Excavation works... pump up the volume

1 February 2002

One of the biggest shafts in the world is being excavated at the Venda Nova hydro plant in Northern Portugal. Janet Wood reports on how the project will bring much needed power to the Portuguese grid, and allow the plant to act as a pumped storage station

At the Venda Nova hydroelectric power station in northern Portugal, engineer Rui Manual Ferriera Carrito is about to start work excavating one of the biggest shafts in the world. It is a vertical surge shaft, which is being rebuilt in a project that will require excavation works totalling 300,000m3.

The surge shaft is a unique part of the project, Carrito says. It will be a full 400m deep. For the 300m at the bottom it will be 4.5m in diameter, while the top 100m will widen to 5.6m. How will the shaft be excavated? 'We are expecting to do the work by drill and blast,' says Carrito. 'We used this technique for other shafts, including a 115m long shaft with a 3.5m diameter that was inclined at 42?. For that we used a Robbins machine, equipped with Secoroc cutting equipment.'

Carrito is managing director of a consortium of three Portuguese companies - Somague, Moniz Damaia Sere & Fortunato (MSF) and Mota et Companhia - formed to complete the civil works of the Venda Nova upgrade. The project is an important one for the power plant's owner, local electricity utility CPPE. It will bring on stream two new 93MW turbines at the Venda Nova dam, bringing much-needed power onto the Portuguese grid. What is more, the plant will be able to act as a pumped storage station between Venda Nova and its companion Paradela dam lower down the river.

With few natural energy sources of its own, Portugal is highly dependent for energy on imported oil. But in the electricity sector the balance is rather different, as hydro power has long supplied around half of Portugal's needs.

The Portuguese electricity supply industry was restructured in mid-1994, when the state-owned producer and distributor of electricity, Electricidade de Portugal (EDP), was broken into operating groups. In the new structure, Grupo EDP consists of the holding company EDP, which co-ordinates the group's operation and strategy; a production company, Companhia Portuguesa de Producao de Electricidade (CPPE); a grid company, Rede Electrica Nacional (REN); four regional distribution companies; and ten more service companies. In June 1997, 30% of Grupo EDP was sold off. Grupo EDP controls 80% of electrical production. The other 20% is accounted for by Tejo Energia, co-generation and small private companies.

Hydro dependent

In 1998, Electricidade de Portugal had an installed capacity of 7532MW, of which 3964MW was hydro power, providing both baseload and peaking power. This dependence on hydro is likely to continue, despite the completion of a US$4.5B pipeline that carries natural gas from Algeria, via Morocco and Spain. The pipeline is designed to feed Tejo Energia, Portugal's first combined cycle gas turbine at Tapada do Ouitero, which is rated at 1000MW. In parallel, Portugal is investing in a number of small scale hydro projects as well as Venda Nova. The country has also been investing in wind power - its first farm, rated at 10.2MW, went into operation in 1996 and five other wind farms are at various stages of development.

Despite its investment in small hydro, gas and wind, Portugal's growing need for both peaking and baseload power underlies the importance of Venda Nova. During the night, or at other periods when electricity demand and prices are low, water that had previously passed through the power house to be stored behind Paradela, will be pumped back up through 4000m of tunnels to the Venda Nova lake, to provide hydroelectric power at periods of high demand. CPPE expects the plant to operate mostly in turbine mode, at least in the early years, generating an average 220GWh annually.

Development of the plant had been delayed for environmental reasons, as it is sited on the borders of a national park - the Vila Real district some 40km east of Braga. But work has been under way in earnest since contracts for the civil works - some US$35.6M, in a project cost totalling US$111.3M - were signed in April 2000 and blasting began in June of that year. The new plant is now scheduled to enter operation in March 2004.

The project's designer and consulting engineer is Hidroruma, and it has employed voith-siemens Hydro as the main electromechanical contractor.

Voith Siemens says the scope of supply includes two radial pump-turbines with governors, two motor-generators with excitation systems and static frequency converter, two spherical valves, a cooling water system, several monitoring systems, penstock and draft tube gates. Each of the two turbines is rated at 92.5MW, with a generator capacity of 103MVA. The maximum planned input in pumping mode is 92.5MW, from a minimum operating head of 400m before it discharges into the Cavado river. The maximum operating head is 437m.

alstom, through its water business unit, is supplying the upper and lower power intakes, and associated auxiliary equipment, and that phase of work is due for completion in March 2003.

