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Workreport 2019-47

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Name:

Results of Monitoring at Olkiluoto in 2018, Rock Mechanics

Writer:

Haapalehto, S. (ed.), Malm, M., Kaisko, O., Lahtinen, S., Saaranen, V.

Language:

English

Page count:

126

Summary:

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 2018 consisted of seismic, GPS, precise levelling, and temperature measurements at Olkiluoto and seismic, displacement, temperature measurements and visual tunnel observations recorded in the 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 with 22 sensors in 2018. In addition, ten temporary sensors deployed for study purposes are running but excluded from the permanent network. The monitoring was operated continuously in 2018, even though individual stations suffered from small operation breaks. Altogether 876 events were located inside the seismic semi-regional area, of which 461 were explosions (ML = -2.9 … 0.8), 166 microearthquakes (ML = -3.0 … -1.2) and 239 miscellaneous or unidentified small events or probable rock falls (ML = -2.7 … -0.8) inside the seismic ONKALO block. In addition, 10 explosions (ML = 0.0 … 1.7) outside the ONKALO block but within the semi-regional area were located. Also, one regional earthquake (ML = 2.1) was recorded. The excavation works of the final disposal facility were more quiet in 2018 compared to the previous year of observation. Also, the amount of induced seismicity was lower compared to previous year and the microearthquakes were 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 seismic ONKALO block area. Despite the quite high number of very small induced microearthquakes, the rock mass has remained stable according to the seismic monitoring in 2018.

The GPS network of Olkiluoto consists of 17 stations, of which 16 stations are operating continuously. 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 0.2 mm/y and mostly below ± 0.1 mm/y for the continuously operating stations. The uncertainties of the horizontal velocities were on average 0.1 mm/y at 95 % confidence level. The relative vertical velocities were mostly within ± 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. For a few stations (GPS3, GPS16) the statistically significant vertical velocities seemed to be unrealistically large compared to the precise levelling results

Levelling observations included ONKALO and VLJ. Larger height changes have been observed in the ONKALO area, where benchmark 06220 has uplifted 3.03 mm since 2006 relative to the fixed BM 03216. Between 2017 and 2018, the largest height change was -0.47 mm at the BM 08201. In the VLJ loop, the largest height change was +0.17 mm at the BM GPS9B. So locally the VLJ area is stable. Since 2006 i.e. during the whole observation history all movements have predominantly been less than 0.5 mm. As a whole, the results of 2018 matched well with the previous results. New levelling fault lines were not found. The largest height change in a benchmark interval from 2017 to 2018 is 0.52 mm. It was observed between the benchmarks 06220 and 06221.

The extensometers installed in the final disposal facility at 7 different measurement areas measured the displacements of the tunnels mainly continuously in 2018. Several downtimes were observed in the parking hall. None of the displacements exceed 0.2 mm in the final disposal facility.

During 2018 temperature measurements were performed in one deep drillhole, in the access tunnel and in the displacement measurement areas. The results of 2018 follow the results from previous years. The seasonal change is observed in the measurements made in the tunnels.

During 2018, 9 visual observations of rock damage were recorded. The number has decreased from 2017 (27 observations). Most of the observations was rock noise.

Keywords:

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

File(s):

WR 2019-47_web (pdf) (7.4 MB)


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