A Caisson foundation is a type of deep foundation that is the most widely used and suitable for bridge structures across the drainage. Here are the types of caisson foundations described used most widely for bridges.
There are three broad categories of the caisson foundations as per their function, viz.
- Box Caissons
- Open Caissons
- Pneumatic Caissons
A Box Caisson is open at the top but closed at the bottom. A Caisson may be constructed of either timber, reinforced concrete, or steel. This type of foundation is suitable for bridges where there are firm bearing strata are available at shallow depth.
The Open caissons are open at both the top and bottom. An open Caisson type of foundation could be constructed from timber or reinforced concrete or steel.
In India, the open caisson type foundations are most widely used for bridges.
Pneumatic caisson, as the name implies, allows air to prevent water from entering the chamber and thus provides a dry atmosphere for working.
Open caissons which are also known as Well foundations could be categorized into the following 7 types as per their shape.
- Single circular shape
Circular shape caissons have more area for the least periphery so sinking the caisson will require the least amount of effort.
- Twin circular
- Dumb-well type
A structure designer could select the shape of open caisson foundation based on the size of piers, the cost, and the care of construction.
The structure designer must also take into the account of potential of tilt and the shift of the caisson during and after the construction.
Parts of an open caisson foundation (also known as the well foundation)
- Bottom Plug
- Cutting edge
- Top Plug
- Well Cap
Forces acting on an open caisson foundation
- Braking and Tractive efforts of moving vehicles on the bridge deck
- Load due to movement because of temperature
- Forces of water current
- Wind forces
- Seismic Forces
You may wish to read
IRC 5-2015 A code on general features of bridges
IRC:6-2017 [Loads and Load Combinations for bridges] PDF Download