Page - 824 - in Book of Full Papers - Symposium Hydro Engineering

Table 2 Automatic measuring parameters for monitoring the behaviour of the dam Type Location No. Measuring parameter Deformation Expansion joint 1 Displacement (x, y, z) Groundwater Uplift pressure caps 4 Uplift pressure Side piezometer 6 Piezometric level, temperature, specific electrical conductivity External loads Reservoir and stilling basin 1 Water level, temperature, specific electrical conductivity Crest of the dam 1 Air temperature, acceleration Inspection gallery 1 Air temperature, acceleration Dam structure 1 Concrete temperature 150 m downstream 1 Acceleration 4. RESULTS OF MONITORING, INVESTIGATIONS AND ACTIONS In general, the results of measurements and visual inspections did not show any abnormalities. However, this was not the case for the erection bay located on the top of the backfilled right bank of the dam, where the results of measurements of vertical displacements showed increasing settlements [6]. By 1999, i.e. over a period of 12 years, these settlements had increased to 22 mm (see Fig. 5). These results were confirmed by measuring the relative displacements of expansion joint D6, located between the erection bay and the machine hall (see Fig. 6). The results of investigations, by drilling three research boreholes (one in the upper courtyard and two in the erection bay) in 1993, and another six such boreholes (four drilled in the upper courtyard and another two drilled in the lower courtyard) in 1996, indicated that the settlements were the consequence of the secondary consolidation of the backfill, and probable also due to scouring of fine material from the backfill. Due to the resulting differential settlements, the crane rail which connects the erection bay to the machine hall, as well as the crane rail which is located along the crest of the dam, became non-functional, and needed height corrections (see Fig. 7). For this reason rehabilitation works of the backfill and of the substratum of the right bank of the dam were performed between September 1999 and August 2000, using 50 m long grouted boreholes (see Fig. 8). This grouting was performed using a combination of water reactive polyurethane and a cement-bentonite mixture [7]. Measurements performed since than have shown that the settlement rate of the erection bay has slowed down slightly (by 2017, i.e. over the last 17 years, the settlements had increased by up to 8 mm, see Fig. 5), but from the point of view of the operation of the two crane rails the settlement process needed to be stopped. 824