Civil works

By late 2001, half way through the excavation work, 4500m of a total 7000m of tunnels has been completed and civil works are a month ahead of schedule.

The civil works are due for completion by September 2003, and over 4000m of tunnels, the cavern that will house the runners and turbines has already been completed. The project's progress has been aided by local geology that is generally favourable to tunnelling, and one that has given few surprises to the team working underground.

'We have mainly granite, with a compressive strength of 20MPa, and the area of the power cavern is very homogeneous,' says Carrito. 'So, although there are some small areas of schists and some zones with fractures and faults, geological problems have not affected the works.'

During drill and blast operations, one of the consortium's two geological consultants classifies the rock in each round. Sections graded one to three, where one represents the best rock requiring the least support to three, have support graded from three to nine bolts per metre. For the weakest rock, graded at four, steel arches are installed instead of bolts. The consortium uses Swellex bolts, and in the tunnel, 3-4m long bolts are adequate. The team's Rocket Boomer can help in weaker areas to get some idea of what the rock is like ahead: it is used to drill a hole 15 or 20m deep and the pressure return on the system provides some data on the rock.

Excavation equipment

To carry out the drill and blast excavation of the tunnel, the consortium has invested in Atlas Copco machines. MSF has a 205 and Somague has two Rocket Boomer LC2s . The choice was partly governed by the LC2's capability and overall economics: 'We have the new generation of machines,' Carrito says. 'They have the Advanced Boom control system so they can drill automatically. It is more accurate so the overbreak is less and it saves the time and cost of spraying concrete.'

Equally important for Carrito is ensuring the availability of the equipment. 'In other civil works we can go away and work on another site, but that is not so in the tunnels,' he says. 'In the tunnels we can't wait because we can only start the next job when this one is finished.' That is why Atlas Copco's ongoing maintenance support is so important. 'Local people don't have any idea how to maintain or repair this type of sophisticated equipment, and if we used them we would be stopped for one or two days each week,' Carrito says. 'But we have an ongoing contract for maintenance and spare parts for the entire project and Atlas Copco keeps one or two people on our site.'

From the start of the project, Atlas Copco has also provided training support. 'Rocket Boomers are familiar in Europe but they were not known in Portugal,' Carrito says. 'To start with our operators were not skilled in working with the machines, they are starting as school leavers effectively from ground zero.' Carrito has worked together to develop a training programme that can last up to six months. 'To start with a drilling round was taking five or six hours,' Carrito says, 'but now it is down to two, or two and a half hours.'

Along with the LC2s, Somague has a small single-armed boomer - a 281 - for tunnels with smaller sections. 'We also use Atlas Copco's Swellex bolts and their injection grouting equipment with the grout drilling tools and grout registers, and we use them for all the drill materials and rock drilling tools,' says Carrito.

For vertical drilling, Somague has a ROC748 and MSF has an ROC D5 and these were used extensively in the power cavern, whose 50,000m3 volume represented almost a third of the project. Excavation was competed in mid-September 2001.

'We used three phases,' Carrito says. 'We had a drift on the top gallery, then two side drifts with a rock pillar in the middle for support. The cavern roof was bolted using one bolt per m2.' The bolts in the roof of the cavern were 9m long and here the company made use of another of the LC2's features: 'One of our jumbos [LC2s] has a BHS system which allows a 9m hole to be drilled,' Carrito recalls. 'Otherwise we would have had to do it manually and that is important because the drilling would be overhead.'

Bolting was followed by spray concrete and the central pillar was then removed. The rest of the cavern was blasted and the spoil removed via the access tunnel. The cavern walls required less support and a 6m bolt was installed in each 2m2 area.

With the cavern complete and work up to schedule, Carrito is considering how best to approach excavation of the surge tunnel. Meanwhile with trained operators, and reliable backup, work is going on steadily and is averaging 80-90m per month, with a site record of 110m per month.

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