250. Timber . - Allowable stresses for timber should be those "for 

 timbers more or less continuously damp or wet. These working stresses 

 may be found in U, S, Department of Commerce publications dealing with 

 American lumber standards, 



251. Stoneo - Usually the availability of stone sources determines 

 the quality of stone used in waterfront structures. However, care should 

 be taken to avoid \ise of stone which may decompose more or less rapidly 

 under wave and water action. Where such stone has been used, the effective 

 life of the structures was decreased considerably. 



SEAWALLS, REVEITMENTS AND BULKHEADS 



252. Types . - Since seawalls, bulkheads and revetments perform over- 

 lapping functions, the structural types will generally be similar. 

 Actually the choice betvieen these three kinds of shore structures is 

 mainly one of noraenclaturej overall design features are determined at the 

 functional planning stage and the structure is named to suit its intended 

 purpose, A seawall at one locality may be a bulkhead or revetment at 

 another , 



2^3, In general, however, seawalls tend to be the more massive of 

 the three because consideration must be given to fill retention character- 

 istics as well as to stability against wave forces. Bulkheads are ordinarily 

 next in size for they must withstand wave forces in addition to retaining 

 a fill. Revetments often are the lightest of the three because considera- 

 tion in their design is given only to their effectiveness in reducing 

 wave-caused erosion, 



25ii. Illustrated in Figure 85 through 93 a^^e various structural 

 types which either have been used, or are typical of those which have been 

 used. Of seawall types, there are four: 



a. Combination stepped and curved face; concrete 



(San Francisco, California) Figure 85 



b. Stepped face; concrete (Harrison County, 



Mississippi) Figure 86 



c. Cellular, sheet pilej steel (Typical) Figure 8? 



d. Stone; rubble mound (Typical) Figure 88 



The first of these, a combination stepped and curved face wall, is a re- 

 latively massive structure, built to resist high wave action and inhibit 

 scour. The second, a stepped face wall, was designed for stability 

 against more moderate wave conditiors* Note in both of these the presence 

 of a sheet pile cut-off wall to prevent leaching of the backfill. The 

 typical cellular sheet pile wall is one type which may be used on a rocky 

 bottom where pile penetration is not adequate for inherent pile stability. 

 A concrete, rubble-mound, or cut-stone masonry (not illustrated) wall may also 

 be used under these circumstances, A rubble-mound wall (Figure 88) is 

 useful where bottom conditions may permit some settlement, 



12 it 



