Lötschberg water inflow explained 07 May 2020

After publishing news last week of high-volume water, sand and silt ingress events into the Lötschberg railway tunnel in Switzerland, the BLS Bern-Lötschberg-Simplon railway owner and operator has provided additional information in the form of answers to a set of questions that further explain the source of the ingress events.

Fig 1. Exploratory drill holes intersecting the water source karst feature
Source: Supplied by BLS

With confirmation of no other known dramatic failure in a tunnel anywhere in the world with a similar drained waterproofing membrane system, engineers in the industry found it incredible that the ingress in the Lötschberg could have been caused by rupture of the waterproofing membrane. It had to be something else and that has proven to be the case.

In written answers to the TunnelTalk questions, Dr Stefan Irngartinger, Construction Officer and Project Manager for the Lötschberg Baseline for BLS, explained that: “During excavation of the tunnel, several pre-excavation exploratory drill holes of up to 80m long were drilled in the crown and ahead of the face in the 500m long karst zone (Fig 1). “One of these drill holes (R37) encountered a karst spring, which drew large quantities of water. Since this bore could not be sealed, the water was collected in a separate pipe and led out of the tunnel. The current ingress site is located exactly at that joint of the tunnel lining where this pipe is led through the sealing system and the insitu concrete lining into the interior of the tunnel. We assume that there is no damage to the waterproofing membrane system, but rather to the passage of the pipe into the tunnel. We assume that the tunnel sealing system has not suffered any damage. A systematic failure of the sealing system through operation of the tunnel was not considered as it is designed for a service life of 100 years. We assume that the damage has occurred where the pipe of the pre-excavation boring leads into the tunnel.”

Karst feature encountered at north end of the alignment
Credit: Geologengruppe Lötschberg-Basistunnel, swisstopo 2013

The decision to the capture of the karst water spring for supply to a village community in the area that had suffered a lowering of the ground water table and its water sources by the tunneling activity, is noted by a geologist involved with the project at the time in a summary geological report of the Lötschberg base tunnel published by the Swiss Geological Survey⁽¹⁾. Translation of the report by the geologist explains that the exploratory drill hole R37 intersected a larger water deposit in the limestone, which initially contained approximately 30 litre/sec of water and washed out several tonne of brown sand. The water remained cloudy for several months. The pressure could not be measured, but 8 bar represents a minimum, since a certain pressure relief took place via a gap.

The water was collected in a measuring basin and led to the Rhone by a pipe. “Sporadically, especially during the melting of the snow, the water was still cloudy, but in 2005 it was hardly detectable. In order to use the water, an attempt was made in early 2005 to drill out the former borehole and to recapture the water. The system now consists of two separate drains at a distance of about 5m, which collect the water at the crown and drain it to a valve chamber in the sidewall on the west side of the tunnel. From this valve chamber (in addition to an overflow into the mountain water drainage pipe in the tunnel floor), the water pipe leads to the cross passage at km 48.53 and from there to the portal of the pilot gallery” (Fig 2). Clarification from BSL is that the water collected from the karst spring is used since 2005 in a concrete plant located at the south portal of the tunnel.

Fig 2. Layout of the excavations at the south portal with water capturing system added
Credit: Base image by fib-ch.epfl.ch Tunnels publication

In providing further information about the recent inflows, BLS confirmed that “inflow of water could not be measured but is estimated at approximately 50 litres/sec to 100 litres/sec, at an estimated 5 bar pressure, and that about 1,000m³ and 700m³ of sand and silt came in with the water respectively during the first ingress event of about 14 days in February 2020 and during the seven days of the second break in March 2020.

"We currently consider the occurrence of a larger cavity behind the tunnel lining to be unlikely. While excavating the tunnel no karst structures were encountered in the immediate excavation area. The karst spring was only encountered in one of numerous pre-exploratory drill holes at about 10m above the crown. We assume that the karst system is a wide network of fissures and karst tubes with an opening width of smaller than 200mm. We therefore assume that the infiltrated sand and silt into the tunnel is exclusively fracture fillings."

