Journal of Advances in Geotechnical Engineering
Volume 2 Issue 3
DOI: http://doi.org/10.5281/zenodo.3461773
Failure Modes of Rocks under Uniaxial Compression Tests: An
Experimental Approach
Sayantan Chakraborty1, Rohan Bisai2*, Sathish Kumar Palaniappan3, Samir Kumar Pal4
1,2,3
Ph.D. Scholar, 4Professor
Department of Mining Engineering, Indian Institute of Technology Kharagpur,
West Bengal, India.
*Corresponding Author
E-mail Id:-rohan.bisai1@gmail.com
ABSTRACT
In present scenario, failure of rock is an important concern in the rock engineering. Failure
modes of rock are very problematic to understand, so the different failure modes of rock need
a complete study. With the help of this research, the design adequacy of an engineering work
can be predicted thoroughly. For this requirement, it is very important to analyze the
different rock failure modes under uniaxial compression test. When uniaxial compressive
strength (UCS) increased for the rock then the different types of failure modes can be seen
accordingly. Generally rock samples fail through one or more extensional planes of the
fracture development in brittle crystalline rock materials. The main reason behind the
different types of failure modes is the microscopic discontinuities in the sample. In this
research it can be predicted that whether the rock sample fails in shear or tension. It is
assumed that different failure modes influence the strength properties of rock samples. The
UCS of rock tested was obtained in shearing along a single plane are the highest value and
the values obtained in Axial Splitting are the lowest.
Keywords:-Rock failure, failure modes, uniaxial compressive strength.
INTRODUCTION
Generally, rock is a medium mainly
consists of cracks, breaks, joints and flaws.
The quality of rock masses depends on the
different properties of rock. Measurement
of the quality of rocks is extremely
important for safety of different civil
structures, i.e., curve dams, scaffold,
docks, and tunnels. Many researchers has
been investigated the failure mechanism
and different failure modes of rock under
various stress conditions with respect to
the failure theories. There are different
types of failure modes are investigated in
rocks. In addition the modes of failure
significantly fluctuate with the impact of
confining pressure. Various stresses and
cracks have been happened in a rock at one
point with the diverse regular conditions.
With this examination valuable data can be
accommodated protected and practical
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plan of different geotechnical structures. If
the failure modes of rock can be predicted
according to the design plan then it will be
very helpful for the economic plan and
safety also. With the present research the
different mode of failure can be
understood.
Some of researchers [1] concluded that in
rock mass there are numerous sort of
infinitesimal discontinuities are there and
for these properties can specifically
influence the rock tests. The fractured size
of rocks depends upon the nature of
loading, the strength of minerals and
texture of rocks [2]. Rock breakage is
inadequate and uncertain, with breakage
under dynamic loads being somewhat
better understood than under static loads
[3]. The location, orientation, size, density
and microscopic discontinuities contribute
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Journal of Advances in Geotechnical Engineering
Volume 2 Issue 3
DOI: http://doi.org/10.5281/zenodo.3461773
to different modes of failure of the rock
sample [4]. Many failure criteria were
utilized in engineering analysis are
primarily based on the process of shear
failure. We can know that a failure
criterion mainly depends on the shear
failure [5]. The types of mode of failure
are depending only on the uniaxial
compression [6]. Failure can manifest
itself in different ways depending on the
microstructure of the rock sample. That
means microstructure affects the rock
sample [7]. When rock samples are tested,
a large range of uniaxial compressive
strength (UCS) and various specimen
failure modes are observed which could be
attributed to micro structural differences
particularly in the form of micro cracks.
That means in various type of UCS value
there are different type of failures are there
and it depends on the micro cracks [8-9].
In case of rock fracture under static
7.
loading conditions, the fracture pattern of
granite, schist and sandstone under
uniaxial compression, Brazilian and point
load tests in relation to corresponding
strength in laboratory scale [10]. As no
straightforward mathematical or numerical
analysis model can ascertain the nature of
fracture development in rock materials, it
has been suggested that physical models of
the rock may provide useful information
particularly when the failure modes are
examined at laboratory scale.
