Geotechnical and hydrogeological diagnosis in Monolithic Church in France

Following a general deterioration of the pillar facings and given its stability conditions and appearance, the City of Auberre-sur-Donne, owner of this monument, mandated a consortium led by Agence GOUTAL, Chief Architect of Heritage Monuments, for a detailed diagnosis of the underground church for different subjects such as geotechnics, hydrogeology, structure, climatology, electricity, landscape, etc.

As a co-contractor in this consortium, Antea Group is in charge of the geotechnical and hydrogeological diagnosis. As part of establishing the inventory of the building, Antea Group's tasks include:

• Collection and analysis of previous studies and works,
• Diagnosis and survey of rock cracks,
• Visual inspection of the filling and the cliff located at the front of the monolithic church entrance,
• Hydrogeological expertise of the site, including water table monitoring and water chemical analyzes;
• Geotechnical survey, including core sampling directly in the pillars and inside the Church, and laboratory tests on limestone samples,
• Installation of a crack-monitoring device consisting of crack gauges and an extensometer from both sides of the vertical facings.

This survey shows that the rock massif, in which the church was built, is relatively homogeneous in terms of lithology, mechanical properties, and hydrogeological characteristics. The limestone massif is very porous (35%) and fed mainly by infiltration from the ground level, but the flow is slow (low permeability), favoring surface seeps and the formation of salts and gypsum crusts.

The monolith is affected by a dense crack network in the roof, linked to the deconfining of the limestone layer after excavation, in relation with pre-existing geological cracks. Major cracks were followed up over a 14-month period. The measurements show the lack of significant movement on the majority of the sensors (annual cyclical movement in correlation with thermal variations) with the exception of one sector that should be monitored (regular movement of crack opening).

At the end of this phase, a sketch phase for the restoration and consolidation of the church will be defined and will include in particular recommendations for the control and management of water inflows on site and the definition of methodologies for strengthening the pillars and the vault.