References
Concarneau dry dock
FRANCE, 2001
Design and construction as main contractor of dry dock, length 130m, width 27m, depth 10.80m.
The target contract for construction of the Concarneau dry dock was performed by a consortium of Soletanche Bachy and Le Pape (excavation and fill). The site was a lagoon fed by a stream and open to the sea. The dock was needed to suit modern requirements, to supplement a boat hoist and slipway already in operation at Concarneau.
Design
The dry dock is 130m long, 27m wide and 10.80m deep, controlled on the seaward side by a trolley-mounted sliding gate. The remote end has a spiral access ramp for more efficient operational use by the commercial companies operating there. A pump room is provided to control washwater and gate leakage. Three pumps can discharge up to 4000 m3 per hour to dewater the dock in four hours when a ship is being docked. There are all the usual fittings conventionally found in harbours works such as bollards, capstans and winches.
Works
One of the challenges facing the consortium was how to deal with the mud covering the lagoon bed to depths of up to 7 metres, considering that the finished dock was to be surrounded by earth platforms for normal harbour operations, with a specified bearing capacity of at least 3 tonnes per square metre. Excavation of the mud would have been difficult and disposal even more problematical, and it was decided to consolidate it in situ by preloading. Apart from the excavation for the dock itself, therefore, all the mud has been left in place. An interceptor channel was dug to divert the river around the lagoon, then the lagoon was emptied to expose the mud. A geotextile was laid over the whole area and covered with the same thickness of free-draining gravel. Strip drains were sunk from this platform down to bedrock in a 1-metre square array. The subsequent weight of the fill gradually expelled the water from the mud through sumps collecting the water in the free-draining layer. Settlement of approximately one metre was observed before construction work proper could commence.
The dock sidewalls were built as diaphragm walls, tied back at the top with passive anchors to sheet piling and fixed at the bottom by the concrete floor of the dock.
The floor is a drained raft to prevent the build-up of uplift. Works on the dock entrance proceeded behind a watertight cofferdam built in the port: pump room, floor under the gate, gate recess (rock excavation with shotcrete and nail support). The contract required a turnkey graving facility, and ancillary works included a perimeter road around the dock, drinking water, electricity and gas supply, fire-fighting system, two-storey control building and all fittings for ship docking (keel blocks, winched cradles, etc.). One of the last operations was assembly of the dock gate, by assembling four caissons to form a single unit 28m long, 11m high and 4m thick, weighing 170 tonnes. The gate was launched by a nearby boat hoist, towed to station and sunk onto its trolleys, standing ready on their rail tracks.
Techniques
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.
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.
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
Voyage au coeur de l'ingénierie de pointe
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