In planning and executing site investigations for dredging works, a port authority or other employer may intend to obtain data on ground and environmental conditions for two separate purposes:
For the design of permanent works with regard to aspects dependent on ground conditions (for example, slope stability, sediment transport, suitability of material for use as fill) and on environmental conditions (for example, channel alignment, mooring forces, berth usability);
For planning and costing of dredging operations, in so far as these are affected by ground and environmental conditions.
This report is concerned with the requirements for planning, execution, interpreting and reporting site investigations for the latter purpose.
Dredging works are major operations which are often complex and costly. They may account for a substantial proportion of the cost of maritime projects. Effective planning and execution of dredging works require knowledge about the material to be dredged and the environmental conditions in which the dredging plant will operate. The high mobilisation and capital costs of dredging plant mean that wrong assessment of basic ground and environmental parameters and consequent selection of unsuitable plant and methods may lead to substantial economic losses. Appropriate ground and environmental surveys are fundamental to the success of dredging operations, from both technical and economic points of view. It is in the interests of both contractor and employer to obtain a full understanding of ground and environmental conditions at the site, in order to ensure the smooth progress of the works and avoid claims and disputes.
The survey techniques applicable to the marine environment are different to those applicable on land. The difficulty of working (sometimes in poor conditions), the cost of the specialised techniques which are required, and the often large areas which need to be investigated for dredging projects probably explain why investigations for dredging works are sometimes inadequate.
Surveys should be based on a preliminary study of existing geological and environmental data against a background of a preliminary design of what is proposed to be dredged. The study should lead to an overall appreciation of the site and provide a focus on the difficulties likely to be encountered in the course of the works, the types of data which need to be collected, and the areas and depths to be investigated. The field investigations are likely to include one or more of the following:
bathymetric surveys to define water depths in and around the dredging and disposal sites;
geophysical investigations to identify obstacles on or under the seabed (shipwrecks, pipelines, debris etc.);
geophysical and geotechnical investigation of the bed to identify the types of soil and rock to be dredged, and to define their physical and mechanical properties;
investigation of the environmental conditions (hydraulic, meteorological, etc.) which affect dredging operations.
Dredging methods and disposal of dredged materials may also be influenced by contamination of the materials to be dredged. The investigation of the extent and degree of contamination is not addressed in this report but attention is drawn to this possibility.