It is a prefabricated cylinder sunk or hollow box into the earth and filled with concrete to some chosen depth, forming a foundation. Deep foundations are commonly employed in the building of bridge piers and other constructions that require foundations beneath rivers and other bodies of water. Caissons are floatable. They can be lifted by crane out of the water and placed on land where they will remain safely until needed again.
There are several different types of deep foundations: caisson, bunker, and cone. Each type has its advantages and disadvantages. Which one is best depends on what you need the structure for.
A caisson is a floating shell used as a shelter for workers during construction projects or as a place of refuge in times of danger. The word comes from French and means'safe house'. These structures are usually built by dredging a site under water and then filling the resulting hole with concrete. Once complete, the casing is lifted out of the water and placed on land. Caissons are typically 12-20 feet (3.6-6.1 m) wide and 30-40 feet (9.1-12.2 m) long. There are also smaller caissons for use in specific projects. A single lift can take a caisson up to 300 feet (91 m) inland or more if necessary. Multiple lifts are often required because each time the casing is moved it needs to be reset to its original position.
Piling is the most popular method of constructing a deep foundation for a bridge. A crane hoists long, thin poles known as piles into the air and drives them into the ground with a massive hammer known as a piledriver (not that piledriver). The pile is topped and knotted after it has reached the desired depth. It can then be used as a base for further construction.
Bridges can also be built on concrete piers or overpasses. These structures use vertical walls called stringers to hold the decking in place. The floor of the bridge is made of wood planks or steel girders. On some bridges, such as those that cross water, the floor is actually raised above sea level.
Major roads have two parallel shoulders on which trees and other vegetation are allowed to grow. This is not always the case; many major streets do not have shoulder widths of their own but instead use portions of adjacent properties as well as parking lots. Shoulder widths vary depending on the road but generally range from 3-20 feet (1-6 m) wide.
Shoulder markers are painted lines on the road's edge indicating where drivers should position themselves when making left turns. They are necessary because so many cars turn left onto major highways from side streets that traffic tends to back up for several blocks.
Many major roads do not have shoulders of their own but instead use portions of adjacent properties as well as parking lots.
Shallow foundations are classified as spread footing (or basic footing), strap footing, combination footing, mat footing, or raft footing. The depth of the base material should be such that it will support the expected loadings on the structure.
The purpose of a shallow foundation is to reduce the cost of the foundation while maintaining the strength and stability of the structure. Shallow foundations can be either pre-cast concrete or steel. Pre-cast concrete shallow foundations are available in standard sizes and shapes for easy installation. These shallow foundations are usually specified by allowing 2 to 4 inches of margin below the frost line to account for shrinkage during curing and drying of the concrete. The depth of steel shallow foundations must be determined by the designer since there are many different types of structures using deep and shallow foundations together. However, in general, 1/4 to 3/8 of an inch thick plate steel will be sufficient for most applications.
Shallow foundations are commonly used on structures where the cost of a deeper foundation would make the project too expensive. This is particularly true for residential construction projects. Also, shallow foundations are recommended for use on compacted soil because the weight of the building above the foundation will cause the soil to compaction.