the structure's reserve stability. The reserve stability of the 100- 

 percent overlay armor is estimated by the following large wave tank 

 results: 



Test 



^D=0 



H£)=^ Reserve stability 





(ft) 



(ft) 



L-1 



3.15 



3.50 1.11 



L-2 



4.02 



4.38 1.09 

 Average 1.10 



Reserve stability of conventional two- layer riprap was determined in the 

 large wave tank by Ahrens (1975) as follows: 



Slope No. of tests Avg. reserve 

 considered stability 



1 on 2.5 9 1.16 



1 on 3.5 13 1.22 



Interpolation of this data for a 1 on 3 slope yields a reserve stability 

 of 1.19; i.e., the riprap still protects the embankment from damage for 

 a wave about 20 percent higher than the zero-damage wave height. By com- 

 parison, the 100-percent overlay has only one-half as much (10 percent) 

 reserve stability as the conventional two layers of armor. 



The number of stones per unit area necessary to have the 100-percent 

 stone overlay condition (all stones touching) is different for boulders 

 and quartzite quarrystone because of stone shape. The coverage fraction 

 (Table 1) shows 100-percent coverage of boulders having a coverage frac- 

 tion between 0.53 to 0.59; the 100-percent coverage for quarrystone 

 (including prototype and small-scale tests) has a coverage fraction 

 between 0.41 and 0.43. This means that to obtain 100-percent coverage 

 about 30 percent more of the rounded boulders would be required per unit 

 area than the blocky quarrystone. 



The variation of coverage fraction between boulders and conventional 

 shape riprap used in these tests is consistent with an evaluation of rip- 

 rap layer thickness reported by Hudson (1958) . This change of coverage 

 fraction with layer thickness for various stone shapes is shown in Figure 

 7. 



The variation of coverage fraction in Figure 7 can be used to make 

 comparative cost estimates of alternative stone types for revetment 

 design. An example of the use of Figure 7 is given in Appendix B. 



2. Conclusions. 



The general conclusion of these prototype and small-scale wave tank 

 tests is that a one- layer stone overlay greatly improves riprap stability 



20 



