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Home My WebLink AboutBLD2007-00591 Geotechnical Report � Ce*esources LLC Ph.253-896-1011 5007 Pacific Hwy.E, Suite 20 Fx.253-896-2633 JAN 2 2 Fife, Washington 98424-2649 ,LU;'.1 NT J January 10, 2008 Olympic Ridge, Inc. 15940 Lindsay Lane Poulsbo, Washington 98370 Attn: Mr. Joe Klinker Geotechnical Report� j � Critical Area Assessment & Design Criteria 0 f7 — 0 5 673 Rainier Lane Jefferson County, Washington +.r�CIC`�''�1� F'dr`� '' ' MLS#:MLA07-00636 Job:OlyRdg.RainierLn.L INTRODUCTION This report summarizes our site observations and conclusions, and provides recommendations and design criteria for the support of the proposed addition to the existing residential structure at the site. We understand that you propose to construct an addition to the northeast portion of the residence (front yard) and remodel the stairway/deck located at the southwest portion of the existing structure. The remodeled timber deck will be elevated and supported on conventional foundation elements. The proposed addition will be situated towards the street, away from the critical slope area. The stairway/deck remodel will be situated in a flat to gently sloping area, also away from the critical area. The purpose of our services is to review the available geotechnical data for the area and provide foundation support recommendations and design criteria for the proposed addition and deck remodel. Mr. Biggerstaff, the undersigned is familiar with the site and surrounding area, having previously provided geotechnical reports in the site area. Our services are provided at your request and based on our review of the available soils and geologic data for the site area, and our site reconnaissance and data review. The project site is situated in an area of existing residential plat development. Vegetation at the site generally consists of grasses with scattered ornamental shrubs and small trees that surround the existing residence. The proposed addition and deck area are in the flatter portions of the lot, greater than 10 feet from the steeper sloping areas. The soils in the site area typically consist of a veneer of glacial soils over weathered basaltic bedrock. The surficial soils are in a medium dense condition. The underlying undisturbed glacial soils and bedrock material are typically in a dense to hard condition. No active groundwater seepage was observed at the site at the time of our site visit. No evidence of erosion or slope instability was observed at the site or the adjacent areas at the time of our site visit. No evidence of foundation distress was observed on the existing residential foundation at the site. CONCLUSIONS AND RECOMMENDATIONS Based on our site observations and experience in the area, the site is suitable for the proposed addition and deck/stairway remodel. The new foundation elements for the addition can be supported on the undisturbed medium dense or denser native soil or on properly compacted structural fill. The new foundation elements can be designed using an Olympic Ridge—Raini�ne January 10, 2008 • Page 2 JAN 2 2 2008 allowable soil bearing pressure of 2,500 psf for combined dead and lon6-term live loads, exclusive of the weight of the footing and any overlying backfill. Where the footings are founded on the undisturbed very dense glacial till or bedrock soils, the allowable bearing pressure may be increased to 3,500 psf. These values may be increased by one-third for transient loads such as those induced by seismic events or wind loading. We recommend a minimum width of 16 inches for isolated footings and 24 inches for continuous wall footings. All exterior-footing elements should be embedded at least 18 inches below the lowest adjacent finished grade. We recommend that any disturbed soils in the footing excavations be removed, or if practical, recompacted prior to concrete placement. We estimate that settlements of footings designed and constructed as recommended will be less than 1 inch, with differential settlements between comparably loaded footings of 1/2 inch or less. Settlements will occur essentially as loads are applied. Disturbance of the foundation subgrade during construction could result in larger settlements than predicted. We recommend that all foundations for the addition be provided with footing drains. Roof drains for the new addition should not be connected to the footing drain system unless adequate measures are taken to prevent surcharge of the footing drains. The new roof runoff maybe directed to the existing stormwater system. Foundation support for the deck/stairway remodel may be supported conventional foundation elements that extend below the surficial topsoil and loose weathered soil materials. We expect that this will require between 12 and 24 inches of embedment. FLOOR SLAB SUPPORT If slab-on-grade floors are utilized, they should be supported on medium dense or denser native soil or on structural fill prepared as recommended above. We recommend