Aldrin Report

8/12/2019 Aldrin Report

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ABSTRACT

Dredgings works have an important

participation in any port or coastal

work, being currently very related to

the artificial feedings of beaches.

His applications are nevertheless

much more diverse, intervening for

example in the conditioning of the

funds, in the extraction of materials

and in the securing of landfills. The

used machinery acquires a special

leading role in this type of works

and is decisive in the final cost of

the operation. There exists a great

variety of dredging equipment,

which differentiate principally in the

way of realizing the excavation.

2 OBJECT OF DREDGINGThe dredging is carried out to

increase the depth of waterway, to

provide sufficient draft for ship or

vessel is the vertical linear

immersion of ship floating in stable

conditions safely. Navigable depth

of water is draft added with vertical

clearance at bottom of ship. If the

navigable depth required for ship is

greater than depth of water available

at site (entrance and basin of

harbour) ship cannot enter into the

harbour. Hence the port authorities

have to decide whether to lose the

sea traffic, or modify the basin

characteristics to be improved at a

harbour is to increase the depth of

water at required locations in basin

by dredging.

3 MAINTENANCE DREDGING

Dredging is defined as excavation

under water and is done by earth-

moving machine called dredger.

These machines are designed to

increase the depth of water by

removing the bed material under

water surface.

The dredging may either be capital

dredging or maintenance grudging.

In capital dredging, the required

depth of water at a particular place is

obtained and it is then maintenance

regularly to have a convenient

harbour. Thus, capital dredging

indicates initial removal of material


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and the material removed may be

anything from rock to slit. In case of

maintenance dredging, the depth of

water is maintained by carrying out

dredging operations periodically and

it is quite clear that the material

obtained during maintenance

dredging will be in the form of fresh

deposits such as sand and silt only.

It may be noted that maintenance

dredging is one of the important

activity at all the harbours and there

is no harbour worth the name where

maintenance dredging is not

required. It may be noted that ports

havebeen shifted in the past simply

because the harbour or the entrance

to it got silted up.

Similarly, many harbours have been

abandoned because of difficulties

involving the overcome of silting of

harbour or for providing more depth

of water to ships of big size.

4 DREDGING METHODS

Currently there are three methods of

sediment removal for inland water

bodies; excavation on the water

body basin, excavation from

shoreline, and hydraulic

dredging. Each of these ideologies

has their respective

limitations. Lets take a moment to

explore each method looking at the

advantages and disadvantages of

each approach.

4.1 Dredging of the Water Body

Basin

This technique involves the draining

of the entire water body, waiting for

the basin to dry, then the employing

of traditional heavy equipment

(excavators). Once all dredging is

complete, then the water body is

refilled.


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Fig.1 Dredging Of The Water BodyBasin

4.2 Dredging from Shoreline

This technique involves the use of

long reach excavators or drag line

cranes to reach out into the water

basin and pull sediment onto shore.

Once at shore the sediment is placed

in trucks and moved to disposal site.Fig. 2 Dredging From Shoreline

Fig. 3 Dredging From Shoreline

4.3 Hydraulic Dredging

This technique uses a barge that

pumps a combination of sediment

and water to a discharge site. The

water then effectively drains fromthe discharge area (spoils pit or

sediment bags) and then returns

back to the body of water.


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Fig.4 Hydraulic Dredging

Fig.5 Hydraulic Dredging

5 DREDGER

A dredger is an earth-moving

machine, which is designed to

increase the depth of water by

removing the bed materials under

the surface.

The choice of a dredger primarily

depends on the nature of sea bed, i.e.

whether it is of loose materials orrocks. It is therefore important to

carry out the preliminary

topographic, geological and

submarine geophysical surveys to

decide the nature of sea bed.

The study of such surveys will help

in determining the following points:

Actual volume of thedredged material

Depth to be attained Length of dredged

channels

There has been considerable

improvement in the dredging

equipment of the present time and

knowing the nature of sea bed and

its characteristics, the most suitable

type can be chosen or recommended.


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The following are some of the

modern types of mechanical dredges

used in modern marine engineering

practice.

Dipper dredge Grapple dredge Continuous bucket

elevator or Ladder

dredge

Hydraulic or Suctiondredge

5.1 Dipper Dredge

It consists of floating vessel strongly

constructed carrying an include

frame Ain the bow to hold the boom

Bby guy wires. Through the middle

of the boom runs a dripper stick,

worked by a rack and pinion

arrangement and to the end of which

is rigidly attached the dipper bucket

K, with a flap. A hoist cable is fixed

to the bucket, to move it up or down.

The vessel is fixed to the bed in

position by means of three stakesduring the dredging operations. The

boom B could be swing horizontally

at the bow

5.1.1 Operation:

The hoist cable is released, to enable

the bucket K to reach the bed when

a continuous force is applied to the

stick, through the rack and pinion

forcing the teeth of the bucket to bite

into the soil. The hoist cable is

slowly pulled resulting in a cut being

made in the bed. The hoisting is

continued and the bucket K is

hauled out of the water, the boom B

is swing round to deposit the

material in the bucket K, into any

scow along side or on to any

predetermined place, by opening the

flap. The boom Bis swing back and

the dipper is lowered, in preparation

for the next cut.

