Unit weight of submerged sand ----- 60 pcf. 



Ka = 0.3 



Kp = 5.0 



For sandfills, Kp = 3.0 



b. Preparation of Earth Pressure Diagrams . Figure 5 shows a typical 

 combined lateral pressure diagram for the bulkhead illustrated in 

 Figure 2. Granular materials are assumed throughout, with the material 

 below the outside bottom assumed as undisturbed soil. The combined 

 lateral pressure diagram is obtained from the diagrams on Figure 6, 

 showing the separate effects of active earth pressure, water lag, and 

 passive earth pressure. The active earth pressure increases at a rate of 

 K^Wg, where K^ is taken as 0.3 and Wg is the effective unit weight of 

 earth. Wg is taken as 100 pcf above the free water surface within the 



backfill and as 60 pcf below the free water surface. Hence, the active 

 earth pressure increases at a rate of 30 pcf above the free water surface 

 and at 18 pcf for the submerged earth. The available passive earth 

 pressure increases at a rate of KpWg but, to allow for a factor of safety 



(Fs) against the outward movement of the lower ends of the sheet piles, 

 the rate of increase in the passive pressure is taken as KpWg/Fg . The 



factor of safety against the outward movement of the sheet piles should 

 be in the range of 1.5 to 2.0. Hence, if Fg is taken as 1.67 and Kp is 



5.0, the rate of increase for the passive pressure, including allowance 

 for the factor of safety is : 



5.0 



1.67 



X 60 pcf (submerged earth) = 180 pcf. 



The free water surface behind the sheet piling for the diagrams on 

 Figure 6 is taken as 1 foot above the outside water level, which for 

 analysis is taken at MLW because the maximum outward forces occur at low 

 water level. The unbalanced water pressure due to this 1-foot water lag 

 augments the outward pressures of the earth. 



Assuming that seawater weighs 64 pcf, the water lag pressure has a 

 constant value of 64 pounds per square foot (psf) from MLW level to the 

 level of the outside bottom at elevation minus 4 feet (-4) . Although the 

 water lag pressure probably decreases from its value of 64 psf at the 

 level of the outside bottom (-4) to a zero value at the bottom of the 

 sheet piles, the assumption is often made that the water lag pressure 

 continues at a constant value to the bottom of the sheet piles as shown 

 by the solid line on Figure 6 (a and b) . This assumption simplifies the 

 computation process in arriving at the combined lateral pressure diagram, 

 because the required depth of penetration for the sheet piling remains to 

 be determined at the time that the construction of the pressure diagrams 

 is in progress. Thus the slope, or rate of increase of the combined 



