Dredging

Revision as of 12:50, 15 November 2016

Introduction

Dredging is the process of excavating and removing sediments and debris from below water level, typically from the bottom of lakes, rivers, harbours, and so on. Dredging is necessary because of sedimentation which is the natural process whereby sand and silt washes downstream and accumulates over time in channels and harbours. Dredging is carried out for many reasons, including the following:

• To increase the depth of the bed to allow boats and ships to pass through.
• To obtain aggregates for manufacturing concrete.
• To obtain other materials for use in civil engineering projects such as roads.
• To excavate trenches for laying pipelines, cables, and so on.
• To increase channel capacity, thereby helping with flood prevention.
• To prevent the spread of contaminants to other areas of the water body, since sediments in and around industrial areas are often polluted.

Another major purpose of dredging is for land reclamation. This is the process of forming areas of land from depositing dredged materials and building them up. One of the most famous projects of this kind was the construction of the Kansai International Airport in Japan, which involved the building of an artificial island in the middle of Osaka Bay.

Considerations and planning

Before commencing with dredging, it is important that site investigations and hydrological surveys are undertaken to assess the likely consequences of removing the material. Likely material movements and the seabed characteristics can be analysed from these surveys.

Some of the main factors which are considered during the planning stages include:

• Site location.
• Depth required.
• Type of material to be dredged.
• If there is no suitable reclamation option, where the dredged material will be deposited.

It should also be considered that if hard rock is likely to be dredged, underwater explosives may need to be used to break them up before being removed by bucket or grab.

Plant and equipment

The plant used for dredging can be broadly categorised as either suction or digging dredgers.

Suction dredgers

These are the most commonly used types of dredger. There are many different variations, such as:

Trailer suction hopper dredger

This is a self-propelled hull capable of loading its own hold using a centrifugal pump which drags along the bed. It is typically used for dredging sand, silt and some clays. Winches or cranes are used to raise and lower the suction pipes, which have a maximum dredging depth of around 35 m. Hydraulically-operated doors in the hopper’s bottom discharge the material.

The advantage of this dredger is that it can operate in adverse sea and weather conditions, and can transport soil over long distances.

Cutter suction dredger

This is moored using anchor legs and combines suction with a powerful cutting action. It uses a revolving cutting head and is capable of breaking up and removing firmer materials, such as hard clay and soft rock. The material is sucked up a pipe and discharged into barges for disposal. They can be used for maximum depths of around 40 m, and have the advantage of being able to produce a fairly uniform and level bed.

Digging dredgers

Bucket chain dredger

This is the common type, where a chain of buckets continuously scoop material out and raise it above water level before tipping it out onto chutes. (See top image.)

Bucket dredger

This is also known as a backhoe dredger since it follows the principles of the land-based backhoe excavator. The advantage of this type of plant is that it can be used to dredge a wide range of materials, including obstacles such as boulders.

Dragline dredger

This is also modeled on the land-based dragline, and is limited to shallow inland waterways. It deposits dredged material on the banks of the waterway.

Floating grab dredger

This uses a grab crane which is mounted on the deck of an anchored or spud-rigged pontoon. The grab is usually a clamshell so as to retain mud and silt. Material is loaded into hopper barges.

Find out more

Related articles on Designing Buildings Wiki

• Aggregate.
• Caisson.
• Civil engineer.
• Coastal defences.
• Dam construction.
• Reservoir construction.
• River engineering.
• Sewer construction.
• Tunneling.
• Watercourse.
• Water engineering.

External resources

• ‘Introduction to Civil Engineering Construction’ (3rd ed.), HOLMES, R., College of Estate Management (1994)