep bluff, where it can dissipate on splash blocks, or it should be tightlined down to Pleasant Harbor. 3 August 2~, 2007• ` T07242/ Bartlemay-Armstrong 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. Fred Rennebaum, L.E.G. Senior Geologist P.O Box 44890 Tacoma, Washington 98448 (253) 537-9400 FERjm TACOUOB FILES\2007 JOB FILES\T072469 BARTLEMAY -ARMSTRONG GEOTECH1T07249 Bartlemay-Armstrong Landslide Evaluation.doc Four copies submitted 4