CROSS OVER AND SPECIAL STRUCTURES

 

THE CROSS OVER:

FUNCTION

Two crossing junctions, called cross overs, are located at one third and two thirds of the tunnel lengths under sea. These structures are equipped with switches to isolate a third of a running tunnel so as to allow maintenance or neutralization in case of any incident during operation.

 

DIMENSIONS

The cross over is 160 metres long, 19 metres wide and 12 metres high.

 

CONSTRUCTION

The method of construction of the cross over on the French side was as follows:

• At PK [Project Kilometer] 12.7 under sea at the site of the future cross over, the 2 running tunnels are built as close as possible to each other and the service tunnel is deviated from its current position downward and outward to the north.
• After passing through of the 3 tunnel boring machines, 11 elementary galleries are dug from the service tunnel by 3 LYNX mobile excavators. These galleries are immediately concreted after digging. They then form a shell that serves as a vault for the work.
• The remaining heart is then excavated, and the running tunnel linings are demolished. A reinforced concrete base slab is then built.
• A concrete separation wall allows the installation of a rolling door between the two railway tunnels; the door remains closed in normal service.

 

SPECIAL STRUCTURES:

 

CROSS PASSAGES

Cross passages between the service and the running tunnels are built every 375 meters to allow evacuation from the railway tunnels to the service tunnel in the event of an accident or incident.

The cross passages are equipped with leak-proof doors. The combined cumulative length of these junctions represents 2.5 kilometers.

 

PRESSURE RELIEF DUCTS

2 meters diameter piston relief ducts connect the railway tunnel every 250 m.

These piston ducts, equipped with butterfly valves, ensure aerodynamic continuity by allowing the exchange of air masses and the balancing of the overpressure/depression related to the movement of the trains in the tunnels.

 

TECHNICAL ROOMS

Other structures, like the cross passages, are spaced at 700 metres apart and are used for electrical equipment.

 

PUMPING STATIONS

The SW3 station at PK 8.8 is one of three pumping stations positioned at the lower points of the tunnel. It collects the infiltration water from the first 14.5 km undersea on the French side. Its construction required the excavation of 11 600 cubic meters of chalk marl, and 5 700 cubic meters of concrete.

The capacity of its sumps is 2400 m3, its pumping rate reaching 22 400 m3/h