DHARM778 GEOTECHNICAL ENGINEERING
However, in terms of lateral stability, a square for rectangular section is better. Circular wells
offer least resistance against tilting, other factors being the same.
The cross section of a pier is such that its dimension in the direction of flow is 3 to 4
times that perpendicular to the flow. Therefore, a single circular well foundation becomes
uneconomical to support large piers in as much as it has to encircle the pier. It also increases
the obstruction to the flow of water, which is not desirable. In such cases, rectangular, twin-
circular, twin-hexagonal, twin-octogonal, or double-D section may be used to advantage.
Twin-circular wells are nothing but two circular wells sunk close to each other to sup-
port a pier or an abutment. Both the wells are sunk simultaneously. Rectifying any tilt is
relatively easy, although translation of the wells towards each other cannot be ruled out.
Double-D wells have more lateral stability than other types. At the corners of the dredge
holes, easy access will not be there for dredging equipment.
Dumb-well and rectangular well with multiple dredge holes are two other types, popu-
larly used for heavy bridge piers and abutments. (Fig. 19.11):
The choice of plan shape depends upon a number of factors. Sometimes it is a matter of
practice—for example, twin-circular wells are widely used in South India, while the Double-D
shape is more popular in North India.
19 .9 Design Aspects of Well Foundati?ns
The basic principle involved in the design of a well foundation, as in the case of any other
foundation, is to satisfy the twin requirements of Stability and Deformation criteria. It is
taken to sufficient depth to ensure adequate factor of safety against the acticipated maximum
vertical load. In addition, it is necessary to ensure adequate lateral stability under maximum
scour conditions. The depth of the well foundation below the local scour level is known as its
‘grip length’. This is chosen in such a way as to ensure safety against overturning moments
caused by a combination of vertical and lateral loads.
(a) Dumb-well (b) Rectangular well with multiple
dredge holes
Fig. 19.11 Additional shapes of wells
The settlement of a well foundation must be within permissible limits. The lateral de-
formation at deck level due to the tilting of the well caused by lateral loads must be within
permissible limits; otherwise the superstructure will be affected. If adequate factor of safety
against overturning is provided, the deformation requirements at deck level are automatically
taken care of.