DRILL & BLAST - COIRE GLAS PROJECT | TECHNICAL
for the support class system to work, it was vital that each advance was properly captured. The agreed- upon Q-value determined by the geologists dictated the ground support to be installed. The geologists were also empowered to instruct any ‘toolbox measures’ - pre-authorised in a Required Excavation Support Sheet (RESS) - if they deemed any features required additional support. The Works were procured on NEC3 Option B
contract, priced with a Bill of Quantities, enabling scaling to match the installation of each support class. Also explained was a rundown of the blasting
methodology in use, of interest to those who’ve not yet tried their hand at hard rock tunnelling. Given the tunnel’s length, it was more economical to use cartridge emulsion explosives, which were initiated with a conventional non-electric system. A standard 3m tunnel advance had 66no. charged
holes, and 4no. void holes around a ‘burn cut’, using roughly 180kg of product. The powder factor here was on the higher side, but given the Shaeff tunnel excavator in use had an apron and conveyor system, good fragmentation was key. Detonators ranged in delays from 0ms to 7000ms, dictated by the blast pattern in use, and all were initiated simultaneously using bunch connectors. More interesting techniques were also required for
some of the different tunnel profiles. A wedge cut was successfully trialled for enlarging an area for a passing bay, with two wider perpendicular advances, followed by a section of slashing. Smooth blasting was also required for some of the in-situ stress testing later in the works - not easy in the highly deformed geology. The larger profiles of the drilling gallery
Design excerpt of support Class-B design
PHOTO CREDIT: STANTEC-COWI
required 140no. charged holes, and upwards of 300kg of product per blast. Tom T took over here to give an overview of the
importance of not just the face mapping, but all of the geological deliverables that were an output of the Adit’s construction. Given the scope of the Exploratory Works, he pointed out, the data collected along the way was as important as the construction itself. Along with the mapping records, completed by both the Strabag and the Stantec-Cowi geologists, there
Above: Geologist
undertaking face mapping PHOTO CREDIT: THRUDUR HELGADOTTIR
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