Final disposal

Research Conducted in ONKALO

The research conducted in ONKALO gives further information on the bedrock and groundwater conditions of the final disposal site, as well as on the impact of the construction on these conditions. Furthermore, it serves the construction process and its planning while the work is underway, as well as all planning and design needs associated with the repository.

The research conducted in ONKALO helps to ensure the suitability of the Olkiluoto bedrock for the final disposal. Research conducted from ONKALO also helps to identify the areas where the construction of the final disposal tunnels is the most cost-effective.

Studies closely associated with construction aim to facilitate the success of the excavation process and the efficiency of the strengthening and sealing measures, and in this way closely contribute to the construction process. Studies conducted during the access tunnel construction also help to test and develop research methods for the future research needs of the actual repository.

Geological mapping is the method for collecting information related to the tunnel walls. Information about the rock to be excavated is obtained by drilling probe and pilot holes. From the tunnel, it is also possible to perform exploratory drillings and to study the groundwater flowing into the tunnel.

Geological Mapping of ONKALO

Geological mapping is used in collecting information for adjusting a geological model as well as for the use of experts who design the strengthening measures and grouting. On the basis of mapping, forecasts are also drafted concerning the characteristics of the rock deeper down.

The geological mapping of ONKALO is divided into two parts:

  • mapping performed in the constructed tunnel immediately after excavation
  • a more precise systematic mapping.

The first phase mapping proceeds simultaneously with excavation work so that the mapped area is located at 10 to 15 metres from the rear end of the tunnel. Geologists determine, among other characteristics, the main rock type in the area, and collect versatile information on the fracturing and quality of the rock.

The systematic mapping follows the excavation work from a distance of some 100 to 200 metres. The most significant difference between the first and second mapping phase is the precision and the amount of detail involved. Almost all geological phenomena that there are to be observed and measured are then determined on the basis of the fractures and the surrounding rock.

Probe and Pilot Holes

Probe holes are bored and pilot holes drilled from ONKALO in order to gather information about the rock to be excavated.

In the rear end of the tunnel, four probe holes are drilled at a distance of approximately 20 metres from each other. Measurements related to leak water, water loss and flowing are performed through the holes. Information gathered on the basis of the probe holes is then employed in the tunnel excavation planning and execution.

From the rear end of the ONKALO tunnel, a number of pilot holes will be drilled at pre-determined locations. These pilot holes will remain within the tunnel profile.

The purpose of these pilot holes is

  • to verify the rock quality at the location the tunnel is going to be before any excavation takes place
  • to locate any water-conducting fracture zones
  • other rock characteristics that may be significant for the construction.

Pilot holes are used for geological, geophysical, hydrogeological and hydrochemical studies. Samples taken include the drill core sample that can be used for determining the rock type and the fracturing of the rock to be excavated.

The length of the pilot holes varies between 50 and 300 metres. By the end of 2008, nine pilot holes had been drilled. According to plans, five more pilot holes will be drilled from ONKALO.

Characterisation Niches

Separate characterisation niches, i.e. short tunnel drifts proceeding from the access tunnel, are also constructed in ONKALO. The first characterisation niche was constructed at 1,475 metres down the tunnel. In total, there are five characterisation niches in ONKALO.

The purpose of the characterisation niches is to house long-term research so that the research work will not disturb the construction. Studies conducted in these characterisation niches are mostly associated with the investigation of the characteristics of the surrounding rock properties and with the monitoring of hydrological and geochemical conditions.

From the characterisation niches, it is possible to drill characterisation holes, i.e. long drillholes into the final disposal depth, which can then be used to examine the rock surrounding ONKALO. These drillholes help in collecting more specific information for planning, for instance, the positioning of the repository.

Groundwater Studies in ONKALO

In ONKALO, the volume of the water leaking into the tunnel is measured and the characteristics of the groundwater studied. Groundwater studies form a part of the monitoring of the consequences of the construction.

Measurement of the volume of leak water is assisted by measuring weirs installed in the tunnel. A visual leak water analysis is also performed for the ONKALO roof and walls two times a year in order to find leak spots and to detect possible changes.

Besides the volume of leak water, the composition of groundwater is also examined in ONKALO. This is facilitated by groundwater stations and monitoring holes.

Rock-Mechanical Studies

The purpose of the rock-mechanical studies is to determine the conditions prevailing inside the rock, such as the direction and intensity of the stress field as well as the strength of the rock. In order to determine these mechanical characteristics of the rock, various types of measurements are performed before the excavation proceeds to the zone in question. The methods can be used both in drillholes and on the excavated surface.

During and after the excavation, the mechanical characteristics of the rock can still be measured. For instance, after raise boring, the shaft profiles experience a minor transformation from round to slightly oval (this cannot be visually detected). The measurement result can then be used to determine the direction and intensity of the stress field prevailing in the area.

Rock mechanical studies are associated with the inspection of an excavation damage zone (EDZ). The development of the excavation damage zone is affected by several factors such as

  • the force of the excavation
  • the geological characteristics of the rock
  • the rock-mechanical conditions.

The excavation damage zone can be studied with various methods. Of these, boring and sawing off rock sections, as well as geophysical measurements, have been used in ONKALO.

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