incident wave and flow environment. The third major prob- 

 lem area deals with failure induced by an eroding founda- 

 tion. Toe buttressing stones and toe berms are suscepti- 

 ble to damage and failure when placed on erodible bottom 

 material. The stones may be sized adequately for the 

 expected level of energy to which they may be exposed, but 

 the exposed bottom material at the outer perimeter of the 

 structure may erode readily under these conditions. Also, 

 an inadequately designed bedding, or filter, material may 

 allow foundation material to leach through it and the toe 

 berm or buttressing armor. Either one or a combination of 

 both of these factors can result in the undermining and 

 displacement of stones that were otherwise stable in the 

 wave and flow environment. 



In summary, a toe failure may stem from any one or a 

 combination of the above. Guidance exists for proper 

 design of bedding (filter) layers based on soil types, but 

 very little guidance is available for the sizing and geom- 

 etries needed for the proper design of toe berms and but- 

 tressing stone for incident wave environments. Most de- 

 sign work done in this area by CE districts 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 likely to fail 

 unless the toe berm is extended to a point where the ener- 

 gy levels are below that of scour initiation. 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 structure toe undermines the berm 

 and bedding material can slough off into the scour hole. 

 This occurrence will provide some armoring to reduce the 

 rate of scour and thus increase the usable, or functional, 

 life of a structure. 



3. Based on survey findings, it was concluded that design guidance is 



seriously needed on the proper sizing and placement configurations required to 



provide adequate buttressing stone and toe berms for rubble -mound coastal 



structures. Existing design guidance for toe berms is based on field 



experience and engineering judgment (weight of toe armor should be at least 



one- tenth the weight of the primary armor ( Shore Protection Manual (SPM) 1984) 



or on research by Brebner and Donnelly (1962) and Tanimoto, Yagyu, and Goda 



(1982) on foundation and toe berm materials lying beneath and/or in front of 



vertical structures, i.e. caissons, timber cribs, etc. 



