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HomeMy WebLinkAboutBuilding Geo-Tech Report 2007-8-11 / /' @ Rug, 162007 10: 17RM P2 Au ~ Cmn 2.COt-- Dl ~ ~ 1:- Branch Engineering, Inc. \, r.wy." '\ FROM :~JCH -, FRX NO. :5416833373 , ' , , 31 0 5th Srr-eet Springfield, Oregon 87477 (541) 748-0837 Fax (541 J 746-0388 August 11 , 2007 Walter Custom Homes 2863 Riv~rwalk Loop Eugene; Oregon 97401 REO: LIMITED GEOTECHNICAL INVESTIGATION NEW SINGLE FAMILY RESIDENCE . LOT 41 - MOUNTAIN GATE SUBDIVISION .'.' cp4z-S ])();q J<;/Z7ifj) SPRINGFIELD, OREGON Introduction A site,'reconnaissance and subsurface ,investigation was conducted on July 27, 2007 for ,the purpose of evaluating the soil conqitions with respect to construC\ing foundations and retaining walls for a single family residence, Proiect Location and Descriotion :rhesubject site is located at Lot 41 off a private accessway on'Dogwood Street in the Mountain Gate Subdivision, The South portion of the lot slopes slightly to the North while the North part slopes steeply (40% - 60%) and'is tree covered, It is our understanding that the building area will be located in the flatter portion of the lot. ' 'Fhe general geology of the area consists of an underlying fractured basalt intrusion. There is no indication of recent gross ground movement or locallandsliding in the immediate area of the site; however,'large ancient landslides exist within the Mountain Gate area, ' Investloation SummalV On July 27, 2007, two exploratory test pits were advanced on the site using a rubber track-mounted Cal D50 excavator, Test Pit 1 (TP-1) was located approximately 55' North of the edge o(asphalt and 15' East of the property line, TP-2 was located 18' off the edge of asphalt and 29' East of TP-1, The depth of each test pit was only ,about 9-inches below the existing ground surface, each met excavation refusal on hard fractured basalt ' The proposed building area on Lot 41 is underlain by native material that consists of about 9- to 12-inches of loose, dry, clayey silt topsoil underlain by fractured, weathered basalt. The basalt became very difficult to excavate with the exploratory equipment; however, larger equipment is expected to be abie to excavate the site several feel deeper, but a hydraulic hammer may be required to complete site excavation depending on the deplh of design subgrade. " CIViL STRUCTURAL TRANSPORTATION SURVEYING ./ FROM :WCH FAX NO. :5415833373 Aug, 152007 10:17AM p~' Lot 41 Mountain Gate Sub. Page 2 No seepage J..as observed in either of the test pits, There ar~ no observed landslides affecting the subject site or springs on or near the site; however; shallow slumps and near surface seepage should be expected on hillsides of this steepness, especially during the wet season, In addition, the Geotechnical Investigation prepared by Boire Associates Inc"dated November 12. 2002, and the subsequent Geotechnical Requirements for Individual Building Lots for phase III~ prepared by K&A Engineering Inc., dated March 28, 2006 have been reviewed, This report satisfies the Levell requlremerits as specified for Lot 41. Conclusions and Recommendations We conclude that Lot 41 is suitable for development of'the proposed single-family reside~ce provided that the following recommendations are incorporated into the design and ,construction of the residence, Site Preparation: The building pad area shall be stripped of surface topsoil and organic debris, OrganicS shall be'removed from the site, The depth of unsuitable topsoil is approximately 10- inches, and after removal of organics, this material may be,used ,in landscape areas on site and to backfill foundations. Filled landscape areas should be moderately compacted to mitigate. erosion. The building pad subgrade should be excavated to a minimum depth of 12.inches to expose a consistent hard, basalt material. We recommend that the subgrade be, observed prior to placement of aggregate fill, or forming of foundations, No on site materials shall be used for structural fill and any imported fill mat~rial to the site shall be granular and approved by the geotechnical engineer prior to import to the site.