DHARM

CAISSONS AND WELL FOUNDATIONS 775

After Installation

Upward force on the Caisson

= 9 × 18 × 9 × 10.2 kN = 14, 871.6 kN,

This is more than the downward force of 10,782 kN; further ballast is required to keep

the caisson in position.

The weight of additional ballast = (14872 – 10782) kN

= 4,090 kN, nearly.

Additional height of sand required =

4090

91822 ××

m = 1.15 m

Thus the total height of sand ballast required is 1.65 m. Stability is automatically en-
sured by the additional weight, which may be verified, if necessary.

The maximum and minimum pressures may be obtained after installation as follows:-

qmax =

W

A

e

B

F 1 +^6

HG

I

KJ

=

45000

918

1 6015

() 9

×

F + ×

HG

I

KJ kN/m

(^2) = 305.6 kN/m 2
qmin =
W
A
e
B
F 1 −^6
HG
I
KJ

45000
918
1 6015
() 9
×
F − ×
HG
I
KJ kN/m
(^2) = 250 kN/m 2
Uplift pressure = 10.2 × 9 kN/m^2 = 91.8 kN/m^2
∴ Net maximum pressure = 305.6 – 91.8 = 213.8 kN/m^2
Net minimum pressure = 250.0 – 91.8 = 158.2 kN/m^2.

19 .8 Well Foundations

Well foundations have been used in India for centuries for providing deep foundations below

water for monuments, bridges, and aqueducts. For example, the famous Taj Mahal at Agra

stands on well foundations.

The construction of a well foundation is, in principle, similar to the conventional wells

sunk for obtaining underground water; in fact, it derives its name owing to this construction

technique. It is a monolithic and massive foundation and is relatively rigid in its engineering

behaviour.

Well foundations are similar to open Caissons referred to in Section 19.3. These are very

popularly used to support bridge piers and abutments in India as they afford a number advan-

tages over other types of deep foundations for such large jobs.

19.8.1Advantages of Well Foundations

The following are the advantages of well foundations over other types of deep foundations

such as pile foundations:

(i) The effect of scour can be better withstood by a well foundation because of its large

cross-sectional area and rigidity.