Tunnels: Mt Terri

Case study: SOFO System

Multipoint optical extensometer
Rock decompression measurement
Crack formation

Project Description

Summary: Tunnel construction with tunnel boring machine, 9-points extensometers, measurement of rock decompression from an existing parallel tunnel

Clients: Jura Canton

Place: Jura Canton (Switzerland)

Period: 1997 - 2000

Installation: ISRF/IMAC - EPFL

Structure's short description: In the framework of an international project investigation the behavior of clay formations, a new tunnel has been excavated. The new tunnel is parallel to an existing one at about 30m distance.

Aim of monitoring: The main aim of the instrumentation is to measure the rock decompression around the new tunnel. To do so, a multipoint SOFO extensometer was installed in a borehole realized from the existing tunnel before the new one was excavated. The sensors were coupled to the surrounding rock by injection of the borehole with expansive grout.

Number of sensor: 9

Main Results: The SOFO sensors were selected because of their reliability, precision, ease of installation and the possibility of accepting large longitudinal and lateral displacements without damage. The 9 sensors had varying gage length according to their distance from the new tunnel (4 x 25cm, 2 x 50cm, 1m, 2m, and 4m) and were pre-assembled together with an injection pipe and delivered in a coil for rapid installation. The measurements were carried out at intervals with an increasing frequency during the passage of the tunnel boring machine (TBM) near the tip of the extensometer. The figure shows the measured strains (deformation divided by the gage length) for the different sensors. Large strains were observed immediately after the passage of the TBM. All sensors measured strains with the same qualitative trend, but with magnitudes decreasing exponentially with the distance from the new tunnel. The first sensors measured deformations in the order of 1cm/m pointing to the probable formation of cracks. The influence zone having significant deformations can be estimated to a few meters. Long-term measurements are planned on these sensors.

Sensor layout. The senosors are installed from the existing tunnel to towards the new one

Recorded deformations for different sensor locations

Bibliography:

SOFO?: Tunnel monitoring with fiber optic sensors , D. Inaudi, N. Casanova, G. Steinmann, J-F. Mathier, G. Martinola, Reducing Risk in Tunnel Design and Construction, ITC publisher, p 25-36, 1998

SOFO?: Structural monitoring with fiber optic sensors , D. Inaudi, S. Vurpillot, G. Martinola, G. Steinmann, Fib Commission Meeting, Institute or structural engineering, University of applied sciences, Vienna, Austria, Vol 40, 9, 1999

Monitoraggio mediante sensori a fibre ottiche , N. Casanova, D. Inaudi, G. Colombo, XX Convegno di Geotecnica, Parma, Italy, Patron Editor, p 395-403, 1999

Globale Bauwerks?erwachung mit Lichtwellenleitersensoren - Funktionsweise des Systems SOFO? und Anwendungsbeispiele , N. Casanova, G. Colombo, D. Inaudi, G. Ballerini, H. Podhradszky, W. Zemann, J. Kling, 13. Sitzung der AG6 - Bautechnik, p 1-12, 1999

Geo-structural monitoring with long-gage interferometric sensors , D. Inaudi, N. Casanova, SPIE, 7th International Symposium on Smart Structures and Materials, Newport Beach, USA, Vol 3995, p 164-174, 2000