Study of collapse occured during the reinforcement work of a vault in a chalk quarry located in the north of Paris (France)

International audienceA vault weighing about ten tons collapsed unexpectedly during the reinforcement works carried out in an old underground chalk quarry situated about sixty kilometers north-west of Paris in France. The quarry exploited during the first half of the 20th century by the method of abandoned rooms and pillars. The extracted hard chalk used for the construction of several buildings and monuments of the region. The exploited layer of chalk is located at a depth of between 15 and 45 m. A mainly chalky roof covered the underground works, and a residual clay-flint formation of varying thickness over-hung the chalky cover. The quarry's right-of-way is mostly under a plateau at an average depth of 45 m. The cover of the quarry is characterized by a singular geological context, due to the presence in the covering of several vertical and horizontal karsts, as well as alteration roots that can reach depths greater than 45 m. The underground works intercept several alteration roots, which are identifiable by their clay material impregnated with numerous blocks of flint of different sizes. These geological objects intercepted at the bottom by the old works of the quarry are sometimes in the origin of a phenomenon of emptying of their filling materials, leading to the appearance of cavities on surface of large diameters (~ 10 m) which may constitute a significant risk to buildings and persons. Natural subvertical to vertical joints are also present at the site. Some are karstified and filled with clay material. They result sometimes in some problems of stability of the lower part of roof. This is notably the case of the 2017 collapse zone, where two major joints were cutting off the roof, forming a vault weighing about ten tons that had collapsed during the drilling of holes destined for the roof reinforcement system by flat beams.This paper describes the accident circumstances, gives some explanations for the accident, and present a simple 3 dimensional model realized to study the stability of the vault taking into account influence of local discontinuitie