51 



Abrasion of boats and floating oltjects 



Leaching action of water on i luniical preservatives 



Action of salt water 



Marine borer attack on tiinijer 



Shell fish and mollusks 

 Mud Loading stresses 



Bending stress in flexible piles near top of substrata 



Bearing on substrata 



Side bearing of substrata — mud, sand, rip-rap, etc. 



Frictional connection with substrata for piles 



Marine borer attack on timber near top of substrata 

 Of these four levels, the air and mud sections present conditions essentiall\' similar 

 to those of land structures. The greatest problems prevail in the two intermediate 

 water sections. Among numerous adverse influences, the air-water section is character- 

 ized by the severe weathering and other deteriorating actions common to conditions 

 where materials are exposed to both water and air, particularly the decay of timber, its 

 attack by the borer Limnoria, when that is present, and the corrosion of metal, such 

 as reinforcing steel in concrete. The water section is characterized especially by borer 

 attack on timber. Metal corrosion, due to the action of sea water, is only less active 

 than in exposure to alternate air and water. Thus if the two most common substructure 

 materials be considered — timber and reinforced concrete — the water section is most 

 critical for the former, and the air-water section for the latter. 



Changes in Water Level 



Alternate wetting and drying of a substructure material may be caused by wa\e 

 spray, or by tidal or seasonal changes in water level. The most adverse condition is 

 that created in ocean exposures by the combination of wave spray and tidal change, 

 which may involve a considerable height of the structure; in protected harbors the 

 tidal change and the wave wash will usually extend over only a few feet. In rivers the 

 seasonal change of level may be as much as 30 or 40 feet, although usually it is much 

 less. While this last case gives a large range, the weathering and corrosive effect is 

 reduced because the change is seasonal and because the corrosive effect of fresh water 

 is less than that of salt water. Lhnder usual conditions, the maximum height of sub- 

 structure sul)jected to alternate wetting and drying is that from the lowest water 

 level to the wharf floor: at San Francisco, where the tidal range is from 5 to 8 feet, 

 wharf floors are built 14 or 15 feet above low water; at Sacramento, on the Sacramento 

 River, the annual change in water level may be as much as 31 feet and wharf floors are 

 about 36 feet above the low level. Thus the height of substructures to be protected 

 against the adverse conditions of the air-water section in the San Francisco Bay 

 region and its tributary rivers, is from about 14 to 36 feet. 



While the range of water levels is relatively stable in ocean and harbor locations, 

 new conditions are often created in rivers and their estuaries. This may be caused by 

 years of unusual drought, by the withdrawal of large quantities of river water for 

 irrigation or municipal purposes, by reclamation projects, by the impounding of head 

 waters, or by locks and dams. De\'elopments which tend to stabilize the water level 

 near the high water ele\'ation to which the wharves are built are desirable, whereas 

 those which lower the level usually create both structural and operating difficulties. 



The falling of the water level may facilitate inspection and repair of substructures 

 to a certain degree, but this advantage is completely offset by the many adverse con- 

 ditions resulting from change in water le\'el. At low tide some structures ma>' be 



