THE FRENCH TUNNELS

 

ORGANIZATION OF THE TUNNEL SUB PROJECT:

Under the authority of its Director, the Tunnel sub project Tunnel was divided into work sectors and functional sectors linked to the corresponding central directorates of Construction France.

Each of the work sectors comprised 300 to 500 people and covered a defined activity: tunnels, shaft-surface, equipment and electric services as well as special works.

Each sector section head reported directly to the Tunnel sub project Director and had significant delegation of power regarding organization of work, methods, cost, quality and safety.

The sub project Director makes sure the policies defined by Construction Management France are implemented sets priorities and that interfaces are taken into account.

The sub project’s 2 500 employees is similar to the workforce on a typical public works job.

 

DESCRIPTION OF THE UNDERSEA TUNNEL BORING MACHINES TBMs:

These machines are ‘mixed’, they can work in closed mode or open mode according to geological conditions.

The shield (outside diameter 8.72 m, 13.75 m long, weight 1250 t) includes the following major components:

• Telescopic cutting head which has 8 arms equipped with cutting wheels on the axle and picks on the edges rotating during digging at a speed of 1.5 to 3 rpm.
• The material extraction device consists of 2 screws in series separated by a compartment of compressed materials.
• An erector with depression suction pad laying the prefabricated segments of lining inside the back shield’s skirt.
• The water tightness is obtained by a sealing device between skirt and lining with 4 rows of metal brushes and injection of grease.

The 800 metric tons and 265 metres long train towed behind the shield of the TBM consists of 16 trailers which carry the following main equipment:

• The piloting of the TBM with computer assistance installed in a cabin on the first trailer.
• Unloading the lining from the pallet-carrying cars and supplying them to the erector.
• Evacuation of the excavated material by conveyor belt from the shield to the filling point of the scrap wagons.
• Installation of the track which supports the train and the tracks leading back to the shaft.
• Supply of the mortar and its injection into the ring vacuum between the skirt and the lining.
• Electrical and hydraulic power supplies, pumping, compressed air, ventilation, dusting…

 

WAY OF OPERATION OF TUNNEL BORING MACHINES:

Digging requires the simultaneous action of the rotation of the cutting wheel and the thrust of the cutting wheel on the ground.

• The rotation is provided by electric motors, the thrust by hydraulic cylinders.

 

IN CLOSED MODE the telescopic head remains retracted:

• During the excavation, the shield advances, pushed by the rear jacks bearing onto the segments of the last ring placed.
• The excavation stops when the shield has advanced sufficiently to allow a new ring to be placed.
• As the shield is immobilized, the rear jacks are retracted and the new ring can be placed.

DIGGING AND LAYING THE RING ARE SUCCESSIVE ACTIONS 

 

IN OPENED MODE, the telescopic head is operational:

• At the start of the excavation lateral hydraulic jacks immobilize the shield and keep it firmly in position.
• During the digging:
  • The telescopic head comes out gradually, pushed by the front jacks supported from the shield.
  • The rear jacks are retracted and the new ring is placed simultaneously.
• As soon as the excavation is over:
• The lateral and front jacks are retracted and the released shield becomes mobile again.
• The shield, pushed from the rear on the lining, advances and retracts the telescopic cutting head.

DIGGING AND LAYING THE LINING ARE SIMULTANEOUS

 

Under land TBMs are not equiped with telescopic heads and operate only in closed mode.

In both modes, digging is done in successive cycles – the length being equal to the length of a ring.

 

 

 

 

 

LOGISTICS IN TUNNELS

The construction of these long tunnels has a specific characteristic in having a significant number of ancillary works that must be fed and served simultaneously without compromising the progress of the TBMs.

As a result, the logistics implemented are powerful so as to ensure the required performance, refined to suit various specificities, flexible to constantly adapt to the changes and inevitable up and downs on such a complex project.

Each tunnel is equipped with two 900mm parallel railway tracks and its traffic is managed in one kilometer cantons equipped with double crossings.

The rolling stock includes 88 tractors of 60 to 415 horse power, 20 staff box cars, 330 scrap wagons and 261 wagons of various kinds.

The entire tunnel train traffic is controlled and managed by the TCC Traffic Control Centre which has large state-of-the-art equipment and uses the SOCRATE information management system developed for the construction site.

The TCC is one of the components of the CCC Central Control Centre installed at the Sangatte shaft which also includes the CSC Central Safety Centre and the CTM Centralized Technical Management.

 

TEAM AND THE WORK RHYTHM:

The excavation of the tunnels is uninterrupted: 24 hours a day, 7 days a week, (except on the Saint’s day celebration of St Barbe, December 4, during which the TBMs are stopped for 24 hours to celebrate it).

The teams work in 8-hour shifts at the front and in 6/3: 6 days of work and 3 days of rest.