Well foundations

A well foundation is a large hollow caisson sunk through river sand and silt to a firm stratum well below the deepest scour. It is the traditional deep foundation for bridges over India's big alluvial rivers, where floods scour the bed many metres deep and loose sand extends far down. Its large size and mass give it high resistance to lateral loads from water current, braking and earthquakes.

A well is built from the bottom up. A steel cutting edge with a sharp lower face is laid at bed level or on an island, a reinforced concrete curb is cast on it, and the steining, the thick wall of the well, is raised in lifts. Soil is dredged from inside, by grab or by divers in difficult conditions, so that the well sinks under its own weight, aided by kentledge when skin friction is high. Once it reaches founding level, a concrete bottom plug is placed, the inside is partly filled with sand, a top plug is cast and a well cap carries the pier.

Sinking must be controlled. Uneven dredging or obstructions make the well tilt or shift from its intended position; tilts and shifts beyond the permissible limits require design rechecks. Sand blows into the well, boulders and buried logs are typical problems.

Design starts with the scour depth, estimated for alluvial beds from the design discharge per metre width and a silt factor using Lacey's relation. Maximum scour around piers is taken as a multiple of the normal scour depth, and the well must reach a grip length below the maximum scour level. Stability against lateral loads is checked by considering soil resistance on the embedded part. In India these rules are given in IRC 78.