Behind the familiar and reassuring name of “Martina” hides a machine of “monstrous” dimensions and power, capable of digging and lining 5,000 metres of tunnel in record time, in the most treacherous stretch of the Apennines, where abrasive rocks, braking clay and firedamp leaks represent real risks, constantly around the corner.
Too cumbersome to cross the Alps and pass the freeway toll booths, it was transported from Baden-Württemberg on the Rhine River and then by sea, rounding Gibraltar to Romagna, where it was used to excavate the twin-tube freeway tunnel in Sparvo, one of the longest and most complex of the entire Variante di Valico between Florence and Bologna. The Sparvo tunnel, excavated and lined in an unprecedented amount of time (19 months for about 5 km of tunnel) with record production peaks (24 linear metres excavated and lined in a single day), is unique in the short history of mechanised excavation in terms of time, dimensions and criticality, and has been the subject of several case studies in international specialist journals.
These exceptional results are also due to the technology of the TBM, specially designed by Herrenknecht engineers with the collaboration of Toto engineers to overcome the many challenges of the terrain to be crossed.
Everything about “Martina” is gigantic and evokes science fiction scenarios. The steel shield (the front part of the machine that directly attacks the excavation face) alone weighs 2700 tons, bringing the entire machine to a weight of about 4500 tons. The shield completely isolates the inside of the TBM from the surrounding soil, allowing operators to perform excavation, ring assembly, and maintenance activities in complete safety.
The shield cutterhead is the component that excavates the ground. On it 76 cutters, 216 knives, 24 scrapers and a central comb, made of special metal alloys, allow to crush any type of soil and convey it into the excavation chamber, behind the head itself. Here the removal of the material is completely automated by means of a 22.70 metre long screw conveyor, powered by 4 motors with a total power of 2,000 kW. The crushed soil ends up on the conveyor belt that accompanies it for the entire length of the TBM, about 130 meters, towards the exit of the tunnel.
Safety is the most important and innovative element of the entire machine. In fact, to avoid the pitfalls of pockets of firedamp that could be encountered during excavation, in addition to sealing the excavation face, the same conveyor belt containing the material was sealed in a containment duct to prevent the spread of gas in areas where workers are present. A control network for the atmosphere inside the machine has also been set up to allow intervention if the gas concentration thresholds are exceeded.
The main transmission unit is driven by 50 hydraulic motors acting on a double-toothed crown which, in turn, drives the movement of the milling head. The weight of the fully assembled main drive unit is approximately 400 tons. A fundamental characteristic for the optimal performance of the excavation is that the machine adopts a counter-thrust to the pressure of the soil being excavated, using the pressure exerted by the crushed soil in the excavation chamber before it is removed by the auger. This is what engineers call “trench face balancing,” hence the acronym EPB.
The use of a TBM also allowed the tunnel lining to be laid at the same time as the excavation work. A powerful hydraulic erector ring equipped with vacuum suction cups, activated at each advancement of the machine, lifts the reinforced concrete segments from the feeder and, remotely controlled by an operator, positions them on the tunnel walls, in the ring formed by nine standard segments and a key segment. The segments were produced by Toto Costruzioni Generali in a 22,000 sq. m. prefabrication plant adjacent to the site. The thickness (70 cm) and weight (16.55 t) of the standard element are the specialty of this plant, one of the largest in the world, capable of “churning out” an average of 80 segments per day, and which was the subject of an essay by Toto engineers, published in the magazine “Tunnels and Tunnelling International” (March 2013).
Revolutionary and unprecedented was also the system adopted to rotate the milling head at the exit of the first tunnel and engage it at the entrance of the South tube. This technique, designed in collaboration with the Palmieri Group of Silla di Gaggio Montano (BO), using a special mobile cradle supported by air cushions, as well as allowing significant cost savings, in just two days allowed the completion of the entire “U-turn” maneuver and placement of the cutter head at the entrance of the second tunnel to be excavated.
Thanks to this feat, Italian engineering has obtained a prestigious international award. Recognition came from the International Tunnelling and Underground Space Association (ITA-AITES) and the British Institute of Civil Engineering (ICE), which in London, in November 2013, awarded Toto Costruzioni Generali as “best contractor” and the Sparvo tunnel as “best project of the year”.
