Rastatt status one year after the collapse 06 Sep 2018

When the leading TBM drive of the new twin tube tunnel on the Rhine Valley rail route collapsed on 12 August 2017, bringing a halt to all rail traffic on the existing surface tracks of the mainline between Germany and Switzerland, intensive work was carried out to repair the damage to the surface railway and to stabilise the area above the new tunnel tubes beneath. The leading east tunnel drive of the parallel TBM single-track tubes of the new rail route, designed to double capacity on the international railway, started four months ahead of the trailing west tunnel drive, with both TBMs launched from the north portal work site in Ötigheim and progressing south to the south tunnel portal in Niederbühl (Fig 1).

Fig 1. Details of the leading TBM drive collapse as provided by Deutsche Bahn

At the time of the collapse, the leading east tunnel TBM was about 300m ahead of the trailing west tunnel drive and had progressed about 40m beyond the surface railway underpass when the segmentally lined tunnel below the surface railway collapsed. Excavation of the 10m diameter TBM tunnel at a reported 4m beneath the surface rail tracks and through soft waterbearing ground was supported by horizontal ground freezing installed from two access shafts about 100m either side of the surface rail route.

About 10,500m³ of concrete was pumped into the collapsed east tunnel and the trailing west tunnel TBM was halted in order to secure the rail line above it against further subsidence. A massive concrete slab of 270m long x 1m thick and comprising about 3,000 tonne of concrete and 540 tonne of reinforcing steel was built to restore the damaged surface tracks. At the same time, lines were installed so that concrete could be injected in the event of any subsequent subsidence.

On 2 October 2017, traffic on the Rhine Valley railway rolled again and the trailing west tunnel TBM continued towards the south portal at Niederbühl. After 3,672m of its 4,030m drive on the 4,270m long tunnels, the trailing west tunnel TBM reached the Rhine Valley surface railway tracks in early December 2017, where it has been in a waiting position ever since.

Divers employed to place underwater concrete base slab in south cut-and-cover work in Niederbühl
Photo: Sebastian Roedig, Deutsche Bahn AG

With investigations into the cause of the collapse still ongoing, excavation of the last 60m and 360m of the parallel east and west TBM drives respectively remains on hold. Nevertheless, construction work continues on other elements of the project.

Cut-and-cover transition works

To the south, the last 200m of underground alignment for the parallel rail tracks is being excavated using the bath-tub cut-and-cover method. The alignment, from the Niederbühl portal to the ground freezing installation access shaft, which doubles as the TBM final breakthrough reception shaft, is first supported and sealed with sheet piling. As the core is removed, the excavation fills with groundwater from below. Once a section of approximately 50m long between lateral sheet-piled walls is excavated, a base slab of underwater concrete is installed with the assistance of industrial divers.

Work on the underwater concreting started at the end of May 2018 with the teams of divers working around the clock for two days, in poor visibility and cold water, to deliver about 4,000m³ of concrete to seal the bottom of each 50m section. Once complete, the water can be pumped out and the next section is concreted.

Also progressing in the south is a 250m long sonic boom dissipation structure. Wide ventilation slots in concrete elements allow the air to escape as soon as the train enters the transition ramp to the surface and prevent the sonic boom as the pressure wave ahead of high-speed trains suddenly escapes at the tunnel exit.

Work is also underway at the north portal in Ötigheim, to connect the northern 50m open-cut bath-tub section of the tunnel portal and its sonic boom dissipation structure.

Removal of the concrete plug to access the backfilled collapsed zone
Photo: Frank Giesen, Deutsche Bahn AG

Works to recover the collapsed heading

As an immediate measure on 12 August 2017, a concrete plug was inserted into the tunnel below the buckled surface tracks to stabilise the area and separate the intact tunnel to the north. The total distance from the concrete plug, through the backup and into the TBM shield to the tunnel face, a distance of about 160m of tunnel and including a muck haulage train of muck skips and locos, was filled with backfill concrete.

At the beginning of February 2018, the first work began on dismantling the 2,000m³ concrete plug after the rails for the TBM supply trains from the north working access portal were dismantled and the invert of the approximately 8.5m i.d. segmentally lined tunnel was filled temporarily with gravel to host a wheel-based excavator and haulage trucks to remove the concrete. After dismantling the concrete plug, about 6m or two rings of the segmental lining, of the backfill concrete was also removed. The distance to the last gantry of the buried TBM backup is about 40m to 50m and the DB spokesman confirmed that no decision is yet made for the method of removing the concrete and uncovering the 93m long TBM.

After dismantling the concrete plug, a shaft was built onto the segmentally lined tunnel to serve as an escape route and as a logistical and supply access shaft of further construction work, including, for example, the construction of a total of nine cross passages in the completed lengths of the parallel tunnels from the north portal, which will also progress to avoid further slippage of the project’s construction programme.

Core drillings should help clarify cause of the collapse
Photo: Charlotte Petrik, Deutsche Bahn AG

Extensive exploration program

Since mid-May 2018, the Ed Züblin/Hochtief Rastatt tunnel construction JV and Deutsche Bahn has been conducting a drilling program to broaden the investigation of the geological conditions in the Rhine Valley railway corridor and in its surroundings. A total of about 60 boreholes of up to 25m deep will be drilled over a length of about 300m of the rail tumnel route, nine of which will become piezometer for groundwater measurements. The boreholes each take between two and four days to complete and the program is scheduled to be completed by mid to late September 2018.

Geologists evaluate the test drillings and send the material for further analysis to a laboratory. On the basis of the drilling results a concept for completing the last 60m of the collapsed east tunnel and the final 360m of the west tunnel under the surface rail tracks and into the TBM/ground freezing reception shaft can be developed.

Evidence-gathering and conciliation procedures

Currently, the evidence collection to clarify the causes of the TBM tunnel collapse is still ongoing, as is the arbitration procedure agreed between DB and the Rastatt tunnel consortium. An end of the procedure, with results on the reasons for the collapse, is not expected before the beginning of 2019, said the DB spokesman.

Fig 2. Assessed economical losses caused by closure of the mainline railway when the new rail tunnel drive underneath collapsed

In addition, Deutsche Bahn has drawn conclusions from the incident in Rastatt and the consequences of the subsequent closure of the Rhine Valley mainline railway for passenger and freight rail traffic on the rail network throughout Europe. In April 2018, a study by the Hanseatic Transport Consultancy, commissioned jointly by two European freight railway associations (ERFA and NEE) and UIRR, the International Union of Road-Rail Combined Transport, reported that unprecedented disruption to rail freight-based logistics chains throughout Europe by the closure of the Rhine Valley railway corridor totalled economic losses of more than €2 billion (Fig 2).

As direct results of this economic impact, DB conducted an internal disruption report, and processes were reviewed, adapted and improved. As part of its study, the European freight railway associations concluded that; “this man-made disruption highlighted the incompatibility of national monopoly-based rail infrastructure management and the increasingly cross-border rail freight traffic that moves across the European Union”. The associations are also collaborating with other European working groups, to develop a handbook for rail infrastructure managers for optimum reaction in the event of an incident and to provide guidelines for uniform processes and better communications for cross-border railway undertakings and other key operational matters. The German railway sector also founded the round table on construction site management to optimise construction processes, construction communication and to improve other processes associated with the country’s programme for modernising its rail network.