The state of Tyrol in western Austria is the setting one thinks of when thinking of the Alps. Sharply rising mountains either side of an emerald valley, streams of water turned pale blue with minerals, traditional cowbells clunking as the animals move in small herds. The energy demands for the region are growing however, and to stay environmentally friendly a new hydropower scheme has broken ground. The Gemeinschaftskraftwerk Inn (GKI) project is owned by Austrian (86 per cent) and Swiss (14 per cent) business interests. It will provide electricity for approximately 100,000 homes in a region with a population of around 700,000.
Scope
Contractor Hochtief selected two identical 6.53m-diameter Robbins double shield TBMs for the job. Tunnelling work is needed to construct a headrace tunnel just under 22km long and 5.8m in diameter. This will be constructed by a 12.1km southbound drive and a 9.4km northbound drive. The tunnel will transport 75m3 of water per second from a 15m-high weir down to a pressure shaft, which will drop down to turbines in a powerhouse. It is expected to come into operation in 2018.
Drill and blast access work
Although TBM technology has been selected for the main drive, drill and blast excavation up to 80sq.m was used to excavate a curved access tunnel that ends in a cavern (for a total 800m length of excavation) and then the 20m southbound TBM’s launching stub. Running parallel to the tunnel back towards the north is another 20m stub tunnel, which at the time of Tunnels and Tunnelling’s visit in October was waiting for the second TBM (a four-month gap between machine launches was in the schedule).
Two access tunnels were originally intended, but due to scheduling concerns this single access excavation was proposed to the client according to Hochtief project manager Stephan Assenmacher. Equipment used for the drill and blast phase included a three-boom Sandvik Axera TII rig, a Liebherr excavator, two Liebherr wheel loaders and a Meyco Potenza sprayer. Girders and mesh were required in early challenging locations, which was reduced to just the plastic fibre reinforced SCL.
Tunnelling conditions
Geology is expected to primarily consist of 70MPa schist at up to 2,000m overburden and there are two fault zones expected. Squeezing ground is one of the major concerns according to Robbins field service technician Andy Birch; the machine can overbore by 25mm. In really extreme cases this can be extended to 50mm, but work would be required to lift the main bearing if this level of overcut is ever needed.
There is also an emergency break out thrust of 57,000kN at 450 bar in case of squeezing ground.
An older style of gripper system has been put in place in for these machines according to Robbins field service engineer Miguel Panteghini. Instead of horizontal opening, the grippers open vertically but with an angular tendency.
They need good rock but allow more space inside the machine "this feels like a larger machine to work in," adds Panteghini. In poor conditions, the machine may run in single mode.
Lining
The tunnel is lined with cage reinforced precast concrete segments in a 4+0 configuration, unbolted and unsealed. Hochtief has reportedly had bad experiences recently with fibres and so prefers cages.
Concrete is from an onsite batching plant operated by Hilte and Jehle. Some 48 Herrenknecht Formwork moulds on a carrousel system equipped with a 55oC steam curer are being used to cast the segments. The segments themselves are designed by Vigl and are 270mm thick, 1m wide.
Segments are supplied to the machine via a multipurpose train consisting of four segment cars, one miscellaneous car, two for pea gravel, 10 for muck and one locomotive. California Switches allow additional muck cars to be on standby near the machines to keep a good advance rate going.
Mucking and environment
Excavated material will be stored on site and used for lining purposes where possible. This is to reduce the amount of material requiring transport by road. Unsuitable material will be transported by an Agir conveyor from the portal to on site storage for later disposal.
A client statement added: "An important aspect when storing the excavated material is […] the possibility of returning the area to agricultural use after the completion of the construction work. For this reason, the fertile topsoil is removed and stored so it can be put back in place. Subsequently, the area covered by the storage area will be replanted."
Journey to the face
The machines were manufactured and assembled (one partially, one full) at the factory in Narni in Italy. They were shipped 800km, a journey that took in the Brenner Pass.
According to Andy Birch, there were a few abnormal loads in terms of weight, but the bulk of physical size of the loads was not a particular concern. The cutterhead were manufactured in two pars and welded on the front side. This should also make the machines fairly easy to disassemble and remove from the tunnel when the job is done, says Birch. When moving the machine to the face, locos synchronised by one operator (in terms of power and revs) pushed the machine while it was being pulled by an excavator. Specialised transport dollies, articulated by a spigot and pin carried the machine.
Lessons Learned
Martin Rauer, project manager for Robbins Europe said that lessons had been learned from the Kargi hydroelectric power plant: "Grout injection techniques, forepoling, peripheral shield lubrication, water inflow control and a large overcut capacity. That is basically what we have taken from earlier work. Essentially bringing NATM principles to TBM technology.
"At Kargi we implemented a canopy drill to allow pipe tube support, increased the torque by 50 per cent, and the advance rate doubled"
