SOLUTION : 



(a) Calculate, 



H 9 



Using Table 7-7, the value of H/H^-q for rough quarrystone falls 

 between 5- to 10-percent and 10- to 15-percent damage, therefore 

 the anticipated damage for a 9-foot wave would be about 10 percent. 



(b) Table 7-7, using a value of K^ = 8.2 for rough quarrystone, 

 shows a percent damage range of 15 to 20 percent. 



(c) From Table 7-7 for D = 30 to 40 percent, 



-H- = 1.47, 



or 



H = 8(1.47) = 11.8 ft. 

 and 



K^ = 12.6. 



Therefore if the structure were designed for a wave height, ^[)=o = 8 

 feet and a no-damage stability coefficient K^, = 4.0 and suDsequently 



attacked by waves H = 11.8 feet, the anticipated damage to the struc- 

 ture's armor layer could be between 30 and 40 percent. On the other 

 hand, if the structure were only designed for a 5. 4- foot wave (H^^^) , 

 but an 8-foot wave could occur, then 30- to 40-percent damage should 

 be anticipated from the 8- foot wave. 



If the structure were designed for H = 8 feet and K^ = 12.6 instead 

 of the no-damage value of K^ = 4.0, 30- to 40-percent damage could be 

 anticipated as the result of the occurrence of an 8- foot wave. 

 ************************************ 



7.375 Importance of Unit Weight of Armor Units . The basic equation used 

 for design of armor units for rubble structures indicates that the unit 

 weight Wj, of quarrystone or concrete is important. Designers should 

 carefully evaluate the advantages of increasing unit weight of concrete 

 armor units to effect savings in the structure cost. Brantzaeg (1966) 

 cautioned that variations in unit weight should be limited within a range 

 --say 120 lbs. /ft? to 180 lbs. /ft? Unit weight of quarry-stone available 

 from a particular quarry will likely vary over a narrow range of values. 

 The unit weight of concrete containing normal aggregates is usually between 

 140 pcf and 155 pcf. It can be made higher or lower through the use of 

 special heavy or light weight aggregates that are usually available but 

 are more costly than normal aggregates. The unit weight obtainable from 

 a given set of materials and mixture proportions can be computed from 



7-179 



