Brig, Switzerland: Road and rail traffic between Switzerland and Italy is funneled through the town of Brig. In Brig, the highway crossed the busy railway, causing long delays. Traffic engineers proposed to build a tunnel for the road traffic, which could then pass under the railway without delays.
Designers chose to drive the tunnel through a shallow layer of alluvial deposits, rather than through the granite bedrock below. Although this would make tunneling easier, it would bring the tunnel crown within 1.5 meters of the track bed. To support the track during tunneling operations, the contractor planned to jet-grout a supporting arch. The immediate risk was that the jet grouting could cause the track bed to heave, resulting in delayed trains or even derailment. And later, when excavation of the tunnel began, there was a risk that the track bed would settle, with the same hazards.
Since Swiss Rail could not halt trains for the duration of the project, real time monitoring of the track was required. EL beam sensors were chosen for this purpose. Two strings of linked beam sensors were installed parallel to the rails. Other beam sensors were installed to monitor tilt across the rails. A CR10 data logger recorded sensor readings.
Two kilometers away from the site, a Swiss Rail operator monitored movement of the track bed, using custom graphic software (now evolved into the MultiMon program). The software displayed a high resolution profile of track movement and included color-coded alarms. If threatening movements occurred, the operator could halt a train before it reached the site. Simultaneously, the system would sound a warning at the tunnel site.
The sensors performed well despite heavy train traffic and temperatures between -l0 and +8 degrees C. Data was verified periodically by periodic optical surveys. As work progressed, the beam sensors were moved to adjacent tracks. The project was completed without incident and beam sensors were left in place for about nine months to monitor any long-term movements.
Thanks to Pierre Auberson of Geotrade SA of Switzerland for providing this story.