15 August 2002
width. Slope the sides slightly inward at the top, but maintain requirements for roadway
and deck layout.
Plan. Make the structure symmetrical about both axes. See Figure 9-2.
Ballast. Ballast is used to control the caisson draft and stability.
Fixed Ballast. Fixed ballast is required to insure stability under all
operating conditions. Fixed ballast at the bottom of a caisson is concrete, with steel
punchings as additional aggregate. Quantity of fixed ballast is determined by the
minimum draft and stability requirements. The minimum draft of a floating caisson must
be such that it can be raised from the sill or seated at mean lower low water.
Water Ballast. The maximum draft of a caisson occurs while the caisson
is being seated at extreme high water. As a general rule, caissons are designed to
allow for seating at mean high water, and the safety deck is located, and dip pipes
proportioned, to suit this condition.
Location. The locations of safety decks depend on necessary water
ballast, adaptability to framing arrangements, and required headroom for machinery.
There are advantages to raising the safety deck higher over the end trim tanks than
over the center tanks of the caisson because it allows use of shorter pump shafts and
valve stems. Locate safety decks at elevations higher than those indicated by
Dip Pipes. Provide dip pipes in each water ballast compartment to limit
ballast water to design level. Dip pipes serve as vents until ballast water reaches the
pipe ends. These pipes are extra long, and cut off to the elevation determined by
Design Analysis. To resist external water pressure when a caisson is
seated and the dock is empty, consider the caisson closure as a rectangular slab
supported at side and bottom edges with free top edge. Assume outside water level
reaches the caisson top.