ABSTRACT

Twenty in situ rock stress measurements using the LVDT-cell method (Hakala et al., 2012) were performed in Posiva Oy's ONKALO investigation facilities during 2009 - 2014. The measurements cover a depth range of 156 m to 420 m below the ground surface. Most of the measurements were performed in the ventilation shaft and access tunnel, two other locations include the rock mechanics investigation niche at the 345 m level and demonstration tunnels at the 420 m level.

The LVDT-cell method was developed as an alternative stress measurement method for hydraulic fracturing and to overcome problems encountered with traditional overcoring methods. The main improvements are the large measurement hole diameter which minimizes the adverse effects of heterogeneity and a mechanical mounting system which eliminated glue related problems such as debonding, drifting and long curing times.

The LVDT-cell method has experienced several improvements after the first shaft measurement at level 265 m. The most important improvements are based on the verification tests in well known in situ stress conditions in the Äspö Hard Rock Laboratory in Sweden (Hakala et al. 2013) and the development of the quality ranking system and a method to estimate measurement location related error by means of stress magnitude and orientation (Hakala et al. 2016).

Two different in situ stress estimates are provided based on the presented LVDT-stress measurement results: The ONKALO area interpretation based on depth gradients for stress tensor components which can be transformed into principal stress magnitudes and orientations. This interpretation is partly questionable as it ignores the recognised effects of geological features and a possible glacial disturbance above a depth of -300 m. The second interpretation is a stochastic approach to define the deposition depth stress state by providing the mean values, confidence limits and characteristics of distribution.

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