imposed by a given site. 



373. Rubble Mound .> - The rubble-mound structure is adaptable to any 

 depth of water and may be placed on practically any kind of foundation. 

 This type of structure -is used extensively tliroughout the United States 

 and almost exclusively on the Pacific Coast (Kaplan and Pape, 1900). .The 

 chief advantages of the random stone structure are: damage is easily re- 

 paired, settlement of the structure results-, only in readjustment of com- 

 ponent stones, rather than the incipient failure of the entire structure, 

 and the reflected wave action is less severe than that from a solid wall 

 structure. The chief disadvantage of rubble-mound construction is the 

 large quantity of material required, which results in a high first cost if 

 satisfactoary mat&rial' is mot available within economic hauling distance. 



37U^ The rubble-mound breakwater is a more or less heterogeneous 

 assemblage of. natural stones of different sizes and shapes either dumped 

 at random or laid in courses. A typical cross section is shown in Figure 

 117. Side slopes and stone sizes are so designed that the structure will 

 'resist the expected wave action. 



37£« ' Composite Earth and Stone . - The composite type of random stone 

 structure is designed for relative economy and rapidity of construction. 

 The principal difference from the rubble-mound breaicwater lies in the 

 substitution of sand and clay or incipient shale for the base and core 

 material. Care must be used in. selecting the clay or earth material with 

 which the sand sub-base is covered. .. This type of construction was used 

 for the 32,000-foot Los Angeles-Long Beach detached breakwater. As 

 shown in Figure II8 this structure consisted of a sand inner core covered 

 by a. hard clay mound, all being armored by a graded rubble-stone covering. 

 If satisfactory material can be secured from an approved or useful dredg- 

 ing area, savings may be eiff acted. 



376. Stone and Concrete . - The stone and concrete type structure 

 is a combination of rubble-mound and concrete-wall types, ranging from a 

 rubble mound, in which the voids in the upper part of the structure are 

 filled with concrete, to massive concrete superstructures on rubble-mound 

 substructures. The rubble mound usually is used either as a foundation 

 for a vertical or nearly vertical concrete wall or as the main structure 

 surmounted by a low concrete superstructure with a vertical, curved, stepped, 

 or inclined face. The use of a composite concrete and stone structure 

 reduces the quantity of material required and may be economical ±^ great 

 depths. 



377. Where the bottom is subject to scour, care must be used to pre- 

 vent the superstructure from being undermined, as storm waves tend, in 

 their recoil down the face, to displace materials of the mound at the 



toe of the superstructure. Kubble-mound foundations require some tiiae 

 to become stable, and should be placed one or more years before construction 

 of the superstructure. Stone and concrete breakwaters, when properly de- 

 signed and constructed, p, ive satisfactory service in ^Jithstanding heavy 



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