Posted Date: 12-Nov-2009
Well foundation is the most commonly adopted foundation for major bridges in India. Since then many major bridges across wide rivers have been founded on wells.
Well foundation is preferable to pile foundation when foundation has to resist large lateral forces.
The foundations may consist of a single large diameter well or of a group of smaller wells of circular or other shaped. The shape of wells may be circular, double D, Square, rectangular, Dum-bell shaped etc.,
The circular bell shapes are used when the bridges has multi lane carriage way. For the piers and abutments of very large size used in the cantilever, cable stayed or suspension bridges large rectangular wells with multi degree holes of square shape may be used. The size of the dredge holes is decided so as to facilitate ease of construction and inspection of the foundation.
While deciding the bottom level of the foundation the following considerations may be kept in view.
a. Normally, a sandy strata with adequate bearing capacity is preferred to a clayey strata.
b. A thin stratum of clay occuring between two layers of sand is not relied upon but pierced through
c. The foundation has to be laid on the clayey layer, a well is rested on stiff clay only.
The size of the well for a particular foundation would depend on a large number of factors. The minimum size of a dredge hole is 2.5m. The thickness of steining should be adequate to permit sinking of the well without excessive knowledge. From considerations of sinking effort and erection of shuttering , a large diameter well is nowadays preferred to a group of small sized wells. From practical considerations, the maximum size of a single circular well should be limited to about 12 m diameter for a concrete steining and 6m diameter for a brick masonry steining. If the wells are to rest on sloping rock, smaller wells may be advantageous for ensuring even seating of the wells on the rock. However, in the case of
Twin circular wells or three wells with a common capping slab have been used in earlier constructions for wide piers and abutments. The wells in these cases had small diameter. Currently, this type is not so popular. Double D and dumbell shaped wells are suitable for deep foundations and double line railways. Multi cellular wells are adopted for deep foundations for long span bridges, as in the Howrah bridge and second Hoogly bridge.
A well foundation consists of the following components.
b. Well Curb
c. Bottom Plug
d. Sand filling
e. Top Plug
f. Well Cap
g. Cutting edge
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