sea action. A cross section of a typical structure is shown in Figure 

 119. 



378, Concrete Caissons. - Breakwaters of this type are built of re- 

 inforced concrete shells, which are f] oated into position, settled upon 

 a prepared foundation, filled with s tone or sand to give stability, and 

 then capped xifith concrete slabs or cap stones. These breakwaters may be 

 constructed with or without parapet walls. In general, concrete caissons 

 are of two typesj one type having a bottom of reinforced concrete which 

 is an integral part of the caisson, the other type not having a permanent 

 bottom. The bottom opening of the latter type is closed with a temporary- 

 wooden bottom which is removed after the caisson is placed on the founda- 

 tion. Stone, which is used to fill the compartments, combine with the 

 foundation material to provide additional r esistance against horizontal 

 movement, A typical section is shown in Figure 120 of concrete caiss on 

 breakwater. 



379 » Caissons are suitable for depths ranging from 10 to 35 feete 

 The foundations must be prepared to support the structure and to withstand 

 scour at the base. This foundation usually consists of a mat or mound 

 of rubble-stone. Piles may be used to support the structure. Heavy rip- 

 tap is usually placed along the caissons to protect against scour, 

 horizontal displacement, or v.'eaving when the caisson is supported on piling. 

 Considerable labor and adequate floating plant are required to prepare a 

 rubble-mound foundation. The top of the mound, where the caisson is to 

 be placed, must be leveled by diver. For bottomless caissons, strips of 

 crushed stone are placed on the longitudinal footings of the caisson and 

 only these strips need be leveled, 



380. Sheet Piling, - Timber or concrete sheet piling has been used 

 for breakwater construction at locations where storm waves are not 

 severe. Steel sheet piling is used for breakwaters in several types of 

 structures, which include a single row of piling with or without pile 

 buttressess; a single row of sheet piling arranged so that the row of 

 piling acts as a buttressed wall,, but requires no more piling than a 

 straight wallj double walls of sheet piling held together with tie rods 

 with the space between the walls filled with stone or sand (usually 

 separated into compartments by cross walls if sand is used); and cellular 

 steel sheet pile structures which are modifications of the double-wall 

 type. Examples of pile breakwaters are shown in Figures 121 to 12[|. in- 

 clusive , 



381. Cellular steel sheet pile breakwaters require little mainten- 

 ance and are suitable for construction in depths up to i|0 feet and 



on all kinds of foundations. Steel sheet pile structures possess, under 

 some conditions, advantages of economy and speed of construction, but 

 during construction are subject to .storm daiaage. GorroSive action is the 

 principal disadvantage in sea water. If abrasive action of sand and 

 water continually wears away the corroded metal, leaving fresh steel ex- 

 posed, the life of the piling May not exceed 10 years. However, if the 



183 



