Tunnelling in the Alps

14 November 2000

Alpina has been working on two hydroelectric plant projects currently under construction in Italy:

Pont Ventoux-Susa hydroelectric plant is a new plant in the northwestern part of the Alps in Val di Susa. Fed by the Dora Riparia’s river it has an installed capacity of 150MW. Construction began in 1995 and is still in progress. The value of the works is approximately 500B lire (US$224M).

The underground works comprise:

•One hydraulic tunnel: 4.75m diameter, 14km long (tunnelling method: TBM, type of rocksoil: fractured micaschists).

•One pressurised hydraulic tunnel: 7bar, 4.75m diameter, 3.5km long (TBM, fractured micaschists).

•One penstock tunnel: section from 995m long - horizontal and 45o slope (drill and blast, fractured micaschists).

•One vertical penstock tunnel: 2.8m diameter, 218m long (raise borer, fractured micashists).

•One large cavern: 20x50m, 52m high housing the power plant (drill and blast, fractured micaschists and gneiss)

•Various service tunnels: sections from 15-60m, total length 800m (drill and blast, fractured gneiss and micaschists)

•One pressurised hydraulic tunnel: 5 bar, 4.7m diameter, 2.5km long (drill and blast, fractured micaschists).

Work on the Premadio II hydro-electric plant concerns the enlargement of the existing Premadio I hydroelectric plant in Valtellina in the Alps. It is fed by the Cancano’s basin water and has an installed capacity of 75MW. Construction work began this year and is in progress. The value of the works is approximately 70B lire (US$31M).

The underground works comprise:

•One pressurised hydraulic tunnel: 0.95Mpa, 3.7m diameter, 190m long (drill and blast, dolomitic rock).

•One pressurised hydraulic tunnel: 0.95 Mpa, 4.8m diameter, 130m long (drill and blast, dolomitic rock).

•One penstock tunnel: slope 45o, 4m diameter, 900m long (TBM, dolomitic and phyllite rock).

•Large cavern: 30m x 30m, 40m high - extension of an existing tunnel for turbine housing (Rodheader - phyllite rock).

The support system of both tunnels is based on DYWIDAG tensioned rebars, of a length between 10-15m. Alpina has developed the complete functional design for both plants including the detailed and construction design for all the underground works. Methods and calculation models used include:

•Finite element structural analysis (FEM) for continuous grounds and structures (2D & 3D)

•Boundary element model (BEM) for underground continuous mass excavation (3D)

•Finite differences element model (FDM) for continuous grounds and structures (2D)

•Distinct elements model for discontinuous rock mass (DEM - 2D & 3D).

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