ABSTRACT
In the Asse II salt mine’s southern flank an array with relatively small pillars and stopes was excavated between 1916 and 1964 in the course of halite extraction. Most of the volume of the chambers (in total approx. 3.5 Mio. m3) was exposed to free convergence until 1995, when a backfilling campaign by pneumatic transportation of a granular salt material started, which lasted until 2003. The barrier to the overburden rocks is formed by rock salt with a minimal thickness of only 15 m in the upper part. The flank dips by approx. 70° in a SW direction. The rock mechanical evolution has been monitored for decades by displacement observations, stress and strain measurements in the pillars, and recording of the backfill pressure built-up in the chambers. Furthermore, microseismic activity in the mine and the adjacent overburden has been recorded. Softening and damaging in the pillars and stopes of the mining horizon have led to stress redistributions into the overburden rocks, where fracturing processes were generated as well. Hence, because of the small dimensions of the bearing elements on the southern flank and the close distance to the overburden, far reaching geomechanical interactions exist. The purpose of the project was the modeling of mining history and of observed rock mechanical behavior aiming at the stability proof and prediction until the end of operating phase. For that a time-dependent elastovisco-plastic constitutive law was used which describes softening, dilation, and creep. The material parameters were found in special laboratory experiments determining the residual strength after exceeding the peak of the stress-strain curves in the domain of dilatancy. In the 3D-modeling course all calculated stresses, deformations, and geomechanical reactions could be confirmed as measured in situ. Therefore, the model can be considered as validated and qualified for prediction.
