be placed and the size of quarrystone used, but should not be less than 

 12 inches to ensure that bottom irregularities are completely covered. 

 It is common practice to extend the bedding layer at least 5 feet beyond 

 the toe of the cover stone. Details of typical rubble structures are 

 shown in Chapter 6, STRUCTURAL FEATURES. 



7.38 STABILITY OF RUBBLE FOUNDATIONS AND TOE PROTECTION 



Forces of waves on rubble structures have been studied by several 

 investigators. (See Section 7.37.) Brebner and Donnelly (1962) studied 

 stability criteria for random-placed rubble of uniform shape and size 

 used as foundation and toe protection at vertical-faced, composite struc- 

 tures. In their experiments, the shape and size of the rubble units were 

 uniform, that is, subrounded to subangular beach gravel of 2.65 specific 

 gravity. In practice, the rubble foundation and toe protection would be 

 constructed with a core of dumped quarry-run material. The superstruc- 

 ture might consist of concrete or timber cribs founded on the core 

 material. Finally, the apron and side slope of the core would be pro- 

 tected from erosion by a cover layer of armor units. The cover layer 

 should have a minimum thickness of two armor units. An alternative 

 method of constructing the superstructure would be to drive a pair of 

 parallel-tied walls of steel sheet piling into the rubble core. 



7.381 Design Wave Heights . For a composite breakwater with the super- 

 structure resting directly on a rubble-mound foundation, structural 

 integrity may depend on the ability of the foundation to resist the 

 erosive scour by the highest waves. Therefore, for design of such 

 structures, it is suggested that the selected design wave height H 

 should be based on the following: 



(1) For critical structures at open exposed sites where failure 

 would be disastrous, and in the absence of reliable wave records, the 

 design wave height H should be the average height of the highest 



1 percent of all waves H^ expected, based on the significant deepwater 

 wave height Ho corrected for refraction and shoaling. (Early breaking 

 might prevent the 1-percent wave from reaching the structure; if so, the 

 maximum wave that could reach the structure should be taken for the 

 design value of H.) 



(2) For less critical structures, where some risk of exceeding 

 design assumptions is allowable, wave heights between Hio and Hi are 

 acceptable. 



The design wave for rubble toe protection is also between Hio and Hi. 



7.382 Stability Number . The stability number is primarily affected by 

 the depth of the rubble foundation and toe protection below the still- 

 water level di, and by the water depth at the structure site, dg. 

 The relation between the depth ratio, d-^/ds> and n| is indicated in 

 Figure 7-99. The cube value of the stability number has been used in 

 the figure to facilitate its substitution in Equation 7-110. 



7-201 



