APPENDIX B-Continued 



Unit weight o( moist sand .... 

 Unit weight ol submerged sand 

 Kp - 5.0 



100 lb per cu ft 

 60 lb per cu ft 



For sand Mis Kp 



3,0 



Preparation ol Earth Pressure Diagrams 



Figure 7 shows a typical combined lateral pressure 

 diagram for the bulkhead illustrated m Fig 2. pari 1 

 (S2) Granular materials are assumed throughout, with 

 the material below the outside bottom assumed as un- 

 disturbed soil The combined lateral pressure diagram 

 IS obtained from the diagrams on Fig 6 showing the 

 separate effects of active earth pressure, water lag, 

 and passive earth pressure The active earth pressure 

 increases at a rate of KaW.. where K» is taken as 0.3 

 and W« IS effective unit weight of earth. W» is taken as 

 100 pounds per cubic toot (pet) above the free water 

 surface within the backfill and as 60 pel below the free 

 water, surface Hence the active earth pressure 

 increases at a rale ol 30 pcf above the free water sur- 

 face and at 18 pcf for the submerged earth. 



The available passive earth pressure increases at a 

 rale of KpW. but, to allow for a factor of safety (Fs) 

 against the outward movement of the lower ends of 

 the sheet piles, Ihe rate of increase in the passive pres- 

 sure is taken as KpWe/Fs The factor of safety against 

 the outward movement ol the sheet piles should be in 

 the range of 1.5 to 2 Hence, it Fs is taken as 1,67 

 and Kp is 5 0. the rate of increase for the passive pres- 

 sure, including allowance for the factor of safety is: 



(5 0,1 67) X 60 pet (submerged earth) = 180 pet 



The free water surface behind the sheet piling for 

 the diagrams in Fig 6 is taken as 1.0 loot above the 

 outside water level, which tor the purpose of analysis 

 IS taken as MLW because the maximum outward forces 

 occur at low water level The unbalanced water pres- 

 sure due to this one-foot water lag augments the out- 

 ward pressures of the earth. 



Assuming that sea water, weighs 64 pcf, the water 

 lag pressure has a constant value of 64 pounds per 

 square foot (psf) from MLW level to the level ol the 



outside bottom at elevation minus four leet (-4.0). 

 Although the water lag pressure probably decreases 

 from its value of 64 psf at the level of the outside bot- 

 tom (-4 0) 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 in Fig. 

 6b and 6d This assumption simplifies the computation 

 process m 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 rale o1 increase of the combined 

 pressure diagram below elevation 4,0 in Fig 7 is sim- 

 ply the difference between the value of 180 pcf for 

 the passive pressure and 18 pcf lor Ihe active pres- 

 sure, giving 162 pcf if the water-lag pressure remains 

 at its constant value of 64 psf to the bottom of the 

 sheet piles 



The diagram for active earth pressure plus water 

 lag shown in Fig, 6d is obtained by adding the pres- 

 sure diagrams shown in figs 6a and 6b. The combined 

 pressure diagram shown in Fig. 7 results from adding 

 the pressure diagrams shown on the two sides of the 

 sheet piles in Fig, 6c and 6d, 



DEPTH OF PENETRATION (Design Step 3) 



The sheet piling must extend below the outside bottom 

 to a depth such thai the total resultant force developed ■ 

 by the passive earth pressure will be of sufficient mag- 

 nitude and so located that, together wilh the tie roo 

 reaction, il will equalize the effects ol the summation 

 ol outward loads produced by the combined active 

 earth pressure and water lag pressure This require- 

 ment IS known as the equilibrium ol moments condi- 

 tion about the tie rod reaction. 



To determine the required depth ol penetration, the 

 forces for the several pressure areas behind the bulk- 

 head, and the positions ol their centers ol gravity with 

 respect to the lie rod level are determined as shown 

 in Fig, 7, The moments of these forces about the tie 



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T»C?^^fc^^/y-'-'.V.,ill»rl' 



Fig. 6: Latftral Pr*»»ur* DIagr 



316 



