Lötschberg plans for long-term water inflow repair 28 May 2020

A new system of higher water-capturing capacity is the planned long-term reparation for the water inflow lining rupture into the Lötschberg baseline railway tunnel in Switzerland. A breach in the lining system of the east tunnel of the twin tube railway, comprising a drained waterproof membrane system and final insitu non-reinforced concrete lining, allowed up to 1,700m³ of sand and silt water to flow into the operating railway tunnel in two ingress events in February and March 2020.

Fig 1. Graphic of how the existing drainage system failed
Credit: SRF Swiss TV

During a recent media conference call, Stefan Irngartinger, a Project Manager of Alptransit for BLS, the company that owns and operates the Bern-Lötschberg-Simplon railway line, explained that the problem is hidden behind the tunnel lining and it is therefore impossible to see exactly what has broken until the lining is removed. From historic records it is known that the section of tunnel that has experienced the lining breach is through a zone of karstic limestone at about 2.5km from the south portal.

During exploratory drilling, geologists intersected a karst spring. To avoid uncontrollable water pressure building up in the host rock, the exploratory drill hole into the spring was left open to channel water in hoses to a distribution cabinet and from there, conveyed it out of the tunnel through a pressure line (Fig 1). Once the system was installed, the waterproofing membrane with its geofabric fleece, was installed and the final lining cast.

It is suspected, according to BLS, that a natural event, such as a mini or set of micro earthquakes, may have changed the water flow paths in the rock to cause more sand to be washed into the exploratory drill hole and block the transition hoses of the open water draining system. The resulting increase in water pressure caused a junction between two of the draining water hoses to break, and for the membrane of the lining system to be damaged and for water and mud to then penetrate into the east tube. The event, explained Irngartinger, is local and is not due to the chosen lining system.

During the presentation, it was confirmed that water also entered the west tube of the twin tube railway, but in a much lower quantity. After clearing the drainage lines, the system in the west tube stabilised.

Fig 2. Layout at the south portal of the Lötschberg railway and its drainage of the karst region

Water and sediment is continuing to flow into the east tunnel and is currently being managed by a provisional repair with the inflow being channelled on the inside of the tunnel to a set of sedimentation tanks and from there out of the tunnel (Fig 2). The location of the breach is also monitored continuously with installed instrumentation and video cameras. During this interim period, trains are required to reduce speed to pass through the repair zone.

The long-term repair, according to Irngartinger, who is the spokesperson engineer for this situation for BLS, is to remove the lining at the location and create a cavern, with a single access point, in the east side of the east tube to continue to capture the ground water and sediment and direct water out of the tunnel. During the conference call, Irngartinger said the cavern system would have a capacity of at least 1,000m³ and explained that the critical location of the draining system should be made accessible, so they can see what will happen there in the future. As the planning of the cavern is not yet finalised, he made no reference to estimated cost of the repair other than to say it would be a “good seven-figure to a low eight-figure amount.”

The system for the long-term solution will continue to dewater the rock and will require ongoing maintenance to ensure the hoses remain clear and to remove build-up of sediment in the capturing cavern. Exactly how sediment washed into the cavern of the system is yet to be clarified.

The proposed reparation is to be developed in detailed design for presentation to BAV, the Swiss Federal Office of Transport, for approval to progress into construction. BLS hopes to have the approval in the Autumn of this year (2020) and to have a contractor engaged to start construction by end of the year. Before approval, the plans of the proposal will be published and BAV must conduct a public consultation through which the national and local environmental agencies, the local state authorities of the Canton of Wallis, and other interested parties can be submitted and considered. According to BAV, this consultation period usually takes one to two years. It is planned that the process for the Lötschberg repair project will be concluded within a few, three or four, months. When contacted, BAV said that it had approved the BLS repair plan in principle, but that it was awaiting the plan in detailed design for the official approval process to begin.

As there is no alternative to removing the current lining of the operating east tube for the repair, it will be closed for several months as work is carried out. Rail traffic in the meantime will use the west tube in single-track, bi-directional operation and on a schedule aimed at maintaining the hourly passenger train services between the cities of Valais and Bern.

Exactly how environmentalists and the BAV officials will react to the permanent drainage of large volumes of water via the solution system is yet to be known. There is also the additional long-term maintenance cost for the BLS operating company to be considered.