Big Scale In-Situ Application of Matrix-stabilized vs. Conventional Salt Grit Backfill with Use of Improved Backfilling Method

With nuclear energy being one of the trending topics in the world regarding energy supply, at the same time the question for safe disposal of the high-level radioactive waste becomes increasingly popular in the public. 

The TU Bergakademie Freiberg wants to provide answers to this challenging task by practical big-scale in-situ underground tests and new backfill material development. The underground mining chair commenced the GESAV (Gefügestabilisierter Salzgrusversatz; engl.: Matrix-stabilized salt grit backfill) project series back in 2013 with the development of a matrix-stabilized salt backfill which solidifies in an early stage by the development of internal polyhalite crystals. The innovative material combines the benefits of a salt backfill material with a stability compared to conventional materials, such as cement like building materials.

The GESAV project series has been finished in 2021 with the successful proof of the big-scale practical applicability of the matrix-stabilized backfill by building several backfill bodies underground with different methods from the conventional underground mining industry.

The goal for the currently going-on SAVER (Entwicklung eines salzgrusbasierten Versatzkonzeptes unter der Option Rückholbarkeit; engl.: Development of a salt grit based backfilling concept with regards to retrievability) project is to build and compare a GESAV material backfill body to a conventional salt backfill body, since in the GESAV II project only the in-situ applicability of the GESAV material in general has been researched. Both of the test drifts in the Sondershausen rock salt underground mine have the same dimensions and are instrumented exactly the same. Moreover, a radioactive waste casket dummy has been developed within the project and placed into the backfill bodies in order to simulate a real-life backfilling process. So far, both backfill bodies have been built with a slinger-vibration-backfill method that already highlighted very high in-situ densities in the GESAV II project (2017-2021). Not only did this validate the applicability of the method on GESAV material but also proved that it is transferable to other salt backfill materials.

During the building process of both bodies, a big sampling campaign was carried out in order to gain an insight into actual possible built-in in-situ densities and other parameters that are relevant for further optimization and understanding of real-life backfilling in a mining scale. Once analyzed, the samples, in combination with the other sensors, can provide a deep insight and further understanding of the geochemical and mechanical properties of both backfill bodies. The sensor data shows that GESAV material highlights better properties regarding settlement than conventional salt grit as well as pressure distribution within the backfill body. The measurement systems will continue to run as long as the project is taking place in order to continuously gain an insight into ongoing processes within both backfill bodies. With the chosen sensor setup as well as the sampling regime it is expected to gain more valuable insights into actual underground in-situ behavior of those materials in the near future. Those insights are very likely to contribute to the ongoing question of developing a feasible concept for high-radioactive waste disposal in rock salt formations.