in the United States. Typical blocks are generally square slabs with 

 ship-lap type interlocking joints as shown in Figures 6-11 and 6-12. The 

 joint of the ship-lap type provides a mechanical interlock with adjacent 

 blocks. 



Stability of an interlocking concrete block depends largely on the 

 type of mechanical interlock. It is impossible to analyze block stability 

 under specified wave action based on the weight alone. However, prototype 

 tests at CERC on blocks having ship-lap joints and tongue -and- groove joints 

 indicate that stability of tongue -and- groove blocks is much greater than 

 the ship-lap blocks. (Hall, 1967.) An installation of the tongue-and- 

 groove interlock block is shown in Figure 6-13. 



6.22 SELECTION OF STRUCTURAL TYPE 



Major considerations for selection of a structural type are: founda- 

 tion conditions, exposure to wave action, availability of materials and 

 costs. The following paragraphs illustrate a procedure for reviewing these 

 factors. 



6.221 Foundation Conditions . Foundation conditions may have a significant 

 influence on the selection of type of structure, and can be considered from 

 two general aspects. First, foundation material must be compatible with 

 the type of structure. A structure that depends on penetration for stabi- 

 lity is not suitable for a rock bottom. Random stone or some type of flex- 

 ible structure using a stone mat or plastic filter cloth could be used on 



a soft bottom, although a cellular steel sheet-pile structure might be used 

 under these conditions. Second, the presence of a seawall, bulkhead or 

 revetment may induce bottom scour and cause failure. Thus, a masonry or 

 mass concrete wall must be protected from the effects of settlement due to 

 bottom scour induced by the wall itself. 



6.222 Exposure to Wave Action . Wave exposure may control the selection 



of both structural type and details of design geometry. In areas of severe 

 wave action, light structures such as timber crib or light riprap revetment 

 should not be used. Where waves are high, a curved, reentrant face wall or 

 possibly a combination of a stepped-face wall with a recurved upper face 

 might be considered over a stepped-face wall. 



6.223 Availability of Materials . This factor is related to construction 

 and maintenance costs as well as to structural type. If materials are not 

 available near the construction site, or are in short supply, a particular 

 type of seawall or bulkhead may not be economically feasible. A cost com- 

 promise may have to be made or a lesser degree of protection provided. 

 Cost analysis includes the first costs of design and construction and 

 annual costs over the economic life of the structure. Annual costs include 

 interest and amortization on the investment, plus average maintenanc costs. 

 The best structure is one that will provide the desired protection at the 

 lowest annual or total cost. Because of wide variations in first cost and 

 maintenance costs, comparison is usually made by reducing all costs to an 

 annual basis for the estimated economic life of the structure. 



6-15 