BLS confirmed that BAV, the Swiss Federal Office of Transport did not send an inspector to see the situation in the tunnel after the first or after the second break ins and that it is in contact with BAV and will send the plan to BAV in Autumn for approval to progress with the permanent reparation works. “Approval by the authorities and procurement of the construction company will then run in parallel so that construction work can begin probably in Winter. The costs of repairing the original damage event are covered by insurance. It is probably not possible to call on the construction warranty, as the maximum warranty period was ten years, and, since there is probably no systematic defect in the sealing membrane, no product liability can be claimed.”

Water inflow through a concrete pour construction joint in the final lining

In confirming details of recent events, BLS explained that: "After the first break on Thursday, 6 February, the east tube was closed. On Friday evening, 7 February, the west tube was closed also for half a day. On 21 February, the east tube was reopened. During the closure, the mud was cleared and interim channels were installed to direct the water into sedimentations basins where the water clears and flows back into the drainage system of the tunnel.

"After the second water break in on Friday, 13 March, alerted to the operators in the operation centre by a camera that was mounted in the tunnel after the first water break in, both rail tunnels were closed from Saturday morning. The west tube reopened on Sunday noon, 15 March and the east tube reopened on Friday, 17 April after being closed for six weeks. During that time, trains could use only the west tube of the south section. The closure of the east tube had no impact on the timetable/traffic since, due to the coronavirus control isolation directives by the Swiss Government, there were fewer trains using the Lötschberg baseline railway." Normally the base railway tunnel, for both directions, carries about 60 freight trains and 50 high speed passenger trains/day.

During the closure after the second break in, the wood basins of the first reparation system after the first break in were replaced with steel basins – five basins of 5m³ each in the east tube and one of 5m³ in the west tube. The third tunnel closure of 30 April was not due to a new water ingress, but to an uneven filling of the sedimentation tanks installed in the tunnel. The sedimentation volume will be increased as a result.

Queries of the construction design of the tunnel confirmed that: “All of the 34.6km of the base tunnel have a waterproofing system, 19.7km with a sealing membrane and drainage layer, and with a drainage layer only in 14.9km, in geological units where no water was found during excavation. The membrane waterproofing system is of the umbrella sheet system with the sealing PVC membrane supported by a drainage layer. In both tubes, there are no known leaks or other problems with the sealing system. In the 500m karst section, there is no steel reinforcement of the cast insitu concrete inner lining. The sealing materials have been tested at EPFL, the Federal University of Technic in Lausanne, and the EMPA Federal Institute for Materials Testing. The installation conditions were surveyed. We have no indications that the quality requirements were not met during the installation of the tunnel sealing. Before the first event, the tunnel drainage system was well maintained. The system became clogged due to the inflow of sand and silt."

In the interim, ahead of the permanent repair works being implemented, BLS technical inspectors will survey the concerned area with the basins in both tubes at least once a week and if necessary clean the basins. In answering the question if there will be any removal of the inner concrete at the point of the water inflow to investigate the situation behind the concrete, the reply was, “this is an option. We are currently evaluating different possibilities and systems to handle the inflow. The planning of the long-term repairs in the tunnel is actually going on. We do consider several procedures. Depending on the selected system, exploratory drillings or measurements of temperature will be carried out."

Layout of the Lötschberg base railway tunnel with plans being developed to complete the twin tube, double track configuration

In concluding, BLS said: “We have been surprised [by the water inflow events] as the tunnel is built to a very high standard and we did not know about any problems with the sealing or drainage system.”

For Heinz Ehrbar who contributed to the first TunnelTalk article about the situation at Lötschberg, and was the Chief Construction Officer for the St Gotthard Baseline railway of the Swiss AlpTransit programme and is currently an independent consultant and a lecturer at the ETH Zurich, there is a lesson to be learned from the incident. He warned, “it is a highly dubious undertaking to catch water through drainage holes directly at the crown of the excavated area and to direct it into the traffic tube of a high-speed tunnel.”

In continuing the reporting cooperation established with BLS with this report, TunnelTalk will plan to publish a news article about the plan for the permanent repair of the tunnel tubes and the construction implementation.