The six varieties in which the rock fails
under uniaxial compression are
1. Axial splitting,
2. Shearing along a single plane,
3. Double shear,
4. Multiple fracturing,
5. Along foliation, and
6. Y-shaped. The schematic diagrams of
each of the failure modes are shown in
Figure 1 [10]
Fig.1: Schematic diagram of failure modes of rock under uniaxial compression
The findings of this study can be useful
particularly in an engineering environment
concerning underground excavation. The
type of laboratory investigations could
provide useful information about probable
rock failure modes as no straightforward
mathematical or numerical analysis model
can ascertain the nature of failure modes.
Similar research has been made on clay
shale which has itself implications in
underground excavations and deep well
drilling. Laboratory studies and theoretical
considerations on brittle rock behavior
HBRP Publication Page 1-8 2019. All Rights Reserved
under unconfined compression revealed
three key stress levels and four behavior
stages. The prevailing micro-cracks has
been perceived at non-linear stress-strain
behavior (stage I) while applying low axial
load onto it. This is followed by linear
elastic behavior in nature (stage II) and the
corresponding linear correlation between
stress vs strain is observed. The presence
of fresh micro-cracks may be due to
increase in axial stress. Crack initiation
stress i.e., damage initiation threshold is
the load in which crack starts propagates.
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Journal of Advances in Geotechnical Engineering
Volume 2 Issue 3
DOI: http://doi.org/10.5281/zenodo.3461773
The initiation stress point generally
coincides linearly with stress volumetric
strain or stress-lateral strain curves. This
does not apply to the stress–axial strain
curve that remains linear (Figure 2) [5]. It
has been shown that for low-porosity
rocks, the onset of cracking is initiated at
30–50% of the short term rupture stress.
Fig.2: Critical stress thresholds and behavior stages for a brittle failing specimen under
unconfined compressive loading
LABORATORY TESTING METHODS
Sample Preparation
Collection and Storage
Test material should be gathered from field
as rough and large blocks, dressed squares
or bored centers (Figure 3). The specimen
is to be checked to demonstrate its unique
position and introduction concerning
guardian rock mass. Tests should be
moisture
sealed
promptly
after
accumulation by waxing. The sample
should be transported deliberately in a
wooden box with saw dust. It might be put
away in shade ensured from exorbitant
changes in humidity and temperature.
Fig.3: Core samples for testing
Sample Drilling
A machine with reasonable clipping
gadget for holding the specimen will be
utilized for drilling. The drill travel will be
adequate to allow consistent keeps running
of minimum 150 mm and ideally 250 to
300 mm without requirement for halting
the machine. The block will be braced
firmly to a solid base to avoid any
development. Clean water will be utilized
for flushing and cooling the machine. For
porous rocks, packed air will be utilized.
HBRP Publication Page 1-8 2019. All Rights Reserved
Lab coring will be finished with dainty
walled rotational diamond bits. The
diameter of the core may fluctuate from 35
to 150 mm.
Sawing and Cutting
A 400-450 mm diameter diamond saw
with a portable is used for cutting as
shown in Figure 4. The huge diamond saw
wheel should be used for specimen cutting.
An exactness cut-off machine with 200
mm width diamond sharpened steel is used
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Journal of Advances in Geotechnical Engineering
Volume 2 Issue 3
DOI: http://doi.org/10.5281/zenodo.3461773
in this machine. For precise cutting, the
precision cut-off machine, if open might
be used. For cross cutting, focus ought to
be fastened in a vee-square opened to
permit passage of the wheel. The inside
ought to in a perfect world be supported on
the two sides of the cut to keep away from
spalling and lip forming toward the end.
Fig.4: Rock cutting machine
Grinding
The work ought to in a perfect world be
done dry with no cutting or cooling
liquids. For edge beating, a gadget post
processor. The unrest ought to be
acceptably moderate say with respect to
300 rev/min. Machine may in like manner
be used for quick end pulverizing of tube
shaped examples. Test ought to be held
explicitly in the throw and turned at 200300 rev/min and the devastating wheel
goes against it (Figure 5). Surface crushing
ought to be used on wide surfaces of
kaleidoscopic guides to achieve nearer
tolerances.
