References

Lille metro, Line 2, section F

FRANCE, 1993 - 1996

Construction* of 2 150 linear metres of a 6.80 m dia. tunnel and three underground stations.

Section F of the Lille underground railway No. 2 Line is an extension for the route connecting Lille city centre with Tourcoing. It comprises:
- three underground stations, Louis Constant, Pavé de Lille and Wasquehal Centre,
- Val section single-tube tunnel between the stations,
- two access and emergency shafts.

TUNNEL
Description
The Val tunnel has an inner diameter of 6.80 m. The longitudinal profile reflects the rolling stock supplier's (Matra) energy-saving layout with a steep descent where the train leaves the station followed by a level run until it climbs up to the next station. Maximum gradients are 7 % (see figure). The first section between Louis Constant and Pavé de Lille stations has some very tight horizontal curves of 150 m radius. Total excavated lengths are:
Section 1:Louis-Constant to Pavé de Lille: 750 m
Section 2: Pavé de Lille to Wasquehal Centre: 650 m
Section 3: Wasquehal Centre to smoke shaft: 750 m
Short tunnels join the lowest points on sections 2 and 3 to the nearby emergency access shafts. These tunnels also provide a route for dewatering the main tunnels, if necessary.

Geology
The three geological formations are, from top to bottom:
- Quaternary ground which can be considered as slightly to highly plastic clays. The stations lie in this formation.
- Upper tufa consisting of alternating very fine silt/clay sand, clean fine sand, and more or less indurated, cemented tufa.
- Louvil clay, thick beds of stiff to very stiff, often indurated plastic clay looking more like a rock.



Tunnel Boring Machine and Associated Items
Tufa sensitivity ranged from 1.5 to 2 bars, so that a slurry shield was selected. The main features were: excavated diameter 7.78 m, length 8 m, thrust 5 200 t, torque 630 t.metre, weight 500 t, tail length 65 m.
Pneumatic-tyred trucks were used to cope with the steep 7% gradients. Boring proceeded in two shifts, the night shift being used to pour the roadway concrete to catch up the day's advance. Trucks passed over the fresh concrete on a 25 m-long steel structure reaching the shield tail.
Instead of the usual telescoping mucking pipelines, a novel system of winding drums and inflatable packers was used, enabling the lines to be extended without loss of slurry.


Segmental Concrete Lining
The most striking innovation on the job concerned the concrete lining. Casting lasted seventeen months with 35 moulds. A Mayrede boltless ring system eliminated the need for bolting, always a difficult and dangerous task with large linings. The segments in adjacent rings were joined with 'Conex' conical connectors, ensuring the segments would remain safely in place if the driving/steering jacks were accidentally retracted. With this system, the rings were installed by the erector operator alone.

STATIONS AND EMERGENCY SHAFTS
The three 50 m long, 16 m wide, 20 m high underground stations and 6 m by 7 m by 30 m-deep emergency shafts crossed the whole geological section down to the clay horizon, entirely within the water table. They were all built on the same principle.
- A perimeter wall of diaphragm panels 1 m thick and 3 m wide was socketed into the naturally watertight formation. They were designed as temporary structures needing subsequent waterproofing which, on completion, will bear only part of the earth pressure. A second in situ concrete inner wall resists hydrostatic pressure and the remaining earth pressure. A water barrier consisting of ahree-ply PVC membrane interliner was stapled to the outer diaphragm wall before fixing the steel and pouring the concrete for the inner walls.
- The station sites were excavated (cut-and-cover for two stations, by mole for the third) inside the outer diaphragm walls, with three concrete struts installed as work proceeded downward.
- The concrete raft was poured, together with the RC inner wall, between 0.50 m and 2 m thick.

The struts were removed as the loads were taken by the raft and intermediate floors. Minor works included:
- Pedestrian entrances, excavated inside slurry trench walls (approximately 30 m by 8 m wide), to receive the escalators.
- Grouted zones, 5 m thick, at each station end, considered as watertight compartments for the tunnelling machine, where the face pressure was gradually applied.

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Techniques

Civil engineering

Regardless of site size, and providing it involves a significant proportion of works in the ground, Soletanche Bachy will handle the complete project, and all aspects of the works, including site supervision, excavations, foundations, civil engineering and all construction operations.

Diaphragm wall

A diaphragm wall is a reinforced concrete wall that is made in situ. The trench is prevented from collapsing during excavation, reinforcing and casting by the use of supporting bentonite slurry. The slurry forms a thick deposit (the cake) on the walls of the trench which balances the inward hydraulic forces and prevents water flow into the trench. A slurry made of polymers can also be used.

Tunnel boring

In the tunnelling field, Soletanche Bachy’s subsidiaries Soletanche Bachy Tunnels and CSM Bessac specialise in management of tunnel sites and also, for the latter, the design and construction of tunnel boring machines.

Ouvrages

Metros

Soletanche Bachy has helped build most of the world's major metro systems, performing soil consolidation grouting and constructing cut-and-cover tunnels with cast-in-situ diaphragm sidewalls, bored tunnels and major underground openings.

Railway works

Soletanche Bachy has developed a number of techniques for use in railway works; cut and cover tunnels, highways, underground stations, viaduct etc.

Shafts

Circular shafts excavated inside a pre-formed cylindrical diaphragm wall only began to develop in a major way when modern plant became capable of digging almost perfectly vertical walls such that the cylinder can be considered self-stable without having to be tied back with ground anchors, or strengthened with struts or reinforced concrete rings.

Tunnels

In the tunnelling field, Soletanche Bachy’s subsidiaries Soletanche Bachy Tunnels and CSM Bessac specialise in management of tunnel sites and also, for the latter, the design and construction of tunnel boring machines.


Voyage au coeur de l'ingénierie de pointe

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