Vízügyi Közlemények, 1973 (55. évfolyam)
4. füzet - Rövidebb közlemények és beszámolók
2.5 Csecskedy, G. and Dr. Kovács, G.: Seepage control and dewatering operations during construction (For the Hungarian text, see pp. 132) With the objective of minimizing the uplift force acting on the foundation and the tailwater apron of the barrage structures, as well as for reducing the velocity of underseepage flow, an impervious cutoff has been envisaged on the upstream side, which fully penetrates the first granular layer and reaches down into the second, impermeable layer. The design and arrangement of the cutoff system has been checked with the help of two- and three dimensional electric analogy models, further by site experiments. The slurry trenches were excavated with the equipment shown in Fig. 2.5 — 1 with the hydraulic clamshell attachment. The total area of the cutoff attained 10,500 sq.m. During construction the materials used and the soils penetrated by the trench have been subjected to continuous analysis. The water-sealing properties of the slurry trench cutoff were checked by using the method illustrated in Fig. 2.5 — 2. This check has revealed a leaking connection e.g. along the powerhouse section. Tests for leakage and pei'meability were also made with radioactive tracer solutions. The reinforced embankment section shown in Fig. 2.5 — 3 has been adopted as one of the control measures agains underseepage along the reservoir. Underseepage is intercepted by the parallel canals running the length of the reservoir embankments on the protected side. Preliminary estimates have yielded for the seepage discharge the figure of about 30 lit'sec per kilometre at 90.50 m A.C.D. retention level. In the interior of the village on the left-hand side of the reservoir tube wells have been sunk as a means for providing pressure relief and drainage of rising groundwater (Fig. 2.5—4). The foundations of the barrage structure reach down into the first pervious layer, the deepest being that of the powerhouse, those of the weir and the navigation lock being 6 to 7 m higher. The fine-sand subsoil belongs to the range where gravitational unwatering is considered possible, but is rather close to the range where dewatering is practicable by vacuum wells only. After preliminary studies groundwater lowering in several stages has been adopted. The groundwater appearing sporadically in 1967 was removed by pumping from open sumps. The ground water-lowering equipment was first operated in 1968 with 60 well points and three machine houses. The progress of drawdown was observed for about a month and the results confirmed the feasibility of dewatering by gravitational methods. In the first stage the dewatering system was completed in the area of the powerhouse only (Fig. 2.5 — 6) and this was extended progressively towards the Tisza River. For a wider margin of safety along the side facing the river a system of relief wells involving 11 deep wells has been envisaged. In the area of the barrage structures and of the tailwater apron the groundwater table in the first permeable granular layer was lowered according to the following schedule: 278
