The process of dredging involves the excavation of materials from under a body of water such as a river, lake, harbor, or ocean using various types of dredging equipment.^[1] Some of the primary reasons for dredging involve deepening and widening water ways, providing fill material in construction or beach and land reclamation projects, raising submerged land above the water table, building dikes and canals, and installing underwater foundations.

Dredging is also a method of  surface mining involving the excavation and extraction of valuable minerals and aggregate materials from underwater. Dredges specifically used in mining are highly specialized and specifically tailored towards mining operations.^[2]

Though dredging is carried out for a range of purposes, the driving objective is usually to recover material of some value or to gather material for other uses.^[3]

Contents 1 History 2 Process 2.1 Dredge Mining 3 Types 3.1 Grab or Grab Bucket Dredge 3.2 Dipper Dredge 3.3 Ladder Bucket Dredge 3.4 Cutter Suction or Bucket Wheel Dredge 3.5 Hydraulic or Suction Dredge 3.6 Trailing Suction Hopper Dredge 4 Equipment Used 5 References

[edit] History

The Dutch invented one of the first apparatus with a function resembling that of a modern day dredge—to remove silt from the bottom of canals. The device comprised a bag attached by a steel ring to the end of a pole mounted off the side of a stationary barge. The bag was dragged along the bottom of the canal and the contents deposited directly onto the barge.^[4]

In 1859 elevator dredges and a thrown-together version of a bucket dredge fabricated from different machine parts were instrumental in building the Suez Canal.^[5] In fact, this was the first massive canal-building construction project where the use of dredges were deployed on a wide scale when digging the canal manually proved to be catastrophic. 

In 1904 dipper dredges, a more advanced type of dredge, were used in the building of the Panama Canal.^[6]

[edit] Process

Dredging involves using a dredge to cut away or loosen in-situ material from underneath the water. This scooping action is accomplished in a similar fashion to the way a land backhoe scoops and digs up dirt. The dredged materials, or spoil, as it is often called, is then suctioned through a large pipe and unloaded or deposited onto a ship, barge, or hopper for relocation and containment.^[7] A dredge is best thought of as a type of floating excavator typically powered by a diesel engine and equipped with either a clamshell bucket, power shovel, or cutterhead in combination with a pipeline for suctioning up spoil.^[8]

Dredges can further be distinguished as mechanical or hydraulic. The three types of mechanical dredgers are grab dredges, backhoe or dipper dredges, and bucket-ladder dredges. The hydraulic category consists of stationary suction dredges, cutter section dredges, and trailing suction hopper dredges.^[9] Mechanical dredges are used for excavating and dislodging material before it is actually raised up to the water’s surface. Material mechanically dredged is this manner is then transported away by barges. The process of hydraulic dredging vastly differs in comparison. Hydraulic dredges use centrifugal pumps to provide a dislodging and elevating force. The dislodged material is then combined with water to create a sort of slurry sediment. The slurry sediment is than disposed through a pipeline into a barge or hopper for transport.

[edit] Dredge Mining

Dredge mining, also referred to as wet mining, involves extracting ores, minerals, or aggregate material from underwater.^[10] One of the earliest materials extracted through dredging was tin ore. Today gold, coal, diamonds, and other minerals are recovered from dredge mining.^[11] Dredges used in mining are commonly a ladder bucket, a wheel dredge, or a cutter or trailing dredge.^[12] Dredges used in mining are sophisticated enough to separate the valuable material from the rest of the spoil to be discarded.^[13]

Aggregate materials such as a sand and gravel used to make concrete are also commonly recovered in the dredging operations and exist as a valuable alternative to land-based aggregate sources. Aggregate dredging can take place in ponds, rivers, and lakes, in addition to inland waters. Similar to dredges used in mining, this dredging equipment is specifically customized for aggregate dredging operations. The U.K. currently occupies one of the largest marine aggregate markets in the world.^[14]

[edit] Types

[edit] Grab or Grab Bucket Dredge

A grab dredge or a grab bucket dredge is a type of dredge comprised of one or more buckets operated by a crane and mounted on a floating vessel or barge, usually located directly on the shore or positioned offshore by anchors or moorings. Grab dredges are designed to scoop up a variety of materials.^[15] The jaws on the grab dredge bucket are usually hinged together. When lowered into the water, the jaws open and face downward. As the bucket sinks into the dredged material, the jaws snap closed to contain the material before it is discharged for removal.^[16] A grab dredge can easily be configured from conventional land-based machines and secured to a pontoon for simple dredging tasks.

