By rosebell
Being published by Unst and Yell Tunnel Action Groups, the -seismic report of Shetland Islands includes site visits in Shetland during June, and reviews earlier investigations into the feasibility of subsea rock tunnels across Bluemull Sound and Yell Sound, while providing the groundwork for the crucial next step, seismic surveys.
As the international engineering consultancy responsible for the design of all subsea tunnels in the Faroe Islands, as well as many in Norway, the Norconsult was responsibile for advancing pre-seismic work.
Involving seabed surveys and geological mapping, this report reviews previous geological studies, carried out between 2000 and 2002, whereas these studies identified potential challenges, including sediment layers, rock quality variations, and fault lines. Based on updated international best practice and site visits, Norconsult has now produced new indicative alignments with a gentler slope of five per cent (compared to eight per cent in older studies).
Despite lengthening the tunnels, this change makes them more suitable for modern design standards and future transport use.
The report highlights that:
- Bluemull Sound has complex geology and adjustments to alignments may be needed to find the best rock mass quality.
- Yell Sound shows deeper rock structures and significant fault zones. The most promising alignment is likely under Bigga Island, though this will need confirmation through further studies.
- On land, refraction surveys are recommended to assess soil thickness and rock mass quality, particularly in tunnel portal areas. Initial surveys on land and sea will inform potential tunnel alignments and entry points. Ground Penetrating Radar (GPR) can identify soil cover but not rock mass quality.
- Existing shallow seismic data is good, but new surveys are required to update and extend the dataset, particularly in Bluemull Sound.
- More detailed mapping, including bathymetric surveys, shoreline geological mapping, and eventually core drilling,will be essential to refine possible routes.
According to Unst Tunnel Action Group joint chair Alec Priest: “The campaigners were delighted to have completed “this important step in the process. It shows how the money raised by our communities is being used and gives everyone the chance to see how the project is progressing. Seismic surveys are the next crucial stage. They will give us the detailed data on the seabed and rock structures that we need before any firm decisions can be made about alignments and help refine costings for potential fixed links.”
He also added: “The groups had most of the funds for consent support work and the seismic surveys. Just a small funding gap remains, which we hope to fill through continued community effort, local business support, and upcoming events.”
Tags: # Shetland Tunnels # Pre-seismic survey # Unst and Yell Tunnel Action Groups # Scotland # Tunneling
