The massivity of a geological formation rests on the concept of layers stiffness that must be evaluated in a morphological and geological context. Parameters being able to play a part in the global stiffness of a formation are either the intrinsic parameters or the surrounding morphological conditions of the calcareous mass over-hanging the layers of iron ore in Lorraine basin. We study the role which the geomorphology can be played on the massivity of the overburden, at first by analytical solutions of a simplified postponed valley, then by numerical simulations carried out in 3D based on the geology and geometrical modeling of geological objects (GOCAD). Geomechanical modelings were carried out by successive excavations of the valley. The results showed that the overstressed zone in excess for the maximum horizontal stress already observed with analytical solutions, appeared at the end of postponed valley for 3D calculations. Sizes of this area in compression are about 250 m by 100 m. Moreover, these overstresses are located only superficially in the twenty first meters of depth. The minor horizontal stress presents, on the contrary, a relaxation due to empty space created by the valley. The vertical stress undergoes a simple relaxation on all the depth of the postponed valley.
Lorraine was an important mining region leaving many empty spaces in the under-ground, responsible for many accidental movements. The consequences on the surface depend not only on the intensity of the phenomenon (magnitude of subsidence and maximum horizontal strain), but also on the kinetics with which these instabilities appear on the surface (Didier & Josien, 2003). The risks of progressive subsidence or sudden collapses remain even after the stop of mining exploitations; indeed the phenomena occurred whereas underground works in the zone had ceased since many years.