c. Floating Docks as Breakwaters . While the Canadian caisson floating 

 breakwater was evaluated from the standpoint of serving as a wharf or floating 

 dock, U.S. manufacturers of floating docks for marinas have recently been 

 investigating the effectiveness of docks as floating breakwaters. The 

 Thompson Flotation Company has had many years of experience in building 

 strong, durable floating structures for use in open-sea conditions, as 

 well as small-boat marinas that are subjected to rough water environments 

 (T. Thompson, personal communication, 1980). Drawing upon this previous 

 experience, the company has developed a floating breakwater (either permanent 

 or temporary) installation which is easily transported. The Thompson Floating 

 Breakwater (Fig. 51) has excellent strength-to-weight characteristics; how- 

 ever, no actual experimental studies or field testing has taken place to 

 quantify the degree of attenuation the system will provide. 



6. Construction Materials and Techniques. 



In the design of a floating breakwater, selecting the appropriate mate- 

 rials is as important as determining the performance characteristics and 

 loading requirements (Stormer, 1979). The density of a material is frequently 

 a critical factor, since structure weight is often a major design considera- 

 tion. Fracture toughness is a measure of a material's ability to absorb 

 energy through plastic deformation before fracturing. Loads or deformations 

 which will not cause a fracture in a single application can result in fracture 

 when applied repeatedly. Fatigue failure is a complex mechanism involving the 

 initiation of small cracks, usually from the surface, that spread under 

 repeated loading. 



Structural materials exposed to seawater must have an adequate resistance 

 to corrosion and to stress corrosion cracking, the fracture of a material by 

 stress and certain environmental factors. Other material properties to be 

 considered include ease of fabrication, weldability, durability, maintenance, 

 general availability, and finally, cost. With several possible modes of fail- 

 ure existing in a floating breakwater, the properties and cost of structural 

 materials should be thoroughly studied before the final choice is made for any 

 specific application. 



Wood and concrete are often suggested for use in underwater work because 

 of their relatively low cost and availability. Concrete has good compressive 

 strength, resistance to corrosion, and formability; however, a disadvantage is 

 its limited tensile strength. The density of concrete is affected by the type 

 of aggregate used. Sand and gravels or crushed stone produce concrete 

 weighing about 150 pounds per cubic foot. Portland cement, magnitite, 

 ilmenite, limonite, and steel shavings yield densities up to about 300 pounds 

 per cubic foot. Protection against chemical attack (sulfates in seawater 

 solution) and leaching of lime from the concrete by the seawater can be 

 provided through the use of high-quality, high-strength concrete. Limiting 

 the tricalcium aluminate content of the cement will increase resistance to 

 chemical attack in the ocean environment. 



A floating breakwater has to endure the worst possible environmental sit- 

 uations (corrosion, abrasion, and freeze-thaw conditions). The structure is 

 partially submerged with parts alternately exposed and submerged. Construc- 

 tion materials for pontoon-type floating breakwaters must be resistant to 

 ordinary solvents, particularly gasoline and petroleum. These structures are 

 inevitably used as docking platforms, whether designed for this purpose or 

 not. Ice, particularly floating and moving ice floes, can cause extreme 



