DRILL & BLAST - COIRE GLAS PROJECT | TECHNICAL
Left: Three geologists face mapping
PHOTO CREDIT: OLLY PAGE
known as Class-SQ, consisted of a fully circular H-beam, excavated as a staggered top heading and bench, with further temporary support from face bolts and overhead spiles installed in every advance. This design was turned around within weeks of intercepting the fault, and allowed for the H-beams and additional support materials to be ordered before the Christmas shutdown. Initially, probing was the only investigation method
available, which successfully located the fault plane dipping down above the tunnel crown, but was unable to estimate the farthest extent ahead of the face. Tom Robinson, Engineering and Technical Manager with Strabag, detailed how more detailed information was obtained. Strabag mobilised one of sister company Zublin’s drilling crews (who were conveniently already lined up for the later GI works) to do an exploratory borehole at the tunnel face. This provided a full long section of the fault zone that could be logged and its characteristics understood, and also samples that could be sent for lab testing ahead of construction. Amberg also assisted at this stage, utilising their Tunnel Seismic Prediction surveying, its first use in the UK, to generate a 3D interpretation of the fault zone. Despite the safe stop, Joe assured us that Strabag’s
tunnelling team were not inactive at this time, and preparations were required underground before works could restart. A section of tunnel behind the fault zone was enlarged for a passing bay and turning niche - needed for more working room. Additional support was installed at the tunnel face, consisting of reinforced ‘Norwegian Ribs’ and additional bolting. Tunnelling workshops were held regularly
during this period, allowing for optimisations in the construction of the Class-SQ lining to be discussed, and roadblocks overcome. The Adit’s alignment was shifted to allow for the initial excavation to be protected by horizontal spiling, preferable to drilling them inclined each advance. An alternative excavator
February 2026 | 15
was brought in, better suited for the new excavation profile, and injection hoses were proposed instead of steel packing behind the H-beams. Everyone’s objective was to get through the fault zone as quickly, and safely, as possible. Tunnelling restarted in early March 2024, the team
utilising a road header and soft ground support more common in London than in the Highlands. As conditions improved, blasting resumed, and it became suitable to transition to a larger horseshoe top heading and a double length invert excavation. In all, 28m of Class-SQ was installed, before transitioning back into a horseshoe- shaped profile and use of regular support. Despite the reduced advance length and time consuming support installation, the team had managed to average more than 1m/day progress through the fault zone; commendable under the circumstances. On most projects, it would be difficult to be positive
about a three month stoppage due to poor ground conditions, but Sean reiterated that one of the key objectives of the Exploratory Adit was to characterise the ground conditions. Therefore, having a better
Left: Driller’s inside view with Boomer in action PHOTO CREDIT: JOHN ZAMMIT
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