In-service rail infrastructure rebuild 10 Dec 2020

The 451m Kuckuckslay tunnel in Germany is a single-tube double-track railway tunnel built in 1871 and located along the River Kyll between Trier and Cologne. The EKT joint venture of Porr and Alfred Kunz was commissioned by the German rail operator Deutsche Bahn DB Netz in 2018 to refurbish the facility and replace the original lining of sandstone blocks backfilled with stones. Construction began in February 2019 and is programmed to be completed by Summer 2021. The €19.15 million contract includes the preliminary work for the closure of one of the rail tracks, while the other stays in service, the enlargement of the cross-section, and the finishing of the construction work.

Enlargement working platform unit and protective shield ready for launch

The in-tunnel rehabilitation method is a tried and tested process by Deutsche Bahn using a movable construction rig installed on its own set of rails to enclose the operating track and allow train traffic to continue as the work progresses. Outside the protective unit, work takes place concurrently to demolish the existing lining, expand the cross section using drill+blast and install immediate support. Once completed, a special formwork will be installed to cast a new inner concrete lining as a follow-on operation. In a presentation for the 2020 Geomechanics Colloquium in Salzburg, Stefan Vetter of DB Netz, Mario Galli of EKT JV and Peter Steiner of Laabmayr Consulting, explained the details of the process.

After mechanical removal of the existing lining and drill+blast expansion of the cross section, a regime of rockbolts and shotcrete was installed for immediate support. Both steps are performed on the same working platform and cyclically at a few metres apart.

Services continued as drill+blast enlargement and rebuild progressed

At Kuckuckslay the EKT JV used a 36m long x 160 tonne tunnel enlargement unit (TEU) fabricated by GTA Maschinensysteme. Similar in design as units used on previous projects, the unit consists of three sections – a shield, the main unit and a backup gantry. The shield has hydraulically adjustable support plates and is always within the existing rail envelope, preventing the existing lining from breaking ahead and falling to the operating track. The main working unit is fitted with demolition hammers and drilling equipment to drilling charge holes and rock support anchors and for spiling presupport as required by geological conditions. The supply devices, such as hydraulic and power units, ventilation equipment and lay-down areas for supplies were mounted on the backup gantry. Excavators removed the demolished lining and excavated enlargement rock from the tunnel.

The portals of the Kuckuckslay tunnel are crossed by a service and cycling route, which was used as a construction road for the works. To set up for the works, a temporary concrete canopy was constructed at each portal using the cut-and-cover method.

A specific ventilation system protected workers in the confined operating railway environment

While the cut-and-cover canopy at the north portal was on a flat foundation, the 26.5m long cover in poorer geological conditions at the south portal had to be supported on a foundation of bored piles either side of the operating rail tracks. After being destroyed and partially restored during the Second World War, there were uncertainties regarding the static loadbearing capacity of the rebuilt sections. There were also logistical and programming considerations and for these reasons, instead of a roof foundation on solid bored piles, an alternative foundation of micropiles was proposed and implemented. A major advantage of the micropile foundation was that the drilling work could be carried out with lightweight drilling equipment instead of heavier bored piling equipment.

Once the TEU working unit was launched, demolition, excavation and primary support work progressed continuously, 24 hrs/7 days a week. Excavation blasts took place during possession outages of the operating track and after close coordination between the construction supervision managers, railway operators and the construction management. Track possessions depended on availability and were as short as possible. As many as 45 trains per day ran on the one bi-directional operating track during the works. Depending on the geological conditions, blast rounds were 1m to 1.5m with the enlargement unit running on rails mounted on temporary reinforced concrete foundations at either side of the rail tracks.

Breakthrough at the North Portal of the Kuckuckslay tunnel in July 2020

In the enclosed and limited working area, occupational health attention was required to protect workers from exposure to air pollutants, particularly to diesel emissions from the continuing train operations and the dust generated during the lining demolition, blasting and shotcreting operations. In the planning phase, a suction ventilation system was provided for the working areas on the working unit, as well as a pressure ventilation system for the main cross-section. The natural airflow was generally from north to south, with the ventilation situation affected also by the bi-directional train traffic.

Due to a requirement of the fire department that any haze, especially from smoke, had to be taken away from the main approach direction of the emergency services, the ventilation plan had to be designed from south to north. This was achieved by installing jet fans on the TEU which generated a vacuum in the main cross section into which dust was sucked from the TEU working areas.

End of excavation for start of casting the inner concrete lining

The ventilation and air quality management system was supplemented by the use of water-spraying devices to suppress dust. The dust conveyed in the direction of tunnelling could also be suppressed with water spray.

With this plan, it was possible to meet the occupational safety limits at all work stages and minimise the use of supplementary personal protective equipment including respirators.

Progress of the TEU, for removal of the existing lining, expansion of the cross-section from 24.10m to 25.47m on the inner radius, and installation of primary support took a total of about 200 days, working two 12 hr shifts per day, 24 hrs/7 days a week and achieving an average advance of 2m per day.