Etude par analyse texturale du rôle de l'humidité dans la déformation des roches salifères. Application au sel du Bassin Bressan et au sel de dôme de la mine d'Asse

摘要

To emphasize the role of humidity in rock salt deformation, several tests using the special test cell of the Centre de Géologie de l'Ingénieur (CGI) were carried out on samples of milky salt and phenoblastic salt from Bresse basin (France) as well as on samples of dome salt from Asse mine (Germany). The method used is based on continuous visual observation of the textural evolution of samples under applied conditions of stress, temperature and humidity. An increase, even weak, of relative humidity ratio always leads to a transient creep phase followed by a stationary creep phase at a constant strain rate. This stationary creep is often faster than the one before the relative humidity ratio increase. The response of the sample texture to humidity and temperature variations determines the predominance of one or another deformation micromechanism. From the microstructure point of view the crystals content of fluid and/or solid inclusions has a strong influence on their ductility. This explains that the milky salt with a high content of fluid inclusions is more ductile than the dome salt where the crystals contain solid inclusions. In the grain boundaries, the humidity penetrates and condenses allowing the development of specific deformation mechanisms. According to grain boundaries morphology and surface state, lubrification and gliding mechanisms act in dome salt while shear and microstylolitisation are the main mechanisms in milky salt. Furthermore the brain films which are re-formed in grain boundaries due to humidity favour the thermomigration of fluid inclusions from one crystal to the next through the saturated boundary. Within the phenoblastic salt the anhydrite which is contained in the microporous matrix is hydrated into gypsum due to humidity and temperature. This transformation brings an important swelling and cracking of the matrix which may lead to material degradation. Taking into account the three essential parameters in salt creep: texture, temperature and humidity, the study tends to reconstitute in a realistic way the conditions which are likely to be gathered near a nuclear waste storage gallery. Therefore the obtained results may help to predict the long term behaviour of a massif of salt.