The pump storage plant on the Vianden reservoir, Luxembourg, was put into operation in 1963 with nine sets of turbines. In the mid-1970s it was extended to include a 10th unit.

Pump storage plants are primarily used for peak load protection. When the National Grid has a surplus of energy the turbines pump water from the lower reservoir via the shafts to the upper reservoir. The surplus energy is thereby transferred into potential energy in the upper reservoir. At peak demand the water is released back through the turbines and the electricity (minus a little loss) is put back in the grid.

The Vianden plant has a capacity of 1,100MW with its 10 turbines. The operator, Societe Electrique de l’Our (SEO) is extending the capacity by 200MW through the installation of an 11th turbine, which uses the existing upper and lower basins, but has its own system of waterways and inlet and outlet structures.

The construction of the facilities to accommodate the 11th machine is under the responsibility of the consortium PSW Vianden Los One, consisting of the companies Zublin, Strabag and Jager. The contract includes, among others, the construction of an inlet and outlet tower in the upper reservoir, followed by a vertical pressure shaft(see figure 2).

Construction of the pressure shaft
The JV awarded the contract for the construction of the pressure shaft to Edilmac dei Fratelli Maccabelli. The pressure shaft extends over 300m from the upper reservoir down to the powerhouse level. After passing through a vertical bend, a 240m long, shallow inclined pressure tunnel connects to the power house. The vertical shaft, manifold and pressure tunnels are equipped with a steel armour, which is designed to accommodate the full, potential outside water pressure. The pressure shaft and tunnel will have an internal diameter of 4.5m.

The project area is located in the clayey to silty fine sand of the Siegen and Ems steeps of the Lower Devonian, whereby generally very low to impermeable ground water conditions were assumed. According to the tender documents, the raiseboring method was selected for the construction of the vertical pressure shaft.

After the contract was awarded in April 2010, Herrenknecht developed the 550 VF raiseboring rig. Here, customer requirements were incorporated. The successful factory acceptance test took place on October 5, 2010. Favoured by the compact, modular design, the rig could be delivered from the factory in Schwanau to the Vianden site with only a few transporters and it arrived on site on 8 October 2010.

The procedure
When using a raiseboring rig for a shaft construction in stable rock, the handling of the rig, over the shaft starting point, takes place in a first step by means of a crawler track or crane. From here the vertical pilot hole starts with the drill bit facing downwards (see figure 3, page 24). If necessary, the pilot hole can be carried out at an angle of up to 45°. The debris is flushed out during the pilot hole excavation by a flushing medium (usually water or air). Depending on the drilling depth, additional drill rods are successively installed until the target depth is reached in the existing lower tunnel or cavern. The pilot hole drill bit is then removed and the reaming head (typically 1-7m in diameter) mounted on the drill string.

The rig pulls up the reaming head with the drill string. Through the rotation of the reaming head, the rock is crushed and falls down where it is removed.

The shaft is reamed from the bottom to the top by the diameter of the reaming head to the target dimension.

Compared with the conventional shaft sinking methods, the use of a raiseboring method significantly increases the safety of the personnel, since they work outside the actual shaft. In addition, significantly higher construction rates can be achieved. The shaft wall is generally smoother and over-excavation is reduced. Removing the muck from within the tunnel is often more efficient than excavating through the shaft being sunk. Since fewer staff are needed to operate the rig than with conventional shaft sinking, significant cost savings can be achieved here.

The RBR 550 VF by Herrenknecht is one of the largest rigs of its kind ever used. It is distinguished by a compact, modular design and has a powerful and highly efficient centre-free drive. Herrenknecht’s variable frequency controlled drive concept llows for variable speed and torque control. The rig is designed for shaft lengths of up to 1,000m. The mechanized drill pipe handling ensures more efficient operations as well as greater safety of personnel during installation and removal of drilling rods.

Drilling Vianden
During on-site assembly, the construction team had to mount the RBR 550 VF rig in a 45m high inlet/outlet tower in significantly confined space. Nevertheless, the on-site assembly was completed in three days.

On 16 November, 2010, the rig was put into operation as planned and the 15in (381mm) diameter pilot hole was drilled. Once the pilot hole was sunk the 5,460mm diameter reamer was fitted and the shaft excavated at 10m per day on average with a maximum of 18m achieved in one day.

With the constantly changing geological conditions of loose to hard rock with maximum compressive strengths of up to 130MPa, the machine control system proved to be particularly favourable, ensuring a failure-free boring process. The time schedule for assembly, pilot boring, reaming and disassembly was met after 40 drilling days. On January 18, 2011 the drilling of the 282m long shaft was completed.

Outlook
The RBR 550 VF successfully completed its premiere drive at Vianden with high reliability. The pressure shaft for the 11th turbine of the Vianden pump storage plant was quickly and safely excavated. The commissioning of the turbine is scheduled for the third quarter of 2013. After its use in Luxembourg the next task for the rig is at hand: a 4,760mm diameter and 280m deep ventilation shaft for a bypass tunnel in Bolzano, Italy. At the construction site in Bolzano the crawler unit of the RBR 550 VF proved its worth, as the location was difficult to reach over tough terrain.


Breakthrough of the Herrenknecht raiseboring rig 550 VF Figure 1, The Vianden pump storage plant is near the German border Figure 2, a schematic showing the shafts and tunnels for the new turbine The 5,460mm reamer head of the RBR 550 VF breaks through Figure 3, raiseboring is undertaken by first drilling a pilot hole then reaming upward