Working in this region is a war against the ground. At different times we have had to deal with different ground conditions such as a mix of lava-breccia, or only lava, or only breccia. It was hard to drive the TBM and work in these conditions,” says Vittorio Lo Faro, a civil engineer at SWS Engineering, who visited the Nesima-Misterbianco and Stesicoro- Airport lots of the Circumetnea rail project.

The project is located in close proximity to Mount Etna, the iconic volcano that dominates the landscape near Catania in Sicily and will upgrade the existing metro from narrow gauge track to a modern, broad gauge electric line. The total length of the completed project will be 10.8km. Tunnelling work was undertaken with a 10.6m-diameter Herrenknecht EPBM, a first in Italy for driving a machine of this diameter through lava.

Due to this paucity of data, the machine was driven entirely manually, without any automation of TBM parameters.

Lo Faro adds, “As well as the geological investigations that were undertaken in the early design phases, and those integrated in subsequent phases, during the actual excavation, further investigations to ascertain the correct geological profile have been required.

Along the tunnel alignment, we have found a large number of geological changes (with huge variability between interfaces) and we had to manage them in order successfully drive the TBM. That was one of the main challenges of this project.”

This is because in mechanised tunnelling, it is usually possible to set the advance parameters of the TBM for homogenous, discrete stretches of about 300-400m of advance without any adjustment required by the operator. However, it was not possible to do this with the high geological variability of Catania. “To overcome this issue, we effectively had to live on the TBM, as many aspects can’t be evaluated from the PLC. You need to be on board,” says Lo Faro.

Nicola Bona, technical director at CMC adds: “At the beginning, the use of a TBM was not expected by the client, but CMC Ravenna tendered and won on this basis. If we had adopted a more traditional method [mechanical hammers – Ed] it would have taken much more time to work through these geological conditions.”

SITE SETUP

The first 75m of the tunnel were excavated with a mechanical hammer to make space for TBM assembly and launch.

It was also necessary to have large areas available for all activities associated with a mechanised excavation including muck storage, segmental lining storage, concrete plant and water supplies.

A site location area was set within the Catania’s commercial district of Misterbianco, which minimises the impact of the jobsite on the population.

NESIMA-MONTE PO SECTION

The first section to be visited by T&T, the Nesima-Monte Po stretch of the project comprises a single-tube tunnel with a double track, which runs for 1.75km.

The tunnel has an excavation diameter of 10.6m, a lining external radius of 10.24m, a lining internal radius of 9.6m and lining thickness of 0.32m.

This project was set to see the first use in Italy of a mixed reinforcement solution of Bekaert-Maccaferri fibres (with a rebar frame) for segments of the final lining. The technology was recently approved by the Consiglio Superiore dei Lavori Pubblici (Italy’s High Council of Public Works). However, due to bureaucratic issues relating to local approval, traditionally reinforced segments have had to be used thus far, with fibrereinforced concrete in use as a temporary lining.

To ensure water-tightness of the tunnel, Fama cast-in gaskets were chosen.

The tunnel will be excavated with a Herrenknecht EPB for the whole length of 1,748.08m. Palmieri supplied the cutters and the launch frame.

The launch shaft is located at the Misterbianco end of the drive in the industrial/commercial area, from which the TBM will navigate to a reception shaft near Nesima. A stub tunnel for the launch of the TBM was under construction as T&T visited site.

Site setup and machine supply

Lo Faro says: “There are six muck storage areas with a total volume of 11,500cu.m, this is spacious enough to avoid stopping the construction of the tunnel once started.

The mixing plant includes several silicates, and the purification plant.

The grouting mix to be used for backfilling the TBM excavation, is a bi-component grout, which includes the A component (retardant) and the B component, accelerant so that there is a reduced gel time during injection.

The mix comprises cement, bentonite, water and the accelerant.

Marti supplied the belt conveyor and its structure, which runs the full length of the tunnel to the muck storage area, going vertical in the shaft with buckets.

In terms of spoil, Lo Faro says, “We use a type of foam, derived from biodegradable surfactants,” he says. “Thus, we need to follow a decay time for these agents. These specific sites are placed near a prefabricated plant and then, we use a tyred vehicle to transport the spoil to a final site 5km from here. The first 75m of the tunnel has been excavated with the traditional method, also relying on several interventions of fibreglass reinforcement and traditional steel ribs double IPN 160. This excavation section is between 120 and 130sqm. The length of this temporary tunnel is necessary to avoid any obstruction issues with the TBM, which needs 75m available in length to work (the full length of the machine is 96m).

“The excavated area area of 120- 130sq.m was required for the siding platform, which needs a mobilisation of cutterhead, placement of counterforce structure to launch the machine. As well as the excavation profile, we needed a bigger section to avoid the obstruction of these external pieces.”

“The geological composition includes lava breccia, which in turn comprises sand, sandy loam and lava,” Lo Faro says. “We have simply excavated with a hammer in cylindrical section and tromboconic.”

“We put at work a Herrenknecht EPB TBM, equipped with a shield of about 10m of length,” Lo Faro says. “We have chosen an EPB because Catania ground is not homogenous. Most of the excavated ground was lava breccia, so this bulk material couldn’t be excavated with other types of TBM. We also have clays and sandy loams of volcanoclastic nature.”

STESICORO

“To date the excavation of the first part of the tunnel, near the launch shaft, is ready and we are going to start TBM assembly,” Lo Faro says.

“The alignment of the main tunnel is entirely underneath the urban city centre of Catania, going underneath many historical building along its 2.3km of length and it finishes towards Stesicoro square, one of the most important square of the city.

“The geological formation and the average overburden of the tunnel is similar to the Nesima project. However, the water table is higher.

The lining of the tunnel is the same of Nesima with a 6+1 segment configuration, a length of 1.50m and 320mm of thickness. A temporary lining has been carried out using fibre-reinforced concrete segments. The temporary lining will be demolished for the construction of the other parts of the work: station enlargement, aeration shaft, and so on.