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<br />footing thickness to rninimize water intrusion below footing <br />level. The excavation above the top of the spread footing <br />should be filled with riprap sized to withstand flood flow <br />velocities. <br /> <br />d. Spread FootinQs Placed on Tremie Seals And Supported On Soil. <br /> <br />o Place the tremie base three feet below the scour line (step 4) if <br />the tremie is structurally capable of sustaining the imposed <br />structural load without lateral soil support. <br /> <br />o Check the design for the superf100d to insure a safety factor of <br />one. <br /> <br />e. For Deep Foundations (Drilled Shaft And Driven Piling) With <br />FootinQs Or Caps. <br /> <br />Placing the top of the footing below streambed a depth equal to the <br />estimated contraction scour depth will minimize obstruction to <br />flood flows and resulting local scour. Even lower footing <br />elevations may be desirable for pile supported footings when the <br />piles could be damaged by erosion and corrosion from exposure to <br />river currents. <br /> <br />a. Repeat the procedure in steps 2 - 6 above and calculate the scour for a <br />superf100d, a flood exceeding the 100 year design. It is ~ecommended that <br />this superflood or check flood be on the order of a 500 year event or a <br />flood 1.7 times the magnitude of the 100 year flood if the magnitude of the <br />500 year flood is not known. However, flows greater or smaller than these <br />suggested floods may be appropriate depending upon hydrologic <br />considerations and the consequences associated with damage to the br;dge. <br />An overtopping flood within the range of the 100 year to 500 year flood may <br />produce the worst case situation for checking the foundation design. The <br />foundation design determined under step 7 should be reevaluated for the <br />superf100d condition and design modifications made where required. <br /> <br />o Check to make sure that the bottom of spread footings on soil or <br />weathered rock is below the scour depth for the superflood. <br /> <br />o All foundations (shallow and deep) should have a minimum factor of <br />safety of 1.0 (ultimate load) under the superfloOd conditions. Note <br />that in actual practice, the calculations for step a would be performed <br />concurrently with steps 1 through 7 for efficiency of operation. <br /> <br />14 <br /> <br />I <br />I <br />~"~'I.' <br />~ <br /> <br /> <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />Irl <br /> <br />~ <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />l_. <br />I <br />