Workreport 2018-23



In Situ Stress Measurements in ONKALO With LVDT Cell – in 2017


Siren, T., Hakala, M.



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Three in situ rock stress measurements using the LVDT cell method (Hakala et al., 2013) were performed in Posiva’s ONKALO investigation facilities during July 2017. The measurements cover three differently oriented tunnels at the depth of -435 m below the ground surface. The measurements were conducted in access tunnels while they were under construction. The locations are fairly close to each other in ca. 50 m diameter.

The aim of the measurements was to expand the cover of the LVDT stress measurements results at the disposal depth and to study if the tunnel orientation has an impact to the stress measurement results.

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 measurements AJYH1 and AJYH5 were of good quality, however the deviating AJYH9 measurement results in a large internal error while other quality parameters suggest the measurement is good. The results of the three measurements of similar magnitude, however the orientation of AJYH9 measurement points 90° from other measurements and the prevailing LVDT stress model for repository depth. The major principal stress magnitude of the reliable remaining measurement results vary from 28.0 to 28.5 MPa, intermediate principal stress from 18.8 to 19.7 MPa, minor principal stress from 13.5 to 14.8 MPa and mean orientation is 284°. Compared to the LVDT stress model for repository the resulting magnitudes are similar and the orientation of major principal stress component is close to the LVDT stress models orientation of 300°.


ONKALO, in situ stress, measurement, LVDT-cell, disposal depth


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