Geotechnical Engineering

DHARM

CAISSONS AND WELL FOUNDATIONS 771

For equilibrium of the forces in the vertical direction,

ΣV = 0

∴ Superstructure load + self weight = Frictional resistance

+ Buoyancy force + Base reaction

Hence, assuming γc = 24 kN/m^3 and γw = 10 kN/m^3 (approx.),

55000

4

+−×^2224

π

()33DDei = π

DD Dππ

ee e××+33 30 4 ××+33 10 4 ×^1800

22

Substituting Di =

De

2

,

π

π

4

33 10 1800

3

4

F ×+ −××33 24 (^2) 33 30 55000
HG
I
KJ
F
HG
I
KJ
DDee+× ×()− = 0
or De^2 + 2.578De – 45.59 = 0
whence De = 5.585 m
Let us adopt the external diameter as 6.00 m.
Feasibility of Sinking
In order to overcome the skin friction resistance for sinking the caisson, we can determine the
internal diameter for giving adequate self-weight, from Eq. 19.3.
π
γ
4
().DDDei c^22 − = f(πD
e.D)
π
4
(. 60022 −×Di )33 24 = 30(π × 6.00 × 33)
(36 – Di^2 ) = 30
Di^2 = 6
Di = 6 = 2.45 m
Let us take the internal diameter as 2.4 m (rounded off to the lower side to provide
sufficient weight to overcome skin friction)
∴ Thickness of the wall =
(. .)60 24
2
−
m
= 1.8 m
Thickness of Concrete Seal
From Eq. 19.4,
t = 059.D
q
i
σc
Assuming σc = 3500 kN/m^2 ,
t = 0.59 × 2.4
1800
3500 m = 1.016 m
A Concrete seal of 1 m thickness may be provided at the base of the caisson.
Example 19.2: An open caisson, 20 m deep, is of cylindrical shape, with external and internal
diameters of 9 m and 6 m, respectively. If the water level is 2 m below the top of the caisson,