Use the following schedule (Table 2) to estimate wave action, where 

 a more precise calculation is not required: 



Table 2. Schedule for estimation of wave action. 



Wave height Minimum height of bulkhead above 



crest to trough high water level, as allowance for 



(feet) wave action (feet) 



1 to 2 3 



2 to 3 4 



By observing the heights of existing structures in a particular 

 locality, experience can often be used as a guide in arriving at the 

 elevation of the finished grade. 



(6) Available Soil Materials . Granular materials, such as sand 

 and gravel, are preferred for backfill behind bulkheads and for base 

 materials into which sheet piles are driven. Granular materials have 

 several characteristics that make them desirable. The active pressures 

 they exert on bulkheads are considerably less than those exerted by silts 

 and clays or even by sands that are "contaminated" with silt or clay. 

 Granular materials can develop a higher passive resistance, permitting 

 the use of shorter sheet piles. 



It is often preferable to bring granular materials to the construc- 

 tion site by truck or other conveyance rather than to backfill with silt 

 or clay. In the final analysis, a judgment is required. A designer 

 must determine whether it is more economical or more desirable to increase 

 the thicknesses of materials in the bulkhead rather than to transport 

 granular materials from far distances. Fortunately, granular materials 

 are quite often readily available near bodies of water. 



Granular material also promotes drainage behind the bulkhead and is 

 less likely to be eroded away through minor fissures in the bulkhead as 

 the water level rises and falls. The drainage is advantageous wherever 

 the backfill is to be used for roadways, sidewalks, parks, yards, etc. 

 Grass and plants will grow better in a soil that is moderately well 

 drained; the granular material will not shrink and swell with alternate 

 drying and wetting to the same degree that clay will. The backfilled 

 area is not likely to resemble a marshy bog where drainage is provided 

 through granular materials where lawns and other plantings are required, 

 a relatively thin layer of top soil san be placed over the granular 

 material . 



Where the material at the shoreline is extremely poor quality, such 

 as silt or clay, the passive resistance will be very low. This will 

 require lengthening the sheet piling to obtain sufficient passive 

 resistance to avert a failure. The greater length of sheet piling 



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