Cavitec refers to the doctrine of State services concerning the management of underground cavities risk. The Departmental Directorate of Territories and the Sea of ​​Seine-Maritime (DDTM76) defined principles for managing the risk linked to underground cavities in the department following the collapses that occurred during the winter of 1995. These principles were formalized and translated into town planning documents from the 2000s. These principles are essentially based on a statistical study carried out in the canton of Yvetot in 1997 by the Bureau of Geological and Mining Research (BRGM). This analysis showed that 98% of the marl pits studied had a maximum radius of 55m. A study carried out in 2001 by the Center for Technical Studies of Equipment (CETE) confirmed this figure.

Based on this study, local rules were put in place in order to best take into account the precautionary principle when issuing urban planning authorizations. They are mainly based on the application of risk perimeters of variable radius depending on the type of underground cavity index:

Doctrine of State services

Drilling carried out as part of the search for marl must reach a minimum depth of 15m from the roof of the chalk. The depth of drilling must adapt to the great variability in the thickness of surface formations (silts and flint clay). They can descend between 25 and 40 m deep.

In the case of “stone quarry”, it is necessary to drill up to 5 m after the chalk roof.

The distance between reconnaissance boreholes must not exceed 3m. However, this distance must be adapted to each case.

(Source: DDTM76)