b . Improper design of toe berms . 



c . Erosion of the bottom material. 



Toe-buttressing stone is used to stabilize the slope armor by preventing 

 downslope slippage of the armor layer and typically is not concerned with 

 scour-related problems, as are items b. and c. The most recent guidance 

 available for design of structures which addresses the three problems listed 

 above is contained in Markle (1989). Generally, sufficient guidance is given 

 for design of bedding or filter layers based on soil type, but very few data 

 are available for selecting material size and geometric configuration for 

 proper toe berm and buttressing design. Although any of the three preceding 

 problems can occur no matter how well the toe of the structure is designed, 

 failure can be expected to occur if the bottom material is exposed to 

 sufficient energy for scour to take place. Additional guidance for design of 

 rubble-mound structures is contained in Pilarczyk (1989) and the SPM (1984). 

 In many cases, the present solution (in some cases impractical) is to extend 

 the berm to some point where insufficient energy exists to displace the bottom 

 materials . 



Figure 6. Scour problems at rubble-mound structures 



23 



