Characterization of fractured crystalline rock: two Swedish in-situ field experiments
Åsa Fransson, Chalmers University of Technology, Sweden
Tomas Lehtimäki, Swedish Nuclear Fuel and Waste Management Co, Sweden
Mattias Åkesson, Clay Technology, Sweden
Construction of a nuclear waste repository demands information and understanding of the fractured
rock. The Swedish concept includes a natural barrier: the rock, and engineered barriers: bentonite and
a copper canister. Important issues are e.g. possible deformation (shearing) of fractures and
deformation zones influencing the location of canisters, and fluid flow resulting in hydration of the
bentonite. This paper aims at presenting two in-situ field experiments: the Bentonite Rock Interaction
Experiment (BRIE) and an experiment referred to as Large fractures. Both experiments can be related
to the different steps included in the detailed investigation program that is under development at the
Swedish Nuclear Fuel and Waste Management Co (SKB). This paper is focused on the
characterization and description of individual fractures and their properties. Investigations include
geological mapping, hydraulic tests and geophysical methods such as radar, resistivity and seismics.
The Bentonite Rock Interaction Experiment (BRIE) is a field experiment which addresses the hydraulic
interaction between the system components of compacted bentonite and the near-field host rock
composed of hard and fractured bedrock. This experiment is also addressed in a joint modeling task
(Task 8) of the Äspö Task forces on Engineered Barrier Systems (EBS) and on Groundwater Flow and
Transport of Solutes (GWFT).
The objective of Large fractures is to further develop strategies and integrated investigation and
modelling methodology for the identification and characterization of geological structures so that the
determination of large (canister position discriminating) fractures or minor deformation zones to a
greater extent can be based on real properties and to a lesser degree on a criterion related to the
existence of a full perimeter fracture – tunnel intersection.
The above experiments will be presented in terms of investigations performed to obtain discrete
fracture descriptions. Comments will be given on fracture properties and flow in relation to hydration
of bentonite (BRIE). Further, fracture size determination and design of grouting works when drilling
an investigation borehole for the experiment Large fractures will be discussed. Results from BRIE
shows that the hydration is uneven and controlled by the main conductive fracture, highlighting the
need for a relevant fracture description. Grouting of fractures intersecting the investigation borehole of
the Large fractures experiment was designed based on field data and a decrease from approximately
200 litres/min to below 1 litre/min was achieved.
Theme: DFN Approach to Fundamental Rock Issues
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Rock mass characterization
Discontinuity characterization methods