height. According to Quinn (1972), a quarry consisting o£ fine-grained 

 basalt may break into relatively small pieces in sizes only about 6 per- 

 cent heavier than 8 tons; however, a quarry composed of hard anorthosite 

 (a type of diorite) , by special drilling patterns has produced relatively 

 large pieces, as much as about 12 percent in sizes between 8 to 15 tons 

 and about 15 percent larger than 15 tons. Twenty-ton stones are about 

 the largest size that can be quarried economically in enough quantities 

 for armor stones. When the design wave height requires a size of armor 

 stone that cannot be quarried economically near the proposed structure, 

 it becomes necessary to use one of the several available types of molded 

 concrete armor units for which accurate stability coefficients have been 

 determined. 



There are several methods of constructing rubble-mound structures in 

 the field, depending on the type of structure, the size of the armor units, 

 the depth of water, the degree of wave protection during construction, and 

 the equipment available for placing the material. When water depths per- 

 mit, the core and underlayer materials that are underwater are usually 

 dumped from scows, the armor units are usually placed individually from 

 a floating plant by crane using a sling, and the underlayer stones upon 

 which the armor units rest can be placed from a scow (if the stones are 

 not too large) or from a floating plant by crane using an orange-peel 

 type of grapple. Special techniques may be used in the placing of stone 

 armor units to increase the stability of the cover layers against wave 

 action, such as placing all long-axis stones with the longer axis perpen- 

 dicular to the face slope, and using special care in the nesting of each 

 armor rock; however, it is usually too expensive to ensure that such 

 placing will be accomplished satisfactorily in the prototype except for 

 that part of the structure that is above water. Thus, most rubble-mound 

 structures are designed and constructed using a random or pell-mell type 

 of placement technique. Certain shapes of concrete armor units can be 

 placed in a regular pattern to greatly increase the stability coefficient, 

 but the required placing techniques for these units are also expensive, 

 and such methods are not presently in common use. The required placing 

 techniques for some of the concrete armor units have been, or can be de- 

 veloped in the laboratory, and as marine contractors accept these special 

 placing tecfmiques, their use should become more frequent. Therefore, the 

 construction of small-scale, rubble-mound structures for use in hydraulic 

 model investigations duplicates as closely as possible the techniques 

 used in the field. 



Scale models of rubble-mound structures are constructed in a wave 

 flume on a sand base. The core material, underlayer stones, and armor 

 units are placed with the flume dewatered (Fig. 6-11). The core material, 

 which usually consists of crushed basalt or limestone, sized and graded to 

 represent the material that will be used in the field using the proper 

 scale relationships, is saturated with a hose and then compacted with a 

 hand trowel to simulate natural compaction resulting from wave action 

 during construction of the full-scale structure. The underlayer material 

 is placed by shovel and smoothed to grade by hand without compaction or 

 the rearranging of individual stones. The armor units are placed by hand 



356 



