Italy’s Pont Ventoux Power Complex

To enhance its hydroelectric production capacity for the next century, the Azienda Energetica Metropolitana (AEM) of Torino, Italy, started in 1996 to build the new 150MW hydroelectric plant, Pont Ventoux – Susa, in Piemonte, Italy, to provide the area with an additional 457GW/year.

For environmental reasons and to achieve the best functional scheme most of the plant was designed to be underground. Situated completely inside the mountain between the two valleys of the Monginevro and Moncenisio passes, only the works for water intake from the river, the reservoir and the water delivery back to the river are not underground.

The plant’s water intake is located near Sauze d’Oulx village, at Pont Ventoux (windy bridge), on the Dora Riparia river at an altitude of 1046m a.s.l. where the flow varies up to 34m³s^-1. The water is diverted through a 14km long tunnel, with a gradient of about 1/1000 supplying daily the Val Clarea reservoir. This reservoir feeds a pressurised 4km tunnel, the upper surge tank and, finally, the 1.3km long, 3.2m diameter penstock tunnel. At the foot of the penstock, at an altitude of 491m a.s.l. and a head of 515m, is the underground power station equipped with two Francis turbines. Each has a flow of 17m³s^-1 and a speed of 750rpm and are both coupled to an 85MVA synchronous generator. After this, downstream surge tanks transfer the water to a 1.6km long outlet tunnel that delivers the water to the downstream Susa reservoir. One of the turbines is coupled with a 13.5m³s^-1 pump (ternary unit: synchronous generator; turbine, pump) that at night transfers water from the Susa lower reservoir back to the Clarea upper reservoir.

AEM awarded the design and the construction work to Pont Ventoux S.c.r.l., an association between Astaldi (Rome), as main contractor, and Eiffage (Paris), who in turn contracted Alpina S.p.A. (Milan) as engineering consultant for the designing of the whole plant and for underground works.

Geology

The geology of the site was established using extensive studies of deep drilling, surface surveys and analysis of geostructural surveys performed in the project’s tunnels during excavation.

The excavation geological data confirmed that the lithotypes found consist essentially of phylladic schistose crystalline micaschists composed of calcite and mica or with decimetric to metric intercalations of marbles and silicates or sometimes with centrimetric to pluricentrimetric intercalations of more or less quartzitic and chloritic gneisses. There are often gradual variations in composition, which are difficult to predict in the rock mass.

Tunnels

The plant consists of 24.7km of tunnels, 22km for hydraulic purposes and about 2.7km for transit.

Most of the hydraulic tunnels (about 15km) were bored using two 4.7m diameter Robbins TBMs (type 148-212-3, 35 cutters, total power of cutter head drive 895 kW, gripper system). The good quality rock allowed a successful TBM performance on most of the drives, with a high average of 15 m/day peaking at 40m/day.

According to the rock quality, bolts were used as provisional supports with steel arches occasionally set whilst passing through fault zones with adverse geomechanical characteristics.

A careful monitoring of the TBM progress parameters [power (kW), energy (kWh), thrust (MN), specific penetration (cm/rev/MNdiscload] achieved both a successful backanalysis of the passed zones and a reliable prediction of the forthcoming ones.

The remaining 6.5km of hydraulic tunnels were excavated by drill and blast due to the high gradient, narrow bending radius, shortness of single sections and changing internal geometry of the tunnels. Also the presence of fault zones and the bad quality of the rock mass made drill and blast the most practical option.

All hydraulic tunnels with a round section had a concrete lining 25-35cm thick, cast by a 36m long iron framework. In the pressurised hydraulic tunnels, liquid mortar injections structurally joined the lining to the rock allowing them to work together supporting the hydraulic loads inside the tunnels.

Detailed modelling of the structural interaction between the rock and concrete showed that the traction working stresses inside the lining were allowable with no need for steel reinforcement. This achieved a big economical saving in comparison with the preliminary foreseen budget, although steel fibres were added to the concrete to avoid microfissures in the lining, ensuring water tightness and a longer durability.

Where a large water head was found inside the rock mass a proper drainage system was set between the concrete and the rock in order to lower the hydrostatic pressure to the load bearing capacity of the lining.

The 220m vertical section of the penstock was drilled by a raise borer allowing work to progress rapidly. The vertical upper surge tank of the plant is 80m high with a 12m i.d. excavated in 2.4m advances by drill and blast followed by casting the 0.7m thick reinforced concrete lining.

The project’s transit tunnels have a U-shaped section between 25m² and 45m² in area and were all excavated by drill and blast. The good quality of the rock mass and minimal finishing requirements led, in most cases, to a single lining of shotcrete reinforced by steel mesh.

Further underground works of the hydroelectric plant include:

• two surge tanks, one of which is 450m long;

• three small caverns, two of which are for underground transformer seating each with a volume of 2400m³, and one for the butterfly valve seating, 2800m³ in volume;

• four steep sloped tunnels connecting the turbines and the pump to the penstock;

• all the underground works for the overflow

  of the upper Clarea reservoir, its flow off drainage and water supply to the pressure tunnel, consisting of several tunnels and caverns;

• a flow off drainage tunnel of the lower Susa reservoir, 49m² in section, 340m long and designed for a maximum flow rate of 180m³s^-1.

