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Home My WebLink About802272003 Geotech AssessmentAspeCtconsu[,ing IN-DEPTH PERSPECTIVE January l3,2003 Mr. Martin Saunders 7375 Bridlevale Boulevard NW Bremerton, Washington 98311- Re: Geological Slope ReconnaisSance Saunders Residence 2364 Elkhorn Lane Quilcene, Washington Project No. 020188-001-01 Dear Mr. Saunders: This letter-report smnmarizes a geologic slope reconnaissance of the residential property located at 2364 Elkhorn Lane in Quilcene, Washington. Aspect Consulting performed this reconnaissance on December 24, 2002. Our services were performed in accordance with our scope of work and cost proposal dated December 31, 2002 and authorized by you on January 2, 2003. This letter-report summarizes our field notes and conclusions regarding the nature of the site slopes, the risk of damage to the slope from the proposed construction, and our recommendations for minimizing landslide and erosion risk to the proposed house at the site. This letter-report is the result of a reconnaissance-level investigation and should be used only for design of project elements as referenced in this letter-report. This letter-report has been prepared for the exclusive use of Mr. Martin Saunders and his agents for specific application to this project.. Within the limitations of scope, schedule and budget, our services have been performed in accordance with generally accepted geotechnical engineering and engineering geology practices in effect in this area at the time our report was prepared. There is no other warranty, expressed or implied. Our observations, ~indings, and opinions are a means to .identify and reduce inherent risks to the owner. Project Understanding We understand that present plans call for constructing an approximately 2,500-square-foot new house at the site. The house will be located on the crest of a gentle'ridge in. the central portion of the 20-acre parcel. We understand the house will be constructed using conventional wood- frame construction with a crawlspace and concrete floors. Our understanding of the project, including current design elements is based on our review of undated and untitled preliminary project plans provided by the builder, Dan Nieman. Mr. Martin Saunders "' ' ' '!! 111 January 13, 2003 ~.;! !!i MAR - 3 2003Pr°J.~e Observations ' c ou rv ucc. 0F COL~MUNI]'YDEVELOPME~f Site Conditions and Topography " T~e project site consists o¢ approximately 20 acres ofuadcve]oped, wooded prope~ i~ the foothills of the Otympic Mountains, west o~ the to~ o¢ Quilce~e, Washi~gtom ~ lot was cowmd wi~ youag, third-~owth fir ~ees with a de~se ~o~d cover ofbe~ bushes, grass and o~er s~bs. ~e topo~apby ofthe site w~ domiaated by a ao~-south ~e~diag with moderately to steeply s]o?~ aodb ~d south fla~s, and steepty slop~g east and west flanks. ~e proposed ~ouse site w~ located o~ thc east ~a~ o¢ the ridge aear the top. At the time o¢ our'field dsit, the top o~ thc ridge ~ad beea removed ~ougb blasting and excavatioa result~g ~a the ~ousc site crest berg lowered by a~pmximately ] 0 veAical [eet. Slope i~cli~atio~s o~ the ~o~ and sou~ fla~ were oa the o~der o~ 5~: iV to 3~: tV (~ofizomal:Ve~ical). Slope ~cli~tio~ o~ the e~t fla~ was oa thc order o¢ l~: iV ~or the upper approximately 200 feet and ~aduaily ~aAe~ed to low ~assland below. ~be west was also o~ ~e order o~ t ~: tV but was located approximately ] 50 hofizoata] fe~t west o~ proposed house locafiom Total elevatio~ change across the s~te was on ~e order o¢ 500 o. 020188-001-01 Drainage No ground water seepage was observed at the site during our field reconnaissance. It is our understanding that the drainfield for the house will be located on the north flank of the ridge, southwest of the proposed house location, and that roof and other storm water drainage will be collected and routed, via tightline pipe down the south flank along the access ~driveway. Vegetation Vegetation on the site cOnsisted primarily of low .shrubs with sparse, young third-growth fir trees. The area appeared to have been logged and partially replanted within the last 10 years. Geology Bedrock was exposed throughout the proposed building site area. Exposures 'in the area of the project were mapped by James C. Yount and Howard D. Gower (1991, U.S. Geological Survey Open File Report OFR 91-147, Bedrock Geologic Map of the Seattle 30' x 60' Quadrangle, Washington) as middle Eocene age, Lyre Formation. The shallow, subsurface conditions encountered in random hand excavations at the site as well as exposures on the flanks of the ridge were consistent with this description. The