placed directly over a sand, but silty and clayey soils and some fine sands 

 must be covered by a coarser sand first. A bedding layer may consist of 

 quarry spalls or other crushed stone, of gravel, or of stone-filled gabions. 

 Quarry spalls, ranging in size from 0.45 to 23 kilograms, will generally 

 suffice if placed over a geotextile or coarse gravel (or crushed stone) filter 

 meeting the stated filter design criteria for the foundation soil. Bedding 

 materials must be placed with care on geotextiles to prevent damage to the 

 fabric from the bedding materials, as well as from heavier materials placed 

 above . 



Filter blanket or bedding layer thickness depends generally on the depth 

 of water in which the material is to be placed and the size of quarrystone 

 used, but should not be less than 0.3 meter to ensure that bottom irregular- 

 ities are completely covered. A filter blanket or bedding layer may be 

 required only beneath the bottom edge of the cover and underlayers if the core 

 material will not settle into or allow erosion of foundation material. Core 

 material that is considerably coarser than the underlying foundation soil may 

 need to be placed on a blanket or layer as protection against scour and 

 settlement. It is also common practice to extend the bedding layer at least 

 1.5 meters beyond the toe of the cover stone. Details of typical rubble 

 structures are shown in Chapter 6, STRUCTURAL FEATURES. In low rubble-mound 

 structures composed entirely of cover and underlayers, leaving no room for a 

 core, the bedding layer is extended across the full width of the structure. 

 Examples are low and submerged breakwaters intended to control sand transport 

 by dissipating waves (Markle and Carver, 1977) and small breakwaters for 

 harbor protection (Carver and Markle, 1981b). 



8. Stability of Rubble Foundations and Toe Protection . 



Forces of waves on rubble structures have been studied by several investi- 

 gators (see Section 7, above). 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 structures. 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 superstructure might consist of concrete or 

 timber cribs founded on the core material or a pair of parallel-tied walls of 

 steel sheet piling driven into the rubble core. Finally, the apron and side 

 slope of the core should be protected from erosion by a cover layer of armor 

 units (see Sec. d and e below). 



a. Design Wave Heights . For a composite breakwater with a superstructure 

 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, it is suggested that the selected design wave height H 

 for such structures 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 Hi expected during an extreme event, based on the deepwater 

 significant wave height H^ corrected for refraction and shoaling. (Early 



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