Anisotropy is a property of rock that results in the rocks strength (or other material properties) being directionally dependent. Unlike isotropic rocks, which have same strength properties in all loading directions, anisotropic rocks can have significantly lower strength when loaded along their weakest orientation

1. Rock Strength Anisotropy
Foliation Fabric: Hiding in Broad Daylight
Julian Watson

2. Definition: Strength Anisotropy
A mechanical property that makes the rock strength directionally dependent.

3. Causes and Types
• Foliated metamorphic rocks: including phyllites, slates, schists and some
schistose gneisses. The temperature and pressure conditions contribute to the
development of preferred orientations of crystalline minerals and platy micas.
• Stratified sedimentary rocks: including mudstones, siltstones, sandstones and
limestones. Bedding and lamination anisotropy in these rocks is related to the
temporal and spatial alignment of similar grain sizes at the time of deposition.
• Igneous/volcanic rocks: exhibit flow structures, such as sheet or exfoliation
joints and volcanic layering, however, generally is less common for the
development of pervasive fabric

4. Importance Foliation Fabric
• Metamorphic foliation fabric is often overlooked or ignored.
• It can be difficult to identify or measure in the field (often no clean, flat
surfaces).
• However, it’s not a new concept – it’s even mentioned in the ISRM standard
for UCS testing. (Why do so many practitioners actively ignore UCS tests that
have been affected by foliation fabric?)
• If included, it’s often confused with jointing – a very different mechanism and
material properties! NOT THE SAME!
• We’re finding foliation fabric has a profound influence on the rock mass
response to mining, including timing, extent, and location of excavation
damage / dilution, and seismicity.
• Blasting?
• Crushing?

5. Causes and Types
Example of typical rock types — Most sedimentary and metamorphic rocks.

6. Foliated Metamorphic Rocks
Schematic of degree of metamorphism
and associated mineral assemblage and
fabric character (The University of the
State of New York 2011)

7. Foliated Metamorphic Rocks
Schematic representation of the
relationship between foliation,
lineation and finite strain

8. Foliated Metamorphic Rocks
Formation of Foliation Fabric in Metamorphosed
Rock Exposed to Strain.
Example of Elongation and Flattening of Quartz Mineral Grains During
Formation of Foliation Fabric.

9. Characterisation: Extent and Intensity
Gneissic rocks (foliation type G) -
representative core photographs
for determining intensity of foliation
• Photo (a): Massive quartz- and
feldspar-rich rock (RMF number 0),
OL-KR8, 223.50 m.
• Photo (b): Gneissic rock with low
foliation intensity (class G1 - RMF
number 1), OLKR24, 288.36 m
• Photo (c): Gneissic rock with
intermediate foliation intensity
(class G2 - RMF number 1), OL-
KR22, 399.20m.
• Photo(d): Gneissic rock with high
foliation intensity (class G3 - RMF
number 2), OLKR28,
• 618.75 m.
Olkiluoto Nuclear Waste Repository, Finland
Olkiluoto foliation classification scheme.

10. Characterisation: Measuring Anisotropy
The influence of the foliation fabric on
the potential for rock failure around a
circular excavation. Note that the
highlighted rock samples around the
periphery will have different strengths
because of the different varying angles
of the peripheral stress relative to the
relatively constant foliation
orientation.

11. Rock Strength Anisotropy: Mine Seismicity
Foliation Included & Modified Stress Tensor Actual Seismicity
No Foliation & Current Stress Tensor Foliation Included & Current Stress Tensor
Sigma 1
Sigma 1 Sigma 1
Best Match
With Actual
Data

12. Characterisation : Measuring Anisotropy
Rock Mass anisotropy and Its effect on
compressive strength tests (Brady & Brown, 2005).
• σc min when β angle is between 30° and 45°
• σc max when β angle is either 0° or 90°

13. Characterisation : Measuring Anisotropy
Select samples with β angles from 30° to 45°, close to 0°, or close to 90 ° for UCS and triaxial testing.

14. Characterisation : Measuring Anisotropy
Ratio of σc max over σc min is
probably the best indicator of
anisotropy intensity.

15. Anisotropy Factor
Modified from Tsidzi 1986, 1987, 1990; Sing et al. 1989; Ramamurthy et al. 1993; and Palmström 1994