pressure diagram below elevation 4 on Figure 5 is simply the difference 

 between the value of 180 pcf for the passive pressure and 18 pcf for the 

 active pressure^ giving 162 pcf if the water lag pressure remains at its 

 constant value of 64 pcf to the bottom of the sheet piles. 



The diagram for active earth pressure plus water lag shown in Fig- 

 ure 6(d) is obtained by adding the pressure diagrams shown in Figure 

 6 (a and b) . The combined pressure diagram shown in Figure 5 results 

 from adding the pressure diagrams shown on the two sides of the sheet 

 piles in Figure 6(c and d) . 



c. Depth of Penetration . The sheet piling must extend below the 

 outside bottom to a depth such that the total resultant force developed 

 by the passive earth pressure will be of sufficient magnitude and so 

 located that, together with the tie rod reaction, it will equalize the 

 effects of the summation of outward loads produced by the combined 

 active earth pressure and water lag pressure. This requirement is 

 known as the equilibrium of moments condition about the tie rod reac- 

 tion. 



To determine the required depth of penetration, the forces for the 

 several pressure areas behind the bulkhead, and the positions of their 

 centers of gravity with respect to the tie rod level are determined in 

 Figure 5. The moments of these forces about the tie rod level are com- 

 puted as shown in the following tabulation: 



Moment 

 (pounds feet) 



-540 



340 



4,850 



2,280 



2,290 6,930 



The location of the zero value (point "0", Figure 5) for the com- 

 bined pressure diagram is at a distance below the outside bottom of: 



334 



f|i= 2.06 feet. 



The required penetration below point "0" is designated by the letter 

 "d". Then the equilibrium of moments condition is expressed by the 

 equation: 



d^ ■ 2 



162 J (8.06 + J d) = 6,930. 



This equation is best solved by trial, where d = 2.92 feet. The 

 total depth of penetration below outside bottom is then 2.06 + 2.92 = 

 4.98 feet. The residual passive pressure intensity at the bottom of the 

 sheet piles is computed as: 



22 



Force 



Arm 



(pounds) 



(feet) 



540 



-1.0 



220 



1.52 



1,190 



4.08 



340 



6.69 



