Mega TBM success at Caltanissetta in Sicily 09 Nov 2017

Patrick Reynolds for TunnelTalk

After two years of tunnelling below an urban area in Sicily and overcoming geological challenges, bored excavation for the Caltanissetta highway tunnel using a single 15.8m diameter EPB TBM manufactured by NFM is complete.

Breakthrough celebration as TBM finishes Caltanissetta Tunnel in Sicily
Breakthrough celebration as TBM finishes Caltanissetta Tunnel in Sicily

The new tunnel is part of two lots let by the national highways authority ANAS to teams led by Italian contractor CMC to upgrade the SS640 motorway, which runs about 62km between Caltanissetta in the middle of the island and Agrigento on the south coast.

Construction work on the northern Lot 2, which includes the mega-TBM bored tunnel and main civil engineering challenges on the entire project, started about five years ago, in late 2012. The twin-bored tunnel is about 4km long and runs under the city of Caltanissetta.

On Lot 2, CMC is leader with 82% within the Empedocle 2 special purpose company set up with Consorzio Integra to execute the upgrade works, including design and partial finance.

The design developed a twin-tube layout with the parallel tunnels at between 35m and 85m apart. CMC Construction Director, Roberto Leonardi, explained to TunnelTalk, as the large TBM was close to finishing its last bore, that the Caltanissetta tunnels run through complex geological and hydrogeological conditions.

Mega TBM completes first road tube at Caltanissetta
Mega TBM completes first road tube at Caltanissetta

These include clay and marl deposits, faults towards the ends of the alignment, squeezing ground including under the city, and a 200m long stretch of expected, highly fractured limestone of calcareous formations of both permeable and non-permeable strata, with the risk of high groundwater inflows. While the overburden was typically up to 120m, cover over the limestone stretch was about 90m with a groundwater head of 60m to 70m.

Dewatering efforts on the alignment through the limestone involved sinking wells between the tunnels, which drew down the groundwater level to a head of approximately 40m. Performance of the 13.45m i.d. segmental lining is designed to withstand the full hydraulic load of the recovered groundwater table for the long term operation of the underground highway. The 14.65m o.d. diameter segmental lining is formed of 2m long rings comprising eight 600mm thick segments plus a key, each with an average weight of approximately 16 tonne.

NFM mega TBM ready for dispatch in 2013
NFM mega TBM ready for dispatch in 2013

After launch on its first drive in June 2014, the mega NFM EPBM, with a cutterhead rotation of up to 1.9rev/min and a maximum torque of 73,300 kNm at 0.9rev/min, ramped up production through the learning curve to achieve peak rates of 28m or 14 rings/day after four months. The first, 3,878m long tunnel drive was completed in October 2015.

In April 2016 the EPBM was relaunched on the parallel tunnel and was half way through the 3,993m drive by October when it met another unexpected calcareous limestone formation, this time 120m long and through which dewatering wells from the surface were not possible to reduce the 90m hydraulic head.

The solution was to use the completed parallel tunnel to bore a series of 40m-50m long, sub-horizontal drainage pipes into the problem zone ahead of the TBM. The groundwater level dropped sufficiently to allow the shield to continue under 5-6 bar pressure.

By February 2017, the TBM had less than 1km to excavate and completed the drive in mid-June, shortly after Leonardi presented on aspects of the Caltanissetta construction experience to the Swiss Tunnel Congress in Lucerne.

Second, and final, breakthrough at Caltanissetta twin tube road tunnel
Second, and final, breakthrough at Caltanissetta twin tube road tunnel

Over the entire project involving almost 8km of tunnel boring, and despite the problems with the limestone zones of 520m in total, across in both tunnels, the contractor achieved a best advance of 16 rings or 32m/day and an average of 18m or nine rings/day, Leonardi reported to the conference. The large TBM was completely dismantled by mid September, and a buy-back option was being negotiated with the manufacturer NFM, Leonardi told TunnelTalk.

With service life of the tunnel being of vital importance in the high hydraulic load environment, the concrete mix design for the segmental lining used basaltic aggregates from lava stone quarries, combined with a calcareous matrix filler to reach a C55/67 performance class, which is greater than the C45/55 class required in the specifications.

Achieving the higher class almost doubled the service life to 90 years. To optimise segment manufacture the single mix design was used for the lining of the full length of the tunnels under different load conditions. Tests illustrated that the high tensile strength produced by the mix reduced segment spalling during handling and installation and that the basaltic aggregate offered stability against high temperatures and would achieve high fire resistance, said Leonardi.


Other mega-TBM highway tunnel projects in Italy:

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