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Work of art

France Grand Palais

A landmark building in Paris is undergoing a complete and complex facelift.

Built on a site near the river Seine, Paris' Grand Palais was constructed on a foundation of 3,300 oak piles, a common practice for waterlogged sites.

Construction began in 1897, to a design by architects Deglane, Louvet, Thomas and Girault for the 1900 World Exhibition, and by as early as 1898 it was decided to retain the building as a home for exhibitions.

However, longevity of timber piles depends on their remaining submerged. Flow management works on the Seine led to a drop in groundwater levels with inevitable consequences to the integrity of the piles, causing differential settlement of between 100mm and 120mm.

Structural problems were first identified following flooding of the area in 1910. From 1930 onwards, a systematic cataloguing was undertaken. A falling rivet in 1993 finally triggered the current rehabilitation programme. The forecast budget is US$135M with completion of the work programmed for 2005.

The Grand Palais is a double skin building, with a steel skeleton encased in masonry.

The first phase of work will make good damaged sections of the steel structure. The outer sections, in dressed stone, will be tackled during a future phase of the work.

The cruciform metallic structure, weighing 7,500t, in turn supports a glass roof, forming a vast hall in the heart of the building. There is also a monumental metalwork staircase that has had to be partially dismantled to allow access to the foundations.

After underpinning the substructure, the roof framework will be renovated and the glass roof restored to its original condition. This includes 3,500m 2ofzinc roofing, 1,300m of lead guttering, and 2,300m of ornamental pressed zinc, as well as more than 14,000m 2of glazing. The elaborate ornamentation of the building is also to be restored.

The contract for the foundations and building fabric phase was won by a consortium led by Soletanche Bachy. Design work is by Setec. The total cost of this first tranche is US$23M. SPIECGPM will act as project manager and will carry out the renovation of the building fabric. Jet grouting will be performed jointly by Spie Foundations, SMET and Soletanche Bachy.

The mixed nature of the subsoil had led the original designers to use piles only under the south side facing the Seine, where they compensate for the instability of the alluvial deposits. The steel support pillars for the glass roof rest on springers set on a bed of millstone grit and concrete. They primarily provide support for the dome, one of the largest in the world.

Two foundation techniques will be used in the restoration project because of the variable nature of the site.

In the piled area, a diaphragm wall will provide a base for the load-bearing steel supports as well as freeing up the interior of the basement. This will provide a space for possible future development. During the construction of the diaphragm wall, temporary restraints for the steel pillars and their springers prevented deflection of the building's metal skeleton from the considerable forces exerted.

Bachy is using a computer controlled low headroom Hydrofraise rig for the diaphragm walls which is excavating the 800mm thick, 2.4m wide panels under bentonite slurry. The panels are formed with tremied concrete and are excavated to depths of between 15m and 19m to found in the underlying limestone bedrock.

Once completed, support beams for the springers are cast on to them. During the foundation work period, structural movements are carefully monitored On the northern side the ground has better load-bearing qualities, and more especially, better homogeneity. Jet grouting will be used here to form 2,100 columns of between 1m and 1.40m diameter to underpin the foundations of the north nave, the Palais d'Antin and the south side galleries. The columns are formed at an angle in order to pass under the existing structures. Three rigs will carry out this work over a 12 month period.

Work on the metal superstructure has been entrusted to Eiffel.

Some 20,000 rivets holding the assembly together will be replaced, heated and forged on site, along with replacement of all damaged parts, reinforcement of the structures, and their protection with three coats of paint matched to the original colours. With the top of the dome at a height of 42m, this task is not an easy one.

Movement of the building has caused some elements to twist, notably the rafters on the south side and the radial support beams for the two crowns. The roof frame assembly is to be raised by 90mm by jacks, supported by micro piled shoring, so that the defective parts can be replaced.

Hinges will be created at various locations to compensate for future movement, using a technique devised by Setec.

The current assembly must be removed in order for the roof frame to be restored, sand blasted and painted. Glazing specialist Dutemple is to replace all 12,800m 2of glazing, which will follow the original design and layout. The glazing will be installed on specially manufactured uncoated aluminium profiles.

The location of the building in the heart of Paris, plus its complex layout, have provided further challenges for the project team. The museum galleries are still open to the public, restricting access to some parts of the site and noise levels have to be kept at 50dB (A) in those areas.

Special provisions have also had to be made for disposal of bentonite.

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