Geotechnical Engineering

DHARM

CAISSONS AND WELL FOUNDATIONS 787

This is nothing but Eq. 19.5 for Rectangular Caissons.

The bottom is made bowl-shaped so as to derive inverted arch-action which reduces
hoop tension in the curb and also provides larger base area. Since underwater concreting is
done, no reinforcements are provided. Cement Concrete, 1:2:4 mix, is used by the Tremie
Method, and the whole plug is laid in one continuous operation. About 10% extra cement is
added to compensate for the washing away of it in water. If the well is to be founded on rock,
the plug may be omitted; but the well should be properly anchored by taking it 0.25 to 0.30 m
deep into the rock bed, and providing adequate dowel bars.

(4) Steining

The thickness of the well steining should be designed in such a way that it is adequate for the
stresses developed during sinking and after installation.

If possible, the thickness should be designed to give adequate self-weight for the well to
avoid the use of additional weight of kentledge for sinking. With this premise, the thickness of
steining for a circular well may be got as follows:

Let the external diameter of the well be De and the thickness of the steining be ts.

Let the depth of penetration be D.

Self-weight of the Well = π (De – ts). ts. D. γc

where γc = Unit weight of the material of the well.

Skin friction resistance = π. De. D. fs,
fs being the unit skin friction.
The self-weight should be at least equal to the skin friction resistance for the well to
sink without kentledge.

∴π(De – ts).ts.D.γc = ∏.De.D.fs ...(Eq. 19.23)

This leads to the following quadratic in ts:

tDt

Df

seses

c

(^2) −+.
γ
= 0
or ts =
Df
D
es
(^2) ec
11
4
−−
L
N
M
M
O
Q
P
.γ P
...(Eq. 19.24)
rejecting the positive sign, since ts cannot be greater than De
2

. (For the solution to be real,

4 f

D

s

ecγ

should be less than unity. Or De >^4 fs

γc

. In other words, the external diameter should be

chosen to satisfy this condition.)

If a kentledge of weight Wk is available, Eq. 19.23 gets modified as follows:
π(De – ts).ts.D.γc = π.De.D.fs + Wk ...(Eq. 19.25)
If, in addition, the well got suspended at a height h above the base, the above equation
gets further modified as

π(De – ts).ts.D.γc = π.De(D – h)fs + Wk ...(Eq. 19.26)