Heave of a sports centre on anhydritic claystone formation

The development of fissures in a building placed at the site of an ancient quarry excavated in a claystone Keuper formation motivated a detailed in-situ investigation of the foundation material. Severe fissures and cracks were present at the partition walls of the basement and ground floor of the building. Swelling and local failures were also observed at the basement floor slab. The structure of the basement floor of the building is solved by means of pillars and unidirectional slabs. The perimeter wall is a reinforced concrete load bearing wall supported by a spread footing. The observed cracks suggested that the damage was related to differential movements of the foundation of the building. Levelling of basement floor, pillars and walls allowed the identification of a slow systematic heave of some pillars. The records of vertical displacement along depth of two continuous extensometers installed at the basement indicated that the heave displacement was associated with the expansion of the four upper meters of the foundation material. Mineralogical investigation by means of X-ray diffraction analysis identified anhydrite and gypsum in samples recovered from boreholes. The vertical profile of anhydrite and gypsum contents could be determined. They helped to understand the swelling mechanism. The most probable expansive mechanism is the precipitation of gypsum in depth, which will most likely take place in fractures of the foundation material. The presence of water found in boreholes favours this mechanism. The opening of fissures in the foundation material was probably associated with the large excavation performed during the excavation of the quarry. Solutions advanced to counteract the heave are discussed in the paper.