Rockfall on the railway line in Gurtnellen
The rock fall that buried parts of the Gotthard Railway Line of the Swiss Federal Railways took everyone by surprise, even though some smaller falls had been registered from a nearby site three months before. The 3000 cubic meters of rock that broke loose on the morning of June 5th caused severe damage to the railway lines and resulted in a one-month closure period of Switzerland’s most important north-south link.
Together with geologists of the Swiss Federal Railways and GEOTEST AG we reached the rock fall site around noon on June 5th. The interferometric georadar, which was still at the rock fall site in Preonzo (TI), was immediately transferred to the Gurtnellen site. Thanks to its simple installation and the independent power supply, we were ready to conduct the first measurements around 8pm that same day – and provide a first assessment of the remaining danger of secondary falls. First evaluations around midnight showed that no imminent dangers prevailed. During the three following days of recovery works we were constantly present, analysing the radar data first-hand so that works in the danger-zone could have been stopped at the first sign of shifts in the wall. With the onset of heavy precipitation on the third day, some movements were detected throughout the talus fan and smaller rock falls were registered. Despite the fact that the radar is incapable of accurately predicting moving rocks in loose debris (since they usually start falling without prior movements), we could still provide a rough identification of the areas prone to movement in order to facilitate the work for local observers.
Once the pressing recovery works had been concluded, the installation of the protective measures could be initiated. All of the potentially unstable rock had been blasted away, the remaining rock face and talus had to be secured.
The Gotthard Railway Line is buried under piles of debris after a security blasting.
During this construction period, several independent surveillance systems were installed:
- Six trifold extensometers were placed in boreholes at depths of up to 40m
- More than ten telejointmeters monitored movements of single blocks and clefts.
- Trigger lines woven into the temporary safety nets. Alarms are automatically triggered (sirens and flashing lights) and are directly sent to the nearby railway control station that would stop all traffic.
- Over 50 geodetic reference points (mirrors) were measured at 15min intervals. This station was operated by Flotron AG, their data was also published on our data portal so as to ensure a central data access for all decision makers.
- Permanent operation of our interferometric radar during the first six months, followed by periodic measurements once a month.
The surpassing of predefined thresholds automatically triggered the sirens at the construction site in order to evacuate workers. At the same time, geologists at GEOTEST were informed via SMS and the live data on our data portal served decision makers in planning further steps. On July 2nd, the Gotthard Railway Line could be reopened to traffic, but the surveillance system will remain in place for some time to come.