Geotechnical Engineering

DHARM

CAISSONS AND WELL FOUNDATIONS 803

Great care must be exercised to sink a well foundation vertically in order to avoid eccen-
tricity of vertical loading. However much care is bestowed, it is common that some little tilt
and shift from the intended position occur. These are to be observed carefully at every stage of
sinking, and remedial measures adopted to rectify tilts and shifts. In this process, a small
degree of eccentricity in the loading, unintended though, is bound to occur. This should also be
borne in mind both in the analysis and construction of the superstructure.

19.11.1 Sinking of Wells

The operation sinking of a well consists of the following steps:

(1) Construction of the Well Curb

If the river bed is dry, the cutting edge over which the well curb is to be built is placed at the
correct position after excavating the bed for about 150 mm for seating. If there is water, with
a depth upto 5 m, a sand island is created before placing the curb. The size of the island should
be large enough to accommodate the well with adequate working space all round (Fig. 19.22).
In case the depth of water is more than 5 m, it is more economical to build the curb on the bank
and float it to the site.

Sand island

Well

Fig. 19.22 Sand island for sinking a well
Wooden sleepers are usually inserted below the cutting edge at regular intervals to
distribute the load evenly on the soil. The shuttering of the well curb is erected—the outer one
with steel or wood and inner with brick masonry. The reinforcements for the curb are then
placed in position, the vertical bars projecting about 2 m above the top of the curb. Concreting
of the curb is done in continuous operation. After the curb is cured and allowed to cure for at
least seven days, the shuttering may be removed as also the sleepers.

(2) Construction of Well Steining

The Steining is constructed with a height of 1.5 m at a time and sinking done after allowing at
least 24 hours for setting. Once the well has acquired a grip of about 6 m into the ground, the
steining can be raised 3 m at a time. The height of any lift is restricted such that the well does
not lose stability.

(3) Sinking Process
The sinking process is commenced after the curb is cast and the first stage of steining is ready
after curing. The material is excavated from inside manually or mechanically. Manual dredging