5.1.2 Advantages of dipper dredge:

Easy maneuverable andhence, suitable for use in


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confined spaces around docks

and narrow channels.

Very powerful and capable ofexcavating in hard soil for

removal of boulders, and

breaking up of heavy objects

like old piles, cribs, etc. this

type of dredge can dig in

depths up to 15m of water and

the dipper capacity varies

from 1 to 4 m3 normally.

These types of dredges of

heavy capacities of about 12

m3are in the Panama Canal.

This type of dredger canexcavate and deposit it either

on the blank or on a floating

barrage.

The dredger is very powerfuland capable of excavating in

hard soil boulder beds and

rocks, which break into largeheavy pieces.

5.2 Grapple Dredge

It consists of a hull, to the front of

which are fixed a frame and a boom

B. the frame is guyed back, by back

legs. A grab bucket K is suspended

by two cable lines HH called the

opening and closing lines.

The boom is fixed at the required

elevation by means of a guy wire

and arrangement to rotate through a

horizontal angle on a pivot at the

lower end. The bucket hoisting lines

pass over two main sheaves fixed to

the upper end of the boom B. the

dredge is moored by the help of

stakes fixed fore and aft.

5.2.1 Operation:

After positioning the dredge, a scow

is brought alongside and tied fast to

it, on the right side. The bucket is

lowered to the bottom, where it bites

the bed and fills itself. Then tension

is put on the closing cable, which

closes the bucket, hoists the load and

swings the boom B horizontally to

the right. When the boom B reaches

over to the scow, the tensions in the


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bucket and the boom B swings back

to the digging position as before.

The hull is generally 30 m to 60 m

long and 15 m across, the booms

being 15 m long. The capacity of the

grab bucket ranges from 8 to 23 m3.

5.2.2 Advantages of grapple dredge:

This dredge is very efficient and

suitable for dredging materials like

sand, clay or mud, but not well for

hard diggings. The bed dredged by a

grab seldom be even and will exhibit

pits and high spots.

5.3 Continuous Bucket Elevator

Or Ladder Dredge

It consists of an endless chain of

buckets, mounted and running round

a ladder, formed in the middle of the

bow of a floating vessel. The ladder

could be is lowered of raised by the

line N. The chain of buckets is

operated, manually or mechanically

by a big wheel E. the buckets are

provided with pronged cutting

edges.

5.3.1 Operation:

Each bucket cuts and brings up

material to the top of the ladder as

chain moves round, where each

bucket inverts on descent and

discharge its contents into special

holds H-H. The vessel movesforward on completion of work at

each section. The size of these

vessels is about 60 m in length and

15 m in width, having a draft of

about 2.50 m to 3 m. these can be

used for digging depths of 9 m to 12

m. The average digging capacity of

such a vessel is about 6000 m3 to

7650 m3per day.

5.3.2 Advantage of continuous

bucket elevator or ladder dredge:

This dredge is very suitable for

handling coarse gravel or sand, hard

clays and even soft or broken stone,

at fairly good depths and beyond the

reach of a dipper.

5.4 Hydraulic Or Suction Dredge


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It consists of a suction pipe, carrying

at the lower end a cutter of some sort

and having a universal joint at top.

This pipe is supported on a ladder

and held in position by an A frame

mounted on the bow of the dredging

vessel. The suction pipe is connected

to a centrifugal pump located

amidships having a long flexible

delivery pipe discharging into

hopper barges alongside the vessel

itself or on to specially selected spot

on shore, needing reclaiming or

filling.

5.4.1 Operation:

The hull V carries the suction pipe

on a ladder in the bow and the

pumping machinery P is housed in

the middle of the ship with its

delivery pipe D discharging into

hopper barges floating behind the

dredging vessel. The cutter is

suspended from a frame in front and

the vessel is moored by anchors. The

cutter rotates cuts and loosens the

soil for quick and easy suction by

the pump. The cutter up materials is

well churned in the operation and

mixed with water to facilitate

suction.

Sea going dredges of this type have

a size varying from 45 m to 140 m

in length and 9 m to 30 m in width.

The dredging depths vary from 4.50

m to 15 m. the machinery is

operated either by stream power or

electric power. The pump is a

special type of dirt and rock brought

out in dredging.

5.4.2 AdvantagesOf Hydraulic Or

Suction Dredge:

This type of dredge is very effective

in beds of sand, silt, mud and clay in

open water and is an excellent

machine to clear sand bars. If big

boulders or tree stumps are meeting

with, they will have to be lifted out

before putting the cutter to work.


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Gravel and soft rocks are easily

reduced by the cutter.

6 DISPOSAL OF DREDGEDMATERIAL

Dredged materials are mainly

disposed off in the following ways.

Conveying out to sea anddeposited far from the siteof accumulation.

