Rennes Convention Centre : Concrete Consecrated Stone

11 February 2016 - Projects update and handover - France

The transformation of the Jacobin monastery into a congress centre in Rennes combines new construction and heritage restoration in a technically complex project. The 14th century landmark, revisited with 21st century expertise, will embark on a new life in early 2017.

Following several years of feasibility studies and discussions, Rennes Métropole, the Rennes authority that owns the historic monument, decided in 2009 to transform it into a congress centre. The conversion of the building, the oldest part of which dates from the 14th century, was a challenge because the site lies within a warren of narrow streets lined with half-timbered houses near the picturesque Saint Anne square and next to the Notre Dame de Bonne Nouvelle church. In addition, the project design had to take on board the presence of local residents at the edges of the property and the clearances required by the local urban develop - ment plan. Rennes Métropole held a competition and in July 2010 selected the project submitted by architect Jean Guervilly in association with heritage restoration architects Perrot & Richard. The works contract, which has an overall value of €75 million, was won by a joint venture led by Sogea Bretagne that includes CMA Entreprises, Degaine, Botte Fondations (VINCI Construction France) Soletanche Bachy (VINCI Construction), Cegelec Clim Ouest (VINCI Energies) and Ineo Atlantique.

In addition to restoring the old building, the project includes the construction of a modular auditorium with a seating capacity of up to 1,000 overlooked by a large number of multi-purpose exhibition spaces as well as several underground conference rooms under the cloister of the existing building. Sylvain Bonaldi, project director at Sogea Bretagne, believes that the project’s bold approach was the decisive factor in the contract award: “The architect’s brilliant idea was to excavate under the monastery to gain space below ground to avoid adding floors to the historic building. This project supports and showcases the monastery, which is a major heritage site in Rennes.” This configuration will transform the nave of the former monastery chapel into a 300- seat hall to breathe new life into the historic space while conserving its character.

"Levitate” the monastery

This design faces the VINCI companies with the methodological and technical challenge of maintaining the cohesion of the historic monument during the earthworks under its foundations. Following an 18-month archaeological excavation campaign, Sogea Bretagne and Soletanche Bachy took over the site in the spring of 2014. They defined a works scenario and submitted it to project management for approval. Their plan is to install a temporary structure made up of a mesh of concrete beams set on a series of piles and micropiles to support the monastery during the works phase. At the same time, Freyssinet will be called in to “regenerate” the old building. This will involve injecting a cementbased slurry compatible with the existing clay mortar at low pressure into the monastery’s rubblestone masonry to consolidate it and prevent it from collapsing or cracking during the earthworks.

Around the building’s periphery, 18 piles and 142 micropiles will be methodically drilled and topped by a solid concrete cap with a weight of up to some 30 tonnes. Freyssinet will step in again to install double flat jacks to transfer loads to the temporary structure. “The company’s traditional expertise consists in inserting a metal pocket between the top of the piles and the base of the building and injecting a cement slurry into it,” says Christophe Houix, sector manager at Freyssinet Nantes. “On this project we innovated by adding a second flat jack into which we injected oil, to be able to measure the forces even better while easing the old building onto its temporary and then permanent bearings.” Some 70 jacking points were used to bear the 10,000 tonne monastery, with each jack supporting about 500 tonnes. During these operations, the historic monument was carefully instrumented to monitor its behaviour. Sogea Bretagne then carried out the earthworks, removing 80,000 cu. metres of soil and gradually freeing the piles and micropiles to a depth of 17 metres, to “levitate” the monastery. Following the construction of three underground levels, the jacks were removed, allowing the monastery to rest on its permanent supports – structural posts placed at the intersections of the main and secondary beams. To complete the process, the temporary piles, micropiles, jacks and concrete caps were removed. “What makes this project particularly complex is the need to coordinate the concrete structural works, which call for engineering and technical capabilities, with the very exacting craft of restoring the old monastery,” says Sylvain Bonaldi. “The fact that Degaine, our VINCI Construction subsidiary specialising in historic monuments, was lead company of the joint venture restoring the old buildings was reassuring for our client.” For the new-build construction of the extension on the western side of the site, adjacent to neighbouring structures, a temporary diaphragm wall was built and stabilised by struts; elsewhere, a shotcrete wall covered with a waterproofing membrane was sufficient to retain the ground during construction of the inner walls. “For the new-build part of the project, the main difficulty consists in putting up the high walls of the main auditorium and the metal frame that covers this large box,” says Sogea Bretagne project methods manager Cyril Courcoux. “Positioning the nine metal beams, each with a 32 metre span and a weight of more than 40 tonnes, required a special study.” Given the absence of clearance around the worksite, temporary concrete platforms will be built on the three first and then the six first beams to enable the crane to move forward above the excavation to position the six last beams in two phases. Three additional levels will be built above this transfer structure to house the exhibition spaces, seminar rooms and restaurants.

Meanwhile, the degaine teams braced all windows and doors at the start of the works to prevent them from shifting during jacking operations and earthworks. “The main focus of the work was on the window and door frames, made of freestone or shale, especially in the cloister,” say Nicolas Back, head of the historic monuments department at Degaine. “The freestone frames had deteriorated over time and we had to join the best conserved stones with mortar and replace the most damaged ones.” Freestone quarries are still operating, but the company had to perform a search to find supplies of shale in order to restore the building according to best practices. To meet the requirement of the Historic Monuments architect – to restore the monastery to its 17th century condition – a number of cloister arches that had been closed were re-opened and windows and doors shown in the original drawings were restored. Once this painstaking masonry work has been completed, the rubblestone walls will be covered in solid white plaster inside the building and in render outside the building, which will leave 15% to 20% of the stones – those that protrude the most – visible. “With about 300 cubic metres of the stone blocks to be replaced and 4,500 square metres of surface area to be plastered and rendered for about €9 million, the Jacobin monastery is a major project and will be a wonderful showcase for Degaine’s expertise,” says Nicolas Back. As lead company for the historic restoration part of the project, Degaine is also coordinating the framing and roofing works packages entrusted to specialised subcontractors: the oak frames of the eastern and western wings will be restored, and the frame covering the nave, which is too dilapidated to be saved, will be rebuilt. The roof will be made of slate sourced in Spain, since the original Angers Trélazé slate quarries have been shut down. To further restore the site’s identity, new stained glass windows like those that existed in the south gallery in the 17th century will be installed, as well as sculptured ornamentation at the cornices and fascias in the cloister. When the congress centre opens in early 2017 after 35 months of works, the cloister will have a central garden bearing witness to its original function as a place for meditation.