Components of well foundation with sketch

A well (caisson) foundation is a hollow, box-like masonry or concrete structure, circular, rectangular or D-shaped in plan. It is sunk through the soil by dredging from inside until it reaches the founding level. It is then plugged and filled, and it carries the bridge pier above. It is widely used for bridges over rivers where scour is deep.

        Pier
     ____|____
    |  Well   |  <- Well cap
    |  cap    |
  --|---------|-- GL / LWL
    |  Top    |
    |  plug   |
    | Sand/   |  <- Steining (walls)
    | filling |
    |         |
    |Interm.  |
    |plug     |
    |  Bottom |
    |  plug   |
    |_Curb____|  <- Cutting edge
     \       /

Components and functions

1. Cutting edge: steel shoe at the bottom that cuts the soil as the well sinks; it protects the curb.
2. Well curb: reinforced concrete wedge-shaped ring above the cutting edge (with an inner slope of 30° to 37°); it helps sinking and transfers the load to the soil.
3. Steining: the walls of the well, of masonry or reinforced concrete; they carry the load to the base and give the weight needed for sinking; usually 0.5 to 1.5 m thick.
4. Bottom plug: concrete plug placed under water (by tremie) at the bottom of the well to seal the base and transfer the load to the soil; its thickness is fixed by design (commonly not less than about half the internal diameter).
5. Sand filling: clean sand placed above the bottom plug to give weight and stability and to transfer the load.
6. Top plug: a layer of concrete (about 300 mm) at the top of the sand filling, to support the well cap.
7. Well cap: a reinforced concrete slab on top of the steining that distributes the pier load uniformly to the well walls.
8. Intermediate plug (if required): a concrete plug placed between the bottom plug and the top plug when the sand filling is not enough, to seal and stiffen the well.
9. Dredge hole: the central opening through which soil is removed by grabs during sinking.

Description

Well foundations resist large vertical and horizontal loads (by their weight and by the passive resistance of the soil on the sides, with a grip length below the maximum scour level). They are suitable for sinking through soft soils, boulders or sand, and are common in Nepal for large river bridges.