DHARM762 GEOTECHNICAL ENGINEERINGIf it is desired to proportion a circular Caisson such that no ballast is required for sink-
ing, the self-weight should be at least equal to the force due to Skin Friction.
This leads us to the Equation
π
γ
4().DDDei c^22 − = f.(πD
e.D)or π γ
4
()DD^22
eic− = f(πDe) ...(Eq. 19.3)
where De and Di = External and Internal diameters of the Caisson
D = Depth of Penetration
γc = Unit weight of the Caisson Materialand f = Unit Skin Friction.
19.2.5Concrete Seal
After the Caisson is placed in its final position a thick concrete layer is placed at the bottom to
plug it. This is known as ‘Concrete Seal’ or ‘Plug’, and forms the permanent base for the Cais-
son. In Open Caissons, the top of the Concrete Seal is carried to a level higher than the bevel-
led portion of the cutting edge (See subsection 19.2.6). In Pneumatic Caissons, the entire working
chamber, filled with concrete, serves as the concrete seal.
During the period of construction, the concrete seal serves to seal off the inflow of water
while placing concrete above it. After construction, it serves as the permanent base.
The thickness of the seal should be sufficient to withstand the upward hydrostatic pres-
sure after dewatering is complete and before concreting of the Caisson shaft is done. The seal
may be designed as a thick plate subjected to uniform pressure due to maximum vertical loads
from the Caisson.
The thickness of the concrete seal, t, may be obtained from the following equations:
For Circular Caissons:t = 0.59 Diq
σc
...(Eq. 19.4)For rectangular caissons:t = 0 866. B (.) 1161q
i
σαc +
...(Eq. 19.5)These are for simply supported conditions.
Here, Di = internal diameter of caisson,
α = Bi/Li,
Li, Bi = internal length and breadth of caisson,
q = net upward pressure on the seal,
and σc = allowable flexural stress for concrete (≤ 3,500 kN/m^2 ).
If H is the depth of water above the base, q can be got from the equation
q = (γwH – γct) ...(Eq. 19.6)
Some designers check for perimeter shear stress also for the seal.