DRILL & BLAST - COIRE GLAS PROJECT | TECHNICAL


understanding of the location and nature of this fault zone prior to Main Works construction will benefit the project in the long run. In fact, SSE were so happy with the data being obtained from the Exploratory Works that, in May, they took the decision to extend the Adit by 300m, and instructed an additional two drilling galleries. The data from the Adit was being reviewed in real-time to verify assumptions against the Reference Design for the Main Works, and the extension provided an opportunity to obtain quality tunnelling GI all the way to the depth of the proposed turbine caverns. There was also a cost-time benefit with the upcoming


underground GI works. An extra couple of months of tunnelling actually reduced the number of long exploratory holes, and opening up more work areas allowed for simultaneous drilling. It also provided a much better spread of locations for in-situ stress testing, and closer to the caverns themselves. While the extension to the Exploratory Adit came as


somewhat of a surprise to the team, it would be just another challenge to overcome, and quickly. Of the 176m of tunnel remaining before the first drilling gallery, 153m of that was constructed in that April alone, the best month on record. Building on from the team dynamic of the fault zone, everyone had their responsibilities, and had to understand each other’s constraints. A new alignment was required, which Stantec-Cowi had to thread through the area of the future main work structures, to avoid later clashes. Strabag had to secure labour, support materials, explosives, and sufficient plant and equipment to facilitate the extra tunnelling. SSE had to coordinate the effort, ensuring they were getting value for money from the extra work. The design ended up including seven different profiles,


up to 8m x 6m, and included a much tighter radius bend to site the last gallery in the optimum position. No matter, just three months later, on 1st August 2024, advance 502 was blasted, the last on the Exploratory Works. Tom T took this opportunity to run through a summary


of all the tunnelling GI that was obtained from the Adit’s construction. The vast majority of the tunnel, almost three quarters, was supported as Class-B (or a variation thereof) with a Q-value of 1.0 to 3.0. Water inflow included strikes up to 30 l/min, but thankfully all but half a dozen were finite, and petered out within an hour. The stitched together LiDAR scans provided an excellent


fly-through of the entire tunnel, in its unsupported state, and can be used for analysis of joint orientations, along with assisting the construction team with space proofing. All the MWD data has also been collated, which shows the relative hardness of the ground throughout the adit. It was Tom R’s responsibility to organise and oversee


the second phase of the works, the Underground GI. This looked very different from its original scope after the extension, but at least the constraints of the drilling equipment had been properly understood while the extension design was evolving. Strabag has the benefit of Zublin, also operating in the UK, and their first rig commenced drilling within weeks of the tunnel’s completion. The scope now included almost 2km of


February 2026 | 17


wireline and conventional exploratory core drilling, along with a host of in-hole geophysical testing, all to target the planned location of key underground structures. Strabag’s geologists continued to be busy underground,


as all the core samples were logged and photographed to exacting standards, before being transported to surface, to minimise disturbance. A large contingent of sub- contractors assisted with the specialist geophysics. There are too many to mention all but Robertson Geo deserve special recognition for their collaboration with Zublin to undertake tests in non-conventional shallow and even uphill inclinations. Plate jack tests were also undertaken, by Sol-Experts, in specifically constructed niches in Gallery-B and Gallery-C. These tests involved putting huge forces into the tunnel walls, through a large hydraulic ram, and measuring displacement within the rock mass.


Class-SQ horizontal spiling sketch


PHOTO CREDIT: JOE COXSON


Above: Tunnel extension hand sketch PHOTO CREDIT:


STEFAN TRENKWALDER


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