Following work by Iribarren (1938, 1950), comprehensive investiga- 

 tions were made by Hudson (1953, 1959, 1961 a, and 1961 b) at the U.S. Army 

 Engineer Waterways Experiment Station (WES), and a formula was developed 

 to determine the stability of armor units on rubble structures. The 

 stability formula, based on the results of extensive small-scale model 

 testing and some preliminary verification by large-scale model testing 

 is 



W H3 



W = (7-105) 



K^ (8^-1)3 cot 6 



where 



W = weight in pounds of an individual armor unit in the primary 

 cover layer. (When the cover layer is two quarry stones in 

 thickness, the stones comprising the primary cover layer can 

 range from about 0.75 W to 1.25 W with about 75 percent of 

 the individual stones weighing more than W. The maximum 

 weight of individual stones depends on the size or shape of 

 the unit. The unit should not be of such a size as to extend 

 an appreciable distance above the average level of the slope.) 



Wj, = unit weight (saturated surface dry) of armor unit, lbs. /ft?, 



H = design wave height at the structure site in feet. 

 (See Section 7.372.), 



Sj, = specific gravity of armor unit, relative to the water at the 

 structure, (S^ = Wy,/wy) . 



w,^ = unit weight of water, fresh water = 62.4 lbs. /ft?, 

 sea water = 64.0 lbs. /ft?, 



6 = angle of structure slope measured from horizontal in degrees, 

 and 



Krj = stability coefficient that varies primarily with the shape 

 of the armor units, roughness of the armor unit surface, 

 sharpness of edges and degree of interlocking obtained 

 in placement. (See Table 7-6.) 



Equation 7-105 is intended for conditions when the crest of the structure 

 is high enough to prevent major overtopping. Also the slope of the cover 

 layer will be partly determined on the basis of stone sizes economically 

 available. Cover layer slopes steeper than 1 on 1.5 are not recommended 

 by the Corps of Engineers. Figures 7-83 through 7-86 provide a graphical 

 solution of Equation 7-105. 



7-169 



