i. Types of Treated Timber Sheet Piling . Sheet piles must have 

 the necessary bending strength together with tight joints between 

 adjacent sheets to prevent loss of sand. Sheet piles in material of a 

 single thickness are desirable from the standpoint of structural 

 strength in bending. 



Figure 7 shows three types of sheet piling. If the single thick- 

 ness strength required is 4 inches or less, tongue and groove joints 

 are used. If the loads are sufficiently great as to require sheet-pile 

 thicknesses of 6 inches or more, splined sheet piles are generally used 

 for single thickness piling. In the event that timber in single thick- 

 ness is not readily available, planks can be assembled into a composite 

 section like the conventional Wakefield sheet pile. This pile has only 

 25 to 50 percent of the bending strength of a sheet pile in a single 

 equivalent thickness, the amount of reduction depending on the size and 

 spacing of the fastenings provided between the three pieces. 



The lower ends of sheet piles should be cut at an angle as shown so 

 that in the driving process each sheet pile is forced into close contact 

 with the adjacent sheet previously driven. 



III. VERTICAL-FACED SEAWALLS OF TREATED TIMBER 



Figure 8 shows a low height seawall for moderate wave action to 

 afford protection to shore properties along a lake or bay front well 

 removed from the attack of large waves. It is normally located so that 

 the line of sheet piles intersects the ground surface somewhat above 

 MLW level. The allowance of a 3- to 5-foot wall height above high 

 water is adjusted to suit the height of wave action anticipated. The 7- 

 foot minimum penetration of sheet piling is based on an allowance of a 

 2- to 3-foot depth of erosion below MLW in front of the structure. The 

 length of the sheet piles is determined in accordance with the tidal 

 variation and the distance allowed above high water to care for wave 

 action. 



The A-frame, which provides lateral support for the top of the sheet 

 piling, is an efficient anchorage for a shoreline that has varying pro- 

 files through the beach with high banks adjacent to the bulkhead line 

 at some locations and wide expanses of low ground at other locations. 

 The A-frame spacing can be adjusted to suit the height of finished 

 grade at the top of the sheet piles. 



1» Design of Timber Seawalls . 



The vertical dimensions of seawalls are established in the same 

 manner as for bulkheads. Due consideration must be given to the antici- 

 pated wave height in determining the wall height above high water level 

 and the allowance for possible erosion of bottom materials in determin- 

 ing the length of sheet piles. 



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