The challenges of geotechnical investigations for ports and harbours requires specialist input, not simply a transfer of land-based techniques to the marine environment.
Ports rely on good quality geotechnical and sample data to provide subsurface information for a range of applications, from initial project planning and consent applications, right through to detailed engineering and design.
However, engineers involved with port development often look to the onshore geotechnical sector to acquire data for a port project. This can result in a site investigation using what are effectively “land-based” techniques over water, often on a jack-up platform.
This may not always be cost-effective or appropriate and can result in higher investigation costs and increased engineering risk. Data coverage with a jack-up may be restricted and the investigation may take far longer. It takes considerable time to re-position and set up for each sampling location and, particularly in busy ports, static jack-up operations may have to be curtailed, and the programme may have to be truncated, to accommodate maritime traffic movements.
Weather can also be a challenge. Weather down-time for conventional terrestrial geotechnical plant can be as much as 100% to 500% of the programme. Instead, what is needed is a complete survey package under a single contract, including environmental and habitat assessments; unexploded ordnance studies; geophysical and hydrographic surveys; ground modelling; volume assessments and foundation design. An integrated approach, typically improves data quality and cost effectiveness significantly.
Furthermore, a specialist marine site investigation contractor will have experienced staff operating under appropriate marine safety systems, plus the equipment to gather the highest quality data.
There is a wealth of techniques for acquiring geotechnical, geochemical or geological data in marine environments, from seabed penetrometer systems (for insitu CPT); to small mini-vibrocores for use on local vessels; standard and heavy duty vibrocorers; underwater rock drills, grab samplers and seabed probes. This marine equipment can be deployed from a crane on a quayside or from the shoreline, saving further cost.
When operated from nearshore survey vessels and platforms, these systems will result in significant time savings compared with systems mounted on a jack-up or barge.
For example, a seabed penetrometer system or vibrocore rig can be mounted on a small multi-cat type vessel. These flat-bottomed craft have a shallow draft, making them ideal for very shallow water.
As seabed-deployed geotechnical equipment is usually not on the seabed for more than 10 to 15 minutes, it is possible to access intertidal areas on a rising tide. In these environments, positioning a jack-up can require a towing vessel, making manoeuvring more onerous and time consuming. Also, if the area dries out at low tide, access to the platform may not be possible.
Employing specialist marine techniques for port and harbour surveys should result in improved and optimised data acquisition. When processed and modelled by engineers specialising in marine geotechnics and geology, this approach will help ensure surveys fully meet the project objectives, and are completed in a safe, timely and cost-effective way.
Marcus Noakes is business development manager at Gardine Geosciences.