Results of monitoring at Olkiluoto in 2012. Rock mechanics

The rock mechanics monitoring at Olkiluoto concentrates on the assessment of potential tectonic movements and stability of the bedrock. The rock mechanics monitoring programme 2012 consisted of seismic measurements, GPS measurements, surface levelling measurements and temperature measurements at Olkiluoto and vicinity and displacement measurements, temperature measurements and visual tunnel observations carried out in the ONKALO. The Posiva's microseismic network consists of 17 seismic stations and 22 triaxial sensors. Six stations are in the ONKALO. In spite of few breaks the network operated continuously and well during 2012. The number of located events was much smaller in 2012 than during the previous years due to the interruption of the excavation. Altogether 337 events were located in the Olkiluoto area of which about half (181) were explosions. Two excavation induced earthquakes were observed at -420 m level and were associated with a known tunnel crosscutting fracture. According to the seismic monitoring the rock mass has been stable in 2012. The local GPS network consists of 19 stations. The whole network was measured twice in 2012. Most of the inner network baselines showed very small motions as in the previous years: 80 % of change rates were smaller than 0.10 mm/a. Roughly one third of the change rates are statistically significant. One baseline was also measured using electronic distance measurements (EDM) and the results fitted well to the times series. The surface levelling network currently consists of 87 fixed measuring points. During 2012 only measuring loops VLJ, ONKALO and Olkiluoto Strait were measured. The results indicated that vertical deformations were small compared to 2011 results. The largest deformations around 0.6 mm upwards existed above the VLJ-repository. The displacement measurements in 2012 consisted of the extensometer measurements in the technical rooms of the ONKALO. Readings were taken continuously once a hour by a datalogger. The results showed that the displacement behaviour was stable during 2012 and no significant changes took place. For the first time temperature measurements were collected from different sources of the Olkiluoto site (surface, drillholes and underground in the ONKALO). All the results were analysed and they indicate relatively uniform distributions of temperature in all depths across the site. Thermal gradient is around 1.4 deg C/100 m below 300 m. Visual observations from the ONKALO tunnels were also collected and analysed. The rock noises i.e. first indication of possible rock damage have been only recorded after the depth of 250 m and often related to the tunnel crossings or intersections. Rock fallouts have been observed at all depths (150 - 450 m) and they seem to be affected more by the weaknesses in the rock structure (foliation, fracturing, rock type contacts) than the tunnel orientation. (orig.)