Geotechnical Engineering

DHARM

CAISSONS AND WELL FOUNDATIONS 773

Concrete

seal

2 m

H = 18 m

1 m

6 m

9 m

Fig. 19.8 Concrete seal for open caisson (Ex. 19.2)

Check for Perimeter Shear

Force causing perimeter shear = γw(H – t) ×

π

4

D^2

i

= 10(18 – 1) ×

π

4

× 62 kN

= 4,806.65 kN

Area taking this force = π × 6 × 1 = 18.85 m^2

Perimeter shear stress =

4806 65

18 85

.

. = 255 kN/m

2

Since this is far less than the permissible value of 650 kN/m^2 the concrete seal of 1 m
thickness is adequate.
Example 19.3: A box caisson, 10 m high, is 18 m × 9 m at the base. The weight of the caisson
is 9 MN and its centre of gravity is 4.2 m above the base. Check whether the Caisson is stable.
If it is not, suggest how it can be made stable. Assume the unit weight of the water at the site
is 10.2 kN/m^3.
If the base is at a depth of 9 m below the water level after installation, determine the net
values of maximum and minimum pressures on the soil. The total weight is 45 MN, which acts
at an eccentricity of 0.15 m.

Solution:

Referring to Fig. 19.9,

Displaced volume of Water,

V =

9000

10 2. m

(^3) = 882.35 m 3
Depth of immersion during floating,
d =
9000
10 2 18 9 .××
m = 5.45 m