By António Freitas da Costa, Maria Eugénia Resende, Manuel Alberto Oliveira, and Vítor Ribeiro
While developing the new 194.2-mw Venda Nova II pumped-storage project in Portugal, Energias de Portugal, S.A. used several innovative civil engineering techniques to design and build the power tunnels, support the powerhouse cavern roof, and excavate the upper surge shaft. These innovations helped EDP reduce construction time and cost for the project.
Development of the 194.2-mw Venda Nova II pumped-storage project in Portugal has provided owner Energias de Portugal, S.A. (EDP) with many opportunities to apply engineering innovations not previously used in the country. These innovations were used with the goal of reducing construction time and cost for the project. During design and construction of Venda Nova II, EDP:
- Used “unlined” tunnels for water conveyance in lieu of a traditional reinforced-concrete lining;
- Adopted an alternative method for supporting the roof of the powerhouse and transformer caverns; and
- Excavated shafts using a raise-boring technique instead of the traditional explosives method.
Background on the project
Venda Nova Dam, a 97-meter-high arch gravity dam on the Rabagão River, was completed in the early 1950s to impound water for the 135-mw Vila Nova powerhouse. A 2,650-meter-long high-pressure tunnel conveys water from Venda Nova Reservoir to an 820-meter-long penstock that feeds the three-unit powerhouse. In an average year, this powerhouse produces about 389 gigawatt-hours (gwh) of electricity. This powerhouse also receives inflow from Paradela Reservoir on the Cávado River. (See Figure 1 on page 24.) The powerhouse discharges its water into Salamonde Reservoir.
The Venda Nova II pumped-storage project takes advantage of about 420 meters of gross head between the Venda Nova and Salamonde reservoirs.
In 1996, EDP decided to build the Venda Nova II project, for two main reasons. First, in 1995 and 1996, the Portuguese Transmission System Operator, a division of the new Public Electricity System, predicted growth in demand for electricity of about 4.5 percent between 1996 and 2000 and 3.2 percent between 2001 and 2005. New power stations were needed to meet this projected growth in demand.
Second, the government of Portugal recognizes that more renewable energy is needed to reduce the country’s dependence on fossil fuel generation. In line with Portugal’s commitment to the Kyoto Protocol and the European Directive regarding the promotion of electricity generation from renewable energ, Portugal’s goal is to have 39 percent of its electrical demand met by renewable energies by 2010. The country plans to meet this target through a combination of decommissioning old thermal stations and building new renewable energy stations.
Venda Nova II was designed to take advantage of about 420 meters of gross head, over a stretch of about 4,500 meters, between the Venda Nova and Salamonde reservoirs. Venda Nova II, which began operating in August 2005, has two reversible 97.1-mw turbine-generating units. Each unit consists of a Francis pump-turbine and a directly coupled synchronous motor-generator, supplied by Voith Siemens Hydro Power Generation. The project produces an annual average of about 220 gwh.
Innovations used at Venda Nova II
This article provides details about the three innovations mentioned earlier.
Using unlined tunnels
Traditionally, the design of hydro facilities in Portugal calls for reinforced concrete lining for underground water supply tunnels. This lining is intended to decrease roughness of the tunnel, protect against falling blocks, and provide waterproofing. However, for Venda Nova II, EDP looked at flexible support structures using fiber shotcrete and rockbolting, also known as “unlined” solutions. In this situation, the rock mass performs a structural function. To compensate for the increased roughness of the walls, the diameter of the tunnels is increased. This also allows for some blocks to fall, without concern for the carrying capacity of the tunnel. This concept is used in Norway.
EDP studied several options to integrate unlined tunnels at Venda Nova II. EDP understood that this concept could save construction time and cost. This unlined tunnel concept is associated with some specific design criteria: to guarantee low flow velocities (1 to 1.5 meters per second), to allow the fall of some rock blocks (units are protected by racks in sandtraps), and to avoid emptying of the system so as not to introduce unfavorable hydraulic gradients to the rock mass.
Based on the quality of the rock mass, four unlined sections were defined, as well as a rigid reinforced concrete-lined section. Total lengths for the rigid lining were around 7 percent in the headrace tunnel and 14 percent in the tailrace tunnel. Figure 2 on page 26 shows the tunnel layout at Venda Nova II.
The option chosen consisted of:
- A 2.8-kilometer-long unlined headrace tunnel with a 15 percent slope and a 6.3-meter-diameter modified circular section, to connect the upper intake and powerhouse. The stretch upstream of the powerhouse is steel-lined and preceded by an upper sand trap;
- A 1.4-kilometer-long subhorizontal unlined tailrace tunnel with a 6.3-meter-diameter modified circular section, between the lower sand trap and lower intake;
- A 4.5-meter-diameter, 420-meter-long vertical unlined shaft to act as the upper surge tank, 500 meters upstream of the powerhouse, which empties into an expansion reservoir at the surface; and
- A 1.5-kilometer-long unlined access tunnel with an 11 percent slope and an 8-meter-diameter cross-section, also used to run the power and control cables from the transformers to the surface support building.
For unlined hydraulic tunnels, first filling is an important operation. During filling, the rock mass hydraulic conditions change because of the water pressures. This process, which results in filling of the fissures and voids in the rock mass, may induce leakage into the caverns and auxiliary tunnels and to the surface.
