Page - 829 - in Book of Full Papers - Symposium Hydro Engineering

SUMMARY The Mavčiče concrete gravity dam, part of the corresponding hydro-power plant, was built on the Sava River, in Slovenia, in 1986. It has a maximum structural height of 38.5 m, and the dam crest has a length of 149 m. The dam structure consists of an erection bay, a machine hall, and two spillways, followed by an embankment dam. Most of the dam is founded on permeable Quaternary conglomerate bedrock, so that a cut-off grout curtain had to be constructed to a depth of up to 60 m below the ground surface, where a layer of impermeable Oligocene marine clay occurs. However, the erection bay, which is located on the right bank of the dam, is founded on a layer of gravel backfill, up to about 25 m thick, which lies on top of the conglomerate bedrock. Long-term manual technical monitoring of the behaviour of the dam began in 1986, and an automated monitoring system was established between 2003 and 2005. 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. By 1999, i.e. over a period of 12 years, these settlements had increased to 22 mm. The results of investigations, by drilling three research boreholes in 1993, and another six such boreholes 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. 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. This grouting was performed using a combination of water reactive polyurethane and a cement-bentonite mixture. Measurements performed since than have shown that the settlement rate has slowed down slightly (by 2017, i.e. over the last 17 years, the settlements had increased by up to 8 mm), but from the point of view of the operation of the two crane rails the settlement process needed to be stopped. Additional investigations, involving the drilling of two research boreholes, as well as appropriate laboratory and field measurements, were performed between November 2015 and March 2016. According to the results of these most recent investigations, the newer settlements were the consequence of additional scouring of fine material from the backfill. In order to achieve a final solution to the problem of the subsidence of the erection bay, additional rehabilitation works of the backfill and substratum of the right bank of the dam, by grouting the permeable zones, would be needed in order to stop both the scouring of fine material from the backfill, as well as any internal erosion of the cavernous conglomerate at the base of the backfill. The latter concerns the long-term stability of the right bank of the dam with potentially serious results. 829