in Table 2 and depicted in Photos 11-14, were similar to those obtained at the 

 lV-on-2H slope. However, a slight decrease in stability was observed. 



22. Tribar tests were initiated with two layers of randomly placed 

 units and an assumed IC of 9. This structure proved to be unstable. Wave 

 heights were progressively reduced to 0.33 ft without achieving stability. 

 Generally, the tribars did not interface well with the larger armor stone at 

 this steeper slope and were prone to downslope shifting. Photos 15 and 16 

 show the structure after attack of 0.38- and 0.33-ft waves, respectively. If 

 stability had been achieved, the corresponding K's would be 4.2 and 2.7. 

 Based on these test results, randomly placed tribars are not recommended for 

 overlaying existing armor stone at slopes steeper than IV on 2H. 



23. Random tribar placement was tested initially because this type is 

 generally the most economical and reliable to construct in the field. How- 

 ever, one layer of uniform tribar placement in concert with large toe but- 

 tressing stone has been used by Pacific Ocean Division (POD). Therefore, it 

 was decided to investigate the stability of this configuration. Based on 

 pod's experience, a buttressing stone weight about 1.3 times greater than the 

 tribar weight was selected. Photo 17 shows the structure before wave attack 

 and Photos 18 and 19 show typical after-testing views. 



24. Results of stability tests for the uniformly placed tribars are 

 summarized in Table 2 along with the previously discussed dolos test results. 

 These data indicate a minimum stability coefficient of 7.0 for the uniformly 

 placed tribars. Figures 4 and 5 present K as a function of d/L and 



H/d , respectively. Similar to results for the lV-on-2H slope, dolos stabil- 

 ity proved to be sensitive to d/L and H/d with minimum stability occurring 

 at the lower values of d/L and higher values of H/d and tribar stability 

 was not significantly influenced by either d/L or H/d . 



25. Minor rocking and shifting of the buttressing stone was observed 

 during some of the tests; therefore, the selected weight (1.3 tribar weights) 

 is recommended as a true minimum. Also, stability results achieved for the 

 uniformly placed tribars are highly dependent on the careful placement of the 

 tribars in concert with the buttressing stone. Specifications should clearly 

 state that gaps between the units are not acceptable as voids invite downslope 

 slippage and may lead to failure of the overlay. 



15 



