Tunnel roof monitor employing an induction coil extensometer

Apparatus for detecting the movement of a tunnel roof structure to warn of potential roof collapse, comprising:

• a detector means including a core element positioned within and inductively coupled to a coil element, said detector means being positioned within a borehole in the roof structure of a tunnel, said core element being secured within a first rock formation and said coil element being secured within a second rock formation such that movement of either of said rock formations with respect to the other results in a relative displacement of said core element with respect to said coil element, said displacement producing a change in the inductance of said detector means, and
• means operatively connected to said detector means to monitor said change of inductance as an indication of movement of said roof structure.