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Workreport 2007-26

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Name:

Rock Stress Measurements in Onkalo Underground Characterisation Facility at Olkiluoto at Depth of 120 m. Working Report 2007-26

Writer:

Fecker, E.

Language:

English

Page count:

178

Summary:

In November and December 2006 overcoring stress measurements have been conducted in the boreholes ONK-PP74, ONK-PP75 and ONK-PP77 in a niche of the access tunnel of the ONKALO underground characterisation facility at the Olkiluoto site.

Measurements have been done using the CSIRO 3D stress measuring cell. This cell is one of the mostly used cells in the whole world for estimation of the state of stress in rock when doing the borehole measurements. The boreholes are at a depth of about 120 m under the ground surface. The rock where the measurements have been conducted is a foliated migmatitic gneiss (subtypes veined and diatexitic gneiss).

Parallel to the overcoring measurements a glue test has been conducted in the laboratory to check the quality of the bonding of the stress cells to the rock. The result showed that the glue makes a good contact between the rock and the stress cell, but air bubbles, which have normally been observed within the glue and at the edges, proved this time to be disadvantageous. Normally such air bubbles have dimensions of about one millimetre, but sometimes certain bubbles may become notably bigger. In the ONKALO overcored probes sawn apart such air bubbles were found both in wet and dry probe conditions.

In the test series eight stress measurements have been provided, three of them failed for technical reasons. At one of these three tests the glue has extruded too early, at the other two tests the overcoring was not conducted deep enough. At the remaining five tests in spite of the glue test results a calculation of the stress tensor could be made. Four of these five measurements can be seen as relatively successful. The results of these measurements show a major principal stress of 14.8 MPa in average, trending northwest – southeast, and with a dipping of 11 degrees in average.

Keywords:

ONKALO, Olkiluoto, in situ stress measurements, three-dimensional overcoring, CSIRO probe

File(s):

Rock Stress Measurements in Onkalo Underground Characterisation Facility at Olkiluoto at Depth of 120 m. Working Report 2007-26 (pdf) (12.4 MB)


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