Workreport 2019-47



Results of Monitoring at Olkiluoto in 2018, Rock Mechanics


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



Page count:



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.


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


WR 2019-47_web (pdf) (7.4 MB)


Share article:
This website stores cookies on your computer. These cookies are used to improve our website and provide more personalised services to you.


To make this site work properly, we sometimes place small data files called cookies on your device. Most big websites do this too.

1. What are cookies?

A cookie is a small text file that a website saves on your computer or mobile device when you visit the site. It enables the website to remember your actions and preferences (such as login, language, font size and other display preferences) over a period of time, so you don’t have to keep re-entering them whenever you come back to the site or browse from one page to another.

2. How do we use cookies?

A number of our pages use cookies to remember your actions and preferences (such as login, language, font size and other display preferences.)

Also, some videos embedded in our pages use a cookie to anonymously gather statistics on how you got there and what videos you visited.

Enabling these cookies is not strictly necessary for the website to work but it will provide you with a better browsing experience. You can delete or block these cookies, but if you do that some features of this site may not work as intended.

The cookie-related information is not used to identify you personally and the pattern data is fully under our control. These cookies are not used for any purpose other than those described here.

3. How to control cookies

You can control and/or delete cookies as you wish – for details, see You can delete all cookies that are already on your computer and you can set most browsers to prevent them from being placed. If you do this, however, you may have to manually adjust some preferences every time you visit a site and some services and functionalities may not work.