induces a higher degree of bending in the piling, requiring a thicker 

 piling than would otherwise be needed. 



Under these circumstances, the designer should consider removing the 

 poor soil and replacing it with a granular material, such as clean sand. 

 The cost is often far less than that of providing thicker materials and 

 more elaborate anchorage systems for the bulkhead. 



(7) Lateral Earth Pressures . The sheet piles are driven into 

 the ground to hold earth on one side of the wall at a higher level than 

 on the other. The pressure exerted against the sheet piles by the 

 retained earth is called active earth pressure. The pressure exerted by 

 the earth on the low side in resistance to lateral movement of the sheet 

 piling is called passive earth pressure. Because cohesive soils can 

 resist far higher forces than they can create, the allowable passive 

 pressure is always higher than the active pressure, occasionally nearly 

 10 times as much. 



The magnitude and distribution of active pressure against a sheet-pile 

 wall depends on a number of factors. These factors include the physical 

 properties of the retained earth, the friction between the earth and the 

 sheet-pile wall, the amount of deflection of the wall, and the flexibility 

 of the sheet piles. Similar factors influence the magnitude and distri- 

 bution of the passive earth pressure. For a discussion of the behavior 

 of sheet-pile walls and the influence of various factors on the active 

 and passive earth pressures, see Terzaghi (1954). 



Customarily, the lateral earth pressures are proportional to the 

 vertical pressure at any given level. If the symbol, P^, designates the 



vertical pressure at any given level (weight of overlying soil), the 

 lateral pressures are expressed as K^^Py for active pressure and KpP for 



passive. The symbols, Ka and Kp, are known as coefficients of earth 

 pressure. In determining the vertical pressure P^ at any level in the 

 soil for bulkhead design, the unit weight is that of moist earth above, 

 and that of submerged earth below, the free water surface. 



Sands are classified as dense, medium, or loose, to approximately 

 describe the density, and as clean or silty to indicate the absence or 

 presence of fine materials. These physical properties influence both 

 the unit weights of the material and the earth pressure coefficients . 

 Some physical properties for clean and silty sand are shown in Table 1 

 (Terzaghi, 1954). 



For designing timber sheet-pile walls backfilled with predominantly 

 granular materials and driven into natural undisturbed deposits, the 

 following average values may be used for the unit weights of sand and 

 for the earth pressure coefficients : 



Unit weight of moist sand ----- 100 pounds per cubic foot (pcf) 



20 



