Posted by A Caisson Foundation is a 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 depths.
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 also known as Well foundations could be categorized into the following 7 types as per their shape.
- Single Circular Shape Caissons
Circular shape caissons have more area for the least periphery so sinking the caisson will require the least amount of effort.
Circular shape Caisson foundation - Twin Circular Shape Caissons
- Dumb-Well Type Caissons
- Double-D Shape Caissons
- Twin-Hexagonal Caissons
Twin Hexagonal caisson - Twin-Octagonal Caissons
Twin Octagonal Type Caissons - Rectangular Caissons
Rectangular shape caisson for bridge foundation
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 (Well Foundation)
- Bottom Plug
- Cutting edge
- Curb
- Stein
- Top Plug
- Well Cap
Forces Acting on Open Caisson Foundation
- Dead Load: The load of superstructure ultimately transfers to the caisson foundation. This is a static load and verticle in direction.
- Braking and Tractive Efforts: Whenever a vehicle accelerates or brakes, it exerts longitudinal forces on the bridge deck.
- Thermal stresses: Shape of bridge superstructure changes with change in ambient temperature. This shape change because of temperature, creates thermal stresses.
- Forces of Water Current: A bridge, built either offshore or on a river, has to experience forces because of flowing water.
- Wind Forces: Wind load acting on superstructure of the bridge transfers ultimately to the foundation.
- Seismic Forces: This load is essentially a lateral load. Seismic load is a dynamic load and caisson foundations should be designed by considering Design Basic Earthquake.
Read Also-
Impact Factors Applied to Bridge Loading
Factors Affecting the Strength of Concrete
IRC 112-2011 Code of Practice for Concrete Road Bridges [PDF Download]
IRC 5-2015 a Code on General Features of Bridges
IRC 6-2017 [Loads and Load Combinations for bridges] PDF Download