Even if the first part of the work, until the end of the summer, will not be very spectacular, it is indeed an extraordinary site which is starting up again these days around the Saint-Michel spire, in Bordeaux, high of 114 m. Following the crumbling that fell from the spire, the historic monument was closed to the public in November 2021, and a security perimeter established. With consequences for the circulation of residents, and the organization of the market organized on the square at the foot of the tower.
“We are now going to enter the operational phase, which will consist of laying the foundations for the scaffolding,” explains Olivier Cazaux, deputy mayor of the Bordeaux south district. These foundations will only intervene after the delivery of the first materials and the construction of a first living base for the workers, in July and August. Some 48 micropiles will be sunk into the ground up to twenty meters deep, to support an extraordinary metal structure of 450 tons, which will itself support scaffolding of 250 tons.
An arrow not strong enough to support traditional scaffolding
“The micropile technique is usually used for new buildings, never for temporary structures,” emphasizes Virginie Zudas, from the building department at Bordeaux Métropole. But for reasons of wind resistance, and because the arrow is not strong enough to support traditional scaffolding, it was decided to build a full-fledged structure, made up of a metal part coming to raise a scaffolding. »
The assembly of this gigantic structure will take time. After the construction of the micropiles, the construction of the metal building which will climb up to 63 meters in height, will not start until October 17, and will last three months. Then will come the construction of the scaffolding, which will come on this structure and will culminate at 114 meters high, from January 2023 and until the end of May, beginning of June 2023.
A living base at 63 m height
Two platforms will also be used, one at 47 m high, the other at 63 m, the latter hosting a living base with offices, toilets and refectory, to avoid masons and other stonemasons who will intervene go back and forth. An elevator and a goods lift will also accompany the staircase leading to the top of the tower. “We will have work in two stages, continues Virginie Zudas, since we will first work on the upper part between 114 m and 63 m, then on the second part between 63 m and 47 m. »
The objective of the work will be to “remove the oxidized metal elements, replace the cement joints with lime mortar joints, change the broken stones and mend all the cracks”, explains Patrick Della-Libera, in charge of monumental heritage and furniture to the city of Bordeaux. The 2.5 ton cross located at the top of the tower will also be dismantled.
Cracks that open “regularly and irremediably”
The first cracks on the building were identified more than twenty years ago, in 2000, following the storm of 1999, recalls Patrick Della-Libera. “Since then, we have been monitoring the building, and we have noticed that these cracks are opening up regularly and irreparably. »
The tower, which has twelve faces, is now cracked on six of them. “And it turns out that the arrow deforms, mainly when there are thermal stresses” continues the heritage manager. Thus, during the heat wave of June 17 and 18, “the electronic gauges placed on the tower panicked everyone because the widening of the cracks exceeded the critical thresholds that had been set”.
Third tallest spire in Europe
Third highest spire in Europe, after Strasbourg and Cologne, the Saint-Michel tower is a listed building. This is why the work had to be submitted for authorization by the Drac (Regional Directorate of Cultural Affairs). They are due to end in 2025.
The total cost of these works amounts to almost eleven million euros. And, to realize the magnitude of this preparatory phase, you have to measure that what concerns only the metal structure, the scaffolding and the protection of the place around the spire, will amount to five million euros, i.e. almost half of the budget.