Tunnelling the Ylivieska railyard
GRK has won a contract competition organized by the Finnish Transport Agency for the restructuring of the environment at the Ylivieska railway station. The fixed-price job on a very tight schedule entails the construction of a subway tunnel under the railway track while the rail traffic is kept running as usual.
“We’ll start work right after May 1st. According to the target set by the customer, it should be finished by the end of the year”, says GRK’s site manager Jarkko Heiskanen.
The requirement of a continuous railway traffic places tight boundaries on the timing and phasing of our work, and naturally also places tight demands on worksite safety. Topping this up with a demanding trench support and a high groundwater level, one can truly speak of skilled structural work.
“Our company does have experience from the soft sections of the Helsinki-Vantaa rail loop. The construction site at Ylivieska provides similar challenges, spiced by the traffic of the regular trains running through the site”, Jarkko Heiskanen ponders.
Controlling the groundwater
The soil of the Ylivieska station area has a low bearing capacity. It consists of soft clay and silty moraine. Shoring up the six-metre deep subway trench with sheet piles of steel is thus one of the tightest challenges at the construction site. Due to the low water permeability of the clay, the temporary lowering of the groundwater table will require extraordinary means.
“The sheet piles in the walls must be assembled very tightly. A 60-meter long permanent tunnel will be built in the trench. It will be cast in-situ with dense steel reinforcement”, says Heiskanen.
The tunnel measures 7 meters in width and 4 meters in height. Its roof and its walls will be cast to a thickness of 0.6 metres. Concrete drainage troughs will be built at the two ends of the tunnel.
Accurate work scheduling
The work will proceed in a tight rhythm, closing only one of the four tracks at a time. The trench will be fully excavated at the start of the job. The sidings will be phased in on tracks 1 and 4.
“The key point is to ensure that train traffic works as smoothly as possible throughout the construction period. Secondly, we must perform high-quality work efficiently and up to schedule”, says Heiskanen.
The value of the contract is about ten million euros. The work site will employ a maximum of 30 professionals, a minimum of 10.