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Last modified
1/25/2010 6:27:19 PM
Creation date
10/4/2006 11:46:12 PM
Metadata
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Template:
Floodplain Documents
County
Statewide
Community
State of Colorado
Stream Name
All
Basin
Statewide
Title
Colorado Flood Proofing Manual
Date
10/1/1983
Prepared For
State of Colorado
Prepared By
CWCB
Floodplain - Doc Type
Educational/Technical/Reference Information
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<br />43 <br /> <br />---- <br /> <br />- <br /> <br />The resultant of vertical forces is called uplift or the buoyant force and designated as Fs and <br />is assumed to act at the center of a horizontal area. Buoyant forces are determined by <br />calculating the volume of water displaced by the submerged or partially submerged object. <br />The resultant buoyant force is simply y times th" volume of displaced water. Figure 6.2 <br />depicts a house with a basement subjected to a saturated soil in addition to a water level <br />surcharge equal to h, I n this saturated condition, the soil particles are ineffect ive in transmit. <br />ting any vertical pressure and the total vertical height of the wall in contact with soil and <br />water is considered submerged, and Fs ~ yAH. <br /> <br /> <br />, <br />~~ <br />"'~ <br />~ <br /> <br />\J <br /> <br />h <br /> <br />. <br /> <br />. <br />, 111l~I\EJllf-ffiE~liE <br /> <br />=~lAT~~~~~~~I~]~"U: <br /> <br /> <br />Fe ' AH~ <br /> <br />A= AREA OF HORIZONTAL <br />, <br />Fl.OOR SURFACE (FT ) <br /> <br />Figure 6,2. Buoyancy force diagram. <br /> <br />The special case of lateral water loading combined with soil loading requires a separate <br />analysis, This situation occurs when 1) flooding is of a duration to allow saturation of the soil <br />or, 2) the groundwater and seepage are above the level of the bottom floor elevation. The <br />most common examples for this condition are structures with basements. The following <br />methodologies to calculate the hydrostatic forces assume saturation of the soil around the <br />structure. Two types of soil conditions are consideredi granular and cohesive. For determina- <br />tion of combined soil and hydrostatic pressure, the Rankine Theory for active soil pressure is <br />used. For a detailed discussion of this theory, the reader is referred to Reference 29, <br /> <br />Under normal loading conditions without flooding, the weight of soil around a basement <br />creates a pressure distribution similar to the hydrostatic pressure distribution for water. Dur- <br />ing flooding conditions, and assuming saturated the effective soils, the effective weight of the <br />soil is reduced due to buoyancy forces on the soil particies and the effective soil pressure is <br />reduced, The resultant horizontal force FH is due to thE' pressure distribution caused by the <br />specific weight of water and the effective saturated weight of soil. The combined specific <br />weight of water plus the effective saturated weight of soil is called the equivalent fluid <br />weight Yeq. The equivalent fluid weight varies based on the water surface elevation in com- <br />parison to the ground surface elevation. Based on a typical basement wall height of eight <br />feet, three conditions are assumed: <br /> <br />Condition One, The surcharge or height of water above ground level (h) is less than 25 <br />percent of the total depth of soil and water loading H (see Figure 6.3). <br /> <br />Condition Two: The surcharge or height of water above ground level (h) s between <br />twenty.five to seventy-five perCE"'t of the total depth of soil and water loading <br />H (see Figure 6.3), <br />
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