Assessing the Condition of the Rock Mass over the Tunnel of Eupalinus in Samos (Greece) using both Conventional Geophysical Methods and Surface to Tunnel Electrical Resistivity Tomography

The surviving Tunnel of Eupalinus comprises one of the greatest engineering achievements of ancient Greeks. The tunnel itself (1036 m long) was carved in the sixth century bc through solid rock by digging from both ends and advancing to the central meeting position. The method for achieving the meeting of the two simultaneously advancing branches is still unclear. The ancient lining shows damage, indicating instability of the rock mass, presumably due to tectonic action. At certain points, the damage becomes more severe and partial collapses are observed. Therefore, the monument is threatened and measures have to be undertaken to restore its stability and secure the safety of future visitors. The geophysical investigations reported here comprise part of a geotechnical study. The aim of the geophysical survey was to image the subsurface from the ground surface to the ceiling of the tunnel and, if possible, thereby provide information on the quality of the rock mass. A variety of well-established methods (VLF, self-potential, seismic refraction and electrical resistivity tomography) were used for this purpose, and also the relatively novel measuring approach of laying out electrodes in a tunnel to surface' mode. The latter method was an attempt for achieving tomographic imaging of the rock mass over the tunnel. Finally, the results of all the methods applied were combined and integrated in order to assess the tectonic regime above the Eupalinean Tunnel. Fractures and shear zones were detected and imaged. Further, the elastic moduli were determined at specific spots. In general, the geophysical interpretation matches well with the visible manifestations of the instabilities of the rock and provides clues for explaining their origin. The surface to tunnel' imaging provided increased resolution, which was a great advantage. Additionally, it is concluded that the construction of the tunnel was chiefly a product of survey method rather than consideration of geological factors. Copyright (c) 2014 John Wiley & Sons, Ltd.