Authors: Tresoldi, G; Hojat, A; Zanzi, L; Certo, A

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Tresoldi, G, Hojat, A, Zanzi, L & Certo, A 2020, 'Introducing G.RE.T.A. – an innovative geo-resistivimeter for long-term monitoring of earthen dams and unstable slopes ', in PM Dight (ed.), Slope Stability 2020: Proceedings of the 2020 International Symposium on Slope Stability in Open Pit Mining and Civil Engineering, Australian Centre for Geomechanics, Perth, pp. 487-498,

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Nowadays there is an increasing interest in introducing techniques capable of monitoring the internal conditions of embankments and slopes in order to develop early warning alarm systems. Geophysical methods have been widely used in geotechnical risk mitigation studies to evaluate the stability conditions of dams, embankments and slopes. Geophysical methods have the advantage of investigating large volumes of subsurface in a non-invasive manner. Among different geophysical techniques, geo-electrical methods are more frequently used for this purpose. In previous decades, electrical resistivity tomography (ERT) technique has been widely used in a variety of problems such as engineering investigations, groundwater studies, landfill surveys, mineral exploration, glaciological applications, hydrogeological risk mitigation (e.g. landslides), and monitoring of embankments, earthen dams and tailings dams. Commercial resistivity instruments are portable devices which are generally capable of investigating the subsurface down to depths of a few hundred metres with high injected currents. These instruments are not customised for long-term installations in remote places. In this paper, we present the design, installation and testing of a new geo-electrical monitoring system customised for long-term monitoring projects. In the first step, the feasibility of using time-lapse ERT method to study the saturation and de-saturation processes and to monitor anomalous seepage paths in a levee was demonstrated. Then, a long-term geoelectrical monitoring system, G.RE.T.A. (Geo REsistivimeter for Time-lapse Analysis), was designed based on the information obtained in the first step. The G.RE.T.A. system is capable of being programmed remotely and working autonomously In order to monitor anomalous seepages in levee structures that can lead to the levee breach, two installations of G.RE.T.A. have been completed along two river embankments in northern Italy. In order to develop the system to monitor landslides, laboratory studies are in progress to monitor rainfall-triggered landslides. The results have proved the capability of ERT measurements in delineating inhomogeneous infiltration zones during precipitation and detecting weak zones before collapse.

Keywords: ERT method, time-lapse, subsurface monitoring, risk mitigation, long-term installation

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