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An innovative multiple extensometer for various applications in tunnelling

Magnetostrictive extensometers measure with high precision the displacements of up to 20 measuring points along a line. Measuring lengths up to 20 meters are possible. Due to the large number of measuring positions along the measuring line a detailed displacement profile can be generated.

Besides the standard applications of extensometers they are used especially fordetermining and controlling of necessary anchor depths in the progressing excavation face of tunnels and as curved multiple point extensometer, embedded in shotcrete to determine the strain profile in the lining.

Principle of operation

The measurement itself is carried out by pushing the flexible magnetostrictive probe into the guide tube, passing all the anchor parts through their centre hole. When all the monitoring points are lined up along the probe, the readout unit stimulates the ferromagnetic waveguide of the probe which generates microscopic distortions of the molecular structure of the waveguide at the location of the measuring anchors, and respectively their magnetic fields (Wiedemann Effect). The generated torsion waves travel along the waveguide and are detected by a sensor in the measuring head. Expressed in a simple way – the time interval from applying a current pulse until the appearance of the torsion pulse in the measuring head is measured. By knowing the transfer speed of the waveguide and the time intervals, the
distances between the measuring head and the anchor parts are calculated.

Our services

We deliver all components in the desired design. Our technicians are also happy to carry out the installation on site. If an automatic measurement data acquisition is desired, we plan and deliver optimized solutions and accompany our customers until commissioning.


  • Monitoring of displacements ahead of the tunnel face
  • Measurements can be performed immediately after blasting
  • Minimised disturbance to excavation activities
  • Low diameter drill hole required
  • Measuring length of up to 20 meters (standard 15 m)
  • Guide tube can easily be cut off after a blast
  • Mobile or permanent use of the probe
  • Fast data acquisition
  • Automated data acquisition and remote data transmission possible on request

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