geophysical investigation

A geophysical survey was carried out at the main tunnel having as a purpose to investigate the thickness of the lining and to provide some evidence about the geological conditions between the tunnel and the ground surface. 

The following methods have been applied: a) electrical tomography at the ground surface, b) ground penetration radar, d) seismic refraction at the ground surface, e) electrical tomography using sensors inside the tunnel and along its projection at the ground surface, f) seismic tomography using geophones in side the tunnel and producing elastic waves at ground surface and g) VLF and SP.

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The thickness of the lining along the tunnel was investigated by both the Ground Penetration Radar and the Electrical Tomography Methods, along two lines at either sidewall. A typical GPR evaluated result presenting the thickness of the lining which varies from 20cm to 50cm is presented above.

Elements of the geological structure between the lined section of the tunnel and the ground surface, which identified from the geophysical survey. The geophysical survey identified zones of poor ground at the two locations where the lining was damaged. Therefore the restoration measures at these locations aimed not only to restore the curved stones of the roof but to strengthen the surrounding ground as well.