e. Armor Stone . Armor stone, chosen to be of nearly uniform size and 

 of compact, sometimes blocky shape, depends on density and mass to resist 

 the force of waves or currents. The voids between the armor rock, to a 

 certain extent, absorb energy through creation of turbulence. To a lesser 

 extent, wave energy is absorbed by wave runup on the sloping outer face. 



f . Underlayers . The underlying rock layers are usually randomly 

 placed and serve to support the armor rock. By size gradation, and some- 

 times in several layers or zones, they may also absorb wave or current 

 energy through turbulence in the voids. Rocks used for these underlayers 

 are considerably less costly per unit volume than armor rock. 



g. Bedding Layers . In breakwaters, jetties, and groins constructed on 

 relatively flat sand or mud bottoms, a bedding layer consisting of smaller 

 well-graded stones is required to prevent the fine bottom material from 

 piping upward through the structures. Piping and subsequent erosion of the 

 foundation soils could result in settlement of the upper layers of rock, 

 particularly the large heavy armor stones. Settlement could in turn cause 

 ultimate collapse of that part of the structure. 



h. Filter Layers . In revetments, seawalls, and bulkheads a layer of 

 filter material is required. While this layer of fine well-graded stones 

 may in part act as a supporting bedding layer for sloped revetment or 

 seawall structures, its primary function is to relieve hydrostatic pressures 

 due to changes in water elevation on the seaward side or changes of ground 

 water elevation on the landward side. It is designed as a true filter to 

 permit the passage of water in either direction, but to prevent the dis- 

 turbance of the sand or clay foundation. The gradation of the filter 

 material depends on the characteristics of the backfill core or beach 

 materials and on the voids of the riprap or armor units. The filter 

 material should be uniformly graded from fine sands, coarse sands, gravels 

 and stones such that it will not wash into the rubble. The material could 

 be in two or more layers. It should also be noted that filter cloth is 

 frequently used in place of a graded granular filter. 



i. Other Categories . Filler stone, consisting of well-graded gravel 

 size material, 6.3 to 100 millimeters (0.25 to 4 inches), is commonly used 

 to fill the voids in core stone. Toe stone is used to protect the base of 

 a coastal structure from erosion or scour. This stone typically ranges 

 from approximately 0.89 kilonewton (200 pounds) to more than 8.9 kilonew- 

 tons (2 000 pounds) in weight and should be reasonably well graded. Chink 

 stone is used to fill voids in riprap or armor stone. Coarse and fine 

 aggregates are used for making concrete. 



j . Fill for Gabions and Cribs . Gabions are wire baskets that can be 

 connected and filled with stone. The baskets must be solidly filled, or 

 wires will be abraded by movement of loose stones. The stones must be 

 large enough, generally 10 to 25 centimeters in diameter, to prevent loss 

 of stone through the gabion mesh. Cribs may be filled with similar stone 

 materials to form a gravity type structure. 



3. Stone Specifications . 



a. General. The character of the stone to be used in coastal struc- 

 tures is of primary importance. Materials may be obtained from any approved 



29 



