Dredging is the operation of removing material from under water. In all but a few situations the excavation is undertaken by specialist floating plant, known as a dredger. Dredging is carried out in many different locations and for many different purposes, but the main objectives are usually to recover material which has some value or use, or to create a greater depth of water. The latter is often associated with navigation, and it is the dredging of ports and harbours that is the most common form of dredging and the one with which most engineers are familiar. But dredging can form part of many other construction activities and it is important to be able to recognise both the capabilities of modern dredgers and the problems associated with their use.
Dredging has grown into a very specialised activity. The plant and equipment involved may have a capital cost of many millions of euro's; the largest of the modern dredgers found operating in the world today are capable of moving hundreds of thousands of cubic metres of material each week. Dredgers can bring great benefits. They allow new ports and harbours to be developed safely. They create the conditions for pipelines to be buried in the seabed, for sandy beaches to be maintained and protected from erosion, for oil to be extracted from deep sea wells and for maritime navigation to take place under safe conditions.
Dredging can be carried out wherever there is sufficient water depth to allow a dredger to operate. Thus dredgers can be found around the coasts, in rivers and in canals. They can be found in lakes and ponds far from the sea and they can be found in exposed offshore locations far from land. Dredgers operate in every ocean of the world, and they can also be found working reservoirs high up in the mountains of the Andes. Dredgers exist in a wide variety of shapes and sizes, ranging in complexity from a simple grab crane on a floating pontoon to some of the most sophisticated and technologically advanced vessels afloat. Some dredgers can be moved from site to site by road transport; others are capable of sailing half way around the world under their own power.
Dredging involves many different skills. Although essentially a civil engineering activity, the work requires knowledge of mechanical and electrical engineering, electronics, naval architecture, the marine environment and many other disciplines. Above all it is a practical subject. Many of the activities are computerised and highly automated, but there is still great reliance on the experience of those involved in both the planning and the execution of the different types of projects.
The operation of dredging involves four distinct processes. The in-situ material must first be disturbed and loosened from its natural state. It then has to be moved from that position to the water surface. These two phases are generally considered to be the dredging operation. Following on are the equally important transport and relocation phases. Transport may take place in a barge or hopper, or as a suspension in pipeline flow. The cost of transport may be a high proportion of the overall dredging cost, especially if the relocation site is at some considerable distance from the dredging site.
Dredging is recognised as having the potential for major environmental impact. Whether in the dredging operation or in the disposal stage, care must be taken to minimise disturbance to marine life. Also, the dredged material should not be regarded simply as waste. Consideration has to be given to the potential for some form of beneficial use of the material. With increasing environmental awareness and tightening legislative control, finding a suitable site to relocate the dredged material can be a major constraint on the implementation of a dredging project.
a. Reasons for Dredging
The recovery from under water of material which has some value or use is the first major objective of dredging. In this instance dredging can be regarded as a form of mining. The dredging of tin ore was an early example of mineral dredging, but gold, coal, rare earths and phosphates are among other materials which are won by dredging. The dredgers involved in this type of mining are highly specialised and custom built for a particular project. They may include some form of on-board processing of the recovered material and are usually owned by the mining company.
A much more common material won by dredging is sand and gravel for use in concrete manufacture. Sea-dredged aggregate is a valuable alternative to land-based sources of these construction materials. The UK's marine-aggregate dredging industry is one of the largest in the world and lands many millions of tonnes of sand and gravel each year. Aggregate dredging also takes place in many inland waters, including rivers, lakes and ponds. Aggregate dredgers are usually specially designed and constructed for the particular operation and are owned and operated by companies whose main activity is construction material supply.
The creation of new land by hydraulic fill is an important use of dredged material. In many coastal situations there is a great shortage of land suitable for development. One way of providing additional space is to raise the existing sea-bed levels by the placement of suitable material recovered from another location. The dredging site may be one where there is a need for deeper water associated with the new land, but more usually is some natural deposit of sediment which can be dredged easily and quickly. After transport to the area to be reclaimed, the material is pumped ashore as a suspension. The sand quickly settles to leave a compact base for such projects as new industry, housing, transport infrastructure and port development. The emphasis is very much on moving large volumes of material as quickly and as economically as possible.
Beach nourishment is another aspect of dredging where the prime objective is achieved by the recovery of suitable material. Where coastlines erode and degrade an alternative to the construction of such hard forms of protection as rock armour and concrete walls is the placement on the shore of natural sands and gravels, perhaps recovered from where the eroded material has deposited. By nourishing or replenishing the beach the natural balance is maintained. This type of work requires dredgers able to place the sand on what is often a shallow and exposed coastline. The creation or enhancement of wetlands by using finer sized dredged material is another potential beneficial use, as is the construction of offshore berms and islands.
The second main objective of dredging is the creation of deeper water. If the natural depths in an area are increased for the first time the activity is known as ‘capital dredging’, ‘development dredging’ or ‘new works dredging’. With capital dredging the full range of geotechnical materials may be encountered. ‘Soft’ material, such as sand, silt and clay, may well be mixed with much stiffer clays, boulders and in some cases rock. This activity covers the construction of new harbours, ports, basins, canals and waterways.
Deepening below the pre-existing bed levels can result in sediment being moved into the deepened area by the actions of water currents and waves. The siltation then has to be removed to maintain the required depth. This type of dredging is known as ‘maintenance dredging’. In some situations maintenance dredging may be required only once every few years. In others it may be needed two or three times each year. And in others it may be a continuous operation throughout the year.
