The section of the new Ferrera oil pipeline from Pavia (Italy) to Aigle (Switzerland) between Chivasso and Grand St Bernardo on the Italo-Swiss border is now under construction. It will replace the old pipeline which has been in operation for more than 30 years and construction has involved unusual problems for the selection of the alignment.

Over certain areas, maintaining an alignment parallel to the existing pipeline was difficult, especially from Valle d’Aosta, where the narrow strip of level ground is bounded by high mountains and where a road, highway, railway, power transmission lines and telecommunications are already located, not to mention towns and villages and the Dora Baltea River.

The difficulties were solved by the use of trenchless technologies employed by the ENI Group for laying flow pipelines, so the construction of this new infrastructure is an interesting example of the application of several different trenchless technologies on one single project. These were chosen according to the structural and morphological problems to be tackled in each separate case.

Trenchless technologies used

The new section of oil pipeline from Chivasso to the Italian state border has the following characteristics:

  • Chivasso-Aosta-Etroubles nominal diameter (ND) 400, length 112km

  • Etroubles-Grand San Bernardo ND 300, length 11km. Five different trenchless technologies were applied on the new alignment:

  • Microtunnelling: two microtunnelling operations crossing racelines beneath the river bed were carried out by Italian contractor ICOD using a Herrenknecht TBM. O.d. was 1100mm over a total length of 75m, comprising two sections of 48m and 27m respectively.

  • Horizontal directional drilling was used by NACAB of The Netherlands to lay the ND 400 pipeline crossing Chiusella Creek over a length of 574m

  • TBM tunnelling: this technology was of major importance because it allowed a tunnel to be bored in rocky terrain in mountainous terrain where open cut techniques would have involved severe operating difficulties, unacceptable environmental impact and high costs. The tunnels driven in this manner extend over a total length of about 3200m, with 2300-3600mm variable boring diameter

  • Raiseboring: to bypass a steep rocky escarpment with a high visual impact, a pit for laying the pipe was constructed by a version of the raiseboring technique in the final section of the new pipeline. Through the adoption of a special application of this technique – raiseboring downwards – a passage through a rocky hill was constructed.

  • Pipejacking: to lay a ND 1200mm steel pipe in the rock over a total length of 105m comprising sections of 56m and 59m, this technique was used to traverse rocky hills at the side of the No.38 provincial road.

    Raiseboring and pipejacking operations were undertaken by Edilmac di Maccabelli, which also manufactured the equipment.

    • Application of the TBM technique

    Using a TBM made it possible to overcome the most difficult terrain and solved some of the major difficulties associated with the project. The TBM was adopted for driving the horizontal Pramotton, Hone 1 and Hone 2 tunnels, and also for the construction of a tunnel with a steep gradient at Montjovet.

    The flow pipeline was laid in the Pramotton tunnels and two pipelines were laid in the Montjovet tunnel – an ND 400 oil pipeline and an ND 450 gas pipeline. Laying the gas pipe in this tunnel enabled the existing pipeline to be moved from an area subject to landslides, which had jeopardised its operation.

    Shallow gradient tunnels

    These tunnels were built by Murer Italia.

    Geological features. Along the route of the pipeline, the valley intersects some of the main structural features of the north western Alps. The tunnels pass through the eclogitic mica schists and minute gneiss in the Austro-alpine Sesia-Lanzo area. The rock that surrounds the Pramotton Tunnel is characterised by three main alignment systems: one faces the north west and lies parallel to the axis of the valley; another, well developed, alignment has a west south west-east south east direction; a third and lesser developed one has a north-south direction.

    The area surrounding the Hone 1 and Hone 2 tunnels consists of alignments transverse to the valley’s axis of the valley. At times, this area is the centre of intense faulting phenomena, which subsequently cause the formation of parallel sub-vertical faults. In general, the surface layer is composed of detritic nappes of extremely variable particle size, accumulated after landslides and alluvial deposits.

    Drilling aspects. Full face TBM excavation had the following advantages:

  • It constructs regular profiles, only rarely moving off course and the final configuration is suitable for laying pipelines

  • Support is not necessary except for lining at various points when required

  • Estimated average daily advance was 10m/day

    Main difficulties of advance were due to:

  • The rock had an average hardness of 200-250MPa, with a maximum of 400MPa, which occasioned the frequent substitution of cutter discs and a consequent slow-down in production

  • The rocky mass being crossed by fractures, sometimes filled with coarse material that fell on to the cutterhead and blocked it, necessitating pre-consolidation

  • In Hone 1, there was significant water and sand ingress requiring the injection of mortar with fibre.

    • Production rates

    Average daily production on the single tunnels was almost to programme. The Pramotton Tunnel achieved 9.4m/day; Hone 1, 10.5m/day; and Hone 2, 10-11m/day. Maximum daily peaks of 29.3m were recorded for Pramotton, 30.3m for Hone 1 and 27.6m for Hone 2. An average of about 0.4-0.6m/h with peaks of 1.2m/h was observed.

    Montjovet Tunnel

    This tunnel was built by Ilbau.

    Geological features. The tunnel was excavated through metamorphic rocks such as amphibolites, mica schists and ophiolitic schists, crossed by different systems of fractures. Rock hardness is high in the amphibolites (more than 490MPa) and lower in the ophiolitic schists (less than 20MPa). The Bieniawski class for the rock mass changes from Class 2 to Class 3, the latter being valid for the more fractured type.

    Drilling aspects. A full section cutterhead of 6350kN thrust, 700kW total power and 540 kNm max. torque was selected for excavating the Montjovet Tunnel. The most difficult part of building this tunnel was in the area around the southern adit caused by the tight work site which was close to a trunk road and the Turin-Aosta motorway.

    After careful preparation of the area, which included construction of reinforced concrete piles under the trunk road and lowering the walkway, excavation began to insert the cutterhead under the road. For space restrictions reasons, tracked shovels were used for muck removal.

    Implementation time. Time taken to excavate the tunnel was 12 days less than programme, with 12.7m/day average daily production and over 30m/day peaks. Because the rock was of good quality, there was scanty rainfall and optimal organisation of the work site, this tunnel, originally considered to present the most problems, was the one where the best performance was obtained.

    Laying the pipeline

    Italian legislation forbids the maintenance of open ducts having pipelines inside without the provision of inspection manholes. However, as an alternative, it is possible to fill the space between the pipeline and the tunnel wall with suitable material, so that the section of pipe inside the tunnel can be considered in the same way as a section of buried pipe of any type. Such an operation is more profitable both in terms of the excavation and operating phases, provided the fill material has these specific features:

  • Easy preparation and installation
  • Speed of consolidation
  • Low voids index
  • Low aggressiveness to lining materials of ducts
  • Low final resistance for when the consolidated material has to be removed for pipeline servicing

    • Different types of fill were used for this pipeline:

  • A mixture of water, bentonite and concrete: for the Pramotton, Hone 1 and 2 tunnels
  • A self levelling mixture composed of water, sand, concrete, aerating additive and fluidiser for microtunnelling of Ferrero 1 and 2 – St Remy and Cesnola raiseboring and pipejacking beneath the provincial road No. 31
  • A self levelling mixture composed of water, loose muck, concrete, aerating additive and fluidiser. This is another version of the above mentioned mixture, for which the excavation material of the tunnel was reused instead of sand in the Montjovet Tunnel.



    • Related Files
    Italo-Swiss border