Answer:
option (c):
Grab
Explanation
Dredger
- It is a marine vessel.
- It has a device or devices to scrap depositions such as sand,
gravel, sediments, and etc. over sea bed.
Types of
dredgers
Dredgers are mainly divided into three types: Hydraulic,
mechanical and other dredgers (dredgers other than Hydraulic and
mechanical dredgers)
(i) Hydraulic
dredger:
- Principle: Addition of large amounts of process water in order
to change original structure of sediments.
- It uses moving water to suction up the deposited
materials.
- Common form: consists of onboard pump to suck material up using
tube lowered to bottom of water and is then pumped through a
pipeline to a different location.
(ii) Mechanical
dredger:
- Suitable for working in confined areas.
- It is used to remove the hand-packed materials.
Mechanical dredgers can be further divided into four types:
Bucket Dredger, Bucket Ladder Dredger and Grab Dredger and Backhoe
dredger.
(A): Bucket
Dredger
- Fixed on anchors (stationary dredgers.
- Movement is possible along the semi-arcs during dredging using
winches.
- It has an endless chain of buckets.
- Buckets are used to scrap off and fill the loose dredged
material.
- The completely filled buckets are emptied into barges by
turning them upside down over barges.
(B): Bucket Ladder
Dredger (modified bucket dredger)
- It has a series of buckets mounted on a wheel used to pick
sediments.
- Limitation: low production, high level of noise
(C): Grab (clamshell)
dredger
- Stationary dredgers fastened on with anchors or using
spud-poles.
- Cutting tool- grab consisting of wire operated two
half-shells.
- Half-shells after being filled get loaded in barges.
- Grab can be mounted on a dragline or a hydraulic
excavator.
- Grabs are of different types: open grab, closed grab and
watertight grab.
- Grabs are useful in: removal of material from corners of dock
and basin, easy excavation close to quay walls.
- Grab dredger is a revolving crane and is fitted with a grab
which is placed on a hopper vessel or pontoon.
- A grab can lowers, closes, and raises a single bucket
using flexible cables.
- It makes use of a bivalve grab suspended from a crane
which can be opened or closed using a cable or by using hydraulic
cylinders. (Thus, the answer is option (c): grab).
- Characteristics of the grab
- Shape of grab – scooper like a circle’s quadrant
- Normal position opening
- When grab is lowered to bottom, Grab dig to the mud and gets
closed with the help of suitable arrangements.
(D) Backhoe
Dredger
- Useful in: harbour maintenance and shallow dredging.
- They are hydraulically driven excavators.
- It consists of a half-open shell or a digging bucket.
- Half-open shell or a digging bucket is capable of digging
across a wide range of materials.