and along the axis of the rubble structure, but can vary from one job to 

 the next. Placement can also vary from one contractor to another. The 

 variables and difficulties in placing armor units one at a time, or dumping 

 by skiff, above and below water, present the engineer with a difficult 

 design problem. The extent of interlocking achieved is unpredictable when 

 using random size (but still within specified limits) quarrystone. 

 Generally, in specifying quarrystone armor units, the dimension of the 

 maximum axis is no greater than three times the minimum axis. This applies 

 only to armor stone, as this ratio was devised to forestall the use of flat 

 or platelike stone that, if laid flat on the structure slope, would 

 be less stable than a more cubic stone. Because of these unpredictable 

 variables all methods of placement, except for uniform and special place- 

 ments, have been lumped together as "random placement" to encompass the 

 range of placement methods. 



Cover or armor stone should generally be placed individually and in a 

 manner to avoid displacing underlying materials, to avoid placing undue 

 impact force on underlying material, and to minimize chipping the stones 

 (U.S. Army Engineer Division, North Central, 1978) (Fig. 6). The 

 stones should be placed with minimum voids and with maximum interlocking of 

 stones. All stone when placed should be stable, keyed, and interlocked, with 

 no overhanging or "floaters." Keying is the wedging and interlocking of the 

 individual pieces of armor stone so that the individual stone is not only 

 firmly seated but is wedged in by the adjacent armor stones. Keying should 

 not be confused with "chinking" by small nondesign stones. These will be 

 removed by the first severe storm and provide little or no stability. The 

 various sizes of cover stone should be so distributed as to produce a uniform 

 well-graded mass. Adjacent stones should be selected as to size and shape and 

 carefully keyed-in to provide a compact and integrated surface course. 

 Smaller stones should be used to fill the space between larger ones, so as to 

 leave a minimum of voids. Equipment used for placing large stone should be 

 capable of placing the stone near its final position before release and 

 capable of moving the stone if necessary to its final position. Dragline 

 buckets and skips should generally not be used for placement of armor stone. 

 Casting or dropping of stone more than 30 centimeters or moving by drifting or 

 manipulating down the slope shoul d not be permitted (U.S. Army Engineer Division, 

 North Central, 1978). Final shaping of the slope should be performed during 

 placement of stones. 



e. Riprap . Stone for "dumped riprap" should be placed on the filter 

 bedding layer or filter cloth in such a manner as to produce a reasonably 

 well-graded mass of rock with the minimum practicable percentage of voids. 

 Riprap should be placed to its full course thickness at one operation and 

 in such a manner as to avoid displacing the bedding material. The larger 

 stones should be well distributed and the entire mass of stones in their 

 final position should be roughly graded to conform to the specified grada- 

 tion. The finished riprap should be free from objectionable pockets of 

 small stones and clusters of larger stones. Placing riprap in layers 

 should not be permitted. Placing riprap by dumping into chutes or by 

 similar methods likely to cause segregation of the various sizes should not 

 be permitted. The desired distribution of the various sizes of stones 

 throughout the mass may be obtained by selective loading of the material at 

 the quarry or other source, by controlled dumping of successive loads 



42 



