Site setup is strange on what was the largest construction contract in Austrian history. The Koralm Tunnel (Kat) Two contract makes up the central 20km of the approximately 33km drive and will be linked to the portals by the completed Kat One section, and the Kat Three section, which will go to tender in early 2013.

There is the possibility of access via the Kat One drive, but due to Kat Two site preparations beginning before the full details for Kat One were finalised, a monumental shaft has been sunk to facilitate all necessary access for the second lot.

Access
The 60m-deep access shaft is a double cell excavation, with each cell measuring at 20m in diameter, giving a figure of eight shape when viewed from above. The top 20m are secant-piled with 1.2m diameter piles, then it is wire mesh and shotcrete reinforced. Excavation was carried out by a heavy excavator then mucked out by a gantry crane.

"The shaft is so huge," says Wolfgang Lehner, general manager in Strabag’s tunnelling department. "You can lift a boomer down it easily, and when the TBMs are running with 1,000t per hour vertical conveyor systems, the whole cross section will be free for lifting equipment.

"It was a major logistics challenge to plan, but once set up it is quite simple."

Tunnels
The Kat Two lot covers works from the access shaft, some 3km from the eastern portal heading west towards Kat Three. Two single tubes (15.5km north and 17km south) will be excavated by two double shield TBMs.

Cross cuts will be constructed every 500m with an emergency station of 1km approximately halfway along the tunnel between the two tubes. This is a setup much in the style of the alpine base tunnels.

A total 4.5km of soft ground work across the Kat Two tubes will be excavated by NATM to launch the TBMs directly into the granite and gneiss.

Assembly caverns will be constructed by drill and blast with systematic rock bolting. Two Atlas Copco boom rigs, a Liebherr excavator and a BASF Meyco Robojet shotcreter will be used. Lehner adds that Liebherr made a dumper for mucking, which is quite unusual for the company.

Reinforcement in the NATM sections has been done with ribs and lattice girders at a 1.3 to 1.7m varied spacing. This is followed by layers of wire mesh with 200 to 250mm of sprayed concrete. Rockbolting was done with DSI self drilling rockbolts.

Lining will consist of precast segments for the main TBM section that will be cast on site to Zublin specifications using mucked material as the aggregate and rebar reinforcement. No poly fibres for fire protection are in use.

The bid
The Strabag/Jaeger JV was the only bidder that proposed a double shield machine, winning the contract in autumn 2010. Lehner said the contractor was surprised by this, but put the explanation down to differing experiences between the bids and methods. As well as the Strabag/Jaeger bid, client OBB received tenders from a Porr-led JV, Alpine and Marti, who all proposed single shields. The client did not permit drill and blast on the main section.

"Our experience on Niagara was probably important," adds Lehner. "Leading to a low cost technique. We also already own a lot of the equipment and we changed the construction programme – applying the final lining after NATM excavation works to improve progress."

Progress so far has been faster than expected due to good ground conditions. No water has been met and waterproofing will not be needed until final lining installation in 2017.

By the end of August, soft ground tunnelling will be completed and the process to assemble the TBMs will begin. The first machine will launch in January 2013, and the second will lag this by some three months.

Ground challenges
The Kat One contract is entirely in soft ground. Kat Two has 1.2 to 1.4km of soft ground, and is then through rock. Kat Three runs through a major fault zone until the third kilometer, then it enters crystalline geology for the remainder.

Lehner concludes, "I am obviously worried by the possibility of poor ground, the 1,200m overburden leads to a low density of investigation. The ground should be good, but there is a chance of rockfall, faults and squeezing ground.

"In terms of the main challenge to come, it will be a test to reach our promised average progress rate of 23m per day. We run two shifts of nine hours, with a six hour shut down for maintenance, probe drilling and services."

If the machines become stuck, a bypass tunnel will be constructed with drill and blast. Provisions have not yet been made for this, the plan is to set up the site properly when approaching a fault.

Regarding the Kat Three tender, he adds, "of course we are tendering, it’s one of the biggest coming projects in the region".