71 



Changes of Mud Line 



Attention must be given to possible changes of the mud line. Where tidal or 

 ri^•er currents are strong, investigations should be made over as long a period as 

 possible to ascertain changes in the bottom. Possible scouring or silting due to changing 

 currents, produced by the contemplated piling or other substructure itself, should be 

 forecast as thoroughly as possible. 



Propeller action often churns away adjacent eml)ankments. The likelihood of 

 future dredging should also be determined. This phase is particularly important 

 where types of protected piles are used in which the protection is carried down onK' 

 to just below the mud line. 



Bottom Penetr.vtion 



Where flexibility is required, yielding substrata are better than hard substrata. 

 The former will support the pile laterally and resist some of the side thrust when the 

 pile is flexed, in this manner distributing the bending stresses in the pile over a greater 

 length. In hard, iniyielding substrata the flexibility and maximum stresses are con- 

 fined to a comparatively short length of the pile above the mud line, which often 

 results in the breaking of piles. When such hard substrata are encountered, a greater 

 number of piles should be dri\'en to distribute the stress. It often happens, however, 

 that piles are broken off at the mud line before the necessity for additional ones is 

 apparent. In some cases rip-rap is placed around the piles to gi\e lateral support. 

 This must be carried out carefully, howe\er, to prevent unequal deposit of the stone 

 around the piles, pressing more on one side than the other, and either forcing the bottom 

 end out of position, or bending the pile and thus creating initial stresses which ma\' 

 result in failure. The more common method is to place the rip-rap first and drive the 

 piles through it. If the piles are supported \ertically to any extent by friction with 

 such rip-rap, any settlement of the latter may cause the piles to settle and create a 

 difficult condition. In any of these cases the rip-rap must not be counted on to protect 

 the timber from marine borers, unless it happens that the voids are completely filled 

 with muil and silt. 



Rel.\tive Depth of Water and of Bottom Penetration 

 Pile design is often determined by the relati\e depth of water and of mud penetra- 

 tion. Below the mud line the \arious types are more or less equal in value, and un- 

 treated timber, which is the cheapest form, will there serve as well as any other. If 

 the water is shallow and the penetration great, it may be economical to applv to the 

 piles a part-length protection, which can be carried only down to a point a short distance 

 below the mud line, the shorter length protected o\-erbalancing their usually greater 

 cost per linear toot as against full-length protections such as creosoting. Part-length 

 surface protections such as the paint and batten protections (see p. 256) are shown bv 

 our extensive service records to be shorter li\ed than creosote impregnated piles. 



Where the water is deep, however, and the penetration comparatively small, a 

 full-length creosote protection is initially cheaper, is longer li\ed, and has the structural 

 advantage that the composition of the pile is uniform throughout its length. 



The drawbacks of partial length protection methods are the difficultv of apph'ing 

 the protection to the correct length of the pile, and the possibility of a changing mud 

 line. The protection must extend a few feet below the mud line to allow a margin of 

 safety, as well as a short distance above high water. This is ditiicult to accomplish for 

 those types in which, as in the paint and batten type, the protection is applied before 

 the pile is driven, particularly because of the uncertainty as to the actual amount of 

 penetration that will occur. The latter must be determined as accurately asjioasible, 



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