Caissons are generally suitable for depths from about 10 to 35 feet. 

 The foundation must support the structure and withstand scour, and usually 

 consists of a mat or mound of rubble stone. (See Section 7.38.) Where 

 foundation conditions dictate, piles may be used to support the structure. 

 Heavy riprap is usually placed along the base of the caissons to protect 

 against scour, horizontal displacement, or weaving when the caisson is 

 supported on piles. 



6.9 BREAKWATERS- -OFFSHORE 



6.91 TYPES 



Offshore breakwaters can also be classified into two types: rubble- 

 mound and cellular steel sheet-pile. Selection of the type for a given 

 location is dependent on the comparative cost which is dependent on the 

 depth of water, availability of material and wave action. For open ocean 

 exposure, rubble-mound structures are usually required; for less severe 

 exposure, as in the Great Lakes, the cellular steel sheet-pile structure 

 may be a better choice. Figures 6-46 and 6-51 illustrate the use of 

 rubble-mound offshore breakwaters to trap littoral material, to protect 

 a floating dredge, and to protect the harbor entrance. 



Figure 6-72 illustrates the structural details of the rubble-mound 

 breakwater at Marina Del Rey, Venice, California. 



Probably the most notable offshore breakwater complex in this country 

 is the 8 1/2-mile-long Los Angeles-Long Beach breakwater built between 

 1899 and 1949. Other offshore breakwaters are located at Santa Monica, 

 California, built in 1934; Venice, California, built in 1905; and Winthrop 

 Beach, Massachusetts, built in 1933. 



6.10 CONSTRUCTION MATERIALS 



The selection of materials in the structural design of shore protec- 

 tive works depends on the environmental conditions of the shore area. 

 Discussions of criteria that should be applied to materials commonly 

 used follow. 



6.101 Concrete . Proper quality concrete is required for satisfactory 

 performance in a marine environment. The quality is obtainable by use of 

 good concrete design and construction practices. The concrete should have 

 low permeability, provided by the water-cement ratio recommended for the 

 exposure conditions; adequate strength; air-entrainment, a necessity in 

 freezing climate; adequate cover over reinforcing steel; durable aggregates 

 and proper type of portable cement for the exposure condition. Factors 

 affecting durability of concrete in a marine environment have been reported 

 by Mather (1957). The requirements for durable concrete, consisting of 

 water-cement ratio, air-entrainment, durable aggregate and type of portland 

 cement are discussed in an engineering manual (U.S. Army, Office, Chief of 

 Engineers, 1971b). Details of reinforcing steel are discussed in an 

 engineering manual (U.S. Army, Office, Chief of Engineers, 1971a). 



6-96 