Shape and Size of Specimens
The sample measurement will not be under
multiple times the most extreme grain size
of the rock and ideally in excess of
multiple times the greatest grain size. Be
that as it may, the prescribed least size is
45 mm and for no situation it ought to be
less than 35 mm, in the last case the
resistances will be correspondingly
diminished. Specimen dimensions shall be
checked during machining with a vernier
caliper. Final dimensions shall be
measured nearest 0.1 mm. Specimen ends
should be flat to within 0.05 mm. They
shall be parallel to each other within
0.002D, where D is the specimen diameter.
The ends shall be perpendicular to axis of
the specimen within 0.001 rad (3.5
minutes) or 0.05 mm in 45 mm diameter
specimen. Different surfaces of the
examples (cylindrical shaped surface on
account of tube shaped example) will be
smooth
and
free
from
sudden
inconsistencies and directly to inside 0.3
mm and the measurements (diameter of
core sample) of the example will not shift
by more than 0.2 mm over the length of
the
example.
Fig.5: Grinding machine for polishing the surface of rock samples
HBRP Publication Page 1-8 2019. All Rights Reserved
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Journal of Advances in Geotechnical Engineering
Volume 2 Issue 3
DOI: http://doi.org/10.5281/zenodo.3461773
Determination of Compressive Strength
After completion of the rock sampling, the
cylindrical rock sample undergone
compressive strength test to determine the
various mode of failure on different rock
samples. The measurements in the present
study involve determination of the ultimate
compressive strength (UCS) of the rock
samples. Ultimate compressive strength is
the uniaxial compressive stress at which
the sample fails i.e.
⁄
Where, Pmax is the load at failure and D is
the diameter of the sample. An INSTRON
make, SATEC series KN model, Universal
Testing Machine (UTM) was used for
these measurements as shown in Figure 6.
Fig.6: UTM machine for conducting UCS experiments
RESULTS
The types of rocks based on modes of failure along with the corresponding compressive
strength of the individual rock sample are summarized in Table 1
Table.1: Classification of the rocks with the mode of failure and compressive strength
Sample No.
Type of Failure
Rock Types
AS 1
AS 2
AS 3
AS 4
AS 5
AS 6
AS 7
SP 1
SP 2
SP 3
SP 4
SP 5
SP 6
SP 7
DS 1
MF 1
AF 1
AF 2
AF 3
AF 4
Y1
Y2
Axial Splitting
Axial Splitting
Axial Splitting
Axial Splitting
Axial Splitting
Axial Splitting
Axial Splitting
Shearing Along A Single Plane
Shearing Along A Single Plane
Shearing Along A Single Plane
Shearing Along A Single Plane
Shearing Along A Single Plane
Shearing Along A Single Plane
Shearing Along A Single Plane
Double Shear
Multiple Fracture
Along Foliation
Along Foliation
Along Foliation
Along Foliation
Y Shaped
Y Shaped
Carbonaceous Shale
Medium to Coarse Grained Sandstone
Sandy shale
Fine- grained Sandstone
Fine-grained Sandstone
Fine grained Sandstone
Intercalated shale to sandstone
Shale
Shale
Fine Grained Sandstone
Shale
Shaly Coal
Shaly Coal
Sandy Shale
Fine Grained Sandstone
Fine Grained Sandstone
Shale
Intercalated Shale
Shale
Fine grained Sandstone
Fine Grained Sandstone
Intercalated Shale
Uniaxial Compressive
Strength (MPa)
15.64 MPa
11.07 MPa
33.78 MPa
10.49 MPa
16.68 MPa
14.48 MPa
14.73 MPa
67.39MPa
50.38 MPa
44.48 MPa
51.60 MPa
22.32 MPa
19.30 MPa
52.25 MPa
17.07 MPa
19.02 MPa
45.93 MPa
34.40 MPa
36.55 MPa
13.20 MPa
27.63 MPa
36.81 MPa
The different modes of failures of rock samples are presented in Figure 7 to 12.
