In 1993, the deteriorating state of the Grand Palais’ metallic roof structure became alarming when a rivet dropped from it, prompting the nave’s closure to the public. Following a technical survey, restoration work began with the most urgent task: consolidating the entire building on firm foundations.
Like many early 20th-century constructions, the Grand Palais was built partly on wooden piles, which don’t rot when they are entirely submerged. But the construction of the Métro, the riverside expressway and the RER lowered the level of the water table, and the tops of the piles supporting the southern half of the Grand Palais were exposed to air. As a result, the gradual rotting of the piles caused the building to settle as much as 12 centimetres in places.
© photothèque Soletanche Bachy
Jet grouting technique
The delicate task of consolidating the foundations
Once the problem had been diagnosed, several techniques were used to replace the foundations without damaging the edifice or its metal and glass roof. The wooden piles beneath the southern part of the nave and dome were replaced by a diaphragm wall.
The wall was anchored in the bedrock, at depth of 20 metres. The foundations of the rest of the building were reinforced with 2,000 jet grouting piles.
Concrete was then poured around the original foundations of the metallic roof structure. Pressing against the diaphragm wall and jet grouting piles, this transfers the weight of the roof structure onto the new foundationsin order the metal and glass roof not to move.
During this work, the movements of the Grand Palais’ structure were constantly monitored by an automated system.
Jet grouting
Jet grouting is a technique used to improve ground. A high-pressure jet of cement (or cement and air depending on the type of ground) alters the structure of the ground during drilling. The eroded ground mixes with the self-setting cement to form columns or piles.
© photothèque Soletanche Bachy
A few figures
The diaphragm concrete walls: 8 900 square metres, 80 centimetres thick, 19 metres deep.
Jet grouting pile: 15 kilometres of jet grouting.