Fugro’s ‘Triple A’ approach reduces mining tunnel risk

Challenge

Our client, Sirius Minerals plc (Sirius), needed Fugro’s Geo-data to safely construct a mineral transport tunnel (MTS) for their underground site, which is the world’s largest high-grade polyhalite development.

Solution

Our solution was based on our ‘Triple A’ approach of acquiring, analysing and advising on the site’s Geo-data. We assessed the site’s geological structure and geotechnical ground conditions to a depth of 500 m and then delivered a robust ground model that provided Sirius and their preferred tunnelling contractor with a better understanding of tunnel alignment, design and construction risk.

Our early involvement in the project, combined with our geotechnical expertise and wireless seismic investigation approach, enabled us to accelerate the programme, confirm the geological conditions, and lessen the impact on the environment and local community.

The wireless seismic survey was a vital part of the integrated ground investigation that informed and optimised the tunnel design for Sirius by providing detailed ground information covering areas that were inaccessible by other methods. By using a lightweight approach, which comprised a cable-free recording system, environmentally benign Vibroseis trucks and a small field crew, the survey was completed ahead of schedule and with minimal disruption to local stakeholders. This approach allowed critical design decisions to be made earlier.

We completed 80 linear km of high-resolution seismic reflection acquisition in often challenging terrain that had multiple surface obstructions. Our wireless system enabled us to obtain continuous survey data while minimising disruption to local communities, farmers, and traffic.

Our seismic imaging results were combined with other geotechnical data such as borehole logs and wireline logging data to produce a more complete ground model along the tunnel alignment. The ground model helped reduce uncertainty in the subsurface and mitigated the risks associated with the design and planning of the tunnel boring machine (TBM) operations, and the eventual construction of the tunnel.

Innovative highlight

On challenging terrain, wireless surveys are faster and more versatile than traditional seismic recording set-ups, which usually require long lengths of cable. We completed 80 linear km of high-resolution seismic reflection acquisition for this project, and our experts also interpreted the data by identifying and characterising fault zones and geological structure, which allowed for a better understanding and management of potential tunnelling risk.

Impact

Our detailed and continuous characterisation of the ground structure and properties along the tunnel route delivered added value at the design, construction, and operational phases by:

• Confirming the overall geological conditions

• Reporting on faulting and other potential geohazards that could affect the progress of the tunnel boring machine

• Evaluating the geological strata in detail at the shaft and cavern locations

• Developing a comprehensive and robust ground model to support the design, construction and operation of the tunnel and shafts

In support of its sustained commitment to a low-impact approach, Sirius subsequently used our seismic data to reduce the amount of intrusive deep drilling work on the project.

At all times, we respected the environmental and the ecological regulations that applied to the national park in which the site is situated, such as employing a wireless seismic reflection acquisition method to minimise intrusive operations.