Last reviewed 16 Sept 2026 · 7 min read
Well foundations and caissons
A caisson is a hollow box or cylinder sunk into place through soil or water and then filled with concrete to become a deep foundation. In India, large masonry or RCC open caissons used for bridge piers and abutments are called well foundations — the traditional choice for bridges over rivers with deep scour and alluvial beds (e.g. major Ganga and Brahmaputra bridges).
Why wells for bridges: large lateral and vertical loads, deep scour, need for a massive, rigid foundation below the maximum scour level; wells resist horizontal forces by soil passive resistance over a large embedded depth and are relatively easy to construct with local labour.
Types of caissons
| Type | Description | Features |
|---|---|---|
| Open caisson (well) | Open at top and bottom during sinking; soil excavated from inside by grabs or dredging; bottom later sealed with a concrete plug | Most common; cheap; bottom cannot be inspected directly; sinking may be obstructed by boulders |
| Pneumatic caisson | Closed working chamber at the bottom kept under compressed air to keep water out; workers excavate in the dry | Allows inspection and removal of obstructions; costly; caisson disease (decompression sickness) risk limits depth to about 35–40 m below water |
| Box (floating) caisson | Closed at the bottom, open at the top; built on land, floated and sunk onto a prepared bed | Used where soil is firm and scour is small |
Shapes of wells
Circular (most common — equal strength in all directions, easy sinking), double-D, twin circular, dumb-bell, rectangular and multi-dredge-hole shapes for large piers. The choice depends on the pier dimensions, flow direction and ease of sinking; circular and double-D wells are standard for Indian bridges.
Components of a well foundation
- Cutting edge — steel edge at the bottom that cuts into the soil.
- Well curb — tapered RCC ring above the cutting edge (inner slope commonly about 30°–37° to the vertical); transfers load and guides sinking.
- Steining — the main wall of the well (masonry or RCC); provides weight to sink the well and resists earth and water pressures; thickness chosen to overcome skin friction and resist sinking stresses.
- Bottom plug — concrete placed (usually under water, by tremie) at the base to seal the well and transfer load to the base soil.
- Sand filling — inside the well above the bottom plug, to add weight and distribute load (sometimes left partly empty).
- Top (intermediate) plug — concrete plug on top of the sand filling.
- Well cap — RCC slab on top of the well supporting the pier.
Depth of well — scour and grip length
Scour depth
Lacey's regime depth of scour below the high flood level:
Normal scour depth (regime depth):
= discharge intensity (m³/s per m width); = silt factor; = weighted mean diameter of bed material (mm).
For unit discharge not known, an alternative form uses total discharge: (m).
Maximum scour depth (for design) = factor × normal scour depth, e.g. about 2.0 at piers, 1.27 on straight reaches and more at bends and noses (IRC 78 / practice values).
Grip length
The grip length is the depth of embedment below the maximum scour level that gives the well adequate lateral stability and bearing. Indian practice (IRC 78) requires the foundation level to be at least about one-third of the maximum depth of scour below the scour level (for wells in erodible strata), subject to stability calculations.