- Structural Fill: Structural fill material shall 'only consist of imported crushed aggregate or other granular material approved by the geotechnical engineer, Fill material shall be compacted to at least 95% of the materials maximum dry density as determined by ASTM Method D-1557. Fill.' should be placed in B-inch thick uncompacted lifts, moisture Donditioned and compacted with suitable equipment.. Fill behind retaining walls requires the same compaction standard, although thinner lifts and lighter compaction equipment is recommended to reduce the force on the wall. . Foundations: Based on our explorations, shallow spread footings may be designed an allowable soil bearing capacity up to 2,500 psf if placedon suitable subgrade at a depth of 12.inches or more below existing surface grade or engineered aggregate fill, The coefficient of friction for concrete , poured neat against fractured basalt 0.45. and 0,50 for concrete on compacted crushed aggregate fill placed on sLibgrade with less that a 10% slope, All up slope found,atier15-Shall have perimeter footing drains that outlet to an approved point Df disposal. Fou~datlOn slabs-on-grade are not anticipated for this residence. F;ROM 'hlCH FRX, NO. ,:5416833373 Rug, 162007 10: 17RM P4 Lot 41 Mountain Gate Sub. Page 3 Retaining Walls: Non-restrained retaining walls are those that are free to rotate about their base. . .-., ' most cantilever walls are in this category. Restrained walls are those that are prevented from, rotation due to comers or floor slabs, The following equivalent fluid pressures are recommended for the 'design of non-restrained and restrained walls with adequate rear wall drainage to prevent hydrostatic pressures behind the wall, High shear' strength granular backfill is recommended with drains Installed at the base of the wall. Table: Recommended Equivalent Fluid Pressures for Retaining Walls Supporting Fractured Basalt Backfill Slope HorizontalNertical Level Equivalent Fluid Pressure I (pcf) for Non-Restrained Wall 30 2:1 40 Equivalent Fluid Pressure (pcf) for Restrained Wall 35 .45 The passive fluid pressure for either wall type may be taken as SOD pcfand'the coefficient for ,concrete poured neat against fractured basalt is 0.45, The upper 1"foot of material shall -be neglected for passive pressure -calculations. The allowable soil beating capacity is 2,500 psf for footings at least1 foot below original ground elevations and the bearing capacity may be increased by 1/3 psf for short term loading, such as wind or seismic, If traffic, compaction equipment, or other type of dynamic toads, is expected within the hori;contal distance equal to the height of the wall, an additional uniform lateral equivalent fiuid pressure equal to 10 pcf should be added to the above stated fiuid pressures acting on the wall. The bearing capacity may be increased by 1/3 for short-term loading such as wind or seismic forces, . . Cut/Fill Slopes: Fill slopes are not recommended for this s~e other than landscape areas where fill should not exceed 2-feet in thickness, Cut slopes oui side the building pad area shall not exceed 4 feet in height with level backfill and final slope grades shall be 2: 1 (horizontal:vertical) unless properly retained. ' Drainage; Surface water shall be directed away from and around the, residence, Surface water. shall not be increased or concentrated 50 as not to cause erosion or flow onto adjacent properties, Roof drainage shall be collected and conveyed via tight-line to the designed storm water system, On-site infiltration of surface water is not recommended, Up slope foundations shall have footing drains and a low point drain shall be provided for the interior of the structure, ,All drains shall outlet to an approved point of disposal. , ' FROM :WCH FAX NO. :5416833373 Aug, 16 2007 10: 18AM P5:1" ... Lot 41 Mountain Gate Sub. . Page 4 Recommendations for GeotechnlcalObservations Ourino Construction Periodic site observations by a geotechnical representative of Branch Engineering Inc are recommended during the constructiofl of the project; the specific phases of construction that should be observed are: ' I Recommended Construction Phases to be Observed by the Geotechnical Engineer At completiDn of excavation to subgrade for fill or'foundatlon Imported fill m'aterial Observation by geotechnical'~nglneer Placement or Compaction of fill material Observation of material or information on, material type and source sent to Branch Engineering Inc ' . Observation by geotechnical engineer or test results by qualified testing agency Observation by geotechnical engineer Placement of retaining wall drains and backfill ' Completion of cut or.fill slopes . 'Observation by geotechnical engineer If you have questions or require additional geotechnical services, pleas~ contact Branch Engineering Inc.. Sincerely, Ronald J, Derrick, P.E.