References

Baudoyer car park

FRANCE, 1995

Design and construction of 5-level 320 place underground car park.

audoyer car park lies under Place Baudoyer between the municipal buildings and the barracks of the Republican Guard.




When planning the excavation work, a number of constraints and engineering problems had to be considered:
- There had to be no disruption of the lives of nearby residents.
- Mature, leaning, closely- spaced trees had to be preserved intact.
- Hard ground would be encountered during excavation.
- The water table lays near the surface.



A further requirement was to provide for an archaeological dig which eventually brought to light remnants of 8th century Merovingian dwellings and tombs from the 12th century Saint Jean cemetery containing skeletons in a remarkable state of preservation.

Description of works
Construction procedure
The retaining wall around the excavation was a diaphragm wall, 0.63m thick. It was built with a Latine Hydrofraise rig, the best suited to trenching in hard ground (coarse-grained limestone) without damaging the branches of the trees, and keeping disturbances to residents from noise and vibration to a minimum.

A grout curtain extending from the toe of the diaphragm wall into the limestone controls underseepage during and after construction (the concrete raft is drained and seepage has to be pumped out).

Part of the retaining wall runs between the front of the municipal building and a row of closely-spaced old plane trees with widely-spreading branches, some of them leaning sharply, for which a half-sphere of 2m radius had to be kept. Construction of the cover slab under the trees consisted of first installing temporary prefounded columns, removing the soil from around the trees inside a mini-Berlin wall, then drilling a subhorizontal support system consisting of three 109/127mm pipes spaced one metre apart over a width of 4 metres.
The pipes rest on steel joists, supported on the prefounded columns at one end and on the ground at the other. Then precast concrete slabs were slipped under the pipes, supported on the diaphragm wall; lastly, a system of longitudinal and transverse beams was installed.
The diaphragm wall is tied back with one row of temporary prestressed ground anchors during excavation to final depth, while the prefounded columns maintain the tree "pots" in position. The five staggered floors were then built from the bottom up.

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Techniques

Civil engineering

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.

Diaphragm wall

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.

Ground anchors

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.

Grouting

Grouting involves the injection of a pumpable product (slurry), which will subsequently stiffen, into the soil or into man-made material (masonry), in order to consolidate the soil or structure or make it impermeable, through filling all the voids it contains. The slurry can fill the voids in the ground, the cracks within rock, solution cavities (it is then referred to as fissure and permeation grouting) and/or displace the surrounding soils through a bottom-up process or by fracturing (compaction grouting or solid injection - see the section on the subject - and strain injection). Grouting with soil displacement may be used to prevent potential damage to the structure brought about by excavations (galleries and tunnels, major urban excavations, etc.) and this is called compensation grouting (see the relevant section).

Ouvrages

Underground car parks

Soletanche Bachy undertakes the whole of a project from feasibility studies, through design to the complete construction on a turnkey basis.

Urban excavations

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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