SEEPAGE AT LES CÈDRES EMBANKMENT DAM (PART 1): SP/ERT SURVEYS AND TEMPERATURE MONITORING

Abstract

Les Cèdres is an embankment dam that guides water from a channel of the St-Lawrence River to a power plant. The 1917-old dam has been plagued with seepage for many years. Water leakage can be observed at dam toe at a few places. Hydro-Quebec, the dam owner and operator, uses the dam to evaluate non-destructive methods that could be employed to detect seepage and assess remediation work. It needs good knowledge of where water flow within the dam to plan an effective grouting program in order to stop seepage. As water flowing through preferential paths can cause streaming potentials and changes in the dam material resistivity and temperature, Self Potential (SP) surveys were carried out along with Electrical Resistance Tomography (ERT) surveys and passive temperature measurements in an existing well through the embankment centered over the perturbed area. The water level in the reservoir on the downstream side of the dam was raised by 5 m during the survey. Therefore, SP surveys were carried out before, during and after the water level raise. SP was also continuously monitored to track any changes possibly induced by pressure and flow rate changes during the impounding. Laboratory measurements of the electrokinetic coupling coefficient were done on material from the dam. The SP maps processed at the three stages show complex structure. However, we observe changes with time/water level rise. The 3D resistivity model of the dam as interpreted from ERT shows that the impervious clay layer on the topside is discontinuous; those discontinuities can promote water infiltration. The vertical temperature profile with time shows that most of the changes in the first 6 m are caused by advection and by conduction in the deepest part. Possible water seepage at a depth of 6m is consistent with changes in soil composition at that depth and particular location. The measured coupling coefficient is in general agreement with published data. The geophysical data indicates complex seepage paths controlled by several sources and internal structure.