Power Cavern

Among all the underground works that make up the Pont Ventoux hydroelectric plant, the most important is definitely the power cavern where the generator-turbine group and the generator-turbine-pump group are set.

This is mainly due to its dimensions (50m long x 21m wide x 21-49m high) together with the structural design and construction systems (the vault and walls of the cavern are self supported by pretensioned rebars).

Location and Orientation

Design of the cavern had to take into consideration problems arising from its location and orientation inside the rock mass between the Cenischia Dora and Ripariathe Valley. In the site chosen for the cavern the foliation dip is between a nearly horizontal direction (10°) and a slightly sloping one (25°), in clockwise direction S-SW.

The joints and the faults detected are few and narrow. These discontinuities do not affect the geomechanical rockmass classification. Joints and faults are closed and without any water flow.

The rockmass appears generally closed, and its geomechanical average is between 2nd and 3rd Bieniawski class. In July 1996 a feasibility study was made, which allowed the official approval of the location of the power plant, and the start of the final design which included a slight move of the plant into a zone with better mechanical characteristics.

The study for the final orientation decision was lead by the construction of spectral curves representing the behaviour of the volume of the wedges as a function of the angle expressed in degrees with respect to the NS axis. From the analysis of the spectrum of the key blocks a range of favourable orientations of the power plant was obtained and identified between 10° and 30° in a clockwise direction from the NS axis, the final value selected was 17°.

Choice of Cavern Shapes

The preliminary design was a mushroom-shaped cavern supported in the long term by a heavy reinforced concrete arch.

The revised cavern location and the new assessments of geomechanical parameters lead the project engineers to re-design the cavern shape to minimilise the rock supports. Possible Egg and U-shape cavern shapes were studied using hybrid FEM/BEM numerical analysis. These generated less decompressed zones and considerably reduced the tensile failure on the walls. From the above considerations, the U-shape design was chosen for the cavern, due to the ease of excavation and better use of space.

Support Design

Firstly the length and spacing of rockbolts and rebars had to be decided. Analysis of the wedge failures and of the length of overstressed zones was useful in designing the extension required for the supports.

The wedge analysis comprised of; potential wedge identification; study of the database of wedges according to location (sides or roof), and physical characteristics (height, volume and safety factors); data collection according to the ‘few sample theory’ and ‘probability density function’ and ‘cumulative distribution function’ analysis; with design of the rebars not exceeding the expected safety factor in the worst scenario situation.

The Alpina engineering team’s idea was to create a double support system with regard to length and installation methods, to differentiate the action of the longer bars from the shorter ones. The short bars, chemical resins injected with short setting times and low prestressing values, have the function of limiting the loosening of the rock caused by excavation, creating a cortical reinforced zone. Tensioned bars, anti-corrosion double coated and cement grout injected, are the final support system of the cavern. In both cases Dywidag bars with high strength steel are used.

After the untensioned bolt installation, a 10cm thick shotcrete lining is applied to limit small wedge failures. In the final lining, the support system is completed by another 15cm of fibre-reinforced shotcrete.

Construction Sequence

The cavern excavation was carried out in stages by blasting. The excavation of the roof cavern took five stages with a maximum opening of an 8m length x 5m height.

Excavation of the cavern benches followed the same method as the roof cavern. A deepening slip road links the upper bench to the lower for mechanised rock removal (see diagram). Untensioned bolts support the cavern in the first phase of the excavation with installation closely following each excavation progress.

Monitoring System

The monitoring system compared computed and read values. The rock mass performance over time was monitored to prevent expected values being exceeded during the construction.

The monitoring instrumentation for rockmass performance comprised; two sections of seven multiple-point borehole extensometers with readings taken by remote data acquisition, optical convergence measurements of excavation, hydraulic pressure cells in the shotcrete and stressmeters on tensioned rebars.

Backanalysis

From the start of excavation, the logical flow chart for the design under construction was outlined by deformation monitoring compared with expected values, adjustment to the design procedure and an under construction design logical flow chart.

The under-evaluation of the displacement of the boundary computed by FEM and BEM models (continuous approach) forced the design team to use distinct elements models.

The numerical method DEM was chosen as the best way to simulate the rock mass performance when compared to the one computed by finite element methods.

The distinct element model, performed by UDEC and 3DEC numerical codes, allowed the study of the displacements at each excavation stage, together with the bolts and the shotcrete lining stresses.

The spacing foliation is the joint generator (method) parameter used for six distinct element models listed below. An automatic generator to create joint patterns based on statistical parameters is included in DEM codes. The main discontinuities were matched on the statistical joint distribution.

The backanalysis models were developed in different zones for joint generation, strength, deformability and boundary condition. Both untensioned bolts and tensioned rebars were included in the numerical models.

The standard Coulomb slip model was used to simulate the joint and the main discontinuities.

Project Update

All civil works on the cavern are now complete, as is excavation of the tunnels with the exception of the 14km long diversion tunnel. This had been split into two 7km long TBM drives with both Robbins machines going to face in 1997.

The first 7km section ran smoothly with a record rate of 40m recorded in one day. Three kilometres into the other 7km drive the second TBM hit a fault zone and boring was suspended.

Completion of the remaining 4kms of tunnel will utilise drill and blast from two faces and is scheduled to take one year.

Related Files
Cavern excavation design