Skrevet av Emne: Unexpected cavern encountered in France  (Lest 1518 ganger)

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Unexpected cavern encountered in France
« p: 23.03.2018 16:51 »
Unexpected cavern encountered in France
22 Mar 2018
Robbins News Release

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.

“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.

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.
Look deep into nature, and then you will understand everything better.
- Albert Einstein -