Surface and borehole geophysics for the rehabilitation of a concrete dam (Penne, Central Italy)

Abstract

In this paper, surface and borehole geophysical surveys are employed to improve the effectiveness of a rehabilitation intervention (cut-off wall) to be executed on the right abutment of a leaking concrete dam (Penne, Central Italy). In a first step, surface geophysical measurements (electrical resistivity and P-wave velocity data), were collected to provide a geophysical model to be compared to the preliminary geological model available before the geophysical survey. Because significant differences emerged between the geophysical and the preliminary geological models, a second phase of borehole investigations was planned to validate the geophysical reconstruction obtained from surface data. The final geological model resulting from the geophysical results differs significantly from the preliminary model available at the time of the geophysical survey. Therefore, the planned cut-off wall needs to be extended both laterally and in depth beyond the position estimated before the geophysical campaign, so that the low-permeability bedrock is intercepted at a depth equal or lower than the critical one. At the end, the low-cost geophysical campaign (1.5% of the entire costs of the intervention) was highly beneficial to this project, minimizing subsurface geological uncertainty to design an effective anti-seepage intervention and stressing the importance of a representative geological model for engineering design.