The Premadio II Project involves the expansion of an existing hydroelectric power plant located near Bormio, Italy (Provinzia di Sondrio, Lombardia). The aim of the project is to guarantee the electricity supply for the region around Bormio, a major Italian tourist area important for the Italian economy. The increase in tourism and the rapid development of the infrastructure of the town and the surrounding area has meant increased demand for energy.

The companies involved in the Premadio II Project include AEM SpA (Italy) as the client, Quadrio Curzio SpA (Italy) as the contractor and Voest-Alpine Bergtechnik GmbH. as the supplier of the excavation machine.

Work started on February 14 this year when the Voest-Alpine Tunnel Miner ATM 105-ICUTROC began excavating a tunnel which is a part of the large construction project

Work on the power plant includes extending the existing generator room to make room for an additional generator. The cavern uses a tubbing support system which is very sensitive to vibration and would be liable to damage by blasting if that had been used in the excavation. Power generation could not be interrupted by the work. Therefore a very smooth method of excavation was required to avoid any damage to the construction of the power house and all its installations. Because of the fact that blasting causes high and hard to control vibrations, also resulting in fracturing of the rock mass surrounding the existing power house cavern the contractor decided to use a roadheader for the enlargement of the new power house cavern.

The project area is located in the geologic formation called ‘Filadi di Bormio’, representing a sequence of metamorphic rocks with significantly developed structural elements like schistosity, joints and faults. The area comprises gneiss, mica-schist and phyllite, but the differences between the individual rock types are not significant. The majority of the rocks show a considerable content of chlorite as well as some quartz veins, but not very intense sericitization. The varying development of schistosity and joint planes is more important for the cutting process than the distribution of the individual rock types within the section to be excavated.

After a detailed analysis of operational conditions, it was decided to use the ATM 105-IC, a roadheader of the 100 tons class with 300 kW cutter head power with a low cutting speed (~1.4 m/s). This can cope with rock of up to 130 MPa uniaxial compressive strength and also high abrasivity values of up to 3.4 CAI (CERCHAR Abrasivity Index).

The generator room cavern extension is 30m high, 31m wide and 23.6m long, a total excavation volume of about 22,000 m³.

Cavern construction

The machine was dismantled and taken to an assembly chamber which crosses the cavern at the top. The cavern had to be excavated from the top to the bottom which means the machine had to cut its own declines to drop from one level to the next. The cavern is separated into nine levels with a height of about 3m. The machine cuts a first central adit about 5m high and from there it excavates each level. The machine will leave the cavern through an existing tunnel crossing the cavern at the bottom. Unusually for a roadheader in tunnelling, the ATM 105-IC is equipped with a short double chain conveyor, to provide as much mobility as possible, necessary for work in the relatively small confines.

Because of delays in completing the assembly chamber, the customer decided to use the ATM to excavate the starting chamber for the TBM which will bore the pressure tunnel for the additional generator. During the excavation of the tunnel the ATM 105-IC produced remarkable operational results. On March 6, it arrived at the chainage of 34m, equal to an excavated volume of 1180 bcm. Up to that chainage, the ATM 105-IC showed a net cutting performance of 125 bcm/h and a specific pick consumption of 0.18 picks/bcm. On the following two days, the ATM made a daily advance of 8m and 7m respectively, working in one shift operation.

Cutting performance

One of the main problems was the highly abrasive rock material (CAI values of up to 3.4) in combination with the high rock strength (uniaxial compressive strength values of up to 130 MPa). The machine was equipped with the 105-GI-57 cutter head tuned to the prevailing rock and rock mass conditions.

The reduced number of picks on the cutter head in comparison with a standard cutter head and the low cutting speed provided higher forces to the picks, which again resulted in a higher cutting performance. The size and quality of the tungsten carbide pick inserts were tuned to the stated rock strength and abrasivity which resulted in significantly lower pick consumption compared to the use of standard picks. Picks are cooled by high-pressure water flushing, which also suppresses dust. The telescopic cutter boom of the ATM 105-IC allows the optimum application of the 300kW of cutting power to the rock mass, and also allows the head to be sumped in without tramming. A vertical face can be achieved, even in high sections. A load-sensing hydraulic system optimises on energy utilisation which also helps reduce operating costs.