[edit] Dipper Dredge

Dipper dredges, also known as a boom-and-dipper assembly, operate somewhat similarly to a land-based power shovel. Also referred to as a backhoe dredge, a dipper dredge is characteristic of an excavator in that excavated material is loaded onto barges or on land.^[17]

[edit] Ladder Bucket Dredge

Ladder bucket dredges consist of a dredge mounted on a self-propelling vessel with a longitudinal well positioned in the middle of the vessel and open to the water. This type of dredge is configured with a long steel frame that is easily lowered and raised and equipped with multiple buckets passing over sprockets situated at each end. The buckets pass through the well opening to scoop material from the bottom and then discharge it through a chute that also shoots out water from the vessel’s side into a hopper moored alongside the vessel or a receiving hopper located on the dredge.^[18]

[edit] Cutter Suction or Bucket Wheel Dredge

The cutter suction dredge or bucket wheel dredge is a type of hydraulic dredge used for loosening hard, compact material. Dredging of the material is carried out through mechanical loosening of some sort in conjunction with suctioning at the mouth. The most common feature of a cutter section dredge is a rotating cutter mounted on the lower end of a ladder that supports both the cutter drive and suction pipe. Once the material is loosened, it passes directly into the suction mouth and up through the suction pipe to a pump.^[19]

[edit] Hydraulic or Suction Dredge

Hydraulic dredges or suction dredges are used to excavate soft, fine materials such as sand and mud. A hydraulic dredge consists of a pipe joined at one end to a centrifugal pump. Various methods, such as suctioning, mechanical loosening, or water jets, are employed to loosen the material. A deep suction dredge is a type of suction dredge used in recovering material from great depths or for extracting sand from clay overburden from depths of 328 feet (100 m).^[20]

[edit] Trailing Suction Hopper Dredge

Trailing suction hopper dredges are self-propelled hydraulic dredge vessels with three large drag arms and a high pumping capacity. They operate similarly to a large vacuum cleaner. In dredging, the arms are extended over the side of the vessel to the bottom of the body of water. The arms suck up a slurry mixture of sand and water from the bottom, which is then passed through drag heads and pipelines before being discharged into a hopper aboard the vessel. This type of dredge is primarily used in clearing channels and offshore sandbars from the mouths of major rivers.^[21]

[edit] Equipment Used

• Backhoe dredge
• Bucket wheel dredge
• Cutter suction dredge
• Dipper dredge
• Grab bucket dredge
• Hydraulic suction dredge
• Ladder bucket dredge
• Trailing suction hopper dredge

[edit] References

1. ↑ Dredge. Absolute Astronomy, 2008-09-29.
2. ↑ Yell Denis, Riddell John. Dredging. Thomas Telford. 1995 pg. 1
3. ↑ Yell Denis, Riddell John. Dredging. Thomas Telford. 1995 pg. 1
4. ↑ Dredge. Answers.com, 2008-09-29.
5. ↑ Dunn, Jimmy. Suez Canal. Tour Egypt!, 2008-09-29.
6. ↑ Yell Denis, Riddell John. Dredging. Thomas Telford. 1995 pg. 12
7. ↑ What is Dredging? Wise Geek, 2008-09-29.
8. ↑ Dredge. Answers.com, 2008-09-29.
9. ↑ Dredging: The Facts. IADC-Dredging.com, 2008-09-29.
10. ↑ Mineral Sands Mining. Iluka, 2008-09-29.
11. ↑ Yell Denis, Riddell, John. Dredging. Thomas Telford, 1995. 2
12. ↑ IHC Mining - Dredging Systems for Wet Mining. Mining-Technology.com, 2008-09-29.
13. ↑ Mining. Encarta, 2008-09-29.
14. ↑ Yell Denis, Riddell John. Dredging. Thomas Telford, 1995. 9
15. ↑ Yell, Denis, Riddell, John. Dredging. Thomas Telford, 1995. 9
16. ↑ Dredge. Answers.com, 2008-09-29.
17. ↑ Yell, Denis, Riddell, John. Dredging. Thomas Telford, 1995. 12
18. ↑ Dredge. Answers.com, 2008-09-29.
19. ↑ Yell Denis, Riddell, John. Dredging. Thomas Telford, 1995. 12
20. ↑ Yell, Denis, Riddell, John. Dredging. Thomas Telford, 1995. 11
21. ↑ Hopper Dredges. GlobalSecurity.org, 2008-09-29.