Geotechnical Engineering

DHARM

788 GEOTECHNICAL ENGINEERING

An inspection of Eq. 19.24 will reveal that, for a given value of skin friction, the thick-
ness of steining required appears to decrease with increasing value of the diameter of the well.
This is, however, contrary to the conventional practice or providing greater thickness of steining
with increasing diameter of the well, as given in Table 19.5.

Table 19.5 Thickness of Steining for Different Diameters of Well

S.No. External Diameter m Thickness of Steining m

1. 3 0.75


2. 5 1.20


3. 7 2.00

Empirically, steining thickness is taken as one-fourth the external diameter for railway
bridge, and one-eighth for road-bridges.

A thumb rule commonly used for the thickness of the Steining is

ts = K

De H

8 100

F +

HG

I

KJ ...(Eq. 19.27)

where De = External diameter of well,

H = Depth below low water level,

and K = a constant (1 for sandy soils, 1.1 for soft clay, and 1.25 for hard clay and boulders).

The design of Steining reinforcement depends upon the skin friction and the unit weight
of the material of the well. It is usual practice to provide reinforcements of about 50 to 60 N/m^3
of the Steining. About 75% of the reinforcement is in the form of vertical reinforcement, and
25% in the form of laterals or hoop rings. The Vertical reinforcement is spread near both the
inner and outer faces. The lateral reinforcement should be checked for the moment developed
due to eccentric kentledge and half the weight of the well at an eccentricity of one-fourth the
width of the well in any direction. This condition is generally critical when the well has sunk to
about the half the designed depth.

(5) Top Plug

The function of the top plug is to transmit the load of the superstructure to the steining. If well
cap is provided, there is no need to provide the top plug. However, it is generally provided as
an extra precaution. Offsets are provided at the top of the steining to provide bearing to the
plug. Cement concrete, 1:2:4 mix, is commonly used for the top plug.

Between the bottom plug and the top plug, sand filling is usually provided. This will
enhance the stability of the well. Sand filling does not contribute to the structural strength of
the well.

(6) Well Cap

The bottom of the cap is generally kept at low water level. It is designed as a slab resting on the
well. The well cap may be extended by cantilever action to accommodate piers of slightly larger
size.