Dredging for deeper water or greater cross-sectional area may be undertaken for many different reasons. The dredging may be part of a water supply or flood relief project. Dredgers can be used to construct reservoirs, deepen floodprone rivers and form irrigation channels. The storage capacity of reservoirs subject to siltation can be maintained by dredging, and hydroelectric power projects can be assisted with their water supplies.
The more common reason for seeking deeper water is to improve navigation. This applies to ships and structures of all types, in the sea, in estuaries and in inland waterways. Navigational dredging is the most common form of dredging activity and is undertaken in ports, harbours and shipping channels throughout the world. In some locations the dredging may be for vessels with deep draughts, such as large oil tankers, bulk carriers and container ships. In others it may be for coasters or inland waterway barges, fishing vessels, naval vessels, ferries or leisure craft. Some of the work may involve increasing the natural depths as ships become larger or new ports are developed. Much navigational dredging is the periodic removal of sediment deposited in the deepened channels. Maintenance dredging is a necessity for almost every navigable waterway and port in the world.
Civil engineering construction work is another activity that can create a demand for dredging. Here the nature of the work is not dissimilar to that undertaken on land. The creation and backfilling of trenches for pipelines and tunnels, and the forming of foundations for structures, are all dredging activities which have their counterparts in the dry. The dredging is generally a specialist subsidiary activity to the main construction with the work often being sub-contracted to the specialist dredging contractor.
An expanding area of dredging activity is that termed remedial or ‘environmental’ dredging. There is increasing recognition of the amenity value of waterways, especially in urban areas. With many rivers, canals, drains and lakes being choked with all forms of rubbish, their clearance has presented new challenges to the dredging industry. Sometimes the sediment is contaminated with pollutants that require special handling to minimize risk to the environment. This is particularly the case in heavily industrialised ports or in rivermouths with industry upstream. The clearance of industrial sludge lagoons and settlement ponds is another highly specialised area of dredging activity, which may even involve the use of a remote-controlled dredger.
b. The Dredging Industry
The many different reasons for dredging illustrate that it is not one single activity. The objectives and requirements can vary widely, as determined by the organization wishing the dredging to be undertaken. For navigation dredging this organization is usually a port or harbour authority or a government department of some form. Reservoir dredging, river dredging and canal dredging are again usually undertaken for the authority which either owns these facilities or is responsible for their operation. The same applies to beach replenishment and to many amenity dredging projects. Reclamation dredging and constructional dredging, on the other hand, may form only a small part of some larger project. Again the dredging may be undertaken directly for a client or on a sub-contract basis to a main contractor.
Organizations with the capability of executing a dredging project are similarly wide ranging. Many organisations with a requirement for dredging operate their own dredgers. This is especially true for maintenance dredging where the work is restricted in location and the opportunity exists to provide a dredger specially designed for that particular project. Thus many port authorities own and operate one or more dredgers, as do inland waterway authorities and government agencies responsible for drainage, irrigation and transport.
The alternative to operating an in-house dredger is to employ a dredging contractor. This is the practice followed by continental ports, such as Rotterdam and Antwerp, as well as by many UK ports. Dredging contractors are very varied in size and capability. Some contractors operate only at a local level, providing a service within a particular river or harbour to one or more clients. Larger contractors may operate on a national basis, or occasionally take contracts overseas. An international dredging contractor will work in any part of the world and will have the plant and equipment able to undertake any type of capital or maintenance dredging project. Today the industry is dominated by a small number of very large companies, mainly based in Belgium and The Netherlands. These companies have the financial resources to keep their dredging equipment in the technologically advanced state required to undertake dredging efficiently and to carry out the vast range of projects found throughout the world.
c. The Dredging Project
Some may regard dredging as a relatively simple activity. But excavating material from below the water surface can present many problems, and if mistakes are made the additional costs can be very high. Dredgers are expensive to purchase and run, and their capital cost and operating rates can only be justified by high utilisation and high production. Production is the key to successful dredging for both client and operator. But the difficulty of working under water – the results of a dredging project are seldom visible – in an often hostile natural environment means that there are many risks. It is thus important that dredging projects are adequately designed and supervised, and that all involved have a proper appreciation of what can and cannot be economically achieved.
Planning the dredging project requires that the work is seen to be practically and economically feasible. Different site conditions and material types and quantities require different types of dredging equipment. Thus there must be the ability to determine those site conditions which affect the likely production of the dredger and hence the cost and time to project completion. Even such relatively familiar construction activities as surveying, setting-out and measurement present many difficulties when the site is under water and cannot be seen. The problems of horizontal and vertical control require special equipment, techniques and skills. Geotechnical investigation to determine the nature of the material to be dredged becomes very important. Wrong information or an inaccurate assessment can result in important production and project delays – probably at considerable cost. It is not always recognised that the prime objective of a dredging site investigation is to allow the feasibility of dredging and the expected production to be assessed. The more usual indications of bearing capacity or consolidation rate are not of major importance.
Every cubic metre of material dredged has to be relocated or disposed of. If the objective is to create deeper water and to recover the material, a suitable disposal site has to be identified. This is becoming increasingly difficult. Licences and approvals are required to dispose of dredged material. The traditional disposal by dumping in the sea may still be the best practicable environmental option, but other possibilities have to be investigated. Disposal of the material may be complicated by the presence of contaminants. These may require special, and costly, techniques for all phases of the dredging cycle. Maintenance dredging of inland waters presents particular problems for relocating the dredged sediments – where can the large volumes of sediment be disposed of ? The environmental impact of the dredging project may require assessment. The possible effects of the dredging and disposal operations will need to be reviewed as well as the potential impact of the completed works. Approvals and permits are required for dredging projects and it takes time, energy and resources to obtain these.
