can be a filter blanket covered by a bedding layer and, if necessary to 

 prevent scour by splash, quarrystone armor or riprap; i.e., an apron similar 

 in design to a toe apron. The apron can also be a pavement of concrete or 

 asphalt which serves to divert overtopping water away from the revetment, 

 decreasing the volume of groundwater beneath the structure. 



e. Toe Berm for Foundation Stability . Once the geometry and material 

 weights of a structure are known, the structure's bearing pressure on the 

 underlying soil can be calculated. Structure settlement can be predicted 

 using this information, and the structure's stability against a slip failure 

 through the underlying soil can be analyzed (Eckert and Callender, 1984). If 

 a bearing failure is considered possible, a quarrystone toe berm sufficiently 

 heavy to prevent slippage can be built within the limit of the slip circle. 

 This berm can be combined with the toe berm supporting the cover layer and the 

 scour apron into one toe construction. 



If the vertical structure being protected by a toe berm is a cantilevered 



or anchored sheet-pile bulkhead, the width of the beirm B must be sufficient 



to cover the zone of passive earth support in front of the wall. Eckert and 



Callender (1984) describe methods of determining the width of this zone. As 



an approximation, B should be the greatest of (a) twice the depth of pile 



penetration, (b) twice the design wave height, or (c) 0.4 d (Eckert, 



1983). If the vertical structure is a gravity retaining wall, the width of 



the zone to be protected can be estimated as the wall height, the design wave 



height, or 0.4 d , whichever is greatest. 

 s 



IV. VELOCITY FORCES— STABILITY OF CHANNEL REVETMENTS 



In the design of channel revetments, the armor stone along the channel 

 slope should be able to withstand anticipated current velocities without being 

 displaced (Cox, 1958; Cambell, 1966). 



The design armor weight is chosen by calculating the local boundary shear 

 expected to act on a revetment and the shear that a design stone weight can 

 withstand. Since the local boundary shear is a function of the revetment 

 surface roughness, and the roughness is a function of the stone size, a range 

 of stone sizes must be evaluated until a size is found which is stable under 

 the shear it produces. 



When velocities near the revetment boundaries are available from model 

 tests, prototype measurements, or other means, the local boundary shear is 



I 5.75 log^Q ^ 

 where \ g 



Tt = local boundary shear 



V = the velocity at a distance y above the boundary 



dg = equivalent armor unit diameter; i.e.. 



7-249 



