What is Caisson Foundation and How is it Used in Construction

A Caisson Foundation is basically a watertight box with the top open. It is sunk into water to gain access to the bed of the stream. It is generally made of wood, steel or concrete depending upon project requirement.

Through this boxlike chamber, the workmen can make construction work at the bottom of water bodies without being hampered by the water. Accordingly, this is mainly used to place foundations at river beds.

Caissons are generally made on shore first. Then, They are launched into the water body. The caisson is then floated to the designated spot. Then they are sunk vertically to the bottom. After ensuring that the caisson is waterproof, the water inside is pumped out to dry.

Types of Caisson Foundation:

1. Box Caisson: This is the most basic type of caisson. It is made of timber, concrete or steel. It is built on shore and floated to the foundation location and sunk to the bottom. It is basically a box without the top.

2. Open Caisson: This type of caisson has neither the top nor the bottom. When made in a cylindrical shape, it looks like a well without top or bottom. It can be built in many shapes - vertical, over, or any other option.

In case of building large bridges, a bathtub-like shape is preferred. Usually open caissons are made of steel plates welded together. RCC can be also used here, as the situation requires.

3. Pneumatic Caisson: This odd type of caisson has an open bottom but closed top. This has to be forced down to the bottom of the water by means of compressed air. Thus the name is derived. A pneumatic caisson consists of a working chamber, a shaft, and an airlock. The caisson is made of inner and outer layer of steel skins. Trusses and girders join them to form a boxlike structure.

The working chamber in pneumatic caissons is 3-4 meters tall only. It is made airtight with a special seal on top. The caisson’s bottom edge has a steel edge that cuts into the riverbed. All this facilitates working within the chamber while the caisson is being sunk. People can access the caison via the airlock.

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Published By
Rajib Dey
www.constructioncost.co
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What is Plane Table Surveying

Plane Table Surveying is used in ad-hoc surveying and designing situations. In this, survey fieldwork and plotting work is done simultaneously. Therefore, there is no need for office work. This is best adapted to situations where high precision is not required. Thereby, it is greatly useful for small-scale mapping operations.

Instruments used in Plane Table Surveying - The Plane Table survey method uses very simple instruments. These are available everywhere.

1. Drawing Board: It is a board made of seasoned wood. They are available in many sizes. Generally, the field work is done in 40 cm by 30 cm boards. They also use the 75 cm by 60 cm boards, for larger work. In the field, a tripod is placed under the board with a ball socket in it. This enables the surveyor to draw on the board from any angle.

2. Alidade: An alidade is basically a ruler with a telescope. This is used on the drawing board to draw lines and rays according to the field. The alidade may also have a spirit level on it.

3. Compass: A tough compass is used to find the magnetic meridian of the place. Generally, the compass used in survey work also has two bubble tubes at right angles to each other.

4. Plumbing Fork: It is a rod with a plumbob attached to one end. This is used to ensure that other equipment is aligned perfectly vertically, when needed.

5. Spirit Level: It is a tube of liquid with a bubble inside it. The tube is slightly bulging in the middle. This lets the bubble stay in the middle of the tube when the level is perfectly horizontal. This is used to make sure the drawing board, or other equipment are placed horizontally.

6. Waterproof cover: A waterproof cover is used to cover up the paper on the drawing board from rain or other splashes and dust. This is generally made of soft polythene.

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Published By
Rajib Dey
www.constructioncost.co
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