A rebuilt Robbins 3.5m diameter main beam TBM has encountered and successfully passed through an unexpected cavern during its drive on the Galerie des Janots water tunnel project in La Ciotat, France. The machine, launched in 2017 by contractor Eiffage Civil Engineering, was 1km into the drive when the cavern, studded with stalactites and stalagmites and measuring 8,000m³ was discovered to the left of the planned route.

Uncovered cavern measures 8,000m³ in size

“We hit the corner of the cavern and had to erect a 4m high wall for the grippers of the TBM to react against as it progressed across the void,” explained Marc Dhiersat, Project Director of Galerie des Janots for Eiffage. A small door allowed access into the cavity, which formed naturally at a point 60m below the surface. The TBM was able to successfully navigate out of the cavern in eight strokes and without significant downtime to the operation.

“It is certainly unusual to come across a cavern of this size, although the geology of karstic and volcanic formations has the potential for underground cavities,” said Detlef Jordan, Robbins Sales Manager Europe. Karst cavities were a known risk during the bore, but the cavern was not shown in vertical borehole reports conducted from the surface along the alignment.

4m high concrete wall allowed the TBM to cross

The TBM has a further 1.8km to excavate before the 2.8km tunnel is complete. As it is possible to encounter further caverns, the machine is fitted with a geotechnical BEAM (bore-tunneling electrical ahead monitoring) system to predict ground conditions using focused electricity induced polarization.

Early in the drive, the TBM and its crews encountered difficult ground conditions consisting of limestone with powdery clays which required resin-anchored bolts and steel arch rings as ground support, topped with a 10-15cm thick layer of shotcrete. After five months of managing through poor ground conditions, Eiffage is optimistic that conditions will improve and the tunnel will be complete in the next four to five months.

Galerie des Janots is one of 14 projects being carried out to save water and protect resources by the Aix-Marseille-Provence metropolis, the water agency Rhône Mediterranean Corsica, and the State Government. The Janots tunnel will replace old deteriorating pipelines located in a railway tunnel which currently contribute to an estimated water loss of 132 million gallons of potable water/year.

The completed tunnel will pass under Le Parc National des Calanques, with cover between 15m and 180m and will increase water carrying capacity. “The current pipes transport 330 litres/sec, which is not enough during the summer period. It is hoped to increase capacity to 440 litres/sec,” said Dhiersat.

After seven previous successful projects, in Europe and in Hong Kong, a 3.5m diameter Robbins TBM is commissioned and launched to undertake its eighth tunnel drive for a water project in France.

Robbins TBM ready for its eighth tunnel project

After a modernization and upgrade rebuild, the TBM was commissioned and launched in early March by French contractor Eiffage Civil Engineering for a 2.7km drive for the Galerie des Janots water supply project in La Ciotat, France.

"The machine will work 24 hours a day for almost 10 months to complete this project,” said Marc Dhiersat, Project Director of Galerie des Janots for Eiffage.

The TBM will bore primarily through limestone to excavate the Janots gallery to improve access to water in the Cassis, Roquefort-la-Bédoule, La Ciotat and Ceyreste communities east of the Aix-Marseille-Provence metropolis. The 2.7km long drive will pass under Le Parc National des Calanques and beneath a cover of between 15m and 180m.

“Limestone is a rock easy to excavate,” explains Loïc Thévenot, Director of Underground Works for Eiffage, “but there is the possibility of being confronted with the phenomenon of karst features.” To cope with the risk, the TBM is equipped with a probe drill and equipment to backfill small karst features. “If the feature is large, we will erect a small parallel gallery,” said Thévenot.

Eiffage celebrates launch of the rebuilt TBM

Since its launch in March, the machine has bored more than 51m and is ramping up to speed as back-up gantries are added.

The Galerie des Janots is part of a project to save water and protect resources being carried out jointly by the Aix-Marseille-Provence metropolis, the water agency Rhône Mediterranean Corsica, and the State Government.

The future Janots gallery will replace existing pipelines currently located in a railway tunnel. These have significant safety and vulnerability deficiencies and currently suffer losses of an estimated 132 million gallons of potable water per year.

“This project is an investment of €55 million,” said Danielle Milon, Mayor of Cassis at a TBM launch ceremony in March. “The objective of the new tunnel is also to increase the water supply capacity from the current 330 liters per second, which is insufficient in the summer period, to 440 liters per second, as well as provide easier access for maintenance compared to the old pipes currently running beneath the railway.”