To ensure that the repository will be ready to be operated in a structured and safe way until the decommissioning applied for begins, the Federal Office for Radiation Protection (BfS) has been monitoring its structural state. The results gained in the past years show only slight and constant deformation rates below one millimetre per year that do not compromise the repository's stability. The geo-mechanical monitoring provides an important basis for the evaluation of the repository's safety.
Under the pressure of the adjoining rock layers, salt rock may be plastically deformed. A slow, fluid movement sets in which is also referred to as "creeping". Therefore, deformations of the rock occur in salt mines. Over many years, this leads to existing cavities closing again gradually. On a regular basis, measurements to monitor the level of deformation are therefore carried out at the surface and underground.
Continuous measurements help in particular observe
- the deformations (convergences) of the cavities in the mine openings,
- changes in length and strains in the rock,
- forming of cracks in the salt rock, and
- changes in height at the earth's surface.
Measurement of deformations (convergences)
At about 230 permanently installed anchor points, it is examined whether there are movements and deformations in the mine openings due to the fluidity of the salt. For this purpose, so-called convergence measuring devices are installed between two anchor points. With these devices horizontal (from wall to wall) or vertical (from ceiling to ground) distances and thus changes in the cavities can be measured. Detected changes are registered with an accuracy of a hundredth millimetre. The results show that in most cases the deformation rates are less than one millimetre per year. As had been expected, larger deformation rates (cf. below) were also observed in the process of the stabilisation measures in the central part.
Measurement of rock movements
At 77 positions, so-called extensometers are installed. They provide information about changes in length and strains in the rock. As had been expected, the largest displacements of up to four millimetres per metre per year were detected in the scope of stabilising the central part (cf. below).
Measurement of cracks in the salt rock
Rock movements cause cracks in the salt rock. So-called fissurometers have been installed at 34 of these cracks. They replace the gypsum marks used previously. Gypsum marks can only provide information as to whether a crack gets larger, the reason for this being that the gypsum mark would then break. With the help of the fissurometers, however, it is possible to carry out three-dimensional measurements. In the past years, the detected changes in the cracks were just in the range of millimetres.
Microacoustics and location seismics
Geo-physical procedures - microacoustics and location seismics – help detect micro-cracks in the rock around the mine and vibrations. Among others, the Bundesanstalt für Geowissenschaften und Rohstoffe (BGR) carries out the computer-aided evaluation.
Measurement of changes in height of the surface
Movements of the rock may cause slight subsidence at the surface. Every two years, changes in height at the earth's surface are measured at two pre-determined points of a comprehensive measuring network. This is done with measuring devices known from road construction. The largest subsidence probably caused by the mine openings is located between the Bartensleben mine and the village of Morsleben, directly above the central part of the repository with a high excavation ratio. Altogether, the subsidence rates in this area have been about 13 millimetre since 1993. In general, the results show that the subsidence rates are in a range where they are barely measurable.
Stabilisation of the central part
While in large parts of the repository just slight and constant deformation rates below one millimetre have been detected, this is not the case in the central part of the Bartensleben mine, where single large pieces of rock fell from the ceiling in the past. This was caused by the high excavation ratio.
To guarantee mining and occupational health and safety and to secure the waste stored, the Federal Office for Radiation Protection has taken measures to stabilise the mine. For this purpose, the up to 140,000 cubic metres large cavities in the Bartensleben mine were backfilled with about 935,000 cubic metres of salt concrete between 2003 and 2011. As had been expected, deformations of up to six millimetres per year have been registered in the past years, caused by the weight and the heat of the setting salt concrete. Based on current results it is assumed that these deformation rates have reached their peak and are now decreasing again. Altogether, the backfilling had the intended effect of stabilising the central part.
State of 2016.06.21