Tunnel planning through geoscanning - case studies from India and Norway

Introduction

Unforeseen, challenging ground conditions are a major obstacle for infrastructure development, including tunnel construction. Addressing this risk with traditional, intrusive ground investigations can be costly, sometime prohibitively so. In this paper, we present airborne geoscanning, a more efficient site investigation method that integrates airborne geophysics with other datasets to produce ground models. We primarily employ helicopter-based time-domain electromagnetics (AEM), a method that images differences in electrical resistivity in the subsurface.

When available, we can combine geophysical data with ancillary datasets for more sophisticated interpretation, an integrated process we call airborne geoscanning. Integration techniques range from simple clustering analysis that support planning of follow-up ground investigations to customised artificial neural networks that automatically detect interfaces like top of rock. Using examples from projects in Norway and India, we illustrate the strengths and weaknesses of using airborne geophysics for tunnelling projects. Using these case studies, we demonstrate three key insights that airborne geoscanning can provide to tunnelling engineers: identify major fractured zones, weaker rock units and rock cover. These insights can be highly valuable for tunnel design and construction projects worldwide.

Conclusion

Using geoscanning in early or late-stage phases of tunnelling projects, ensures that valuable insight into study areas can reveal potential zones for follow-up investigations. The four case studies presented here have shown the possibility to detect major fracture zones, weak rock units, and insufficient rock cover. This led to better planning of drilling campaigns and earlier optimizations and re-design tunnel alignments and tunnel support design. By having the opportunity to address these issues early in the planning phase before large investments have been incurred reduces the risk of cost overruns. Even when projects are in a later stage or tunnel alignment modifications cannot be avoided, a more precise estimation of costs and geological risks are still valuable for ensuring financially sound and safe construction.

Dedication and Acknowledgement‍

We dedicate this paper to the memory of Mr. Sanjeev Malik who went to his heavenly abode due to COVID-19 during the preparation of the manuscript. Mr. Malik, Executive Director of India’s National Highways and Infrastructure Development Corporation Ltd, was forward thinking with a keen interest in EM methods and the possibilities to use data in large infrastructure projects. We know Mr. Malik as a very intelligent, pleasant, and kind man. He will be sorely missed, and the loss is felt heavily by all.

We thank Bane NOR (Norwegian national railway infrastructure company) for permission to publish results for the case study in 3.1, and extend special thanks to Agnethe Hoff Finnøy, Inger Lise Ullnæss, and Hiruy Ghidey Hishe for their assistance in preparing the geotechnical data.

References

The full paper can be requested through the download link above or found directly at www.iopscience.iop.org.

Rasmussen, A. H., Linares, G. M., Christensen, C. W., Malik, S., Skurdal, G. H., & Pfaffhuber, A. A. (2021, October). Airborne geoscanning as a site investigation tool in large-scale tunnelling projects: A synthesis of case studies from Norway and India. In IOP Conference Series: Earth and Environmental Science (Vol. 861, No. 4, p. 042039). IOP Publishing.