About dredging


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. If the objective is to create deeper water and to recover the material, a suitable relocation site has to be identified. This is becoming increasingly difficult. Licences and approvals are required to relocate dredged material. The traditional relocation by offloading in the sea may still be the best practicable environmental option, but other possibilities have to be investigated. Relocation 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 relocated ? The environmental impact of the dredging project may require assessment. The possible effects of the extraction and relocation 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.