TECHNICAL | DRILL & BLAST
2,000,000
1,800,000 1,600,000 1,400,000
1,200,000 1,000,000 800,000 600,000 400,000 200,000 0
Total 2024
Total 2023
Above: Comparative tunnelling by contractors in Norway over 2023-2024 INFORMATION COURTESY OF THE NORWEGIAN TUNNELLING SOCIETY (NFF) The Eurock paper adds to this work, primarily by
drawing upon more tunnelling project data. Two new projects were added to the database, with their specific charging (kg/m3
per blast and cut design). The key difference with these projects, though, was to widen the D&B conditions in the database by adding different geology, equipment, operational crew, tunnel cross-sections, and face lengths. The data came from tunnelling contractors via their reporting systems to authorities. The paper also notes that project data, which
considered a total of 353 individual and new blast rounds, show the same trends as found in the previous research: current blast design differs from the principles in the NTNU model by using more charged drill holes per round; plus, higher specific charging. It notes that for road tunnel the increase in specific
change as doubled in some projects, irrespective of contractor, drillhole diameter, drill length or the rock Blastability Index (SPR). A main hypothesis is, as in the prior research, the view that use of bulk slurry emulsion as a type of explosive in Norwegian tunnelling these days is susceptible to “unintentional over-consumption” or waste, compared to cartridge explosives which were previously dominant, around 20 years ago.
) and drilling data (No of charged holes
More project-based research is needed though. But, even before then, it notes that with the current
practice in blast design “seemingly” not regarding the SPV is a “matter of big concern”, and that contractors “elect to use more explosives regardless of rock blastability or rock type.” It queries the possibility that a contribution to this apparent approach is how tunnelling contracts are organised, “favouring high production rates and low cost”. The paper says that higher specific charge should
lead to more rock fragmentation, which may help mucking out but rock quality and usefulness for re0use might be affected. It adds that, if this is so, the NTNU model could be adjusted to be more in line with current practice by adjusting the SPR, appropriately with size distribution compared to the original in the model (half to be less than 250mm sieving size). More broadly, the paper says that the project data
studied gives a “rather large variation envelope” and is bigger that the possible output envelope offered by the current NTNU models. However, the paper notes that the NTNU model deals with averages, not projects and their variations - which is a caution urged upon those who might use the model “for risk analysis or a as a guideline in legal disputes in specific projects in the future.”
REFERENCES ● Gjengedal, S., Jakobsen, P.D. & Nilsen, H. R. F. (2025) ‘Current practice D&B tunnelling versus the
NTNU prognosis model for D&B blast design - a case study’. Eurock 2025, Trondheim, June 2025
● Jakobsen, P.D., Grøv, E., Bruland, A.; Gjengedal, S. (2024) ‘Validity of the NTNU Prediction Model for D&B Tunnelling’. Rock Mech Rock Eng 57, 781–791.
https://doi.org/10.1007/s00603-023- 03585-9
● Jakobsen, P.D., Grøv, E., Bruland, A.; Gjengedal, S. (2024) Validity of the NTNU Prediction Model for D&B Tunnelling. T&TI, Feb 2024
16 | August 2025
m3
Skanska
AF (inc Ghella) Haehre
Veidekke Implenia Metrostav LNS
B&G Tunnel Sotralink Eiffage NCC
Birkeland
Ottar Dvergsdal Aurstad
VTG-Anlegg Flage Maskin BMO
Bleikvassli Hywer
PA Entreprenør Mesta
Spilda Entr. Hardangermaskin
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