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Impact of Earthquake on Rock Bolts at Konrad Repository

Plastic state of emplacement drift

Vertical displacement (cm) of bolt

Considering the operational phase of the Konrad repository, earthquake resistivity of safety-relevant technical components – e.g. pipes for hydraulically transported cementitious backfill for the emplacement drifts – has to be demonstrated according to the technical safety guide-lines of the Nuclear Safety Standards Commission (KTA). Some of the technical components are fixed to the drift contour by rock bolts. Therefore, it was investigated numerically whether rock bolts are additionally strained by an earthquake impact due to bedrock fluctuation.

The model area is at a depth of 858.5 m to 935 m and covers the cross-section of one emplacement drift as well as an axial section. At the Konrad emplacement level, the sedimentary rock has an inclination angle of about 25°, and as a consequence, the bedrock shows an anisotropic behavior. To verify the suitability of the rock mechanical parameters, the history of the emplacement drift from its excavation to the present time was modelled, and the calculated convergence behavior was compared with convergence measurements. As the agreement between calculated and measured values was adequate, the simulated plastic state of the calculation model was used as initial state for the earthquake calculation.

The calculation results were evaluated at two rock bolt positions in the roof of the emplacement drift. As the length of the individual rock bolts is 1.80 m, the displacement difference between the drift contour and 1.80 m depth in direction of rock bolting was determined, because a displacement difference causes additional strain in the rock bolts. The calculation results showed that the rock bolts are subjected to fluctuations in displacement. The additionally induced strain is negligible. Thus, the empirical experience that earthquakes do not cause significant damage in the deep underground is confirmed by the numerical results.