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POSIVA Report 2016-20

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Name:

In Situ Stress Measurements in ONKALO with LVDT - Cell

Writer:

Hakala, M., Siren, T., Ström, J., Valli, J., Hakala, V., Heine, J., Sireni, S., Kemppainen, K., Savunen, J.

Language:

English

Page count:

186

ISBN:

ISBN 978-951-652-263-3 / ISSN 2343-4740

Summary:

Twenty in situ rock stress measurements using the LVDT-cell method (Hakala et al. 2013) 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 ventilation shaft ONK-KU2 and the access tunnel (ONK-VT1), two other locations include the ONK-TKU3 (POSE) rock mechanics investigation niche at the -345 m level and demonstration tunnels ONK-DT1 and ONK-DT2 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 vulnerability regarding 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 measurement at level -265 m of ONK-KU2. 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.

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.

The in situ stress interpretations described in this report are in relatively good agreement with the semi-integration stress model (Ask 2011), presented in the last Olkiluoto Site Description as Stress Model 1 (Posiva 2013). The orientations of the horizontal stress components are approximately the same, although the LVDT-method based models yield magnitudes that are approximately 5 MPa higher. At deposition depth the maximum in situ principal stress is almost horizontal and the 90% confidence limits for the magnitude and orientation are 28 MPa - 35 MPa and 283° - 323°, respectively, resulting in an ESE trend. Despite the clear identification of good mean values for repository design, especially regarding the orientation of maximum compression, it is also clear that considerable variation within the deposition horizon is expected.

Keywords:

ONKALO, in situ stress, measurement, interpretation, LVDT-cell.

File(s):

In Situ Stress Measurements in ONKALO with LVDT - Cell (pdf) (20.8 MB)
Posiva_ONKALO_LVDT_Appendix_1 (pdf) (13 MB)
Posiva_ONKALO_LVDT_Appendix_2 (pdf) (6.1 MB)
Posiva_ONKALO_LVDT_Appendix_3 (pdf) (687.4 KB)
Posiva_ONKALO_LVDT_Appendix_4 (pdf) (39.3 MB)


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