Geomechanical response of an abandoned chalk mine to multi-annual water table fluctuations

The Paris basin contains 35,000–40,000 mines, most of which are abandoned, having mined chalk, a rock very sensitive to water. Although they have been excavated above the groundwater level, some of the mines can be temporarily or permanently flooded, due to (some) exceptional rainfall events. We have thus instrumented the chalk mine of Estreux (Nord department) to measure the impact of the oscillations of the water table on the stability of one of its pillars. It is a mine dug around 20 m deep by the chambers and pillars method. Now abandoned, it is temporarily flooded when the water table, located a few meters below the mine floor, rises due to heavy precipitation, which occurs on average every 8 years. From 2004 to 2012, a pillar was equipped with a convergence rod and two extensometers. There is a correlation between the vertical and horizontal deformation rates of the pillar and the periodic rise in the water table. Chalk samples were also taken in situ and their geomechanical properties characterized in the laboratory. On this basis, a first 3D mechanical simulation of the behavior of this pillar has been carried out taking into account elasticity, plasticity and creep. The calculated mean values of deformation generally match the measurements, and the model succeeds in reproducing the annual fluctuations of these parameters in relation to those of the level of the water table. However, compared to the measurements carried out in situ, the current model exaggerates the vertical deformations compared to the horizontal deformations. However, with this first model, we can observe that the variations in the water table induce elastic and reversible deformations of the pillar, given the absence of plastification of the chalk. These deformations are complementary to those due to the creep induced by the weight of the overlying grounds. If we want to model the long-term behavior of the pillar, particularly under the effect of the fluctuations of the water table induced by the climate change in progress, it is necessary to improve this model, taking into account the hydromechanical couplings and the flows in unsaturated condition. In conclusion, it is proposed to use these data later in a complete mechanical model, still under development at INERIS.