After driving into uncharted natural caverns on its 2.8km drive, a Robbins 3.5m diameter gripper TBM broke through to complete the drive in early April. Twice during its progress the TBM ran into naturally occurring voids, the largest of which measured a voluminous 8,000m3 in size. The TBM was operated by French contractor Eiffage Civil Construction to excavate the Galerie des Janots water conveyance tunnel below the town of Cassis in Aix-Marseille-Provence, France.

Two natural caverns, studded with stalactites and stalagmites, were encountered by the 3.5m TBM

Located in an area of limestone known for its groundwater, karstic cavities and voids, the combination of limestone and powdery clays made for difficult excavation following the machine’s launch in March 2017. At 1,035m into the drive, the TBM encountered the first cavern, studded with stalactites and stalagmites. The crew had to erect a 4m high concrete wall on the left side to provide a stable foundation for the TBM gripper to thrust against. The TBM was then started up and was able to successfully navigate out of the cavern in eight strokes without significant downtime to the operation, the whole process taking about two weeks.

The first cavern, while the largest, was not the most difficult encountered. At the 2,157m mark, the machine grazed the top of an unknown cavity that extended deep below the tunnel path. The structure measured 22m long x 15m wide x 14m deep, and about 4,500m3 in open space volume.

Eiffage crew celebrates breakthrough after overcoming the drive challenges

To this point the machine was averaging 20m to 22m in two shifts/day, with a dedicated maintenance night shift. On the five-day/week production cycle, the machine was achieving about 400m per month. As part of the cycle, a combination of probe drilling and geotechnical investigations using the BEAM electricity-induced polarization method, were used to detect anomalies ahead of the TBM. On meeting the second large cavern, crews probed in front of the cutterhead and began work to stabilize and secure the cavity beneath with foam and concrete. They also excavated a bypass gallery.

“After filling about 1,500m³ of the cavity, our biggest difficulty was to ensure secure foundations for the gripping of the machine,” said Marc Dhiersat, Project Director for Eiffage. “We needed six bypass galleries and four months of work to reach the end of this challenge. For the last 600m, we were finally in good rock. Despite the obstacles, we are proud to have led a motivated and conscientious team to the end of the tunnel who worked well and without accidents in the face of the many technical difficulties.” Overall rates for the project averaged 18m/day in two shifts, and topped at 25m in one day.

Rebuilt Robbins TBM ready for launch in March 2017

Dhiersat thought positively of the TBM throughout the ordeal “This has been the best machine for the job due to all the geological difficulties,” he said. Detlef Jordan, Business Manager Robbins Europe said: “The cooperation, professionalism and commitment to the project by Marc and his team was very good. For us, it was satisfying and motivating to see that, by working together and joining the efforts of all partners on the project, the best and most successful outcome can be achieved. This commitment is at the heart of success in the tunnelling industry, but is not always been observed on other projects.”

Once completed, the Galerie des Janots will replace existing pipelines currently located in a railway tunnel. These original pipes have significant deficiencies with estimated water losses of 500,000m3/year. The new tunnel will increase water conveyance capacity to 440 liters/sec.

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.”