Before the tunnels were filled, EDP performed surveying to identify all infiltration points into the tunnels and caverns. A filling velocity was then defined, considering the geological and hydraulic conditions, to avoid any damages caused by high pore pressure.
To monitor the tunnels for leakage during first filling, EDP installed piezometers supplied by Tecnasol FGE and performed visual inspections of the most significant leakage points. Due to the very large water pressures in the headrace tunnel, EDP installed a high-precision digital manometer in the penstock, close to the protection valve in the powerhouse, to measure water level in the tunnel.
Filling started from the tailrace tunnel, which is shorter than the headrace tunnel and has a much lower final water pressure. This filling took place in one step.
Headrace tunnel filling was carried out through controlled periodic opening of the intake gate. The duration of opening was adjusted to control the rate of water level increase inside the tunnel (about 10 meters per hour in the first 250 meters and about 5 meters per hour in the last 150 meters). Monitoring steps were established at about 100 meters and later adjusted, depending on the behavior of the measured leakages. Leakages out of the headrace tunnel were evaluated by the water level variation inside the tunnel, using the manometer. When the leakages were considered acceptable and the rate was reduced with time, filling continued.
Supporting cavern roof and walls
For most hydro projects developed in Portugal prior to Venda Nova II, the practice for supporting the roof and walls of an underground cavern consisted of a cast-in-place reinforced concrete arch structure. However, EDP decided to use a support system consisting of cement-grouted rockbolts and fiber shotcrete. This is the same type of support structure used for the unlined tunnels.
This arrangement was possible because EDP considered the geotechnical characteristics of the rock mass during selection of the powerhouse location. The utility was looking for good overburden conditions, as well as compatibility with the most important geologic formations. To choose an approximate location, EDP drilled four 350-meter-long vertical boreholes from the surface. The utility then performed cross-hole seismic tomography (Lugeon) tests to define geologic conditions. In addition, EDP carried out laboratory tests on the samples to evaluate mechanical characteristics of the rock.
The field tests and displacements measured during the different excavation phases led EDP to the conclusion that the vertical stress in the powerhouse area was almost equal to the weight of the overburden. In addition, the horizontal stress normal to the powerhouse axis was about 2.5 times greater than the vertical stress. These values correlated closely with those accepted for the initial studies of this type of support structure for the cavern roof and walls.
The excavations of the powerhouse caverns were simulated, in the design stage, using the 3D numerical model FLAC3D, supplied by HCItasca. EDP used this model to evaluate the induced ground stresses, as well as the loads transmitted to the support system. Predicted displacements in specific points in the rock mass around the cavities for the powerhouse and transformer hall were used to control the real displacements measured by extensometers installed during excavation.
The powerhouse cavern for Venda Nova II is about 350 meters underground. It is 20 by 60 meters, with a maximum height of 40 meters. A smaller cavern adjacent to the powerhouse cavern contains two power transformers.
Excavation of the two caverns was performed in two stages:
- First, the vault was excavated using two side-drift tunnels with four transverse sections connecting these tunnels. The contractor installed extensometers in the transverse sections. Then the contractor completed excavation of the main cavern arch and installed the shotcrete and rockbolt support structure.
- Second, the contractor excavated the rest of the cavern using the bench blasting technique.
Using raise boring for shaft excavation
Excavation of the vertical shaft for the upper surge tank was a huge challenge because of its great height (420 meters). Because of concerns about the environment and construction safety, EDP decided that the excavation would be carried out by mechanical means, instead of using explosives.
The raise-boring technique adopted for this shaft at Venda Nova II involved opening a downward pilot borehole 0.3 meter in diameter, then reaming this borehole to the final diameter of 4.5 meters. The main civil works contractor, Consorcio Somague/Moniz da Maia Serra & Fortunato (MSF)/Mota & Companhia, performed this work.
A crucial aspect was to guarantee verticality of the pilot borehole axis, given its long length and its link to the headrace tunnel. The directional control system adopted, supplied by Edil-mac of Italy, featured a built-in drilling head with automatic position reading and self-correction equipment. This method proved very efficient, with a deviation of only 13.9 centimeters at the bottom of the borehole.
Other smaller shafts at Venda Nova II also were constructed using the raise boring technique. These include the vertical shaft of the lower surge tank, with a diameter of 5.6 meters and a height of 58 meters, and two slanted shafts connecting the powerhouse and transformer caverns to the ventilation and safety tunnel. These slanted shafts each are about 100 meters long and have diameters of 3.5 and 2.1 meters.
Development of Venda Nova II began in 1997, with construction of roads and the access tunnel to the powerhouse. The main civil works began in mid-2000 and were completed in 2004. The project began generating electricity in August 2005. s
Antonio Freitas da Costa is project manager with Energias de Portugal, S.A. (EDP). Maria Eugénia Resende is structural engineer, Manuel Alberto Oliveira is project management engineer, and Vítor Ribeiro is hydraulic engineer with EDP. The authors were members of the Venda Nova II design and project management team.
The authors may be reached at Energias de Portugal, S.A., Rua do Bolhâo 36, Porto 4000-111 Portugal; (351) 220013191 (da Costa), (351) 220013174 (Resende), (351) 220013268