Research and Development on Geological Disposal of High-Level Radioactive Waste

3-4

Regional Estimate of Fracture Permeability－ Hydrogeological Investigation of Sedimentary Rocks －Permeability = Log

Permeability (Fracture+rock matrix)
Permeability (rock matrix)
100

108Yuchi Fm & Koetoi Fm (upper)

Brittleness of rock

Brittleness of rock100

108Koetoi Fm (main)
With major fault
Without major fault
Wakkanai Fm
With major fault
Without major fault

Koetoi Fm (main)
Wakkanai Fm

1-2-1

0

123

4561

7-2-1

0

Permeability of rock mass2134567Permeability of rock mass

Elevation (km)Fig.3-12 Relationship between brittleness of rock and permeability of rock mass
When the brittleness is lesser than 8, the permeability of a rock mass is low even if fractures/faults develop.

0

Zone 2
Zone 1
Zone 3
Zone 2-1

Zone 1

Zone 1

Zone 2

Zone 20

Zone 2

Yuchi Fm
Koetoi Fm
Wakkanai Fm

1 km

Brittleness Permeabilityof rock (BRI) of rock mass
Low
Zone 3
BRI≦2
Low
Zone 2 2 ＜BRI＜8High
Zone 1
8≦BRI-1

Fig.3-13 Distribution of brittleness of rock and permeability of rock mass based on geological map
Permeability of rock mass (including fractures) can be inferred from brittleness of rock from geological map.

The fracture permeability of a rock mass is an importantparameter for retardation assessment of radionuclide
migration and, consequently, for the design and constructionof an underground facility. Therefore, the regional distributionof the fracture permeability should be understood before
determining the location and layout of the facility. However,
it is impossible to measure the entire fracture permeability;
hence, some estimation of the permeability from the limited
available data is required. In sedimentary rocks, the matrix
permeability is often estimated by geophysical surveys orstratigraphic analyses, but it is difficult to estimate the fracturepermeability.
The relationship between the strength and stress state ofrocks and the fracture permeability was studied to develop
a technique for estimating the regional fracture permeability
in sedimentary rocks. Usually in the formation of fractures,a brittle deformation tends to result in high-permeability
fractures, whereas a ductile deformation tends to result in lowpermeability fractures. Whether the deformation is brittle or
ductile depends on the strength, stress state, strain rate, andtemperature at the time of the deformation. Considering thestrength and stress state, a softer rock or a higher confined
stress relative to the pore pressure is likely related toductile deformation. For the depth (>300 m) at which highlevel radioactive wastes are disposed of, the strengths andstress states of rocks are the most sensitive and importantparameters; i.e., it is assumed that the fracture permeability isclosely associated with the strengths and stress states of rocks.
Geological, hydrological, and rock mechanical tests of
sedimentary rocks distributed in and around the Horonobe
Underground Research Laboratory (URL) revealed that, when
the brittleness of a rock is less than 8, the permeability of therock mass is low even if fractures/faults develop (Fig.3-12:
the brittleness is defined as unconfined compressive strength ÷effective vertical stress × 2). This threshold is consistent with
deformational theory assuming elastic material. The regional
brittleness can be estimated using a geological map. Fig.3-13
shows the distribution of the brittleness and permeabilityof rock masses in and around the Horonobe URL. The
relationship between the brittleness and permeability of therock masses facilitates estimation of the regional fracture
permeability in sedimentary rocks.

ReferenceIshii, E. et al., The Relationships among Brittleness, Deformation Behavior, and Transport Properties in Mudstones: An Example from the Horonobe
Underground Research Laboratory, Japan, Journal of Geophysical Research, vol.116, issue B9, 2011, p.B09206-1-B09206-15.

44JAEA R&D Review 2013