Conveyed and deposited inswampy inland areas or

adjacent to the shore to the

reclamation of land.

Disposal site selection for dredged

material is one of the most important

and challenging parts of planning a

dredging project. The most common

dredged material disposal methods

are ocean placement, beach

nourishment, confined disposal

facilities (CDFs), flow-lane and

within-banks placement, and capped

disposal

6.1 Ocean Placement

Ocean Dredged Material Disposal

Sites (ODMDS) are primarily used

for material coming from inlets,

coastal entrance bars, or main

coastal navigation waterways.

Typically, in ocean placement, a

hopper dredge or towed barge sails

to a designated area in the ocean,

where the hull (bottom) of the vessel

is opened. The dredged sediments

drift to the bottom. Only clean

dredged material may go to ocean

sites.

6.2 Beach Nourishment

Beach nourishment is the placement

of dredged material on or near the

beach, usually to replenish an

eroding beach or protect an eroding

wetland. The dredged material is

generally sand coming from inlets,

coastal entrance bars, or main

offshore waterways. Both hopper

dredges and pipeline dredges can

use beach nourishment sites. When

hopper dredges place sand offshore

along the beach, natural processes


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carry it onto the beach over a long

period of time. Only clean dredged

material can be used for beach

nourishment. This method of

dredged material disposal is

considered a beneficial use of

dredged sands. It is used in coastal

areas all around the country.

6.3 Confined Disposal Facilities

(CDFs)

In Confined Disposal Facilities

(CDFs), dredged material is placed

behind dikes, which contain and

isolate it from the surrounding

environment. There are three types

of CDFs: Upland, Shoreline, and

Island. A mixture of dredged

material and water is pumped into an

area that is divided into several

smaller areas, called cells. As the

water moves between the cells, itslows, the dredged material settles

out, and finally, clean water is

discharged from the site. The

difference in the three types of CDFs

is their location. Upland CDFs are

on land, above the line of high waterand out of wetland areas. Shoreline

CDFs are constructed over the sea or

lake bottom and are attached to the

shoreline on at least one side. Island

CDFs are constructed offshore, but

in relatively shallow water.

6.4 Flow-lane and Within-banks

Placement

Some waterways are in high-energy

river systems with rapidly flowing

water and strong currents. The

energy of the water causes shoals

made of coarse sand to form, move

along the bottom, and re-form

relatively quickly. In these systems,

flow-lane or within-banks disposal

may be used.

6.5Capped Disposal

Sometimes, but not often, dredged

material is contaminated with

metals, chemicals, or other

substances. If the contamination is

bad enough, it might be harmful to

the environment to put the material

in a typical in-water site or a typical

CDF. In these cases special handling


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will be used to prevent the

contaminants from re-entering the

environment during dredging and

disposal.

7 ENVIRONMENTAL

IMPACTS

Dredging can create disturbance

to aquatic ecosystems, often with

adverse impacts. In addition, dredge

spoils may contain toxic chemicals

that may have an adverse effect on

the disposal area; furthermore, the

process of dredging often dislodges

chemicals residing

in benthic substrates and injects

them into the water column.

The activity of dredging can create

the following principal impacts to

the environment:

Release of toxic chemicals(including heavy

metals and PCB) from

bottom sediments into

the water column.

Short term increasesin turbidity, which can affect

aquatic species metabolism

and interfere with spawning.

Secondary effects from watercolumn contamination of

uptake of heavy

metals, DDT and other

persistent organic toxins,

via food chain uptake and

subsequent concentrations of

these toxins in

higher organisms including

humans.

Secondary impactsto marsh productivity

from sedimentation

Tertiary impactsto avifauna which

may prey upon contaminated

aquatic organisms

Secondary impacts to aquaticand benthic

organisms' metabolism andmortality

Possible contamination ofdredge spoils sites

8 LIMITATIONS OF

DREDGING

Can be very expensive


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Need a disposal site Need large nearby

handling area

May be significantresiduals

May need to over-dredge and remove

debris

May be significantlosses to resuspension or

volatilization

May be difficult to meetState WQS

May be disruption tocommunity

9 CONCLUSION

Dredging is necessary for the ships

to entering into the harbour. For

entering, the depth of water

available at the harbour must be

greater than the navigable depth

required for the ship. By discussing

Dredging in harbour, we got the

importance of dredging in harbour,

method and disposal of dredged

materials and clear idea about what

types of dredging should be used in

different situation and area.

REFERENCE

http://www.wikipedia.org http://www.dredging.com http://www.dscdredging.com http://www.vmi-dredges.com http://www.pla.co.uk R. Sreenivasan (2005),

Harbour Dock and Tunnel

Engineering.

Environmental SolutionsLimited (2001).

Environmental Impact

Assessment: Port Antonio

Yacht Club and Marina

Project. Report submitted to

Port Authority of Jamaica,

Kingston, Jamaica.

H.P.Oza & G.H.Oza (2005).Dock and Harbour

Engineering

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Aldrin Report