Question

In: Civil Engineering

A _____________ dredger essentially consist of a graph suspended from cable. Select one: a. Hydraulic b....

A _____________ dredger essentially consist of a graph suspended from cable.

Select one:

a. Hydraulic

b. Bucket

c. Grab

d. Ladder

Expert Solution

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.

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