hot asphalt mixture and tends toward early structural failure of the section 

 seaward of the beach berm crest. Failure results from lack, of resistance to 

 normal seasonal variability of the shoreface and consequent undermining of the 

 structure foundation. Modification of the design as to mix, dimensions, and 

 sequence of construction may reveal a different behavior. See Asphalt Insti- 

 tute (1964, 1965, 1969, and 1976) for discussions of the uses of asphalt in 

 hydraulic structures. 



2. Selecti on of Type. 



After research on a problem area has indicated the use of groins as prac- 

 ticable, the selection of groin type is based on varying factors related to 

 conditions at each location. A thorough investigation of existing foundation 

 materials is essential. Borings or probings should be taken to determine the 

 subsurface conditions for penetration of piles. Where foundations are poor 

 or where little penetration is possible, a gravity-type structure such as 

 a rubble or a cellular-steel sheet-pile groin should be considered. Where 

 penetration is good, a cantilever-type structure made of concrete, timber, or 

 steel-sheet piles should be considered. 



Availability of materials affects the selection of the most suitable 

 groin type because of costs. Annual maintenance, the period during which 

 protection will be required, and the available funds for initial construction 

 must also be considered. The initial costs of timber and steel sheet-pile 

 groins, in that order, are often less than for other types of construction. 

 Concrete sheet-pile groins are generally more expensive than either timber or 

 steel, but may cost less than a rubble-mound groin. However, concrete and 

 rubble-mound groins require less maintenance and have a longer life than 

 timber or steel sheet-pile groins. 



3. Design . 



The structural design of a groin is explained in a number of Engineer 

 Manuals (EM's). EM 1110-2-3300 (U.S. Army Corps of Engineers 1966) is a 

 general discussion of the components of a coastal project. A forthcoming EM 

 (U.S. Army Corps of Engineers (in preparation, 1984)) is a comprehensive 

 presentation of the design of coastal groins. The basic soil mechanics 

 involved in calculating the soil forces on retaining walls (and, therefore, 

 sheet-pile groins) are presented in EM 1110-2-2502 (U.S. Army Corps of 

 Engineers 1961). EM 1110-2-2906 (U.S. Army Corps of Engineers 1958) discusses 

 the design of pile structures and foundations that can be used in the design 

 of sheet-pile groins. Wave loading on vertical sheet-pile groins is discussed 

 by Weggel (1981a). 



VII. JETTIES 



Types. 



The principal materials for jetty construction are stone, concrete, 

 steel, and timber. Asphalt has occasionally been used as a binder. Some 

 structural types of jetties are illustrated in Figures 6-60, 6-61, and 6-62. 



a. Rubble-Mo und Jetties. The rubble-mound structure is a mound of 

 stones of different sizes and shapes, either dumped at random or placed in 



6-84 