HBRP Publication Page 1-8 2019. All Rights Reserved
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Journal of Advances in Geotechnical Engineering
Volume 2 Issue 3
DOI: http://doi.org/10.5281/zenodo.3461773
Fig.7: Axial splitting type fracture in rock samples under uniaxial, monotonic compressive
loading (AS 1, AS 2, AS 3, AS 4, AS 5, AS 6, AS 7)
Fig.8: Shearing along a single plane (SP 1, SP 2, SP 3, SP 4, SP 5, SP 6, SP 7)
Fig.9: Double shear (DS 1)
Fig.10: Multiple fracture (MF 1)
Fig.11: Fracturing along foliation (AF 1, AF 2, AF 3, AF 4)
Fig.12: Y-shaped failure (Y 1, Y 2)
HBRP Publication Page 1-8 2019. All Rights Reserved
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Journal of Advances in Geotechnical Engineering
Volume 2 Issue 3
DOI: http://doi.org/10.5281/zenodo.3461773
DISCUSSION
Recognition of the different types of
failure
pattern
of
rocks
under
monotonically
increasing
uniaxial
compression is another aspect of the study.
Photographs are taken for the broken
samples and analyzed. Few common types
of fracture patterns were identified from
the different pictures of the fractured
samples under monotonic uniaxial
compression. In the case of uniaxial
compression tests, the six varieties of
failure patterns have been categorized viz.
1. Axial splitting,
2. Shearing along a single plane
3. Double shear
4. Multiple fracturing
5. Fracturing along foliation
6. Y-shaped failure.
100 samples were tested to recognize the
various fracture patterns of the failed rocks
under uniaxial compressive test. The
failure pattern depends on the crack
propagation direction and the spatial
distribution of the relatively weaker zones.
The most common types of failure patterns
that were evident in the study were
shearing along a particular plane and axial
splitting. Pre-existing micro-cracks, under
a uniaxial state of stress get closed as the
applied compressive stress reaches a
particular level known as the crack-closure
stress. The cracks generated by the tensile
stresses induced by compression are
known as the wing cracks and they align
themselves parallel to the maximum
principal stress. When microstructure of a
specimen does not hinder the propagation
of wing cracks, the specimen fails in axial
splitting mode. Rocks failing by axial
splitting have numerous cracks parallel to
the loading direction which get joined and
cause multiple failures in axial direction.
Multiple fracturing is essentially an
irregular breakage or crumbling of the
specimens which is likely to take place in
order to release the strain energy in any
feasible way when systematic coalescence
of adjacent wing cracks even becomes
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restricted. Multiple fractures occur in hard
rocks where the existing micro-cracks are
scanty. Shear failure occurs when the
adjacent wing cracks or cracks in close
proximity coalesce together.
CONCLUSIONS
On the basis of present experimental study
the following conclusions are drawn.
Uniaxial compressive strength (UCS) tests
on cylindrical rock samples are conducted
that failed in different modes are
presented. Fine grained sandstone shows
axial splitting at the UCS of (10.49-16.48)
MPa, Shearing along a single plane type
failure at the UCS of 44.48 MPa. The
same material (fine grained sandstone)
shows Double Shear at the UCS of 17.07
MPa and Multiple Fracture at the UCS of
19.02 MPa. Shale sample shows Axial
Splitting at the UCS between 15.64MPa to
33.78 MPa and it shows Y Shaped failure
at the UCS of 27.63 MPa. From the Table
1 it is evident that the uniaxial
compressive strength (UCS) of rock tested
was obtained in Shearing along a Single
Plane are the highest, whilst the values
obtained in Axial Splitting are the lowest.
Therefore, the modes of failure are found
to be dependent on UCS of the rock.
It shows that different failure modes are a
result of the discontinuities in rock tests
instead of assortments in example course
of action, test strategy or boundary
conditions. This present paper makes we
can understand that the rock samples failed
in tension or in shear or in coupling.
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Cite this article as: Sayantan
Chakraborty, Rohan Bisai, Sathish
Kumar Palaniappan, & Samir Kumar
Pal. (2019). Failure Modes of Rocks
under Uniaxial Compression Tests:
An Experimental Approach. Journal
of Advances in Geotechnical
Engineering,
2(3),
1–8.
http://doi.org/10.5281/zenodo.34617
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