How do you characterise ground conditions for underwater tunnels?

Fugro’s Chris Pressdee examines the problems that can surround characterising the ground conditions for infrastructure tunnels that pass underwater.
How do you characterise ground  conditions for underwater tunnels? How do you characterise ground  conditions for underwater tunnels? How do you characterise ground  conditions for underwater tunnels? How do you characterise ground  conditions for underwater tunnels? How do you characterise ground  conditions for underwater tunnels?

Fugro’s fleet of specialist testing equipment includes tracked CPT trucks and jack-up barges

Chris Pressdee

This article aims to highlight how specialist expertise can address the challenges associated with characterising the ground conditions for underwater tunnels, which can also be relevant for other linear infrastructure including cables and pipelines traversing land, inter-tidal/coastal and marine environments.

The possible challenges

Planning and acquisition of geo-data for tunnels is critical for the development of a suitable ground model and effective management of what is known as ‘ground risk'. There are clear and tangible benefits of an integrated, iterative and predictive ground model approach for tunnels using multiple phases and types of site characterisation techniques.

The challenges of designing and delivering a pre-construction dynamic ground model and integrating multiple sources of data, are amplified, when the tunnel alignment crosses underneath a body of water (river, estuary, lake, sea). Access for people and equipment across the shallow transition zone from land to water frequently presents elevated safety risks and in regions with high tidal ranges, it is extremely challenging to acquire a continuous line of data and produce a ground model with no gaps or inaccuracies.

A summary of the survey and geo-data acquisition challenges posed by linear infrastructure that cross the shallow transition zone is:

  • Difficult access - too wet for land rigs, too shallow for survey vessels
  • Safety - soft mud, tides, emergency access, UXO
  • Environmental - protected birds, protected wetlands, archaeology
  • Data management - the fusion of land and marine data
  • Data gaps - estimated parameters, conservative design or omission of risk mitigation
  • Ground risk - variable soils, liquefaction and lateral spreading, paleo-channels, weathered rock stability, faulting, shallow cover.

Tunnels can be challenging to build and remediation of poor performance is generally very expensive and highly disruptive to heavily used assets. The gearing or return on incremental investment of performing the right site characterisation is huge - often 50 times or more. Poorly defined subsurface conditions leave the way open for inadequate or inappropriate design, resulting in a reduced return on investment from that expected or in unnecessary claims due to unforeseen but potentially foreseeable ground conditions.

An engineer or owner responsible for specifying a site characterisation for an underwater tunnel project may decide to take a traditional approach and parcel the contracts into land and marine and open the market to competitive pricing procurement practices. The resulting data may prove to be inadequate to provide enough overlap in the transition zone and unnecessary risks are taken by both parties in tidal sites when trying to achieve sufficient coverage.

What are the solutions?

A preferred solution would be to seek advice from an expert with innovative equipment borne from decades of experience. Specialist amphibious vehicles and tracked land plant with the ability to drill and sample the subsurface safely in shallow water could offer an integrated solution alongside nearshore survey vessels, able to undertake sub-bottom profiling in the same environment.

In environments where waves in the surf zone can be unpredictable and present a significant operational and safety hazard, it would be advantageous to be able to work from a stable platform which can ‘walk' between sampling and testing locations without the need for a tug or support vessel.

When the seabed slopes steeply from the coast the acquisition of accurate geo-data needs a range of specialist vessels capable of working in harsh marine conditions in busy shipping lanes.

Fugro has developed a team of people and has a fleet of resources equipped to do all the above and offer clients considerable benefits in terms of risk reduction, schedule acceleration and savings on their total costs.

On the Eurasia tunnel in Istanbul, Fugro characterised the ground conditions along the entire alignment, including underneath the Bosphorus Strait, using bespoke techniques and highly skilled analysts able to combine geophysical data from both the marine and land environments. Our data supported a safe design and with the ability to withstand earthquakes of magnitude up to 7.5 on Richter scale. A 45ft (13.7m) diameter Herrenknecht mixed-shield slurry TBM was used to mine the tunnel under the Bosphorus without serious difficulties.

There are several marine tunnel mega-projects planned to be built in the next 10-15 years, including FINEST from Helsinki to Tallinn, the Malta to Gozo road link tunnel and maybe one day we will see a tunnel connect Europe and Africa across The Gibraltar Strait. Each of these projects could realise significant benefits by engaging with specialist organisations with experience of safe and efficient geo-data acquisition from such challenging environments.

Chris Pressdee is the commercial director, Tunnelling & Underground Space at Fugro