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Workreport 2017-47

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Name:

Results of Monitoring at Olkiluoto in 2016, Rock Mechanics

Writer:

Haapalehto, S., Malm, M, Saari, J., Lahtinen, S., Saaranen, V.

Language:

English

Page count:

98

Summary:

The rock mechanics monitoring at Olkiluoto is aimed to identify potential tectonic movements and estimate the stability of the bedrock. The rock mechanics monitoring programme 2016 consisted of microseismic, GPS, precise levelling and temperature measurements at Olkiluoto and of microseismic, displacement and temperature measurements and visual tunnel observations recorded in ONKALO.

One new seismic station inside the ONKALO was integrated to Posiva’s seismic network in March 2016 and one surface station was permanently disconnected from the seismic network in October 2016. At the end of the year, Posiva’s permanent seismic network consists of 17 seismic stations, which have 21 triaxial sensors. Five stations are in the ONKALO. The network operated continuously except for the one hour and 25 minutes period due to power failure in Olkiluoto. Altogether 764 explosions and earthquakes were located inside the seismic semi-regional area and in its vicinity in 2016. About 68% (513) of the 755 observed explosions happened inside the seismic ONKALO block, and total of 581 explosions were located inside the seismic semi-regional area. The magnitudes of the explosions inside the semi-regional area range from ML = -1.4 to ML = 2.0. Four microearthquakes were located inside the seismic ONKALO block in 2016. The magnitudes of the events were from ML = -2.0 to ML = -1.6. The estimated peak slip values of the earthquake are from 0.5 μm to about 9 μm. The estimated radiuses of the faults range from 6.6 m to 20.5 m. According to seismic monitoring, the rock mass has been stable in 2016.

The GPS network of Olkiluoto consists of 18 stations of which 17 stations operate continuously. The non-permanent station at Pyrekari Island was also successfully measured in 2016. The station velocities were estimated relative to the mean velocity of all stations. Some of the stations had still data for too short time span for reliable velocity estimation. The estimated horizontal velocity components were at maximum 0.3 mm/y. The vertical velocities were ± 0.6 mm/y for all other stations expect for GPS3 having -1.4 mm/y velocity. The rms of the residuals varied between 0.3 – 0.8 mm in horizontal and 1.8 – 5.5 mm in vertical. The change rates of the baseline lengths were also analysed from the daily coordinate time series. Roughly 75 % of the rates were below 0.2 mm/y and the maximum rate was 0.4 mm/y (GPS1-GPS4). The rates were on average slightly smaller compared to 2015.

The precise levelling network consists of 87 fixed measuring points. In 2016, only the ONKALO- and VLJ-loops were measured. In the ONKALO-loop, changes were very small. The largest changes, -0.20 mm were measured at the benchmark 10301. The benchmark 06220 continued its deformation rate: the height has been changed 2.24 mm since 2006. In the VLJ-loop the largest change measured was +0.19 mm at the benchmark 06215.

Before March 2016, 8 new borehole extensometers (with three reversed-head extensometers) were installed in the ONKALO for the monitoring of the excavations of parking hall 2 and vehicle connection 17. Three of them installed in the profile of parking hall 2 were excavated away. By the end of 2016, a total of 20 extensometers were installed in 7 different areas in the ONKALO. The displacements measured were minor. Largest displacements were measured near the excavations but still remained below 3 mm. The temperature and humidity sensors of the extensometers of the parking hall 1 and the vehicle connection 13 measured incorrect values until November, when the sensors were changed in the parking hall 1 and corrected in the vehicle connection 13.

Temperature measurements collected in 2016 confirmed the previous results and indicated relatively uniform distributions of temperature in all depths across the site. Thermal gradient is around 1.3 - 1.4 ºC/100 m below 300 m.

During year 2016, 18 visual observations of rock damage were recorded. The increase comparing to previous years (two in 2015, three in 2014) is due to high excavation activity and a new systematic reporting of the visual observations. Most of the observations were rock noise and rock fall.

Keywords:

Monitoring, rock mechanics, Olkiluoto, seismic network, microearthquake, safeguards, GPS, precise levelling, extensometers, temperature measurements, rock damage, visual observations.

File(s):

Results of Monitoring at Olkiluoto in 2016, Rock Mechanics (pdf) (10.2 MB)


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