This condition must be given special attention in the design of small 

 isolated structures. However, because of the flexibility of an ice 

 field, pressures probably are not as great as those of a solid ice 

 sheet in a confined area. 



Ice formations at times cause considerable damage on shores in 

 local areas, but their net effects are largely beneficial. Spray from 

 winds and waves freezes on the banks and structures along the shore, 

 covering them with a protective layer of ice. Ice piled on shore by 

 wind and wave action does not, in general, cause serious damage to 

 beaches, bulkheads, or protective riprap, but provides additional pro- 

 tection against severe winter waves. Ice often affects impoundment of 

 littoral drift. Updrift source material is less erodible when frozen, 

 and windrowed ice is a barrier to shoreward moving wave energy, there- 

 fore, the quantity of material reaching an impounding structure is re- 

 duced. During the winters of 1951-52, it was estimated that ice caused 

 a reduction in rate of impoundment of 40 to 50 percent at the Fort 

 Sheridan, Illinois, groin system. 



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 attached members such as wales, and then being 

 forced upward by a rise in water stage or wave action. 



7.7 EARTH FORCES 



Various texts on soil mechanics such as Andersen (1948), Hough 

 (1957), and Terzaghi and Peck (1967), adequately discuss the subject. 

 The forces exerted on a wall by soil backfill depend on the physical 

 characteristics 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 indetermin- 

 ate because of the factors noted, approximations of their influence must 

 be made. 



7.71 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 active 

 eax'th fovoe. Retaining walls are generally designed to allow minor rota- 

 tion about the wall base to develop this active force, which is less than 

 the at-vest fovae exerted if no rotation occurs. Coulomb developed the 

 following active force equation: 



wh^ 

 2 



CSC d sin (0 — <j>) 



V sin (0 + 6)' + /sin ( + 5) sin (0 - ij 

 sin (0 - i) 



V 



(7-116) 



7-208 



