c. Other types of Dredger

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.

Augur suction dredgers operate on the same principles as a cutter suction dredger, except that the mechanical cutting tool is a rotating Archimedean screw placed at right angles to the suction pipe. The screw dislodges material, which is fed to the centrally placed suction pipe. Most units have a shroud over the cutting screw which reduces the spread of the plume of disturbed bed material which normally escapes from all dredgers. The augur suction dredger advances.into the cutting face by hauling itself along a wire deployed directly ahead. Very accurate horizontal and vertical dimensions can be achieved.

Pneumatic dredgers work on the ‘evacuator’ principle. A chamber with inlets for bed material is pumped out with the inlets closed. The inlets are then opened and water and material drawn in. The mixture is then pumped out and the cycle repeated. The unit is generally suspended from a crane on land or from a small pontoon or barge. The dredging action is intermittent and suitable only for easily flowing material.

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.

WATER INJECTION DREDGER



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.