References
Harlay car park
FRANCE, 1997
Design and construction of 10-level 295 place underground car park.
Harlay car park is an extension of the Harlay-Pont Neuf underground car park (3 basement levels, 553 places) on the Ile de la Cité island in the Seine. It lies 7m from the river at its nearest point, and fits snugly between the Palais de Justice (law courts), Maison de l'Avocat, Place Dauphine and the existing car park of which it forms part (and which had to be kept open to customers during the construction work).
It is a cylinder of 33.50m inside diameter, excavated to a depth of 27m, with a 35cm thick cover slab carried on 70 x 70cm beams. The interior structure is a spiral floor supported on the diaphragm wall and a central core housing the lifts, stair, ventilation system, etc.
The main difficulty concerned the intersection of the new car park with the old (especially as the third level was subject to flooding and the old and new car parks could only communicate at levels 1 and 2). This led to significant preparatory work for partially demolishing the existing car park where it encroached on the new:
- The existing structure and diaphragm wall had to be cut back in the three redundant parts (13m deep, of which 5m was below the water table).
- Filter points were installed in the existing car park to be able to cut the concrete below the water table.
Temporary concrete struts were needed during the work to support the existing cover slab carrying, among other things, the diaphragm wall construction plant.
Description of works
Construction procedure
The retaining wall around the excavation is a circular diaphragm wall needing no support by reason of its cylindrical shape. It is 0.82m thick and 52m deep, extending down to a relatively impervious pseudo-clay bed. It was built with a Hydrofraise rig fitted with powerful (8000 m.kg) motors. This is the best type of machine for digging through hard ground (18m of coarse-grained limestone) and the only one capable of achieving the 0.3% verticality specified in the contract with reference to the compressive stress in the diaphragm wall concrete.
After completing the cap beam at the top of the diaphragm wall and joining the extension to the existing structure, excavation proceeded to final depth while the parts of the existing car park lying within the circular wall were concurrently demolished. Spoil was disposed of by way of the nearby river Seine, the barges being loaded via a portal crane straddling the Quai de l'Horloge road.
Construction of the internal structure began with the central core (less than 10m in diameter), followed by the spiral floor, circling the core from the bottom up.
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.
Ouvrages
Soletanche Bachy undertakes the whole of a project from feasibility studies, through design to the complete construction on a turnkey basis.
Major urban excavations are one of the specialist activities of Soletanche Bachy. Such excavations are required for basements to buildings that are usually part of the foundations of the structure and also used for under-street car parks, cut and cover tunnels for roads, rail, metro and storm water tank...
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