Survey works form a very important part of dredging projects. They are required at the initial stage of the project while preparing the pre-feasibility study and at the end of the dredging project. At the initial stage, survey works help in determining site conditions and the initial depth of the seabed, while at the end of the project, they ensure that the required dredge levels have been achieved. Repeated surveys are also required over the life of a port to ensure that the required clearance depths are maintained. Further, an in-depth study of the environment around the dredging site is required to ensure that the dredging activity does not harm the surrounding area.
A typical port development project comprises wharfs, a turning basin, breakwater, approach channel, etc. The project may include land reclamation using the dredged material or may require the dredge spoil to be disposed of at suitable dumping sites.
Planning must also include oceanographic work to understand the restrictions imposed by waves, currents and tides on vessel movement and subsequent sediment transport. Regardless of the project details, it is essential to fully understand the nature and composition of the seabed and avoid costly project delays subsequently.
Deciding the location of the port infrastructure is a very complex task, especially in the case of greenfield port development. Given the existing site information and the purpose of dredging, specific site investigation needs to be carried out. This should comprise a balance of geophysical (remote sensing) and geotechnical (physical sampling) studies. An appropriate combination of the two investigation categories – geophysical and geotechnical – reduces the overall cost of the investigation. Incomplete knowledge of the site conditions from the feasibility study has various possible consequences – if the port invites bids for a dredging project based on inadequate surveys and on a few scattered boreholes, the information about the site can be misleading. For instance, if the contractor assumes that there are hard materials, whereas none actually exist, the quotation may be inflated, to cover risks. Another possible consequence of inadequate investigations is that hard (undredgeable) materials may be missed, leading to project delays and litigation at a later stage. Investigation will generally include the measurement of water depths, analysis of sub-bottom geology and assessment of the texture of the sea floor. To investigate shallow seabeds, a “boomer” has historically been the most common geophysical tool, since the 1970s. The boomer works well in soft sediments; however, the results may be inadequate in shallow water when there is cemented material on the seabed, if the seabed contains gas or if the sub-bottom boundaries are indistinct. Some of the latest geophysical investigation techniques are electrical resistivity, seismic refraction and multichannel reflection. The refraction method measures an actual in situ property of the seabed – the compressional wave velocity – and this has proven to be the best parameter to correlate with ease of dredging.
The way forward
Thorough investigations must be undertaken before awarding a dredging contract. In soft conditions, the cone penetration test can collect the data economically. However, a problem arises if there is a hard layer which prevents penetration. Boreholes will then be required to determine the conditions. Appropriate lab testing of the samples collected during geotechnical studies should be undertaken. For small areas, such as piling locations for a wharf, boreholes may be sufficient; however, for larger regions, geophysical surveys will be essential.
To investigate a shallow seabed, it is critical that the geophysical survey precedes the geotechnical work. This will allow optimisation of the sampling locations and greatly improve the final result. Thus, sufficient time must be given in the project planning schedule to carry out and report on the geophysical work.
To conclude, survey and investigation form a crucial part of dredging projects. To undertake a successful project, the importance of a carefully designed and detailed site investigation cannot be ignored. w
Based on a presentation by Jim Anderson, Principal Geophysicist, Fugro Survey Pty Ltd