TECHNICAL | DRILL AND BLAST
2 D&B PLUS TRADITIONAL MINING AT MONT CENIS
An immense amount of drill and blast, and traditional, tunnel excavation is needed to help create the 57.5km-long Mont Cenis Base Tunnel, which is the core feature of the Lyon-Turin project crossing the Alps across the French-Italian border. The rail project is a strategic link in the growing trans-Europe rail network (TEN-T). About 67km of drill-and-blast and traditional tunnelling will be undertaken on the project and of that total almost 14km, or one fifth, had been completed by early January, project client Tunnel Euralpin Lyon Turin (TELT) tells T&T. The 67km represents nearly 40% of all excavations on the project, it adds, and is to be completed by 2028. The majority of the drill-and-blast and traditional tunnelling works
are focused on a number of key locations on the project, with totals (and progress) given by TELT, as follows: FRANCE ● Portal: Saint-Julien Montdenis = 6km (0) ● Adit: Saint Martin la Porte = 18.5km (6.5km) ● Adit: La Praz = 9.8km (2.8km) ● Adit: Villarodin-Bourge/Modane = 21.2km (4.2km) ITALY ● Adit: Chiomonte = 11.2km (0.2km) ● Portal: Susa = 4km (0)
While these tunnelling works continue across the multiple sites, with excavation cross-sections typically ranging from 127–147m2 (but some are much smaller while a few are far larger), the project lots will have significant tunnelling also performed by a further seven TBMs. The shields will start boring from launch caverns opened up by drill and blast. At peak tunnelling activity, the project is to have 15 excavation faces, including the TBM drives. Further, more localised works for the traditional methods include
carving out 22 interchange niches, each 30-40m long, at the sides of the existing La Maddalena exploratory tunnel in Chiomonte, Italy. The 3m-deep niches are needed after a main construction plan changed by shifting tunnelling starts from Susa to Chiomonte. The La Maddalena tunnel was previously bored by a Robbins open gripper TBM. Total tunnelling to create the twin tubes is 115km, and including
the earlier exploratory adits the whole project will have 164km of tunnel excavation.
Three major tunnelling lots were awarded last year on the French side and one lot remains to be awarded in Italy. All four lots involve significant amounts of drill and blast, and traditional, tunnelling as well as mechanised boring.
Above left: Around 40% of the Lyon-Turin tunnel is by drill & blast, and traditional methods... SOURCE/CREDIT: TELT/ CAROLINE MOUREAUX Above right:… and around a fifth (14km) of those works was completed by early January 2022 SOURCE/CREDIT: TELT/ CAROLINE MOUREAUX
“For projects adopting an unlined design concept,
the final design can only be settled during the construction stages, from stress measurements inside the tunnels,” says Ødegaard. “Hence the focus on the stress measurement techniques in such settings.” But the principles apply not only to hydraulic pressure
tunnel linings but indeed to all types of project, tunnels and excavation methods, as well as tunnel depths, he tells T&T, for the central idea is to measure stresses at far more locations. “Relying on overburden as an indicator of minimum
principal stress has – over and over – proven to be inadequate for pressure tunnel design – he says. “I have developed the RSRT in an effort to facilitate greater numbers of tests, such that we can measure stress, rather than guess it.”
40 | February 2022
The key, says Ødegaard, is to have many more
sampled data points distributed in the rock mass along the alignment axes. The result is improved interpolations and interpretations of the stress state between the data points, from borehole tests, he adds. Typical test regimes presently use more advanced
test protocols, with relatively few testing locations, or alternatively a variety of informal hydraulic jacking test protocols to estimate the minimal principal stress. These supplement the broad estimates of rock stress based on overburden depth. The RSRT method, instead, uses an ordinary jumbo to drill 1 – 3 boreholes at regular intervals along the tunnel and extending at least two times the tunnel diameter to get beyond the near-field stress caused by tunnelling to reach unaffected rock mass. Testing can be close behind
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