OVERLAY OF LARGE, PLACED QUARRYSTONE AND BOULDERS 

 TO INCREASE RIPRAP STABILITY 



Bruce L. McCartney and John P. Ahrens 



I. INTRODUCTION 



Conventional riprap (underlayer stone) revetment design requires an 

 armor layer thickness 1.5 to 2 times the median diameter of the armor stone. 

 The armor layer completely buries the underlayer so the revetment derives 

 minimal benefit from the underlayer stone stability. This report presents 

 the results of an investigation of revetment stability to wave attack for 

 a one-stone-diameter-thick armor overlay. The armor overlay could also be 

 used to upgrade existing riprap or as a cost-effective initial design. 



The reduced armor layer thickness concept was proposed by the U.S. 

 Army Engineer Division, Missouri River, as a means of upgrading an exist- 

 ing riprap slope on the Oahe Reservoir in South Dakota. The slope was 

 damaged by unusually high storm waves in 1967. A case study of the Oahe 

 Reservoir riprap repair is presented in Appendix A. 



II. WAVE TANK TESTS 



1. General . 



Wave tank testing to determine stability against wave attack for single- 

 layer quarrystone and boulder overlays, for both 100- and 67-percent surface 

 coverage, was conducted at the U.S. Army Coastal Engineering Research 

 Center (CERC) in both the small and large wave tanks. The 100-percent 

 coverage is defined as all overlay stones touching; the 67-percent cover- 

 age used two-thirds as many stones per unit area as the 100-percent cover- 

 age. These general overlayer coverage definitions were converted to a 

 dimensionless parameter (coverage fraction) which provides a quantitative 

 measurement of coverage. The coverage fraction is the overlay stone weight 

 per unit area for a specific overlay percent and stone shape. Cover frac- 

 tion (C.F.) is defined by: 



C.F. = 7-TT73— , (1) 



■ , r 

 r I 



where C is the overlay weight (pounds per square foot) , Wj^ is the lonit 

 weight of the stone (pounds per cubic foot), and W is the average stone 

 weight (pounds) . 



Fourteen model tests were conducted at a model to prototype scale of 

 1:10 in the small wave tank, 1.5 feet (45.7 centimeters) wide, 2 feet 

 (61 centimeters) deep, and 72 feet (21.9 meters) long. The small wave 

 tank tests were used to evaluate the stability of a single-layer overlay 

 of rounded boulders and angular quarrystone. Two prototype (full-scale) 

 tests, using a single layer of angular to blocky quarrystone, were 



