References
Rivière des Pluies viaduct - Saint-Denis, La Réunion island
FRANCE, 2006-2007
Foundations of the viaduct and protection of the embankment works.
he Rivière des Pluies viaduct is located on the southern boulevard of Saint Denis. The structure is a curved bridge, 190m long, supported by two abutments and three intermediate piers set in the river bed. The bridge deck is made up of two transverse caissons for a total width of 31m, which provides passage for motor vehicles, bicycles, pedestrians and the future tram-train.
Solétanche Bachy was awarded the contract to build 33 barrettes under the various supports and also a 210m long anchored diaphragm wall to protect the embankment near the right-hand abutment.
Special features
Most of the work was carried out directly on the bed of a river with highly variable and unpredictable water behaviour. The water flow in dry weather (<< 1m3/s) can suddenly increase a thousand-fold to reach a torrential flow rate of 500 to 1 000m3/s in some conditions.
The surface water hydrology and the hydrogeology change completely after heavy rain and make it difficult to set the platform levels for building the diaphragm wall and the barrettes.
Geology
The Réunion Island is a relatively recent volcanic formation with a highly complex geology. The main difficulty faced was the highly heterogeneous, indurated and cemented nature of the ground, which also contained basalt blocks of up to 2m in height. Trepanning was used virtually systematically during excavation. The presence of a large unweathered basalt slab and the need to embed the barrettes through the slab meant that the ground surrounding the 8 barrettes under the P1 pier had to be pre- fractured by blasting.
Phasing
Phase 1: start of diaphragm wall works on the right bank.
Phase 2: simultaneously, construction of barrettes under the P3 pier.
Phase 3: construction of barrettes under the A4 abutment on the right bank and under the P2 pier.
Phase 4: continuation of the diaphragm wall in the A4 area and installation of the barrettes under the P1 pier.
Phase 5: completion of the diaphragm wall in the A4 area and installation of the barrettes under A0 on the left bank.
Techniques
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.
A ground anchor is a load transfer system designed to transfer the forces applied to it to a competent stratum. An anchor is said to be temporary if it has a lifespan of under two years and permanent if the lifespan is over two years.
Ouvrages
Soletanche Bachy has developed a number of techniques for use in road structures; cut and cover tunnels, viaduct etc.
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