cost for 100 feet (30.5 meters) of structure as a function of dolosse weight, 

 structure slope, and concrete unit weight. Each point in the figure represents 

 a solution to the design problem. One solution (example 1 in Fig. 2), using 

 the curves for Kq = 13.6, is that a structure with a 1 on 2 slope having a 

 concrete unit weight of 160 pounds per cubic foot requires a 5.2-ton (4.77 

 kilonewtons) dolosse for armor against the 18-foot design wave. The cost for 

 100 feet of structure armored with a 5.2-ton dolosse is about $618,000. Another 

 solution to the design problem (example 2 in Fig. 2) would be to use a 7-ton 

 (6.42 kilonewtons) dolosse having a unit weight of 155 pounds per cubic foot 

 (25.1 kilonewtons per cubic meter) placed on a 1 on 1.75 slope. The cost of 

 this solution per 100 feet of structure is $565,000. 



When the stability coefficient is increased to % = 25.0, the family of 

 curves to the left in Figure 2 represents solutions to the design problem. The 

 required dolosse weight has been nearly halved for equivalent conditions of 

 structure slope and concrete unit weight. The cost per 100 feet of structure, 

 however, has not changed appreciably; e.g., using Kp = 25.0 for conditions 

 cited in example 1 above with a structure slope of 1 on 2 and a concrete unit 

 weight of 160 pounds per cubic foot, the required dolosse weight has been re- 

 duced from 5.2 to 2.8 tons (4.77 to 2.51 kilonewtons) but the cost only 

 decreased from $618,000 to $612,000 per 100 feet of structure. In example 2, 

 the required dolosse weight is now only 3.7 tons (3.39 kilonewtons) rather than 

 7 tons but the cost has only decreased from $565,000 to $550,000 (2.7 percent) 

 per 100 feet. In fact, for some conditions of structure slope and concrete unit 

 weight the cost actually increases for the larger stability coefficient and 

 smaller armor units. This generally occurs for flatter slopes and higher values 

 of concrete unit weight. 



The explanations for the relatively small change in cost with smaller armor 

 units are that (a) the cost of the armor layer may represent a relatively small 

 percentage of the total cost of the structure, especially for flat-sloped struc- 

 tures that have large quantities of core material, and (b) the relative cost of 

 labor compared with the cost of materials used to construct armor units is high 

 and results in an increase in the cost of armor. Labor costs in casting concrete 

 armor units are sensitive to the number of units that need to be formed, stripped 

 from forms, reinforced (if necessary), transported, and placed on the structure. 

 The cost of materials on the other hand is simply proportional to the amount of 

 materials needed. As the size of armor units decreases, the number of units 

 required to cover a given structure surface area increases and, along with it, 

 the cost of labor to form, strip, reinforce, transport, and place the units. 

 The amount of concrete, reinforcing, etc., required to cover a given area in 

 armor will decrease with decreasing armor unit size. Whether or not a cost 

 saving is realized by decreasing armor unit size depends on whether the savings 

 achieved by using less materials exceed any increase in labor costs resulting 

 from using more armor units. The relative cost of labor versus materials is 

 thus an important factor in establishing the optimum size armor unit. As the 

 relative cost of labor increases, it becomes more economical to design using 

 fewer, larger units, i.e., overdesigning the armor. 



The way in which the foregoing factors influence a design is through selec- 

 tion of a design level, i.e., by selecting a design wave height which will 

 result in the most economical structure by balancing the structure's first cost 

 against annual maintenance costs, repair costs, and benefits foregone to achieve 

 an overall least-cost design. Obviously, for a given armor unit shape, its 



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