It should be noted that Pp is applied below the normal to the structure 

 slope by an angle -6, whereas the active force is applied above the 

 noimal line by an angle +6. (See Figure 7-101.) 



For the Rankine conditions given in Section 7.71, Equation 7-118 

 reduces to 



^ = 



— tan^ 145"+- 

 2 \ 2 



(7-119) 



Equation 7-119 is satisfactory for use with a sheet-pile structure, 

 assuming a substantially horizontal backfill. 



NOTE: Angle of friction should be reduced by about 5 degrees if the wall fill will support 

 train or truck traffic. The coefficient of friction, /i would equal the tangent of the 

 new angle, 8. 



7.73 COHESIVE SOILS 



Sections 7.71 and 7.72 dealt with forces in cohesionless soil. A 

 cohesive backfill may be necessary, and reduces the active fcrce. How- 

 ever unless the soil can move continuously to maintain the cohesive 

 resistance, it may relax. Thus, the wall should be designed for the 

 active force in cohesionless soil. 



7.74 STRUCTURES OF IRREGULAR SECTION 



Earth force against structures of irregular section such as stepped- 

 stone blocks or those having two or more back batters may be computed by 

 Equations 7-116 and 7-118 by substituting an approximate average wall 

 batter or slope to determine the angle, 9. 



7.75 SUBMERGED MATERIAL 



Forces due to submerged fills may be calculated by substituting the 

 unit weight of the material reduced by buoyancy for value of w in the 

 preceding equations, and then adding to the calculated forces the full 

 hydrostatic force due to the water. Values of unit weight for dry, 

 saturated, and submerged materials are indicated in Table 7-13. 



7-212 



