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The large prefabricated box or cylindrical type structures that can be sunk through soft ground or water and then filled with concrete thus forming a foundation are called as Caissons.

The caisson is generally a box like structure with an open bottom and open top some times. The structure keeps water out of the construction area while its open bottom allows workers to place foundations and piers in the sea bed or riverbed. An open caisson can be used in shallow water; its open top allows light and air to enter from above the water line.

For deep-water construction, a pneumatic caisson has a closed top; pressurized air is pumped in, and personnel enter and leave through an airlock. Both types have sharp inclined lower edge, which allows the caisson to be deeply embedded in the ground.

Before putting the caisson in its place, engineers look for stable seabed like rock. But if they don’t find this layer they make artificial layer to provide safety to the caisson. In pneumatic caissons, an airlock allows access to the chamber and the pressurized air flow must be constant to ensure regular air changes for the workers and prevent excessive inflow of mud or water at the base of the caisson.

• Caissons are similar to pile foundations but installed using different ways.
• These are actually deep foundations constructed above ground level & sunk into earth to the required level by excavating material within the caisson. 
• Caissons also provide dry space to carry out other construction works. 

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Dredgers may be broadly classified into these main groups or types depending upon the method used to transport loosened material from the sea-bed to the water surface. These are:

1. Mechanical dredgers;

2. Hydraulic dredgers. 

 3. other types

Mechanical dredgers use  grab or bucket to loosen the in-situ material and raise and transport it to the surface.

A stationary dredger, fixed on anchors and moved while dredging along semi-arcs by winches. The bucket dredger is one of the oldest types of dredging equipment. It has an endless chain of buckets, that fill while scraping over the bottom. The buckets are turned upside down and empty moving over the tumbler at the top. The dredged material is loaded in barges.

The dredging action starts when a bucket reaches the bottom of the ladder, where it loosens and scoops up a quantity of material. This material is carried in the bucket to the top of the ladder where, at the highest point of the chain, the bucket overturns and the contents are discharged. The material falls into drop chutes and into a barge moored alongside the dredger. Each bucket then returns empty on the underside of the chain to the bottom of the ladder where the cycle begins again. The size of a bucket dredger is usually described by the capacity of the buckets, which is in the range 100-900 litres.

Bucket ladder dredgers are able to dredge almost any material up to the point where blasting is required, and if fitted with ripper teeth may even be directly able to dredge weak rock. A minimal amount of water is added to the dredged material during careful use of the buckets. This is advantageous to production and costs, especially when dredging in silt and mud.

In operation, a bucket ladder dredger is held accurately in position by up to six moorings or anchors and the bucket ladder moved from side to side to excavate material.

A stationary dredger, moored on anchors or on spudpoles. The dredging tool is a grab normally consisting of two halfshells operated by wires or (electro)-hydraulically. The grab can be mounted on a dragline or on a hydraulic excavator of the backhoe type. 

Grab dredgers, sometimes called clamshells, can exist in pontoon and self-propelled forms, the latter usually including a hopper within the vessel. The pontoon type grab dredger again comprises a rectangular pontoon on which is mounted a revolving crane equipped with a grab. The dredging operation consists of lowering the grab to the bottom, closing the grab, raising the filled grab to the surface and discharging the contents into a barge or, if appropriate, onto the adjoining bank. The size of this type is determined by the capacity of the grab bucket, which can vary between 1.0 and 20 m3 , depending upon the crane power.

The self-propelled grab hopper dredger is basically a ship which has one or more dredging cranes mounted around a receiving hopper. It is easily moved from site to site under its own power and also transports the dredged material to the disposal area. The size of this type of dredger is expressed in terms of the hopper capacity and can range from 100 to about 2.500 m3. The smaller vessels have a single crane, but some of the larger craft have up to four. Production depends upon crane and grab size, water depth and, in the case of the self-propelled variety, on the distance to the material disposal site.

Grab dredgers are usually held in position while working by anchors and moorings but a few are fitted with a spud, or pile, which can be dropped onto the bottom while the dredger is operating.

A stationary dredger, moved on anchors or on spudpoles. A spud is a large pole that can anchor a ship while allowing a rotating movement around the point of anchorage. Small backhoe dredgers can be track mounted and work from the banks of ditches. A backhoe dredger is a hydraulic excavator equipped with a half open shell. This shell is filled moving towards the machine. Usually the dredged material is loaded in barges. This machine is mainly used in harbours and other shallow waters.

