A site investigation collects data that characterises the material at the dredging site and at the borrow area and the disposal site.
Site investigation data applied
The data gathered will answer questions as:
- What types of soils and material are present?
- Are these materials dredgeable?
- Are these materials suitable for reuse?
- If contaminated, will these materials require special treatment or disposal arrangements?
- What type of equipment and plant will be needed?
- What will the wear and tear on plant be?
- Is the stipulated budget feasible for the work to be carried out?
Dredged Material Characterisation
The characterisation of types of materials to be encountered in a project is made by the use of subsurface exploration, using geophysical, chemical and biological techniques in which a representative set of samples is taken throughout the potential or actual project dimensions.
Sampling of the sub-seafloor should cover three aspects of ground examination:
- geological and geotechnical evaluations;
- bathymetric surveys; and
- environmental assessments.
Dredged material can be distributed through the water both vertically and horizontally. This means that samples are normally taken vertically to a level at least one metre below the dredging depths planned for the project.
Samples are visually observed at the time of sampling, but also should be subjected to laboratory investigation. Sampling is conducted twice:
- during the feasibility or planning phase and
- again in more detail during the design and construction phase.
The difference in the level of detail between the feasibility stage and the design stage and the need for two occasions of exploration must be emphasised to all parties involved, management and technical staff at both the client and contractor.
The costs of a two-phase site investigation, although not insignificant, are relatively minor in comparison with the overall project costs. The expense is well worth avoiding the risk of large and unpredictable cost overruns if unexpected materials are encountered during the operation.
Tests and analyses of samples
Tests and analyses are then performed on the samples to determine:
- particle size distribution,
- soil consistency/water content,
- organic content,
- settlement and consolidation characteristics,
- shear strength characteristics,
- in-situ density,
- particle specific gravity,
- permeability and so on.
These data may then be supplemented with field tests such as standard penetration tests (SPT) and cone penetration tests (CPT).