Some abrasion of timber or concrete structures may be caused, and 

 individual members may be broken or bent by the weight of the ice mass. 

 Piling has been slowly pulled by the repeated lifting effect of ice freezing 

 to the piles, or to attached members such as wales, and then being forced 

 upward by a rise in water stage or wave action. 



VII. EARTH FORCES 



Numerous texts on soil mechanics such as those by Anderson (1948), Hough 

 (1957), and Terzaghi and Peck (1967) thoroughly discuss this subject. The 

 forces exerted on a wall by soil backfill depend on the physical character- 

 istics of the soil particles, the degree of soil compaction and saturation, 

 the geometry of the soil mass, the movements of the wall caused by the action 

 of the backfill, and the foundation deformation. In wall design, since 

 pressures and pressure distributions are typically indeterminate because of 

 the factors noted, approximations of their influence must be made. Guidance 

 for problems of this nature should be sought from one of the many texts and 

 manuals dedicated to the subject. The following material is presented as a 

 brief introduction. 



1. Active Forces . 



When a mass of earth is held back by means of a retaining structure, a 

 lateral force is exerted on the structure. If this is not effectively 

 resisted, the earth mass will fail and a portion of it will move sideways and 

 downward. The force exerted by the earth on the wall is called aati,ve earth 

 force. Retaining walls are generally designed to allow minor rotation about 

 the wall base to develop this active force, which is less than the at-rest 

 force exerted if no rotation occurs. Coulomb developed the following active 

 force equation: 



P - wh 

 a 2 



esc 6 sin (9 - <))) 



/sin (6 + 6) + / sin (4. + 6) sin ((}) - i) 

 y \ sin (6 - i) 



(7-143) 



where 



P = active force per unit length, kilonewtons per meter (pounds per 

 linear foot) of wall 



w = unit weight of soil, kilonewtons per cubic meter (pounds per linear 

 foot) of wall 



= height of wall or height of fill at wall if lower than wall , meters 

 (feet) 



= angle between horizontal and backslope of wall, degrees. 



= angle of backfill surface from horizontal, degrees 



= internal angle of friction of the material, degrees 



7-256 



