Table 7-16. Coefficients and angles of friction. 



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

 will support train or truck traffic; the coefficient \i would then 

 equal the tangent of the new angle 6 . 



3. Cohesive Soils. 



Sections 1 and 2 above have briefly dealt with forces in cohesionless 

 soil. A cohesive backfill which reduces the active force may be advan- 

 tageous. However, unless the soil can move continuously to maintain the 

 cohesive resistance, it may relax. Thus, walls should usually be designed for 

 the active force in cohesionless soil. 



4. 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 estimated using 

 equations (7-142) and (7-144) by substituting an approximate average wall 

 batter or slope to determine the angle 6 . 



5. Submerged Material . 



Forces due to submerged fills may be calculated by substituting the unit 

 weight of the material reduced by buoyancy for the 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 7-15. 



6. Uplift Forces . 



For design computations, uplift forces should be considered as full 

 hydrostatic force for walls whose bases are below design water level or for 

 walls with saturated backfill. 



7-260 



