MTBM takes on Europe’s largest wild river

Tours Métropole Val-de-Loire contracted SADE to carry out trenchless tunnelling works to cross the River Loire using an MTBM.

The construction of the tunnel and backflow station is an important part of an €8,000,000 (US$8.8 million) infrastructure development in the region. Once active, several systems will connect with the tunnel, include those for wastewater treatment, drinking water and optical fibre.

After launching works in June 2016, the project took a total of 28 months to complete, and the River Loire was crossed using a 2,200 mm inside diameter (ID) under river utility tunnel laid with an 800 mm ID pipe, along with other systems, for the new backflow station.

The project comprised two packages. The first was concerned with the creation of the 10 m diameter, 8 m deep backflow station guaranteeing a peak flow of 2,500 m3/h. The scope of the second package included:

  • laying the 250 lm gravity system to supply the backflow station
  • creating the 2,200 m diameter tunnel using a MTBM
  • laying an 800 mm diameter wastewater backflow pipe, a 250 mm diameter cast iron water supply pipeline and five optical fibres in the utility tunnel
  • constructing a P4 work shaft using a 4 m diameter vertical kirving, a 2,200 m diameter MTBM work shaft, a P3 stormwater outfall and a surge tank connecting to the mains supply.

A risk assessment indicated that the project risks to be considered were essentially concerned with the risk of alluvium compaction, the presence of timber piles at an uncertain depth and the presence of obstacles such as bridge abutment and railway lines requiring crossing.

Crossing in complex conditions

The German-designed MTBM was a reinforced, pressurised slurry-shield Herrenknecht AVND2000 AB model with a hydraulic slurry circuit. The 620 lm, 2.2 m ID utility tunnel was excavated using an MTBM, allowing the River Loire to be crossed from La Riche to the backflow station located on the opposite bank in Saint-Cyr-sur-Loire.

The MTBM tunnelled section crossed the largest wild river in Europe and has become the highest diameter microtunnelling drive bored in France, with a significant radius of curvature and a railway crossing at the end of the tunnel.

Any railway track crossing requires the preparation and submission of a full technical pack for approval from the French national railway company engineering department; this MTBM needed to tunnel under the railway lines and avoid a bridge abutment with timber piles.

To ensure these requirements were satisfied, a 5 m ‘safety zone’ was added around this structure when plotting the MTBM’s route, as the foundations of the abutment were not clearly identified.

Once the MTBM crossed the tracks approximately 12 m down, the machine needed to pivot up at a gradient of more than 11 per cent to reach the exit shaft at the right elevation. This 8 m deep secant pile shaft was part of another package for the construction of the backflow station, which also acted as an MTBM recovery shaft.

These constraints all resulted in the implementation of a complex three-dimensional curved drive (horizontal curve and altimetrical curve) with a radius of curvature of 314 m.

A first for France

Prior to this project, the implementation of a 314 m radius curved drive and the use of 3 m pipe jacking pipers were incompatible as curved drives would cause angular deflection between each pipe jacking nozzle. Despite being a new process, it worked as required and allowed for the handling of different route constraints while using 3 m long pipes to optimise work rates.

A hydraulic seal sized by the design and engineering department and pressurised at the factory is inserted between each pipe joint; once the pipes are placed under stress, the incompressible fluid allows better spreading over the tube’s entire circumference.

Vital utility tunnel

The utility tunnel works consisted of laying a sleeve using the MTBM with an inner diameter of 2.2 m followed by several systems – wastewater treatment, drinking water and optical fibre – being laid in the tunnel.
To ensure the safety of the staff, metal walkway gratings were installed in the lower section of the utility tunnel as well as custom cradles to support future pipes, which were fixed into the upper section of the utility tunnel.

Once the construction of the utility tunnel was finalised, pipelaying operations in the tunnel began with the 800 mm diameter glass fibre reinforced plastic wastewater backflow pipe, 250 mm diameter cast iron water supply pipe and five optical fibres being installed with the same technique.

The principle behind the installation of the utility tunnel is a primary drive to reduce the overall costs of operations as well as the environmental impact of system activities. Additionally, this method of implementation results in a solution that allows future pipe installation via a monorail fixed to the utility tunnel’s arched roof.

Project conclusions

Despite the complex execution constraints flagged in the risk assessment, the microtunnelled wastewater treatment site allowed the implementation of several specific operating procedures and the deployment of technical innovations to navigate these issues.

The modernised MTBM allowed low radius curved drives using special hydraulic seals to reassess the route of the structure that was already under construction and permitted innovative progress within the project that relied on team efficiency and close monitoring to achieve successful completion.

This article was featured in Winter Edition of Trenchless International. To view the magazine on your PC, Mac, tablet, or mobile device, click here.

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