In its most simple form this type consists of a pontoon able to support a pump and suction pipe and to make the connection to the discharge pipe. More sophisticated vessels have separate suction and delivery pumps, water jets at the suction inlet and articulated suction pipes. While working, a dredger may be held in position by one or more spuds or, in deeper water, by a complex system of moorings. Plain suction dredgers are mainly used to win fill material for reclamation, with the material being placed ashore through a floating pipeline. Very long distances can be pumped by the addition of booster pumps in the line. Material may alternatively be loaded directly into barges moored alongside. The normal measures of size are the diameter of the discharge pipe, which can vary between 100 and 1000 mm, or the installed horsepower.

Modern suction dredgers can recover material from great depths and can also extract sand from below a clay overburden. Known as a deep suction dredger, this type offers the potential to recover fill material from depths up to 100 m. Production is very dependent upon the permeability of the material dredged and is best in clean sands.

When the in-situ material is too compact to be removed by suction action alone, some form of mechanical loosening must be incorporated near the suction mouth. The most common method is a rotating cutter; the main feature of the cutter suction dredger. This is mounted at the lower end of the ladder used to support the cutter drive and the suction pipe. The loosened material then enters the suction mouth, passes through the suction pipe and pump (or pumps) and into the delivery line.

Cutter suction dredgers operate by swinging about a central working spud using moorings leading from the lower end of the ladder to anchors. By pulling on alternate sides the dredger clears an arc of cut, and then moves forward by pushing against the working spud using a spud carriage. A generally smooth bottom can be achieved, and modern instrumentation allows profiles and side slopes to be dredged accurately. Some of the larger cutter suction dredgers are self-propelled to allow easy movement from site to site.

Cutter suction dredgers are mainly used for capital dredging, especially when reclamation is associated with the dredging. Smaller vessels can be dismantled into sections and moved by road or rail for work in inland waterways, sludge lagoons, reservoirs and similar isolated areas. Large heavy-duty cutter dredgers are capable of dredging some types of rock which have not been pre-treated.

An alternative form of loosening is the use of a rotating bucket wheel at the suction mouth. Bucket wheel dredgers are most commonly used in mineral dredging operations and to date have not found general favour among the major international dredging contractors.

Specialized types of dredger are usually of small size and output. They include simple jet-lift and air-lift, auger suction, pneumatic and amphibious dredgers.

Jet-lift dredgers use the Venturi effect of a concentrated high-speed stream of water to draw the adjacent water, together with bed material, into a delivery pipe. The jet head has no moving parts so blockage by wires and other dock debris is minimized. These dredgers are relatively small units and some can be manoeuvred on spuds alone.

Air-lift dredgers are very similar to the jet-lift dredgers but the medium for inducing water and material flow is high pressure air injected at the month of the suction pipe. As with jet-lift dredgers there are no moving parts in the flow system. Hard or other difficult to loosen materials cannot be dredged.

Amphibious dredgers have the unusual feature of being able to work afloat or elevated clear of the water surface on legs. They can be equipped with grabs, buckets or a shovel installation.

All the above specialist types of dredger (and others) have been developed for specific situations and generally for small scale work such as narrow canals, industrial lagoons and reservoirs. Some types have been developed to handle contaminated sediments with minimum disturbance. They are not normally employed for large scale maintenance or capital dredging work.

A further type of dredger is the plough or bed leveller. This consists of a blade or bar which is pulled behind a suitable tug or work-boat. The method can be used for direct dredging over short distances and for levelling off the bed to the desired depth when a trailer or grab dredger is operating. It may also be used to pull material from close to quay walls and other places where a trailer cannot reach into a more accessible area. Sometimes the trailer itself operates the level1er if no tug or work-boat is available.

A self propelled dredger which brings the sediment to be excavated into suspension with waterjets. This suspension is denser than water. It will be carried away by gravity and currents. Water injection dredging is mainly used for maintenance in harbours.

A relatively recent development in dredging equipment is the water injection dredger. This can be very effective in some material in order to fluidize it and create a turbidity current of higher density than the surrounding water. The bed material then moves in its own current. The system works best in mud and fine sand beds and has been used successfully in a number of port areas. Careful assessment must be made of the likely destination of the turbid water.

The common materials which are usually employed for the construction of a caisson are as follows: