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Going rate

Turin, capital of Italy's Piedmont region is making a major investment in renewable energy

A significant portion of Turin's electricity comes from hydroelectric schemes in the Alps, centred 40km west of the city in the Susa Valley. But existing facilities at Susa and Chiomonde date from early last century and are in need of replacement.

The Pont Ventoux scheme, currently under construction, will see hydroelectric capacity of the region increase from 27MW to 150MW. However it is the new scheme's impressive peak generation rate of 458 Gigawatt hours per year that makes the project attractive to client AEM, Turin's municipal electricity board.

Two recent political changes make this impressive generation rate essential to the economic viability of the scheme, certainly in the Italian marketplace, explains AEM's Enricco Basso.

First, renewable energy grants have been reduced, which has increased the effective unit cost of hydroelectric power. Secondly, electricity supply companies are now free to buy and sell power on the open market. Without the ability to generate in response to peak demands, when prices are highest, it would be difficult to make the scheme pay.

Unsurprisingly then, Pont Ventoux, where construction started in 1995, has been designed to maximise power generation rates, not total capacity. Pay back period of the Lira 370bn (US$ 193.5bn) scheme is calculated at around 20 years.

In outline, a new earthfill embankment dam will create a reservoir in the Val Clerea basin of the River Doria high up in the valley, some 500m above the power generating turbines housed in a rock cavern deep within the mountain.

The basin will have a storage capacity of 560,000m 3which will drive the maximum flow rate of 33m 3/s through the turbines. The basin will be filled every night and emptied daily through tunnels, generating electricity as it passes along the elaborate tunnel system, through turbines. Below, water is discharged into a holding reservoir created by a 420,000m 3gravity arch dam in the Gorge della Dora, designed to regulate the outflow into the Doria river.

Civils work associated with the project is colossal. In addition to the two dams, it includes 14km of 4m diameter tunnel, 12m diameter tunnelled surge tanks and an 18m wide, 40m high by 50m long rock cavern to house the turbines.

Excavation of the cavern is nearing completion. Its construction has involved more than 50 stages. Geology in the cavern area is favourable with strong competent metamorphosed rock. Rock quality, assessed using Bienawski's rock mass classification system, is between classes 2 and 3.

Primary support in the cavern is provided by a combination of 6m long nails designed with a capacity of 338kN, together with 1252kN capacity post tensioned rock anchors and fibre reinforced sprayed concrete and steel grids.

Anchor capacity is achieved in three different anchor configurations varying in length from 6m to 15m, with fixed lengths of between 3m and 8m.

Nails and anchors are installed on standard grid layouts, with nails at 2m spacing and the anchors at 2.8m. The basic construction sequence for each stage is to excavate the rock, install the nails, apply sprayed concrete and then install the post tensioned anchors.

Performance of the cavern during excavation and the effectiveness of the support system have been monitored throughout using a combination of instruments. These include multipoint extensometers which measure movements up to 15m from the cavern, strain gauges for measuring tension in the sprayed concrete lining, pressure cells which pick up the loads on the anchors and optical targets which provide an overall control for the monitoring programme.

Movements recorded to date are well within those predicted, with 16mm vertical movement at the cavern crown and 9mm on the sides where the crane gantry is to be installed.

While excavation of the cavern has been achieved without hitch, tunnelling has experienced mixed fortunes. The mica rich calcareous schist is highly favourable for tunnelling, so good progress was anticipated and risks were considered to be relatively low.

General Contractor, a joint venture between Astaldi, Sae and Cogei set up two drives using Robbins TBMs, operating without shields. Boring upslope on a 0.1% gradient more than 7km was achieved in the first year of tunnelling, with daily progress rates of up to 70m.

This confidence proved to be short lived when the tunnellers intercepted an unexpected shallow inclined tectonic discontinuity. The feature was not identified during the site investigation, or from geological surface mapping.

Tunnellers were then surprised to encounter a major fault infilled with very soft plastic material, aligned in an orientation that would intercept a considerable section of remaining tunnel.

To overcome the problem, the turnkey contractor introduced an alternative tunnel layout. The extra design and construction effort involved was considered a sufficient trade-off against potential difficulties arising from the problematic ground.

The redesign involves rerouting the penstock, which adds to construction costs but will not, assures Basso, affect the operational efficiency of the power plant.

Despite this hitch, work on the project is progressing well. Tunnelling below the power house cavern is complete and lining, which is expected to take around a year, will begin soon. This will help control high water inflows which are currently managed by forming local sumps and pumping at up to 300 litres per second.

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