Shani Wallis, TunnelTalk

Confidence in the ability of mega TBMs to excavate multi-lane highway tunnels is celebrated today with final breakthrough of the 15.62m diameter EPBM on the Sparvo project in Italy. As well as several technical advances, the breakthrough confirms the decision to apply TBM technology as an alternative to open face methods for the 2.5km long twin tube tunnel through particularly demanding geological conditions.

Giant Sparvo machine breaks through (left tube) to complete main excavation

Leading Italian politicians and members of the client organisation, Autostrade per l'Italia SpA, joined senior managers of tunnelling contractor Toto Costruzioni Generali and its JV partners, Vianini Lavori and Profacta, as well as representatives of TBM manufacturer Herrenknecht AG, and teams of tunnellers and project workers, to mark the milestone occasion. The cutting-edge TBM at 15.62m diameter remained the largest in the world during its Sparvo project application. The final breakthrough is celebrated eight months after starting the second 2,450m drive in November 2012, and less than two years after starting the first tube in August 2011.

It was after award of the Lot 6-7 contact on the new highway between Bologna and Florence that Toto promoted mechanised TBM tunnelling as an alternative to open face excavation for one of the contract's two twin-tube tunnels. The €420 million, 6.6km long contract includes two major twin-set viaducts between the tunnels, and half of the second 3.8km long Val de Sambro tunnel with another contractor working from the opposite portals.

Final breakthrough success

Geological conditions presented significant demands for both open and closed-face tunnelling methods. In a region characterised by major landslides, the geology comprises predominantly clays, claystone, sandstone and limestone with 'floating' ophiolite intrusions recording up to 300MPa in UCS. Added to this is a high concentration of methane gas in the ground.

Where the open face Adeco method is being applied, using excavators in the soft materials, for the longer Val di Sambro tunnel tubes, mechanised tunnelling using closed-face EPB technology and a precast concrete segmental lining was promoted as being safer and more assured of steady progress for the two 2.5km drives of the Sparvo tunnel. To accommodate three 3.75m wide highway-grade traffic lanes and a narrow hard shoulder in each tube, a TBM of record-setting dimension was required and ordered from Herrenknecht. The 15.62m diameter EPBM has ordered in 2010, delivered to site after full factory testing, and assembled for a start on the first tube in August 2011.

"Coming to grips with the shear size of the TBM and dealing with the methane gas hazard were the two greatest challenges for the TBM operating crews," explained Jens Classen, recently appointed Director of the Mechanized Tunnelling Department for Toto. "The TBM is methane explosion protected and it operated at all times in the closed EPB mode to help control the gas."

A 24m long x 1.6m inner diameter screw conveyor, one of the largest ever used in an EPB machine, is one of the central technical features of the TBM. Another is an independent ventilation system that fed fresh air into the outlet of the screw conveyor to the ex-proof conveyor belt channel to dilute high concentrations of methane. "This machine was intended to set new standards," explained Herrenknecht Project Manager Alexander Ell. "Together with Toto and with the support of local authorities, and the Universities of Bologna and Turin, Herrenknecht developed a complex safety system which includes also a fully-enclosed transfer belt conveyor across the backup to the continuous haulage conveyor, a permanent fresh air supply to all areas of the machine, and permanent monitoring of gas concentrations. The system has worked just the way we have anticipated," he said.

Table 1. Technical data of EPBM Shield Martina Excavation diameter: 15,615mm Shield diameter: 15,550mm Installed power: 12,000kW Cutterhead power: 315,000kN Torque: 94,793kNm Cutting tools: 76 disc cutters, 216 cutting knives, 24 buckets, 1 center cutter Screw conveyor: 24m long x 1.64m i.d. Total weight: 4,500 tonne Total length: 130m Segment length: 2m

On reaching final breakthrough, Toto reported top advance rates for the second drive of 24m/day, 126m/week and 406m/month, building rings of 2m wide x 700mm thick precast concrete segments as it progressed. Maintaining finely tuned soil conditioning and haulage of more than 4,200m³ of soil per tube were further challenges for the operating crews and site managers.

After a protracted learning curve, the first drive broke through in July 2012, just short of 12 months from the start. The enormous TBM then had to take a U-turn to drive the second tube in the opposite direction. For the 3,000 tonne shield and four trailing gantries, this was a complex operation achieved in just 15 days.

Working with engineers from tunnelling equipment manufacturer Palmieri of Italy, the solution was a patented air-propelled transporter built of a series of individual air cushions, supplied by banks of air compressors. Once the shield was advanced onto the transporter cradle, the massive load was air lifted by just millimetres and ready to be pulled and guided by trucks across the water covered, welded steel plate surface into its new position at the eye of the second drive. Each gantry was moved in the same way and within 15 days, the machine was reassembled and ready to begin the second drive.

Innovative airmover for the TBM U-turn

Behind it, supply lines for power, soil conditioning, water and the bi-component annular grout mix, came through the first drive and turned to follow the machine back along the second drive. Segments were also transferred through the first tube for relay to the MSVs (multi-service vehicles), which carried a full ring set of segments, at about 180 tonne total, into the TBM. The continuous muck-hauling conveyor system also carried muck from the second drive through the first tube and to the main stockpile for onward disposal.

Now at the end of its second drive, the TBM, named Martina, is ready for new challenges. The €50 million machine and its associated equipment, including segment moulds, remains the property of Toto and its application on other contracts on Italy's extensive programme of highway extensions and upgrades is being actively promoted. "The machine is in very good order," said Classen. "It has been maintained well through its operation and there is little wear on the machine body. The main bearing will be inspected of course as part of the refurbishment process and with little further major expense the machine would be in good shape to start work again. It has proven itself and the TBM method as an alternative at Sparvoand there are current opportunities for new challenges."

For Toto, its next tunnelling project is on the island of Sicily where it is engaged to build a section of new railway partly below the famous resort town of Cefalù, including a new underground station for the ancient city. The contract includes TBM driven twin tube tunnels for a total contract length for the two-track section of about 12km. Having completed the Sparvo TBM project with a high degree of success, Toto has established a reputation as being among the leading TBM tunnellers of the world.

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References

Giant Sparvo EPBM completes first drive - TunnelTalk, August 2012
Giant TBM accepted and heading for Italy - TunnelTalk, December 2010
Record size TBM for highway tunnels in Italy - TunnelTalk, June 2010
Russia confirms order for largest TBM ever - TunnelTalk, August 2011
Tracking the world's mega-TBMs - TunnelTalk, July 2011

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