Lötschberg inflow through lining system failure 30 Apr 2020

Ingress of water and mud into the 13-year old Lötschberg baseline railway tunnel has raised alarm in Switzerland by those who have criticised the railway owner and rail authorities for not taking the event seriously enough. BLS, the Bern-Lötschberg-Simplon railway company, and the Swiss Federal Office of Transport (BAV) have reacted to the event as a situation that has been managed and that the root causes of the incident require no additional immediate investigation.

High volume water and mud ingress halted traffic for cleanup and a temporary repair
Video footage: Supplied by BLS and published by SRF
Image source: Stefan Irngartinger, BLS AG and published by SDA

For Heinz Ehrbar, former Chief Construction Officer for the St Gotthard Baseline railway of the Swiss AlpTransit programme, of which Lötschberg is also part, this event is a “clear failure of certain elements in the lining system. Such a water inflow in a tunnel, sealed with a high technology sealing and drainage system, is not expected by anybody and that must be investigated and in combination with a well-defined risk management process for immediate management and permanent repair of the Lötschberg tunnel. All underground facilities in the world using the same final lining design should know the reason of this failure in order to be able to assess their own risks.”

The facts of the situation in the Lötschberg, as published in Swiss national media and as confirmed in calls by TunnelTalk to the BLS and BAV media relations representatives, state that the situation started on 6 February when a water inflow, rising to more than 100 litres/sec, broke through the lining in the crown, carrying with it significant fines to create a thick mud on the invert. The water ingress became known when a Swiss SBB passenger train emerged at the south portal covered in mud. Traffic on the line was stopped "for about a day", according to the Lötschberg media relations representative, while the situation was assessed, and a short-term solution applied. Suction was used to clean the invert and a steel channel was installed under the leak to convey inflow into a sedimentation tank for water only to flow into the installed drainage system.

First indication of a problem in the tunnel in February 2020

Some 37 days later, on 14 March, a second inrush at the same location overwhelmed the initial reparation system. A camera installed at the failure zone alerted the tunnel operating control room of the inrush, and train traffic in both tunnels was stopped. On this occasion, the closure of the affected east tube was for about six weeks with traffic diverted to the bidirectional traffic through the parallel west tube and onto the existing railway over the mountain, to clean the invert and upgrade the initial reparation measures. Criticism of the tunnel operator BLS, and of the Swiss Transportation Office, is that ten weeks after the first event, research into the causes of this failure in the tunnel lining remained a low priority. In a statement released on 22 April and in the telephone interview by TunnelTalk, BLS said that “the clean-up is complete, the drainage pipes have been flushed, the vault is cleaned”, and that, as of 5pm Friday 23 April, the tunnel is “completely accessible again”. Calls to BLS and BAV also confirmed the statement that to control the situation permanently, BLS is developing different approaches “with the goal to create a volume to accommodate future events whereby the solid material can be captured and cleared without overly affecting the rail traffic”. Plans are confirmed to be submitted by BLS in Autumn 2020 for approval by BAV to advance a proposed permanent repair solution. BAV is the Swiss Federal Government authority that approves all transportation project applications in Switzerland, both new builds and rehabilitation projects. In the call to the BAV press office it was unknown if BAV inspectors have visited the Lötschberg situation. "The BAV is working with the Lötschberg engineers and is confident that they are managing the situation," said the BAV media relations officer.

Figs 1 and 2. Leakage occurred at about 2.5km from the south portal of the east tube of the twin tube baseline railway

The BLS media relations representative confirmed in a phone call that the inflow occurred at about 2.5km in from the south portal of the twin tube tunnel through a known reach of karst limestone conditions and under an overburden of about 400m (Figs 1 and 2). At this point, the tunnel is designed and built with a drained waterproofing system using a plastic membrane in combination with a geofleece layer and is finished with an in-situ concrete final lining, with the membrane directing water to the drainage system running longitudinally in the centre and on both sides of the invert (Fig 3). The final concrete vault was cast using 12m long formwork shutters with stopends. With a continuously welded waterproofing membrane draining around the tunnel, waterstops between pours was not considered necessary. One of several technical questions submitted to BLS and awaiting responses is to know if the insitu inner lining is reinforced in the karst zone or unreinforced as per the base design. Ehrbar, who was involved with the Swiss AlpTransit baseline railway programme at the time, said that the waterproofing membrane materials and the drainage system was developed specially for tunnels by industry suppliers with the criteria and specification to meet the 100-year design life of the tunnel without significant repairs required. Tests, he said, submitted the materials and system to simulated environments of high rock pressures and high rock temperatures.

A drained membrane waterproofing system is applied to only specific reaches of the Lötschberg, to about 40% of its alignment. The reach through the karstic limestone at the south end was one of those reaches, and the need was confirmed during drill+blast excavation when high volume, high pressure water ingress was encountered, indicating extensive karst systems in the limestone. A review of as-built documents and reports also indicates that the water ingress during excavation was diverted to the invert with geo materials that were subsequently covered by the primary shotcrete.

Fig 3. A high quality fleece and membrane was installed to create a drained waterproofing system to comply with the 100-year design life specification

Video footage of the inflow makes it evident that the water ingress is at one of the cast lining radial joints. Both BLS and BAV agreed that something has changed in the mountain and suggest that a "mini earthquake might have changed the geological environment behind the lining." For Ehrbar, this is highly unlikely for a tunnel designed to pass through the Alps and for a 100-year design life. Questions for further thorough investigation include:

• What was the quality of the installation of the waterproofing membrane;
• Were there metal pipes installed to drain the karst during construction that have since pierced the waterproofing membrane;
• Were the initial water diverting ducts installed during excavation intended to be kept cleaned and open as part of the permanent waterproofing system;
• Is the radial joint ingress at the end of a section of installed waterproofing membrane, or is the leakage indication of a radial fault in the waterproofing installed membrane;
• Has blockage of the drainage channels caused a build-up of water pressure behind the lining to the point of causing a breaking in at the weakest point of the lining system;
• Was the waterproof membrane installed of a double layer design with injection points available for subsequent grouting of the system to repair any membrane leaks;
• Was the installed membrane of the correct quality to comply with the specified 100-year design life of the railway infrastructure;
• If so, how was potential failure of the waterproofing system of the tunnel lining included in the safety and risk management programmes of the long-term operating railway.
These questions have been put to BLS and a reply is pending.

These forensic investigations will contribute to the rehabilitation works of the infrastructure to meet the remaining 87 years of the 100-year design life specification and to design the repair to avoid a repeat failure in existing but unknown weak points.

The fact that the temporary fix of the initial February ingress was overrun by a second, large volume inflow in March, is evidence that the karst system of the limestone geology is under very specific hydraulic and hydrogeological conditions.

“As a civil engineer, and as a public servant who was involved on the AlpTransit projects in Switzerland since 1996, I believe these investigations are critical and that they must be undertaken without any delay,” said Ehrbar. “It is not understandable that the Swiss Federal Office of Transport does not insist on an investigation by BLS, based on standard risk management assessments, and publication of the root cause of the inflow, not only for the benefit of the future of the Lötschberg Basetunnel, but for other long and deep tunnel projects under construction and for the future. Safety first must be guaranteed always. Railway systems are only safe when the basic principles of risk management are respected without compromise.”