An Update on the Importance of Site Investigations for Dredging Projects
The development of a successful dredging project relies on a thorough site assessment as the foundation for all other project activities, including developing budgets, schedules, and proposals for bids. Without an understanding of water depth, currents, sediment composition and topography of a site, the owner is essentially working in the dark, sometimes hundreds of feet beneath the surface. The risks of potentially encountering unknown factors can be costly and time-consuming, potentially bringing a project to a halt, in addition to damaging equipment and risking the safety of contractors.
The success of a project requires a thorough site assessment in order to gather enough data to characterize what may be encountered and where, calculate the amount of material being removed, and to determine potential sediment movement that may hinder a successful operation.
Roles, Responsibilities, and Risk Allocation
It is essential that the project owners understand that they are ultimately responsible for anything that happens during a project. The project owner is ultimately responsible for understanding their site and for transferring that information to the contractor in an understandable manner.
Therefore, the thoroughness and accuracy of the site assessment and the ability to transfer that information to the contractor is critical in order to fulfill obligations.
Site Investigation Essentials:
· What types of material are present?
· Are there contaminants present in the material?
· At what locations does the material change?
· Are there any obstacles, such as pipelines, UXO’s etc?
· Are there any critical habitats in the dredge area?
· What is the best methodology for material removal and transport?
· What are the risks associated with material removal, i.e. sediment movement via currents, etc.?
Geotechnical & Sediment Investigations
Dredging projects are often bid on a unit rate basis, thus the contractor must be provided information regarding the location of the various materials to be dredged, both in area and elevation, in order to establish the relative quantities of material that will be removed. The thickness and area of each material layer is the basis for the dredging quantity. In addition, it is critical to understand what contaminants may be in the sediments to be dredged and in the new surface left after dredging is complete. Contaminants can have a significant impact on costs – with high concentrations of metals or organics forcing contractors to dispose of material at upland sites or treated in-water using complex engineered remediation techniques. Consequentially the sediment data is essential for making the decisions of how dredged material will be treated and transported. For barge transportation, the primary driver is unit weight and bulk, while for hydraulic transportation via pipeline, particle side and distribution are critical data.
In order to calculate the above information and to understand what types of equipment that will be needed and the stresses on that equipment, it is necessary to understand the physical and mechanical properties of the material present to determine if the sea-bed contains cohesive or non-cohesive soils and rock. In addition, it is important to know the particle size distribution, strength, plasticity, in-situ density, mineralogy, particle specific gravity, permeability, calcium carbonate content and organic content.
All of the above information typically involves the taking of soil borings or vibracores to obtain samples for material investigation and testing. Classification and testing can be done both in-situ during the subsampling process or in a laboratory. These results are critical for project owners and they are included in subsequent bid packages.
All dredge projects require an understanding of bathymetric features of the site to the greatest extent possible, including water depths and sea floor topography. Bathymetric surveys establish water depths and how water levels are affected during various tidal events. This results in the formation of “isobaths curves” and digitized cross sections of site topography. These underwater surveys provide detailed topographic data of the bottom surface. Bathymetric mapping also supports the evaluation of underwater erosion dynamics, and to support sediment and hydrodynamic modeling programs for several private and government entities.
Bathymetric surveys are also critical for determining access or equipment operational constraints for dredging projects. This information is provided to contractors and available throughout project activities in order to assist contractors to maintain efficient on schedule projects.
In order to establish an efficient sampling protocol and assist the project owner with developing a dredging plan, an environmental assessment is key for most dredging projects. This includes meteorological, hydraulic, sediment transport information, and biological habitat assessments. Wave and current data are necessary to predict best operational times and any possible sediment transport events that may occur due to tides and weather events. Regulatory agencies may require complex environmental impact assessments (EIAs) prior to project initiation in order to evaluate impacts on sensitive marine ecosystems and obtain the necessary permits for project activities. In addition, EIAs may need to evaluate the site for archeological resources, sound impacts, and other impacts to both natural and human systems.
Gravity Marine is an experienced team of scientists with the critical skills necessary to assist project owners in the development of successful dredging projects. They possess decades of experience in marine navigation, sampling and modeling methods, technology, data analysis, and an in depth knowledge of construction processes and regulatory requirements. Gravity has been a key player on hundreds of dredging projects and has the understanding necessary to assist clients through critical steps in order to create cost efficient, safe, and successful projects. Contact them today!