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HomeMy WebLinkAbout948200013 Geotech Assessment GeoResources, LLC Ph 253-279-1023 5113 Pacific Hwy E., Ste. 1-I Fx 253-638-8992 Fife, Washington 98424-2642 Olympic Ridge 15940 Lindsay Lane Poulsbo, WA 98370 A~n: Mr. Joe Klinker · Geotechni~l Consultation Single-Family Residential Lot Edgewood Village, Lot 13 Jeffe~on County, Washington INTRODUCTION This repod summarizes our site obse~ations and ~nclusions, and provides our recommendations for the suppod of the proposed residential ~m~ure for Lot 13. We unde~and · that you propose to ~n~m~ a ~nventional residential ~m~ure and ddveway at the site. In general, grading for the proposed site development will ~ minimal and is generally completed. The pu~ose of our se~i~s is to evaluate the soil renditions at the s~e and provide foundation suppod re~mmendations and design cdteria for the proposed ~m~ure. Our se~ices are provided at your request and based on our review of the available soils and geologic data for the site area, and our site re~nnai~ance and our site ex, rations. The proje~ site is situated in an area of e~ablished and newer residential concretion. Vegetation at the site generally ~nsi~s of grasses w~h ~Eered brash in the flaker podion of the lot and small trees and brash on the sloping and rear podions of the lot. The proposed building area is in the fiat ~dion of the lot, greater than 10 feet from the toe of the sloping area. Two backhoe te~ pits were ex,rated at the toe of the slope to evaluate soil ~nditions. The soils ~nsi~ of a thin veneer of weather~ bedrock and topsoil over basaEic bedrock at approximately 1.5 to 2 feet below the exi~ing ground su~a~. The weathered soils were in a medium dense ~ndition. The underlying bedrock matedal was in a ve~ dense to hard condition. No a~ive groundwater seepage was obse~ed at the site at the time of our site visit. CONSLUSIONS AND RECOMMENDATIONS In general, footings found~ on undisturbed medium dense or denser native soil or on propedy ~mpacted ~m~ural fill ~n ~ designed using an allowable ~il beadng pressure of 2,500 psf for ~mbined dead and Iong-te~ live loads, exclusive of the weight of the footing and any overlying ~ckfill. Where the footings are founded on the undiluted bedrock soils, the allowable ~adng pressure may ~ increased to 3,500 psf. These values may be increased by one-third for transient loads such as tho~ indued by seismic events or ~nd loading. We re~mmend a minimum width of 16 inches for isolated footings and 24 inches for ~ntinuous wall footings. All e~edor-footing elements should be emb~d~ at lea~ 18 inches below the Iowe~ adja~nt finished ~rade. We recommend that any di~u~ed soils in the footing ex,rations be removed, or if pra~i~l, re~mpa~ed prior to concrete pla~ment. We e~imate that seElements of footings designed and con~m~ed as re~mmended will be le~ than 1 inch, with differential se~lements be~een comparably loaded footings of 1/2 inch or less. Se~lements will occur e~entially as loads are applied. Distu~ance of the foundation su~rade dudng con~ru~ion ~uld result in larger settlements than predi~ed. Olympic Ridge July 10, 2002 Page 2 We recommend that all foundations be provided with footing drains. Roof drains should not be connected to the footing drain system unless adequate measures are taken to prevent surcharge of the footing drains. FLOOR SLAB SUPPORT Where slab-on-grade floors (including garage 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 matedal 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 barder and immediately below the slab to protect the polyethylene liner dudng steel and/or concrete placement. A subgrade modulus of 250 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) above the water table, and 160 pcf for saturated soils. 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 propedy prepared. Footings bearing on undisturbed native soils or structural fill as described above can be designed using an average allowable bearing value of 1,600 psf with a maximum toe pressure of 2,500 psf, when the adjacent downhill slope is 4 to I (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 fdction of 0.35 to calculate fdction 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 backslope 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 Io~ r"~'., ';~ ,~. [.' JUL 16 DEPT. OF CO~MUNII'Y DEVELOP~,!ENI' Olympic Ridge July 10, 2002 Page 3 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 LSI/ADaPT Engineering OlympicP~ C C