2. Dredging is an excavation activity usually carried out
underwater, in shallow seas and in freshwater with the
purpose of gathering up bottom sediments & widening.
It is a technique, often used to keep waterways
navigable and creates an anti-sludge pathway for boats.
It is also used as a way to replenish sand on some public
beaches, where sand has been lost because of coastal
erosion.
Dredger is an apparatus for bringing up objects,sand or
mud from a river or a seabed by scooping & dragging.
3.
4. 1) Capital : Dredging is carried out to create a new
harbour, berth or waterway ,or to deepen existing
facilities in order to allow larger ships access.This
work is usually done by using a cutter suction
dredge or large trailing suction hopper dredge.
2) Preparatory: This excavation work is prepared for
future bridges, piers or docks/wharves, often
connected with foundation work.
5. 3) Maintenance: Dredging maintain navigable
waterways or channels which becomes silted with
the passage of time, due to sedimented sand and
mud,makes them too shallow for navigation. This
is often carried out with a trailing suction hopper
dredge. Mostly dredging is used for this purpose.
4) Land Reclamation: Dredging to mine sand, clay or
rock from the seabed and using it to construct
new land elsewhere.This is perform by a cutter-
suction dredge or trailing suction hopper dredge.
5) Harvesting materials: Dredging sediment for
elements like gold, diamonds or other valuable
trace substances.
6. 6) Seabed mining: It can be used probably in future
for recovering natural metal ore nodules from the
sea abyssal’s plains.
7) Construction materials: Dredging sand and
gravels from offshore licensed regions can be
used in construction industry, principally for use
in concrete.
8) Beach Nourishment: Mining sand offshore and
placing on a beach,is used to replace sand
eroded by storms or wave action. This is done to
enhance the protective function of the beaches.
7. 9) Flood Prevention: This can help to increase
channel depth and therefore increase a channel's
capacity for carrying water.
10) Contaminant Remediation: To reclaim the areas
affected by chemical spills, storm water surges
(with urban runoff) and other soil contaminations,
including silt from sewage sludge and from
decayed matter, like wilted plants. Disposal
becomes a proportionally large factor in these
operations.
8. Dredgers may be broadly classified into two main
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.
MECHANICAL DREDGER
Mechanical dredgers come in a variety of forms, each
involving the use of grab or bucket to loosen the in-situ
material and raise and transport it to the surface.
9. A Bucket Dredger is 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 is used to scrape over the bottom.
10. A Grab Dredger is a stationary dredger, moored on anchors or on
spud-poles. The dredging tool is a grab normally consisting of two
half-shells operated by wires or (electro)-hydraulically. The grab can
be mounted on a dragline or on a hydraulic excavator of the
backhoe type.
11. A Backhoe Dredger is a stationary dredger, moored on anchors or on
spud-poles. A spud is a large pole that can anchor a ship. 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.
12. Hydraulic dredging is most efficient when
working with fine materials, because they can
easily be held in suspension. Coarser materials
and even gravel can be worked but with a
greater demand on pump power and with
greater wear on pumps and pipes. The
principal feature of all dredgers in this category
is that the loosened material is raised from its
in-situ state in suspension through a pipe
system connected to a centrifugal pump.
Dredgers of this type is now widely used.
13. A Suction Dredger is a stationary dredger used to mine for sand. The
suction pipe is pushed vertically into a sand deposit. If necessary
water jets help to bring the sand up. It is loaded into barges or
pumped via pipeline directly to the reclamation area.
14. A Cutter Suction Dredger is a stationary dredger which makes use of
a cutter head to loosen the material to be dredged. It pumps the
dredged material via a pipeline ashore or into barges. The cutter
head can be replaced by several kinds of suction heads for special
purposes, such as environmental dredging.
15. A Trailing Suction Hopper Dredger is a self-propelled ship which fills
its hold or hopper during dredging, while following a pre-set track.
The hopper can be emptied by opening bottom doors ,offloading by
pumping its load off ashore. This kind of dredger is mainly used in
open water: rivers, canals, and the open sea.Trailing suction hopper
dredgers, commonly known simply as ‘hoppers’ or ‘trailers’.
16. Barge unloading dredgers are used to transfer material from hopper
barges to shore, usually for reclamation. A barge unloader is
basically a pontoon supporting a suction pump for the unloading,
and a high pressure water pump used to fluidise the barge contents
by jetting. The mixture is then pumped through a pipeline to the
point of reclamation or relocation.
17. The activity of dredging can create the following principal
impacts to the environment:
1. Collection of heavy metals lead left by fishing, bullets, 98%
mercury reclaimed [natural occurring and left over from gold rush
era].
2. Short term increases in turbidity, which can affect
aquatic species metabolism and interfere with spawning. Suction
dredging activity is allowed only during non-spawning time
frames set by fish and game (in-water work periods).
3. Secondary impacts to marsh productivity from sedimentation.
4. Tertiary impacts to avifauna which may prey upon contaminated
aquatic organisms.
5. Secondary impacts to aquatic and benthic
organisms' metabolism and mortality.
6. Possible contamination of dredge spoils sites.
7. Changes to the topography by the creation of "spoil islands" from
the accumulated spoil.
8. Releases toxic compound TributyltIn , a popular biocide used
in anti-fouling paint banned in 2008, back into the water.
18. According to a Rabobank outlook report in
2013, the largest dredging companies in the
world are in order of size-
1. China Harbors Engineering (China).
2. Jan De Null (Belgium).
3. DEME (Belgium).
4. Royal Boskalis Westminster(Netherlands).
5. Van Ord Dredging and Marine
Contractors(Netherlands).
6. Dredging Corporation of India Limited
(Vishakhapatnam, India).
19. FROM THIS MINI PROJECT WE CAME TO KNOW
ABOUT DREDGING,ITS USES,ITS TECHNIQUES,ITS
TYPES,ENVIRONMENTAL IMPACTS AND MAJOR
DREDGING COMPANIES.
NOW- A-DAYS,THERE ARE A LOT OF DREDGERS
ARE USED IN CIVIL ENGINEERING PROJECTS.
THANK YOU!
