The building was built on recent alluvium from the Seine, and has been subject to settlement since it was built. The building has become increasingly fragile over the years, to the point of having serious safety problems, which led to the cellar (lower room) being partially filled from the floor up. As part of an operation to consolidate and modernise the building, Soletanche Bachy was awarded the work to underpin the foundations enabling the cellar to be cleared.

The chosen solution consists in underpinning:

– each pillar of the cellar (two rows of 16 pillars), by two micropiles sealed in a block created under each pier (100% of the load is transferred by these micropiles),

– the building’s peripheral walls, by inclined micropiles sealed directly into the brickwork (only a proportion

of the load is transferred by these micropiles, and the existing foundations continue to play their role).

The work was accomplished from the building’s cellar, with a low ceiling height and in a fragile and narrow setting.

322 x 15m micropiles

The cellar pillars

Micropiles (64) were firstly constructed around each column base. A metal collar, supported by the new micropiles,

was then installed to clear the bases of the columns. This system enables the existing foundation stones to be removed, and a foundation block to be built under each pillar. The heads of the micropiles are then connected to the block by metal plates.

The walls

The micropiles (258) were bored from the ground floor or, in certain cases, from recesses, whether existing ones or ones which were created specially, and also from the outside through the buttresses.