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FOUNDATIONS AND RETAINING WALLS <br />step 2: Determine the passive reaction and its point <br />-, of application?~ <br />step 3: Find the vertical forces against the base. <br />These forces are the weights. W„ of the <br />areas shown in Hgure 10.2]. Find the cen- <br />troid of each area and the moment arm, r„ <br />from the centroid of the zth area to point <br />G..3 <br />G <br />p~, min <br />t <br />~' <br />~~. <br />~; <br />B <br />Figure 10.21 Elements Contributing <br />to Vertical Force <br />hheel) <br />step 4: Find the moment about point G of all the <br />vertical forces and the active pressure. <br />M~ _ ~W;r; } RA,hrp 10.80 <br />step 5: Find the location and eccentricity of the ver- <br />tical force resultant. The eccentricity is the <br />distance from the center of the base to the <br />vertical force resultant. Eccentricity should <br />be less than B/6 for the entire base to be in <br />compression. <br />rR = M~ 10.81 <br />W, + RA,V <br />e=rR-2B 10.82 <br />pv, max <br />Figure 10.22 Resultant Forces on Base <br />lals <br />step 6: Check the factor of safety against overturn- <br />ing by summing moments about the toe. <br />The moment arms x and y must be mea- <br />sured from the toe.'4 The factor of safety <br />should generally exceed 1.5 for cohesionless <br />soils. and 2.0 for cohesive soils. <br />~ W';x; + RA,,,x,a.,, 10.83 <br />Fovenurning = <br />RA.h yA.h . <br />step 7: Find the maximum (at the toe) and mini- <br />mum (at the heel) foundation pressure on <br />the base. <br />~ L1'~; B Ra.v r ~ 1 <br />Pv :m axr Pv,min = 11 - J <br />10.84 <br />The maximum pressure should not exceed <br />the allowable soil pressure. <br />step 8: Calculate the resistance against sliding. <br />Disregarding the passive pressure, the active <br />pressure must be resisted by the shearing <br />strengi,h of the soil or the friction between <br />the base and the soil. Equation 10.85 is for <br />use when the wall has a key, and then only <br />for the soil to the left of the key. Equation <br />10.86 is (or use with the soil to the right of <br />a key, and for Hat-bottomed walls. <br />R,= (~W;+RA,,,)tan¢+cnB 10.85 <br />~~ The passive pressure is usually disregarded on the assumption R, = I ~ W; + RA,v) tan 6 } ea B ll).86 <br />that the bxkfill will be in place prior to the front 511, or that the <br />front fill will be removed at some future date for repairs to the <br />wall. <br />34 Other interpre[ations may include RA•v as a negative term in <br />PJ Moments can also be taken about the tce. the denominator. <br />PROFESSIONAL PUBLICATIONS INC. • P.O. Box 199. San Carlos, CA 94070 <br />