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

 creates turbulence within both the structure and the underlying soil that may 

 draw the soil into the structure, allowing the rubble itself to sink. 

 Revetments and seawalls placed on sloping beaches and banks must withstand 

 groundwater pressure tending to wash underlying soil through the structure. 

 When 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 settlement. This settlement, in turn, 

 can provide a stable foundation; but a rubble structure can be protected from 

 excessive settlement resulting from leaching, undermining, or scour, by the 

 use of either a filter blanket or bedding layer. 



It is advisable to use a filter blanket or bedding layer 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 rubble structure is founded on cohesionless soil, especially 

 sand, a filter blanket should be provided to prevent differential wave 

 pressures, currents, and groundwater flow from creating a quick condition in 

 the foundation by removing sand through voids of the rubble and thus causing 

 settlement. A filter blanket under a revetment may have to retain the 

 foundation soil while passing large volumes of groundwater. Foundations of 

 coarse gravel may be too heavy and permeable to produce a quick condition, 

 while cohesive foundation material may be too impermeable. 



A foundation that does not require a filter blanket may require a 

 protective bedding layer. A bedding layer prevents erosion during and after 

 construction by dissipating forces from horizontal wave, tide, and longshore 

 currents. It also acts as a bearing layer that spreads the load of overlying 

 stone (a) on the foundation soil to prevent excessive or differential 

 settlement, and (b) on the filter material to prevent puncture. It interlocks 

 with the overlying stone, increasing structure stability on slopes and near 

 the toe. In many cases a filter blanket is required to hold foundation soil 

 in place but a bedding layer is required to hold the filter in place. Grada- 

 tion requirements of a filter layer depend principally on the size character- 

 istics of the foundation material. If the criterion for filter design (Sowers 

 and Sowers, 1970) is used, Y>^^ (filter) ■'■^ less than or equal to 



5Doc /£ J ^- \ (i.e., the diameter exceeded by the coarsest 85 percent of 

 85 (foundation) ' ■' '^ 



the filter material must be less than or equal to 5 times the diameter 



exceeded by the coarsest 15 percent of the foundation material) to ensure that 



the pores in the filter are too small to allow passage of the soil. Depending 



on the weight of the quarrystone in the structure, a geotextile filter may be 



used (a) instead of a mineral blanket, or (b) with a thinner mineral 



blanket. Geotextiles are discussed in Chapter 6 and by Moffatt and Nichol, 



Engineers (1983) and Eckert and Callender (1984), who present detailed 



requirements for using geotextile filters beneath quarrystone armor in coastal 



structures. A geotextile, coarse gravel, or crushed stone filter may be 



7-241 