outcrops consisted of highly deformed (folded), gray and tan, pebble conglomerate. The high density of the rock makes this material relatively impermeable to surface water infiltration, and well suited for foundation support. In places, the bedrock was mantled with approximately 1 to 2 feet of sand and gravel that was the result of random deposition and erosion of much younger glacial outwash. The house location consisted of a flat building pad comprised-of the blasted rock and debris from lowering the top of the slope. The contractor informed us that 1 to 6 feet of fill was present across the area including the proposed house footprint. The fill had been graded smooth and was retained along the south side by a makeshift rockery from the larger blast rock. The rockery was on the order of 6 feet high. Page 2 Mr. Martin Saunders i j j'~': ~; January 13, 2003 !' i MA]:{ 3 2003 lPmj'elct No. J£F-FERSOh G0tJN!Y S/ope Stability ~.~)'r. OF COMMUNITY Two types of slope instability are prev~i~'s~o~-~'~F[A~i~'G}F-b'~-~:6'ck like the slopes on the subject site. Shallow debris or coliuvial landslides usually involve movement of a thin s]dn of loose and weathered soil and vegetation that mantles steep slopes. Landsliding of this type usually consists of the colluvial layers sliding or flowing over the more dense unweathered bedrock below. Other common types of landslide failure on rock slopes are rock-toPple, wedge-failure and block-failure and exfoliation. These types of mass-wasting consists of toppling or spalling of blocks of relatively intact rock from a steep slope or bluff. These blocks typically fail along high angle (typically near vertical) fractures present in the rock/soil units on near vertical rock outcrops. Exposures at the s~te exhibited near vertical bedding planes, as a result of tectonic uplift that occurred as the Olympic Mountains were formed. Toppling can occuralong these bedding-planes where exposed and can also occur along fractures parallel and near the surface of the rock as a result of differential expansion and contraction from freezing and thawing. 020i88-001-01 We noted evidence of small-scale (less than 10 square feet) rock toppie and exfoliation west of the house location. The exposed, vertical rock-outcrops on the east flank of the ridge are at risk for this type of ongoing mass-wasting. However, due to the anticipated design-life of the structure and the relatively long recurrence interval and observed small scale associated with these types of instabilities} the house site has a low risk of being damaged by landsliding and mass-wasting of this type. If slope and runoff conditions are Changed in an adverse manner during or after construction, the potential for surflcial landslides and slope damage could increase. Seismic Design Considerations Recent studies suggest that several east-west and north-south trending faults project near the project site. North-south faults are inferred to be present in the Agate Pass/Port Orchard Bay waterway between the Kitsap Peninsula and Bainbridge Island, and in Hood Canal between the Kitsap Peninsula and the Olympic Peninsula. East-west faulting has been delineated along the Seattle fault to the southwest and is inferred in the southern Kitsap Peninsula. According to U.S. GeolOgical Survey studies, the last large movement of the Seattle fault system, which is located in the vicinity of the south end of Bainbridge Island, occurred about 1,100 years ago, resulting in over 20 feet of surficial displacement. The displacement resulted in uplift of a beach and wave-cut marine bench that is presently visible as the terraces along Alki Point in West Seattle and Restoration Point at the south end of Bainbridge Island. Aspect Consulting recognizes that much higher earth, quake accelerations (measured as a proportion of the force of gravity [g]) than currently used in building codes have been postulated as possible for earthquakes associated with the Seattle fault. These higher acceleration values are associated with a much lower frequency of occurrence than those specified in the current Uniform Building Code (UBC). The current UBC, which has been the basis for design of single-family homes in the region since 1997, uses a seismic acceleration . selected to have lesS than a 5 percent chance of being exceeded in 50 years. Some local Page 3 Mr. Martin Saunders January 13, 2003 jurisdictions, notably the City of Seattle and in geologically hazardous areas, resulting in M^R -3 2003 Project-N0. 