that floor slabs at the site be underlain by a 4-inch thickness of uniformly graded gravel or sand containing no more than 3 percent fines to provide a capillary break. The capillary break material should be placed in one lift and compacted to a minimum of 95 percent of the MDD. The capillary break material should be connected to a suitable drain outlet to provide an exit for any accumulated seepage. A vapor barrier, such as a polyethylene liner, is also recommended. A thin layer of "clean" sand may be placed over the vapor barrier and immediately below the slab to protect the polyethylene liner during steel and/or concrete placement. A subgrade modulus of 400 kcf (kips per cubic foot) may be used for design. We estimate that settlement of the floor slabs designed and constructed as recommended, will be 1/2 inch or less over a span of 50 feet. LATERAL RESISTANCE Lateral loads may be resisted by friction on the base of footings and floor slab and as passive pressure on the sides of footings. We recommend a coefficient of friction of 0.35 be used to calculate friction between the concrete and soil. Passive pressure may be determined using an equivalent fluid weight of 300 pcf (pounds per cubic foot). This assumes that structural fill is placed against the sides of the footings and that the top of the fill is confined by either a concrete floor slab or pavement. A safety factor of 1.5 is conventionally applied to these values. RETAINING AND SUBGRADE WALLS Retaining and/or subgrade walls are not expected at this site. Deep basements are not recommended. However, in the event that shallow subgrade walls are required (stem- type walls), they can be supported on shallow footings founded on dense native soils or structural fill, if properly prepared. Footings bearing on undisturbed native soils or Olympic Ridge—RainillOne . January 10,2008 Page 3 structural fill as described above can be designed using an average allowable bearing value of 2,500 psf with a maximum toe pressure of 3,500 psf, when the adjacent downhill slope is 4 to 1 (horizontal to vertical), or flatter. Lateral loads on conventional retaining structures founded as described above may be resisted by friction on the base of the wall footings and as passive pressure on the sides of footings. We recommend using an ultimate coefficient of friction of 0.35 to calculate friction between the concrete and dense native soils or on structural fill. Passive pressure may be determined using an equivalent fluid weight of 300 pcf. This assumes that structural fill is placed against the sides of the footings. A safety factor of 1.5 should be applied to these values, for sliding and overturning. The lateral active soil pressures acting on reinforced concrete retaining walls depend on the nature, density and configuration of the soil behind the wall. We recommend that walls supporting horizontal backfill be designed using an equivalent fluid density of 35 pcf for a level back slope behind the wall. This pressure is based on backfill placed within 2 feet of the wall being compacted by hand-operated equipment to a density of 90 percent of the MDD and consisting of clean sand or sand and gravel. The recommended pressure does not include the effects of surcharges from surface loads. LIMITATIONS We have prepared this report for use by Olympic Ridge and members of the design team, for use in the design of a portion of this project. The data used in preparing this report and this report should be provided to prospective contractors for their bidding or estimating purposes only. Our report, conclusions and interpretations are based on data from others and limited site reconnaissance, and should not be construed as a warranty of the subsurface conditions. A contingency for unanticipated conditions should be included in the budget and schedule. Sufficient monitoring, testing and consultation should be provided by our firm during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during the work differ from those anticipated, and to evaluate whether earthwork and foundation installation activities comply with contract plans and specifications. Our recommendations are not intended to direct the contractor's methods, techniques, sequences or procedures, except as specifically described in our report for consideration in design. If there are any changes in the loads, grades, locations, configurations or type of facilities to be constructed, the conclusions and recommendations presented in this report may not be fully applicable. If such changes are made, we should be given the opportunity to review our recommendations and provide written modifications or verifications, as appropriate. !I J A N 2 2 2008 _ J Olympic Ridge-Rainieene • January 10, 2008 Page 4 We hope this satisfies your current needs. If you have any questions regarding this letter or need additional information please call me. Yours Very Truly, GeoResources, LLC Bradley P. 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