Behind the familiar and reassuring name of “Martina” lies a machine with “monstrous” dimensions and power, capable of digging and covering 5000 metres of tunnels in record time, in the most treacherous part of the Apennines, where abrasive rocks, braking clays and firedamp joints represent real risks, constantly around the corner.
The Tunnel Boring Machine S-574, this is its factory name, was designed and built by the German company Herrenknecht in the explosion-proof EPB (Earth Pressure Balanced) version. Martina, at the time of excavation, the largest milling machine ever produced in the world with a diameter of 15.6 metres, was purchased by Toto Costruzioni Generali for a total investment of 65 million euros.
Too bulky to cross the Alps and cross the motorway tollbooths, it was transported from Baden-Württemberg on the Rhine River and then by sea to Gibraltar and finally to Romagna, where it was used for the excavation of the double barrel motorway Sparvo tunnel, one of the longest and most complex of the entire Variante di Valico, between Florence and Bologna.
The Sparvo tunnel, excavated and covered in an unprecedented time interval (19 months for about 5 km of tunnel) with peaks of record production (24 linear meters dug and covered in a single day), constitutes a unicum, both in terms of timing, size and criticality, in the short history of mechanized excavation, so much so that it has been the subject of several case studies in international specialized journals.
These outstanding results are also due to TBM technology, especially designed by Herrenknecht engineers in collaboration with Toto engineers to overcome the many challenges of the terrain to be crossed.
Everything in “Martina” is gigantic and evokes science fiction scenarios. The steel shield (front of the machine that directly attacks the digging face) in steel, weighs alone 2,700 tons, bringing the entire machine to a weight of about 4,500 tons. The shield completely isolates the inside of the TBM from the surrounding terrain, allowing operators to perform excavation, ring assembly, and maintenance tasks in complete safety.
The milling head of the shield is the component that excavates the ground. On it are 76 cutters, 216 knives, 24 scrapers and a central comb, made of special metal alloys, which make it possible to break up all kinds of soil and convey it to the excavation chamber behind the head. Here, the material is removed completely automatically by means of a 22.70 metre long auger fed by 4 engines with a total power output of 2,000 kW. The shattered ground ends up on the conveyor belt that accompanies it for the entire length of the TBM, about 130 metres, towards the tunnel exit.
Safety is the most important and innovative element of the entire machine. In fact, in order to avoid the pitfalls of firedamp pouches that could be encountered during the excavation, in addition to the sealing of the excavation front, it was also provided with proper sealing, through a containment duct, of the conveyor belt containing the material, to prevent the spread of gas in the areas where the workers are present. A network has also been set up to control the atmosphere inside the machine, which makes it possible to intervene in case the gas concentration thresholds are exceeded.
The main drive unit is driven by 50 hydraulic engines which act on a double-toothed crown, which in turn drives the movement of the milling head. The weight of the main transmission unit, fully assembled, is approximately 400 tonnes. A fundamental characteristic for the optimal excavation is that the machine adopts a counter thrust to the pressure of the soil that is excavated, using the pressure exerted by the crushed soil present in the excavation chamber before it is removed from the auger. This is what technicians call the “balancing of the excavation front”, hence the English acronym EPB.
The use of a TBM also made it possible to install the lining of the tunnel at the same time as the excavation work. A powerful hydraulic erector ring equipped with vacuum suction pads, activated every time the machine moves forward, lifts the reinforced concrete ashlars from the feeder and, remote-controlled by an operator, places them on the tunnel walls, in the ring formed by nine standard ashlars and a keystone. The ashlars were produced by Toto Costruzioni Generali in a special prefabrication plant of 22,000 m³, adjacent to the site. The thickness (70 cm) and weight (16.55 t) of the standard element are the specialty of this plant, among the largest in the world, capable of “churning” an average of 80 ashlars per day, and which has been the subject of an essay by Toto engineers, published by the magazine “Tunnels and Tunnelling International” (March 2013).
Revolutionary and unprecedented, it was also the system adopted to roll the milling head at the exit of the first tunnel and engage it at the entrance of the southern barrel. This technique, designed in collaboration with the Palmieri Group of Silla di Gaggio Montano (BO), using a special mobile cradle supported by air cushions, in addition to allowing considerable cost savings, in just two days has allowed to complete the entire manoeuvre of “U-shaped inversion” and placement of the head of the cutter at the entrance of the second tunnel to be excavated.
Thanks to this endeavour, Italian engineering has obtained a prestigious international recognition. The award 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 “best contractor” and the Sparvo Tunnel “best project of the year”.