After two breaches of water and sediment ingress through a failed joint in the final lining of the east tube of the 13-year old Lötschberg twin tube high-speed rail link in Switzerland in early 2020, construction of a CHF15 million (about US$16.7 million) permanent solution has been completed. Starting in early September 2020, BLS, the the owner and operator of the Bern-Lötschberg-Simplon rail line, gave notice to its contractor Frutiger to begin excavation of the parallel water and sediment management cavern solution designed by its design consultants srp engineers, Emch + Berger and Pini Swiss Engineering, and its geotechnical consultant Kellerhals + Haefeli.

Fig 1. Design of the permanent management works

The permanent solution for continuing safe operation of the railway and management of the geological ground water hydrology within the overburden is to breakout of the existing east tube running tunnel at two points and excavate a cavern to house a set of water and sedimentation settling tanks. The cavern, with its sloping invert, is 89m long x 11.40m wide x 5.7m to 11.50m deep and the settling basin has a capacity of more than 2,000m³ from which settled water will continue to be channelled to the surface outside the south Raron portal of the railway and from there into the River Rhone, which passes about 400m from the portal. Sediment will be cleared regularly as it builds up (Fig 1). As an immediate remedy in the east tube, BLS installed a steel sedimentation tank in the affected area to permit rail traffic until closed again for the cavern excavation works.

Under its negotiated construction contract of about CHF12 million, Frutiger began with the two breakouts, of about 25m wide each, from the main running tunnel in early September and completed about 10,000m³ of drill+blast excavation and the casting of the internal concrete works by end of January (Fig 1). Works included excavation of a cavity of about 8m x 4m x 2m above the crown of the east running tunnel at the point of the lining breach, in which karst water is to be collected and piped to the sedimentation basin.

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  Works begin with breakout of the east tube lining

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  Drill+blast excavation progress

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  Mucking out into the main tunnel working area

The contractor for the M&E technical equipment, including measurement and control technology, and water pipe systems is Rhomberg Bahntechnik and InfraTech working to designs completed by consultant eprotraffic and the BLS specialist services. Completion of all works is expected by end of February. This will be in time and in readiness for the Spring snowmelt on the mountains above. About 2,700m of 400mm diameter steel pipework with an internal mortar coating has been installed as part of the works, with another 300m of 250mm diameter Inox-tubes in the cavern.

Primary support in the cavern topheading

Works train in the main running tunnel

Access provided via the closed east tube from the Raron portal of the railway

During the works, the east tube of the operating railway was closed, with trains either diverted to other lines or running in a managed bi-directional operation in the west tube alone, providing 24hr/day, 7 day/week access for the works. Drill+blast excavation of the cavern progressed on a topheading and bench sequence with muck transported out via railbound works trains through the Raron portal about 2km to the south. The existing operating tracks at the works site were covered with temporary backfill to permit excavators and drilling equipment traffic. “It was a logistical challenge to get all the materials in and out of the tunnel by train,” reported Stefan Irngartinger, the works Project Manager for BLS, “but with experience, logistics ran very well.” According to Irngartinger, there were only minor water inflows from joints during excavation, with a 10cm karst joint encountered at the north end of the cavern. Blasting vibrations had to be well controlled to permit train operations in the west tunnel at about 40m parallel.

After main excavation works, the plan was to open the east tube to traffic and use engineering hours to complete finishing works. BLS however decided to keep the east tube closed until all work was completed. The works programme includes rehabilitation of the east tube to repair its final lining, to finalise the junction of the two permanent adit openings into the sedimentation cavern, and to reinstall the overhead catenary power supply and radio and the signalling cables, which were removed for the construction work. An intense cleaning of the east running tunnel was also then required before its reopening. “The original plan was to carry out this work during night shifts in order to be able to resume rail traffic in the eastern tube during the day,” said Irngartinger. “However, since there was a good experience of the closure of the east tube and the single-track bi-directional operation in the west tube, permitted by reduced train traffic - not least due to the corona pandemic and with redirection of traffic through the Gotthard baseline railway - it was decided to continue full closure of the eastern tube until the end of February to carry out these final works more efficiently on 24hr/day operation.” There are no doors in the two adit openings in the running tunnel. As Irngartinger explained; "if there were doors, ventilators would have been required for the cavern’s climatization. The main openings are left open so that the cavern is ventilated by suction and pressure of the train traffic."

