Dredging

Dredging is defined as excavation of bed below water.

• This technique is often used to keep waterways navigable.
• It is also used as a way to replenish sand on some beaches, where too much sand has been lost because of coastal erosion.
• Dredge or dredger, is the equipment that carries out dredging work.
• To know where dredging is necessary, hydrographic survey is performed in the entire basin.

PRIMARY DREDGING

• If the dredged materials are to be removed only once, it is known as primary dredging.
• Dredged materials are removed only at the start or opening of harbour.

CAPITAL DREDGING

• Initial removal of material to obtain required depth.
• This depth acquired is maintained regularly to have a convenient harbour.
• Materials removed includes hard rocks and even silt.

MAINTENANCE DREDGING

It is a dredging to deepen or maintain navigable waterways , which are threatened to become silted with the passage of time.

• Materials removed will be in the form of fresh deposits (sand and silt) only.
• It is carried out periodically to maintain the navigable depth.

Use of dredging

1.For ensuring proper depth at the harbour or ship channel

2.For the maintenance of the harbour

3.Land reclamation- dredged materials used to construct new land.

4.Beach nourishment.

5.Seabed mining- recovering natural metal ores.

6.Construction materials- dredged sand and gravel can be used in construction industry.

7.Contaminant remediation- to reclaim areas affected by chemical spills,etc

Dredger.

• Earth moving machine to remove bed material from under water, to increase the water depth.
• i.e. for dredging
• Dredgers can be classified into two:

                                  Mechanical dredgers

                                 Hydraulic dredgers

Mechanical Dredgers

   Dredger which excavate the material by physically  lifting it

   The main types of mechanical dredgers are:

      1.Bucket ladder dredger

      2.grab or grapple dredger

     3.dipper dredger

Hydraulic Dredger

• They suck the material
• Loosening the material by cutter or jet if required.
• Transporting it using water as the carrier.

Comparison between Mechanical and Hydraulic dredgers

     Mechanical dredgers

           Removes the material by physically lifting it.

           Advantageous in getting high density of dredged spoil.

    Hydraulic dredgers

          Removes the material by sucking.

          Used to handle loose strata.

          Highly efficient

          Great variation in type, size and mode of operation.

consists of a large suction pipe which is mounted on a hull and supported and moved about by              a boom

         Has a mechanical agitator or cutter head which churns up earth in front of the pipe

         centrifugal pumps mounted on a dredge sucks up water and loose solids.

BUCKET LADDER DREDGER

• It consists of an endless chain of buckets mounted & running round a ladder,formed in the middle of the bow of a floating vessel
• The ladder could be lowered or raised.
• Chain of buckets is operated manually or mechanically by a big wheel
• Suitable for handling coarse gravel or sand, hard clays and soft or broken stone

OPERATION

• Each bucket cuts and brings up material to the top of ladder as the chain moves round
• It then discharges its contents into special holds
• The vessel moves forward on completion of work at each section
• Its capacity is 6000 cu.m to 7650 cu.m per day

ADVANTAGES

• Suitable for handling coarse gravel or sand, hard clays and even soft or broken stone at fairly good depths.
• The bed dredged is the least disturbed

Dipper dredger

• A backhoe/dipper dredge has a backhoe like on some excavators

Grapple dredger

For sand, clay or mud, not for hard diggings

 

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Dock Engineering