Aktio – Preveza Immersed Tunnel Technical Description

Technical Description

The works consist of an immersed part 1.100 m long placed at max 28 m depth and having a small curvature. The immersed parts were constructed in a temporary dry dock which was constructed adjacent to Aktio exit. The 500 m long 150 m wide dock, with its bottom level at -11 m was kept dry by a surrounding system of pumps reaching the water table of -40 m. The dry dock was communicating with the sea by a trench supported by diaphragm walls on each side and separated by it by steel door-dams.

The concrete elements were appr. 140 m long, 10 m wide and 7 m high and weighed appr. 14.000 tons each. No cracking concrete was used, the concrete mix design was prepared by Taylor Woodrow Lab in UK. The concrete was cooled during pooring procedure by water pumped through a serpantine pump surrounding each concrete element. After removing the formwork any trivial cracks were mapped in 1:1 scale and, after following their behaviour for a while, were closed with special admixtures.

To tug the elements into the sea the pumps were closed, water was let in through pipes at the doors, and the dry dock was flooded. then the doors were completely removed. The elements were tagged by using in the area of the dry dock a system of winches placed accordingly around the dock and the trench. Gradually, in the trench area, the dry dock winches gave the elements to tug boats and winches on pontoons anchored in the crossing.

To control the float of the elements the concrete blocks were sealed by steel frames and sheets water tight. Big water tanks were constructed in the four corners of each element that communicated with pumps placed outside, on top of the elements. The system was designed in a way so when the tanks were full of water sunk to the bottom of the sea or the dry dock, when the tanks were emptied the sealed elements were floating and could be tugged. So, initially the tanks were full and remained at the bottom of the flooded dry dock, then, one by one, the tanks of one element were emptied, the element floated, was tuged in place, the water tanks were filled again with water, the element sunk, and connected with the adgacent element. The first element sunk was connected with the cut and cover section already constructed, the last one as well. Then concrete was placed inside the elements, to finish the bottom surface, weighing more than the weight of the water in the tanks. Thus, the tanks were emptied and permanently removed and the elements permanently placed at their position. A sophisticated system of winches allowed very slow, very controlled, smooth and horizontal movements of the elements in order to be placed properly. The relevant diving operations were undertaken by a Ukraine team of construction divers. To control the presision of these operations measurements were taken for two years at the site regarding the water’s salinity, temperature and speed of currents.

The joints between the elements were sealed with “omega” joints, and … Also, to eliminate relative movement between the elements, …(dontia)

The tunnel was founded on a well prepared ground. A trench was excavated at the bottom of the sea in which the tunnel was finally placed. Stone columns were constructed, 1,20 m diameter, 24 m deep, in a 3 m by 3 m array pattern. The stonecolumns allowed for replacement of ground material in the foundation area of appr. 25 %. The stone columns improved the ground conditions considerably and prevent the phenomenon of liquifaction since the ground has a big proportion of silty sand.

Gravel beds in various layers were placed above and accurate leveling techniques were utilized to prepare the bed for the elements. The elements had “bags” tied below them which were connected with pumps on the surface. After being placed cement was injected from above in the bagw, with controlled pressure, so as to fill any gaps that had renained below the element and assure that the element is well based on all its surface.

At various points instumentation was placed in order to follow the tunnels’ behaviour and differential settlements during the years but also after seismic events. The tunnels design was based on specific measurements of the coefficient of seismic accelerator at the tunnels area.  

The tunnel was covered by various layers of ground material and on top a rip-rap protection was placed. The tunell is designed to stand the load of the biggest ship allowed to pass the crossing in case of sinking. It is also designed, the cut and cover sections as well, to stand the expected magnitude of a tsunami. Explosives and transportation vehicles carrying gas, etc are not allowed through the tunnel.

The life period of the project is 100 years.

The tunnel, near Preveza side was connected from within with the service building. There through an automated control panel, drawing information from various monitoring points and a CCTV circuit, the every day operation of the tunnel takes place. The strictest international specifications regarding monitoring CO, etc were applied.

The project was completed with the construction of more conventional works, such as, cut and cover sections near the shore lines, highway construction, service roads, toll station, various buildings. a new dock for the ferry boats,, automated and up to date electromechanical works (lightning, air control, sound control, etc) and especially fire fighting works. At the exits persides are placed rotating automatically with the use of specific sensors as designed by National Technical University of Athens (Metsovion

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