PART III: DISCUSSION 



32. In general, there appear to be three major problem areas with 

 rubble-mound coastal structure toes. One of these pertains to the proper 

 sizing and placement of toe buttressing stone. The purpose of the buttressing 

 stone is to stabilize the onslope armor by preventing downslope slippage of 

 the armor layer. For these stone to function properly, they must be of suf- 

 ficient weight and placed in such a way that they are stable in a wave and/or 

 flow environment. The second major problem area concerns toe berms. A toe 

 berm's primary function is to protect a structure placed on an erodible bottom 

 from being undermined by wave- and/or flow-induced scour and to resist down- 

 slope slippage of the armor. For a toe berm to function properly it, like the 

 toe buttressing stone, must be composed of materials and be constructed in a 

 geometry that will be stable in the incident wave and/or flow environment. 

 Thirdly, toe buttressing stone and toe berms are susceptible to damage and 

 failure when placed on an erodible bottom material. The stone may be sized 

 adequately for the level of energy to which they are exposed, but the exposed 

 bottom material at the outer perimeter of the structure may readily erode 

 and/or an inadequately designed bedding material may allow the foundation 

 material to migrate through it and the toe berm armor. Either one or both of 

 these factors can result in the undermining and displacement of stone that 

 were otherwise able to withstand the wave and flow environment but failed 

 because of undermining induced displacement. 



33. In summary, a toe failure may be the result of any one or a combi- 

 nation of the above. Guidance exists for proper design of bedding (filter) 

 layers based on soil types, but very little guidance is available for the siz- 

 ing and geometries needed for the proper design of toe berms and buttressing 

 stone for incident wave environments. Most work done by the districts in 

 these areas is based on field experience and engineering judgment. A scouring 

 bottom is a problem in itself. No matter how well a toe is designed, if the 

 local bottom materials (sands, silts, clays, etc.) are exposed to sufficient 

 energy levels for scour to occur, the toe of the structure is doomed to fail- 

 ure unless the toe berm is extended out to a point where the energy levels are 

 below those which will initiate scour. In most cases this is not practical or 

 feasible. In these instances, sufficient toe berm material, that in itself is 

 stable for the wave and/or flow environment must be placed so that as the 



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