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CHAPTER VI 



MARINE CONDITIONS AS AFEECTIXG MARINE CONSTRUCTION 



Exposure 



Marine structural iirohlems are largely conditioned by the nature of exposure, 

 which may range from that of the highly protected harbor to that without protection 

 from the violence of the ocean. While the structural requirements may be identical 

 with regard to loading, depth of water, or condition of substrata foundation, differing 

 exposures will require different procedures. All adverse conditions are accentuated in 

 ocean exposures, both by the chemical effects of sea water and by the destructive 

 impact forces of waves. A method of timber pile protection which may serve perfectly 

 in calm water may be worthless in locations subject to severe wave action. In general, 

 ocean exposure will require extreme precautions, with the elimination of details in- 

 capable of withstanding severe conditions. The various destructive influences and the 

 manner of their efTect on various materials are discussed in the following pages. 



Influences Operative at Various Substructure Levels 



Substructure materials are subjected to a series of influences at different levels 

 which may be designated, from top to bottom, as the "air," "air-w^ater," "water" and 

 "mud" sections. The "air" section is that above high water; the "air-water" section 

 is that alternately exposed to air and water either from wave action or from tidal or 

 seasonal changes in water level; the "water" section is that constantly submerged in 

 water; the "mud" section is that below the mud line. A single unit which passes through 

 these four levels must be designed to fit conditions prevailing in each, which may be 

 briefly outlined as follows: 



Air Connecting to superstructure 



Bracing 



Loading stresses 



Decay of timber 



Corrosion of metal 



Abrasion of boats 



Fire hazard 

 Air-Water. . . .Loading stresses 



Current stresses 



Impact of waves 



Effect of temperature changes 



Abrasion of boats and floating objects 



Corrosion of metal 



Decay of timber to mean tide level 



Leaching action of water on chemical preservatives 



Action of salt water 



Marine borer attack on timber, especially that of Limnona 

 W'ater Loading stresses 



Current stresses 



Impact of waves 



Bending stress in flexible piles 



