During the whole 2019, the intensive works were continued on excavation of the headrace tunnel for the future hydropower plant Dabar (L = 12.20 km) within the hydrotechnical system “Gornji horizinti”.
The works were simultaneously executed at four points, in two and three shifts. Up to 250 workers are engaged at works daily, and the modern machines are used as well.
The executed excavations by attack points (NM) are the following:
-NM1 (from direction of the entry structure): l = 251 m;
-NM2 (upstream of access tunnel Straževica): l = 2,015 m;
-NM3 (downstream of access tunnel Straževica): l = 3,104 m;
-NM4 (upstream of the access tunnel Vodostan): l = 2,423 m;
TOTAL: L = 7,793 m
During previous years two access tunnels were excavated: Straževica (l=652 m) in the medium part of the headrace tunnel and Vodostan (l=91 m) at the end side of the headrace tunnel. Access portals were excavated at starting and ending side of the headrace tunnel.
During the winter the works will be continued with less intensity due to high groundwater and surface waters (seepage water) that flood the construction site each year. The works at NM1 will be suspended from the end of November 2019 until the spring 2020. The works at NM2 and NM3 will be continued under more difficult conditions and the advancement will be slower than in the summer, while at NM4 no significant problems caused by the inflow of water into the tunnel are expected.
From technological point of view, connecting of excavated parts of tunnels is planned at two points:
-between NM1 and NM2: there is remaining l = 2,005 m to be excavated;
– between NM3 and NM4: there is remaining l = 2,326 m to be excavated.
Connection between NM3 and NM4 is expected to be excavated in June 2020, after which the concrete lining of the tunnel will be made.
The time of connection of NM1 and NM2 will depend on the length of winter break due to flooding.
At the site of excavation of the headrace tunnel there are mainly limestone rocks of various composition and characteristics. Besides, during the works numerous caverns have been found, filled with clay and rock. Caverns require additional works and more complex types of bracing. (Caverns are underground structures in karst that are formed by chemical actions of water that enlarge the existing cracks.)
The main characteristics of the headrace tunnel for the future hydropower plant Dabar:
– the length l= 12,216.00 m
– altitude of the beginning of the tunnel H=813.00 m
– altitude of the end of the tunnel H=787.06 m
– longitudinal inclination i=2.139‰;
– circular diameter of the clear opening D=4.6 m at length l=7,853.00 m
– horseshoe section of the excavation height H=5.20 m at length l= 4,272.00m;
– built using the new Austrian method.