Workreport 2018-47



Results of Monitoring at Olkiluoto in 2017, Rock Mechanics


Haapalehto, S., Malm, M., Kaisko, O., Lahtinen, S., Saaranen, V.



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The rock mechanical monitoring is done at Olkiluoto in order to measure potential tectonic movements, estimate the stability of the bedrock and to implement safeguards-work. The rock mechanics monitoring programme 2017 consisted of seismic, GPS, precise levelling, and temperature measurements at Olkiluoto and seismic, displacement, temperature measurements and visual tunnel observations recorded in final disposal facility.

Posiva runs a microseismic network in ONKALO and its surroundings for monitoring the structure and long term stability of the bedrock and as a part of their safeguards programme. The permanent network consisted of 18 seismic stations at the end of 2017, when a new station was deployed and integrated into the network in September 2017. In addition, ten temporary stations deployed for study purposes are running but excluded from the permanent network. The monitoring was operated continuously in 2017, even though individual stations suffered from small operation breaks. Altogether 1634 events were located inside the seismic semi-regional area, of which 1130 were explosions (ML = -2.4 … 1.5), 213 microearthquakes (ML = -2.8 … -0.8) and 278 miscellaneous or unidentified small events or probable rock falls (ML = -2.8 … -0.9) inside the seismic ONKALO block. In addition 13 explosions (ML = 0.0 … 1.6) outside the ONKALO block but within the semi-regional area were located. Also four regional earthquakes (ML = 1.2 … 3.3) were recorded. The excavation work of the final disposal facility was more intense in 2017 than in the earlier years of observation. Also the amount of induced seismicity was significantly larger but the microearthquakes were still of small magnitudes and no visual observations or tremors directly related to these events were observed. Most of the microearthquakes were connected to construction operations and no natural earthquakes could be identified in the area of the seismic ONKALO block. Despite the increased number of very small induced microearthquakes, the rock mass has remained stable according to the seismic monitoring in 2017.

The GPS network of Olkiluoto consists of 17 stations, of which 16 stations are operating continuously. One station was decommissioned during the year 2017 due to upcoming construction work. The station velocities with uncertainties were estimated for each station from the daily coordinate solutions taking into account the seasonal signals and discontinuities. The relative horizontal velocity components of the stations were maximum 0.3 mm/y and mostly below ± 0.1 mm/y. The uncertainties of the horizontal velocities were on average 0.1 mm/y at 95 % confidence level. The relative vertical trends were mostly below ± 0.5 mm/y. The uncertainties of the vertical velocities were clearly larger compared to the horizontal estimates, on average 0.7 mm/y at 95 % confidence level. At a few stations (GPS3, GPS16) a significant negative trend was estimated.

Levelling observations included ONKALO, VLJ, and GPS stations on the Island and near the Olkiluoto Strait. Land uplift rates were computed using the observations 2003-2017.  Relative land uplift rates at the GPS stations are within ± 0.08 mm. Due to short time span GPS-stations 20 and 21 were not included into the comparison. The largest movements relative to the previous year exist in the ONKALO area. Height change relative to the fixed BM 03216 was +0.59 mm at the BM 06220. That benchmark has uplifted 2.83 mm since 2006. The largest height change in the VLJ loop was -0.14 mm at the BM 06216. Since 2006 i.e. during the whole observation history all movements have been less than 0.5 mm. So locally the VLJ area is stable. The benchmark interval over Olkiluoto Strait was stable in relation to the previous measurements, which were performed in 2015.

The extensometers installed in the final disposal facility at 7 different measurement areas measured the displacements of the tunnels mainly continuously in 2017. Several downtimes were observed in the surface extensometers and the extensometer of the vehicle connection 13 and most of them occurred during January and February. One reading head was changed in the parking hall 1 in October. The largest displacement about 0.5 mm was located in the vehicle connection 13. Also one extensometer in the parking hall 1 has a decreasing trend during the whole year with a cumulative displacement of 0.3 mm. Rest of the displacements did not exceed 0.2 mm in the final disposal facility.

 During 2017 temperature measurements were performed in six deep drillholes and two B-drillholes, in the access tunnel and in the displacement measurement areas. The results of 2017 follow the results from previous years. The seasonal change is observed in the measurements made in the tunnels and in some of the measurements done in the drillholes, the thermal effect of the tunnels is perceptible.

During 2017, 27 visual observations of rock damage were recorded. The increase comparing to previous years (18 in 2016, two in 2015, three in 2014) is due to large amount of excavations and related microseismic activity as well as the new systematic reporting of the visual observations. Most of the observations were rock falls.


Monitoring, rock mechanics, Olkiluoto, seismic network, microearthquake, induced seismicity, safeguards, GPS, precise levelling, extensometers, temperature measurements, rock noise


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