TECHNICAL | DRILL & BLAST - COIRE GLAS PROJECT
Q&A The speakers took questions following their presentation.
Q: Were you able to target all of the important underground elements required for the Main Works? Sean: The Exploratory Works targeted structures based on a preliminary Reference Design, which will be refined for the Detailed Design. We have a good body of information for the Adit alignment, but also have the findings from the surface and underground GI. We’re confident we’ve carried out the most detailed GI possible at this stage.
Q: Is it anticipated that the fault zone encountered could intersect with any underground structures? Sean: Review of the ground modelling is still ongoing, but with the fault discovered and its character understood, the position of structures can be reviewed to avoid interacting with it, or appropriate support designed.
Q: Are you doing long-term convergence monitoring? In the SQ section, was immediate support required? Joe: Yes, quarterly monitoring is ongoing. During construction a regime of daily, weekly and monthly monitoring was completed, based upon support class. We continue to have had little movement since the monitoring arrays were installed. In the fault zone, we saw 2mm-3mm of convergence, especially at the base of the horseshoe, before the invert was fully closed, implying immediate support was required.
Q: Were there concerns about tunnel collapse during construction? How did you manage the risk? Joe: Up to the fault zone, there was a lot of confidence that the ground support was suitably robust, given the support class system in use. The fault zone was a genuine concern, given the poor rock, and even with robust initial support installed, the Designer required additional measures for long term stability.
Q: What does Class-SQ stand for? What will the Exploratory Adit be used for long term? Sean: SQ was a code without a specific meaning that stuck. Given the tunnel’s position, the current idea is it will be useful for access, but exactly how is in planning.
Q: Why was a reduced diameter borehole used for the exploratory drilling? Tom R: Borehole diameter was dictated by the size of the rigs in use. The diameters used in vertical drilling on surface weren’t possible with rigs suitable for underground use. We considered HQ (63.5mm-diameter core size) to be sensible, given the equipment, and the distances of drilling required almost entirely in hard rock. This was agreed early on with the Designer, and did not affect the lab testing.
Q: Did surface GI findings affect decisions? Was it possible to interpret the fault zone thickness? Sean: This was a separate package of works, but the Designer received data from both the surface and
tunnelling GI, which was being assessed in real-time, so was able to influence their decisions. The fault zone wasn’t encountered from the surface boreholes, given the location that it intercepted the steep hillside above. Tom R: Some of the underground boreholes were used to target the fault zone, to ascertain its position, with some success.
Q: What type of rock bolts have been installed? If it becomes a permanent adit, will it have to be re-lined? Joe: Almost all rock bolts were fully grouted galvanised steel dowels, ranging from 25mm-32mm diameter, and 3m-3.6m in length. Some self-drilling hollow bars were utilised in the softer rock of the fault zone. GFRP bars were used in some instances, where longer bolts were required, given their reduced weight. Sean: There is no plan for the Adit to be used as a permanent hydraulic tunnel. If it were to be used long-term as an access tunnel, a design review would be carried out.
Q: Have investigations been carried out on the stockpiled material, for re-use? Sean: Yes, we have intentions to re-use the stockpiled material. We have conducted laboratory testing to understand its properties.
Q: Why do you think there was so little groundwater inflow during probing? Is the inflow from surface water? Tom T: The most persistent inflows early on are possibly connected to streams on surface. All the other water encountered would have been from localised pockets, as it dried up after a short time. Joe: Water strikes occurred during the underground GI. Two of the longest boreholes remain open to record inflow. Geological monitoring is ongoing, e.g., with piezometers, extensometers.
Q: What is the expected design life of the Exploratory Adit? Sean: Given the scope of the works, the current Adit design life is short-term.
Q: How far from the Adit was drilling for overcoring tests? Stress regime? Tom R: Overcoring too close to the Adit was avoided, to ensure valid results, which as a rule was taken to be a horizontal depth greater than the excavated span in that location. The testing also had to be targeted to areas that were unfractured, and the shallower the testing the easier the method. Depths were agreed with the Designer on site. The K-ratio and overall orientation were difficult to determine, given the geological setting. Results varied, and need to be reviewed in conjunction with the other underground geophysics, which is currently ongoing by the Designer.
Q: Were boreholes drilled from surface to tunnel level? Sean: Surface GI included a steeply inclined borehole to 650m depth, in the planned location of the surge shaft, drilled from a compound on the top of the mountain, only accessible by helicopter.
20 | February 2026
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