Page - 827 - in Book of Full Papers - Symposium Hydro Engineering

The results of the vertical displacement measurements at eight points in the lower courtyard, since the end of rehabilitation works in 2000, showed settlements up to 8 mm, which is similar as in the case of the erection bay. Since 2000, the settlements at ten points in the upper courtyard did not reach 8 mm, whereas at the remaining eight points the settlements reached locally even up to 32 mm. Additional investigations, involving the drilling of two research boreholes in the upper courtyard, 28.0 m and 30.0 m in depth, as well as appropriate field tests (soil and rock classification, groundwater level, standard penetration test) and laboratory tests (particle size distribution curve, water permeability coefficient) carried out on 21 samples, 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 [8]. 5. CONCLUSIONS In the years following the Mavčiče Dam completion in 1986, the results of measurements of vertical displacements showed increasing settlements of the erection bay at the right bank of the dam, and therefore the differential settlements according to the adjacent machine hall, which resulted to uselessness of both crane rails, which needed height corrections. After investigations, involving the drilling of research boreholes, as well as appropriate field and laboratory tests in 1993 and 1996, the rehabilitation works of the backfill and the substratum of the right bank of the dam were performed in 1999 and 2000. Since even after the rehabilitation works, the settlements of the erection bay continued to increase, additional investigations, involving the drilling of two research boreholes as well as suitable tests (soil and rock classification, groundwater level, standard penetration test, particle size distribution and water permeability coefficient) were carried out in 2015 and 2016. The results of these investigations showed that the settlements measured in the recent years were the outcome of the additional internal erosion of 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. 827