020188-001-01 less s~ingent desi~ basis. The 2000 International Building Code (IBC) has been published but not yet adopted by Washington State. The IBC code presents higher design values that are based on a minimum 2 percent chance ofexceedence in 50 years. Existing UBC code requires design tO 0.3g acceleration, while the proposed IBC code would require design to 0.5g acceleration, resulting in a more stringent design basis. This information is presented to inform current and future owners of the property that the general state of practice for single-family structure construction in Jefferson County is to design to the required UBC requirement of 0.3g acceleration. Owners and designers who prefer a lower level of risk may wish to design to the more strict IBC recommendation of 0.5g acceleration. If the IBC is legislated into municipal code in the future, local code may require design to the higher value. Conclusions Geotechnically, development of the site is feasible at the proposed 30-foot setback from the tOp of the eastern slope provided that the owner accepts the fact that there will always be some risk of slope instability and potential damage to the structure. Shallow site soils consist of blast- rock fill and very dense bedrock. No obvious evidence of past, historical deep-seated slope .instability was noted at the site. The site topographic, geologic, and surface and ground water characteristics suggest that the site has a low potential for slope instability under existing conditions. Improper hillside grading and poor storm water handling practices could increase the risk of slope failure. The risk of landslides increases considerably during a local seismic event. The recurrence interval of large earthquakes associated with movement along the shallow fault systems in the project area is still unknown, although it is hypothesized to be on the order of several thousand years. The February 28, 2001, 6.8-magnitude earthquake was a different fault system than the system that is located in the vicinity of the site. This earthquake; did not appear to affect slope stability at the project site. Due to the suspected long recurrence interval of earthquakes that would affect the site, the potential for surficial ground rupture or seismically induced landslides, as a result of movement on this fault, is considered low during the expected life of the structure.' In our opinion, the project is feasible provided the following recommendations contained in this report are followed and properly implemented. We understand that unweathered bedrock is present within approximately 4 feet of the existing blast rock surface in the vicinity of the proposed house location. Under construction conditions, the potential for surface erosion within the disturbed area is considered moderate and care should be taken in the excavation storing and moving of soil and rock. Recommendations and Design Considerations It is our understanding that significant filling in excess of 3 to 4 feet has occurred as a result of blasting and regrading at the site. We do not recommend any additional filling of any kind Page 4 Mr. Martin Saunders January 13, 2003 Project No. 020188-001-01 above any unsupported slopes steeper than 2H: 1V at the site. We recommend that all footings penetrate through the blast rock fill and are founded directly on undisturbed bedrock. Excavation depths on the order of 4 feet below the existing ground surface will be necessary. A maximum allowable soil bearing capacity of 3,000 pounds per square foot (psf) may be used for footings founded entirely on undisturbed bedrock. This value can be increased by up to one-third for transient wind and seismic loading. We recommend maintaining the minimum 30-foot setback distance proposed by the contractor. Drainage and storm water from hard surfaces (downspouts and driveways) should not be allowed to discharge on the site slopes or flow over unprotected slopes. All storm water from the house should be tightlined to a suitable, low-energy discharge point where erosion will not occur. Under no circumstances should storm water be allowed to flow over site slopes. We recommend the use of butt-fused High Density Polyethylene (HDPE) pipe for the tightline and appropriate energy dissipation at the discharge point. Thank you for the opportunity to provide these services to you. If you please do not hesitate to call. Sincerely, MAR -3 2003 Aspect consulting, LL¢ ; i ~' , John L. Peterson, P.E. Robert F. Cousins, P.G. Project Geologist rcousins~aspectconsulting.com Associate Geotechnical Engineer jpeterson@aspectconsulting.com V~cc: . Dan Nieman Construction (2 copies) WS_GEOTECH\020188 Saunders Residence\020188-001-01 Saunders Residence Ur. doc Page 5