APPENDIX B-Continued 



Hieh Wotar Level V 



Wal.r Log 

 low Waltr L.v.l (DatumjS^ 



]-« Unbolonced Pr.iiufe 



Oi..nb""o' > 

 Mgd Lin. D.ogrom 





^^-— - 



Fig. 4: Bulkhead or seawall, basic vertical dimensions. 



by passage ol water through cracks in the bulkhead 

 .Because the soil slows down the How of water some- 

 what, the ground-water level behind the bulkhead is 

 seldom the same as the tree-water level on the sea- 

 ward side As the tide rises, the ground water level 

 rises, but at a slower rate Similarly, as the tide recedes, 

 the ground-water level falls, also at a slower rate The 

 distance between these two water levels is called the 

 water lag (Fig 4) 



When the free water is higer than the ground water 

 there is no particular design problem The water pres- 

 sure IS resisted by the soil backfill But when the 

 ground-water level is higher than the free water, there 

 Is ah outward pressure that is resisted only by the 

 wall and its anchorage system. The net effect of the 

 ground-water pressure is similar to that of the backfill 

 against the bulkhead. The outward pressure of both 

 water and soil must be considered in the design of a 

 bulkhead wall 



To determine the ground-water level, dig a hole 

 behind the sheet piles of any nearby bulkhead Water 

 will rise and fall to measurable levels If no bulkheads 

 exist in the vicinity, use a minimum value of one foot. 

 or a maximum of one-half the tidal variation, lor the 

 water lag. 



Table 1: Estimated Wave/Bulkhead Heights 



Wave Height 

 Crest to Trough 



Minimum Height of Bulkhead 

 above high water level, as 

 allowance for wave action 



1 10 2 ft 



2 to 3 ft 



As the water level recedes during ebb tide, the water 

 in the ground behind the sheeting will escape through 

 any openings left in the wall, carrying away the finer 

 particles of the soil backfill During numerous tidal 

 interchanges, large quantities of backfill can escape 

 in this manner, unless tight joints are provided between 

 sheet piles. Figure 5 shows joints used to minimize 

 losses. 



FINISHED ELEVATIONS (Step 1e) 



Figure 4 indicates the criteria used to determine the 

 basic vertical dimensions of a bulkhead The finished 

 elevation of the bulkhead and backfill were established 

 by allowing for wave action above the high water level 



Wave height is a function of wind velocity and dura- 

 tion, of expanse of water (fetch) over which the wind 

 blows, and of water depth. Reference' 2 describes , 

 methods for estimating wave height and penod under 

 various site conditions 



In protected areas, where treated timber bulkheads 

 have their best application, the length of fetch and 

 water depth are limited Consequently in most cases 

 the expected wave heights are in the range of 1 to 

 2 feet 



Use Table 1 to estimate wave action, where a mora 

 precise calculation is not required. 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 



AVAILABLE SOIL MATERIALS (Step If) 



Granular materials, such as sand and gravel are 

 preferred for backfill behind bulkheads and lor base 



Table 2: Unit Weights of Soils an 



dCoel 



cients 



Earth Pressure 









Type 

 of 

 Soil 



Unit Weight ol Soil 

 Pounds per cubic loot 



Active Earth Pressure 



Passive Ea 



th Pressure 



Moist Submerged 



Coefficient. K. 



Angles 



ol 



Friction. 



Degrees 



Coefficient. K. 



for 

 Soils in Place 



Angles 



of 



Friction, 



Degrees 



Mm Max 



Mm. Max 



Soils 

 Backfill Place 



Clean Sand 

 Dense 

 Medium 

 Loose 



110 

 110 

 90 



140 

 130 

 125 



65 

 60 

 56 



78 

 68 

 63 



020 



025 



35 30 



38 20 

 34 17 

 30 15 



90 

 7.0 

 50 



38 25 

 34 23 

 30 20 



Silty Sand: 

 Dense 

 Medium 

 Loose 



110 

 95 

 80 



150 

 130 

 125 



70 

 60 

 50 



88 

 68 

 63 



0.25 



030 



50 35 





7.0 

 50 

 3.0 





313 



