References
Voie des Mercureaux - Head of Bois du Peu Tunnel
FRANCE, 2004
Anchored walls to reinforce side embankments and portals of a double-tube tunnel head.
In the construction of the RN57 Besançon bypass, all that remains is to cross the Bois du Peu anticline, the South-Western extension of the La Citadelle buttress. The boring of a double-tube tunnel requires significant fitting work at the two portals of the tunnels, because of the mediocre quality of the covering soils and the instability of the drainage basin. As a result, the tunnel heads have had to be covered in a specific contract valued at over 12 million, representing half of the cost of the tunnels overall.
Works at the Doubs tunnel head are essential before the tunnel is bored. Anchored walls will have to be constructed to strengthen the lateral embankments and the tube heads.
Geology
Under a layer of scree are alternating banks of marl (loam rock) and limestone. These layers are inclined and cut through with a series of faults. The compartments produced by the faults present net slips.
The tectonic structure is complex, as demonstrates the significant division of the cliffs overlooking the Doubs Valley.
CARRYING OUT THE WORKS
Anchoring
The support system relies on a number of geotechnical procedures: HA steel nails 40 or 50mm in diameter, glass fiber nails, 4T15, 5T15 and 6T15 anchor ties, and finally pipes.
The drilling is carried out with an in-hole drill 110mm or 150mm in diameter according to purpose. The 110mm diameter is used for 40mm steel nails, glass fiber nails and pipes. In the scree and highly fractured facies, the drilling frequently has to be tubed. The drilling parameters (speed of progress, pressure on tool, torque) are systematically recorded and analyzed at each drilling, to verify compliance of the soil with the calculation hypotheses.
Two revolving cranes with 25m and 40m booms respectively operate on the site, and are used for delivery of anchorages to the required points and for lifting and positioning them inside the bore holes. The nails and ties, which can reach 27m in length, are thus introduced relatively easily into the downward bore holes.
The seals, meanwhile, are produced using cement mortar and not cast, because of the heavy fissuring of the limestone rock and the presence of karsts, which necessitate the use of a thick product in order to limit over-consumption. The mortar is injected under controlled volumes and pressures.
Sprayed concrete
The excavations are made by means of successive passes, 1.50m high. The sprayed concrete is 250mm thick ground the nails and 360mm thick ground the anchor ties. It is reinforced with two layers of steelmesh. An Enkadrain B10-25 drainage level is placed on the interface between the concrete and the soil. The dry-cast material has a water/cement ratio of between 0.40 and 0.45.
Results
On this site, located in an area of difficult weather conditions, work has been rapid because suitable means have been applied. The deadlines have therefore all been met. The anchored wall works employed 23 executives, technicians and specialist operators.
Techniques
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.
A retaining shell is a slim shell structure used during vertical excavations. Unlike retaining walls, the weight of a retaining shell has little effect on its capacity to balance the pressures.
Ouvrages
Soletanche Bachy has developed a number of techniques for use in road structures; cut and cover tunnels, viaduct etc.
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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