CONVENTIONAL AND MODERN SEISMIC INVESTIGATIONS FOR ROCK QUALITY DETERMINATION AT A DAM SITE - A CASE-HISTORY

Refraction seismics and modern tomographic surveys were employed at th e Platanovrissi dam site on the Nestos river in northern Greece in an effort to investigate the dynamic elastic properties and rockmass cond ition of gneiss-schist outcropping in the area under investigation. Si x seismic refraction traverses were run on the ground surface, inside the exploratory tunnels and between tunnels and the ground surface. Th e results provided information about the low-velocity surface layer pa rameters and the stress relief zones on the walls of all the tunnels i n which seismic tomography tests were performed. Ultrasonic tests, con ducted both in situ and in the laboratory, enabled the determination o f the ratio V-p/V-s and the Poisson's ratio calculation. From the tomo graphy data analysis and the geological interpretation of the results, three main velocity zones, ranging between 3500 and 4000, 3000 and 35 00 and less than 3000m/s, were detected in each tunnel. High-velocity zones were found to be compatible with the hard rockmass of gneiss-sch ist material, while moderate velocities are closely related to jointed and fractured zones. Finally, low-velocity zones were well correlated with weathered gneiss-schist material.