Blast Tunnels with Centimetre Precision
Published 2010-01-29 15:16:44 | Updated 2010-01-29 15:16:44
At the Äspö Hard Rock Laboratory near Oskarshamn, tunnels are built with centimetre precision in order to test the technology for terminal storage of spent nuclear fuel. Bergteamet has built an 80 metre long test tunnel at a depth of 450 metres.
- We have selected bedrock which we believe has crack zones in contact with sea water. This creates a very high water pressure of 45 Bars. We are investigating methods for building a water-tight tunnel meeting high security and documentation requirements, says Rickard Karlizen, SKB.
The tunnel is built to meet the requirements concerning repository tunnels for terminal storage of spent nuclear fuel. The subject is highly topical as politicians are currently debating the future of nuclear power. This year SKB will choose a terminal storage location and will then submit its application. The application is assessed by the Environmental Court of Sweden and the government. The test tunnel has a key role in the assessment of the method’s safety.
- Precision and documentation requirements in this project were very high, but consequently the working pace was relatively low. Because of this, it was a real art to keep staff motivated. We are used to working at a very high pace when we carry out our mining and tunnelling work, says Stellan Arvidsson, site manager.
During the project procurement, great importance was attached to the proficiency and experience of the site manager and the staff. Björn Stjernström, from Zinkgruvan and Daniel Lignell, Örebro, both are highly experienced Bergteamet employees, were appointed as jumbo operators.
- Qualification was important for us because we had to document the process and the planning of the work in order to correctly analyse and assess the applied methods, says Rickard Karlzen.
Mining and tunnelling
Mining and tunnelling was initiated by injection of sealants through fan-shaped bore holes in order to fine seal the cracked bedrock. The sealant we used was Silicia Sol, a siliceous substance that hardens thanks to a salt. The pressure of the sealing materials was 90 Bars, two times higher than water pressure.
During the next stage, the bedrock was sealed via horizontal drill holes in longitudinal direction within the tunnel outline. Drilling outside the tunnel profile was completely avoided in order to prevent affecting the bedrock as much as possible. Centimetre precision was applied when positioning the drilling rig. Manual alternate measurements were made afterwards to check the drill holes and inspect if there were any deviations before loading.
The tunnel area is 19 sq m. Up to 97 drill holes with a length of up to 4.6 metres per round were used for blasting. The hole diameter used was 48 mm, a rough dimension where the holes were adjusted to the cartridge charge. The result was a careful “soft blast”. The goal was to drill a tunnel with no more than 30% overbreak, i.e. breaking outside the tunnel profile. The high precision resulted in 15% overbreak, which is a good result.
- We would like to have a tunnel profile which is as smooth and even as possible, says Rickard Karlzen.
- We have a good record with SKB and we regard the assignment as an important reference job for Bergteamet, says Stellan Arvidsson.
Leave a comment