or one-fifth the weight of the overlying armor units. The second under- 

 layer for this part of the structure should have a minimum thickness of 

 two quarrystones; these should weigh about one-twentieth the weight of 

 overlying quarrystones (1/20 x W/10 = W/200) . The first underlayer for 

 that part below -1.5H should have a minimum of two thicknesses of 

 quarrystone; these should weigh about one-twentieth the overlying second- 

 ary armor unit (1/20 x W/15 = W/300). The second underlayer for that part 

 below -1.5H, and the core material, can be as light as W/6,000, or 

 quarry- run. 



For a graded riprap cover layer, the weight of the first underlayer, 

 if required, should be about Vl^^/lO to prevent the material from washing 

 through the voids of the cover layer. 



h. Bedding Layer or Filter Blanket . Foundation conditions for 

 marine structures require thorough evaluation. Wave action against a 

 rubble structure may scour the natural bottom and the foundation of the 

 structure, even at depths usually considered unaffected by such action. 

 A rubble structure may be protected from excessive settlement resulting 

 from leaching, undermining, or scour, by the use of either a bedding 

 layer or filter blanket. Depending on the weight of quarrystone used, 

 a plastic filter cloth may be used instead of a bedding layer or with 

 a thinner bedding layer. 



It is advisable to use a bedding layer or filter blanket to protect 

 the foundations of rubble-mound structures from undermining except; (a) 

 where depths are greater than about three times the maximum wave height, 

 (b) where the anticipated current velocities are too weak to move the 

 average size of foundation material, or (c) where the foundation is a 

 hard, durable material (such as bedrock). 



When the foundation is a cohesive material, a filter blanket may not 

 be required. However, a layer of quarry spalls or other crushed rock or 

 gravel may be placed as a bedding layer or apron to reduce scour of the 

 bottom or settlement of the structure. Foundations of coarse gravel may 

 not require a filter blanket. When the rubble structure is founded on 

 sand, a filter blanket should be provided to prevent waves and currents 

 from removing sand through the voids of the rubble and thus causing 

 settlemento 



IVhen large quarrystones are placed directly on a sand foundation at 

 depths where waves and currents act on the bottom (as in the surf zone) , 

 the rubble will settle into the sand until it reaches the depth below 

 which the sand will not be disturbed by the currents. Large amounts of 

 rubble may be required to allow for the loss of rubble because of settle- 

 ment „ This, in turn, can provide a stable foundation. 



Gradation requirements of a bedding layer depend principally on the 

 size characteristic of the foundation material. However, quarry spalls, 

 ranging in size from 1 to 50 pounds, will generally suffice. Layer thick- 

 ness depends generally on the depth of water in which the material is to 



7-200 



