Lille metro - Leulier and JP Sartre branches
Ground freezing with liquid nitrogen for construction of two emergency access shafts.
There are two shafts and connecting tunnels for the emergency services access to Section F of Line 2 on the Lille underground railway. The Leulier and JP Sartre shafts were excavated inside a diaphragm wall and in situ reinforced concrete inner wall, with tunnels measuring 2.50 m by 4.50 m connecting them to the main tunnel 10 m below ground level. The connecting tunnels lie in the Louvil clays and tufa formations, very fine silt clay sand with a 15-20 m head of ground water, and the ground had to be rendered watertight and competent before driving. The solution was a frozen ground arch which would exclude water and transmit the water pressure to the clays.
Horizontal boreholes were drilled from the access shaft, using a blow-out preventer to counter ground water pressure. These boreholes had:
• Freezing pipes around the crown contour, spacing 1.00 m approx.
• Thermocouples passing through the frozen arch.
• A monitoring drain within the tunnel section.
As the tunnels were driven, heat insulation was placed around the walls to prevent the ground arch from thawing.
The freezing process proceeded in two steps. The ground was brought down to the required temperature by permanently circulating liquid nitrogen for 24 hours a day, then, when the ground had been frozen, the nitrogen was circulated only outside daylight hours, for four to eight hours each night.
The thermocouples were provided to:
• Check that freezing temperature had been reached in the ground.
• Detect any anomalies in the frozen ground arch.
• Determine what nitrogen circulation was needed to maintain the required temperature, and check that it was effective.
The drain hole provided a cross-check that the arch remained frozen. Steel rib support was installed as excavation proceeded, before building the concrete lining on completion of the driving, and contact grouting the annulus.
Ground freezing works on the principle of freezing the water in the soil pores, thus rendering the soil impermeable and of greater strength.
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.
Soletanche Bachy has developed a number of techniques for use in railway works; cut and cover tunnels, highways, underground stations, viaduct etc.
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.
Voyage au coeur de l'ingénierie de pointe
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