Last edited 22 Sep 2020



[edit] Introduction

A caisson is a box-like structure commonly used in civil engineering projects where work is being carried out in areas submerged in water. Such projects might include:

Caissons differ from cofferdams in that cofferdams are removed after completion of the work, whereas caissons are built to remain in place as a part of the completed structure.

Caissons can be made of materials including timber, steel, masonry and reinforced concrete, and may be constructed onshore then floated to the required location, where they are sunk into place, enabling access to the bed and excavation of foundations to the required depth.

They are particularly suitable for the construction of underwater foundations or where the water is deep, as they are strong enough to withstand significant vertical and horizontal loads, as well as lateral forces such as waves.

[edit] Types of caisson

There are several different types of caisson, the choice of which will depend upon the structure for which it is being built as well as the required depth. Some of the more most common types are describes below:

[edit] Box caisson

This is a watertight timber or reinforced concrete box with a closed bottom and an open top. The caisson is cast and cured on land and then sunk into place, or it can be rested on top of a pile formation. Sand, concrete or gravel is used to weigh down and sink the caisson. This is most suitable for areas where the bearing strata is reasonably level and no excavation is required, although it is possible for some dredging to further level the base if required to avoid the tilting of the caisson once in place.

This type of caisson is generally relatively economical but may not be suitable if the bearing strata requires compacting and/or levelling.

[edit] Open caisson

This is a timber, steel or concrete box that is open at both the bottom and the top. The walls are heavy and made with sharp edges that facilitate the sinking process. There are three different types of open caisson:

[edit] Single wall

As opposed to box caissons which are prefabricated, open caissons are formed in situ. Wherever possible, a temporary cofferdam is constructed to keep the working environment dry and to enable the excavation of a pit in which the caisson is to be sunk, either by its self-weight, by concrete or by the use of hydraulic jacks if the skin friction prevents it from sinking. The soil from the space inside the caisson can then be removed by a clamshell excavator bucket on a crane.

The sinking proceeds at the rate necessary to allow each section to be concreted to a sufficient strength. Once sunk to the required depth, the base is plugged with a thick seal of concrete and once set the internal water is pumped out, to be replaced with sand, gravel or concrete.

[edit] Cylindrical (or well)

This is a cylindrical shell made of timber, masonry, steel or reinforced concrete. The wall must be sufficiently thick so that when the soil from inside is excavated, it sinks under its own weight. A cutting edge on the base of the walls and water jets can be used to assist sinking and reduce the level of skin friction. Once sunk to the required depth, the bottom is sealed with concrete.

[edit] Open with dredging wells

This method is commonly used for the deepest foundations, such as bridge piers and abutments. The outside walls, as well as inside divider walls which make up smaller sections (or wells), are made of reinforced concrete. Dredging the soil out through the wells enables the caisson to sink. Once sunk to the required depth, a concrete seal is installed and the walls filled with sand or concrete.

[edit] Pneumatic caisson

Pneumatic caissons are closed at the top but open at the bottom, with the water forced out using compressed air, creating a working chamber which is airtight in order for excavation to be carried out. This is suitable when it is not possible to excavate wet ground in the open.

To facilitate the internal work, the caisson has two air locks that consist of a steel chamber and two air-tight doors. One air lock allows labourers to enter and exit, and the other is used for removing the excavated material. It is important that the air pressure must be carefully monitored at all times, with slow raising and lowering as the labourers enter and leave the chamber. Otherwise, there is the risk of labourers suffering with caisson disease which results from the expansion of air bubbles trapped in joints and muscles.

Although this method is suitable for difficult locations, such as depths ranging from 25-40 m, it is a complex, slow and expensive procedure.

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[edit] External references

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