A press release from BLS stated that, "with the implemented cavern solution and its large settling basins, BLS reacted quickly to the natural events of the Spring last year and within a year. In the future, this solution should prevent mountain water from entering the main running tunnel carriageway, and direct it rather from the crown area of the running tunnel into the sedimentation basin. If the water is mixed with sand, it settles in the pool and the cleaned mountain water is drained out of the tunnel. However, BLS admits, the geological situation still harbours risks, which is why BLS is investigating the effectiveness of the system after the resumption of rail operations."

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  Works begin on the internal structures

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  Settlement basin structure

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  Completed settlement basin and its pipework

In response to questions asked by TunnelTalk, BLS said that a detailed measurement of water inflow was not possible during the construction period and that monitoring and measurement instrumentation has been installed as part of the operational equipment of the basins. “We estimate an inflow of 20 litre/sec to 40 litres/sec.”

The basins have a sedimentation volume of about 2,000m³ (2 x 1000m³) and BLS said that it is impossible to estimate the volume of sediment washed in during an incident. "We will start clearing sediment from the basin when it is half filled. If there are no larger incidents, we will clean the basins once a year. We need to gain experience with the sediment washed in during the Summer and Winter for future operation of the basin. Maintenance works in the basin cavern will be carried out by loaders and works trains during the eight hours of engineering periods in the operating railway on Sunday/Monday night each week."

With completion of the excavations works, questions remain about the long term draining of the ground water and washout of sediment from the mountain and the potential consequences. The permanent works also raise queries about the environmental regulations for management of natural water regimes around excavation projects both in Switzerland and internationally. Regulations to protect the natural environment have been strengthened in recent decades and complying with these regulations are significant and important processes to be managed by owners and developers of new underground infrastructure.

Excavation of a 150m long purpose-designed cavern, aligned parallel to the running tunnels and connected with two adits to the east tube of the twin tube Lötschberg railway tunnel in Switzerland, is at the heart of the works planned to solve the water inflow, final lining, ruptures in February and March of this year (2020).

Plan of repair works and renovation excavations
Credit: Stefan Irngartinger, BLS

The plan is for water in the karst system of the zone of the initial water break-in, in both the east and the west running tunnels, in March, to continue to be captured and piped out through the southern portal of the east running tunnel tube for on-going commercial resale to businesses in the Raron district of the Wallis Canton.

In removing the inner lining, a cavity of 8m x 4m x 2m is to be created above the crown of the east running tunnel at the point of the lining breach, in which karst water is to be collected. From there, flow is to be piped through the 30m long adit into sedimentation ponds in the cavern and arranged in series of 45m long x 11m wide x 5m deep series and with a 2,500m³ capacity (Fig 1). The clean water is to be piped out of the sedimentation cavern, through the second 30m adit and out vie the 2.5km of the east tube to the Raron portal.

Accumulated sand in the settling bays will be dredged and transported out of the tunnel as part of scheduled night-time engineering maintenance periods. The cavity in the vault above the east running tunnel, in the karst zone that is the source of the ground water, is also accessible for on-going monitoring and maintenance.

Water and mud inflowed in through a concrete construction joint of the 13 year old operating rail tunnel in February, March and April 2020

The system of the new works, estimated at about CHF 15 million (USD$ 16 million), is designed to manage more than double the capacity of the 1,000m³ of water and mud that flowed into the east running tunnel during the breaches in February and March. BLS, owner and operator of the Lötschberg railway, installed steel sedimentation tanks in the east tube of the tunnel as an immediate reparation measure to manage the inflow. Water has continued to flow into these intermediate sedimentation tanks, with some cubic meters of sand being deposited in the east tube and the pools being emptied once a week. In a press release, BLS states that “no water has penetrated the less severely affected west tube of the tunnel, since the second entry of water on 13 March. The temporary settling tanks in the east tube will remain in place until the new cavern and its facilities are into operation.

The long-term remedial plan developed by BLS, has been submitted to the Swiss Federal Office of Transport (BAV) for approval. The objective for BLS is to complete the renovation works as quickly as possible and before the next Spring snow-melt in 2021. If approved by BAV without undue delay, work could begin in September with a programmed four-month construction period progressing to mid-December. Preparatory works are planned to start in August. During the works the east running tunnel of the railway tunnel would be closed with traffic running on a bi-directional schedule through the west tunnel.

According to a press release statement from BLS, the project is to be financed through the performance agreement with the Federal Government.

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.