The Confining Effect of End
Roughness on Unconfined
Compressive Strength
Zig Szczepanik
Doug Milne
Chis Hawkes
Dept. of Civil and Geological Engineering
Background to Current Research
• Long term loading tests on hard rock
were conducted to determine if failure
significantly below the rock UCS could
be achieved (Szczepanik et. al., 2003)
• Sample end conditions were varied on
small samples of hard granite. A
significant variation in rock strength was
measured (Szczepanik et. al., 2005)
• This paper presents additional results
on 2 sample sizes and 2 types of granite
under various sample end conditions.
Sample Information
• 28 samples –
– eleven 61mm diameter
– seventeen 35mm diameter
• All length to diameter ratios were between 2.0 to 2.5
• Medium grained grey granite - northern Manitoba
– 4 samples - Group 1 P-wave velocity (3161 to 4373 m/s)
– 24 samples - Group 2 P-wave velocity (4496 to 5134 m/s)
– There was no major difference between the two groups of
samples
Sample End Preparation
•
•
Sample preparations were within ASTM
standards
Rough Sample End Conditions
– Multiple passes of a 1.5cm wide grinding
wheel provided sample roughness with an
asperity amplitude less than the ASTM
recommended 25 x 10μm
•
Standard Sample End Conditions
– Single pass with a grinding wheel
•
Smooth Sample End
– Obtained by polishing standard sample
ends with a thin section polishing wheel
Platen End Conditions
•
•
•
•
•
Three platen end conditions were used in
the testing:
Polished platens were finished on a thin
section polishing wheel
Smooth-striated platens were prepared on
a fine grinding wheel
Grooved platens were prepared with
concentric rings with an amplitude in
excess of ASTM standards.
9 samples were tested with lead foil
between the sample and loading platen
Platen End Conditions
• Smooth platens were prepared on a fine
grinding wheel. It was initially hoped these
platens would be the standard for reduced
end effect. Shallow sharp grooves or
striations produced high apparent friction
Platen End Conditions
• Concentric grooved platens were prepared.
It was initially hoped these platens would
show high apparent friction, however the
rounded grooves acted like bearings and
produced low apparent friction
Measuring Roughness
Average roughness, Ra
(after Hebert,
2004)
Profile length
j
a
b
Roughness
average (Ra)
c
d
e
f
g
h
Centreline
i
k
l
m
n o p
y a + yb + y c + K + y n
Ra =
n
Roughness measurements
Pass length = 12.5 mm
Diameter = 35.1 mm
Measured Roughness
Platen Type / Sample End
Friction
Polished platens
Average
Roughness
0.17μm <Ra<0.21μm
Smooth/striated platens
0.8μm <Ra<1.0μm
Concentric grooved platens
4.0μm <Ra<4.6μm
Rough sample ends
3.8μm <Ra<4.3μm
Standard/smooth sample ends
2.4μm <Ra<3.0μm
Polished sample ends
0.6μm <Ra<1.2μm
Strain Measurements
• All samples were strain gauged with
circumferential gauges at the sample midpoint and 1cm from each sample end
• The 35mm diameter samples were gauged
with 14 or 90mm long gauges and the 61mm
diameter samples were gauged with two 60
mm long strain gauges at each location
• The ratio of mid strain to end strain, at 50%
of the sample UCS, was a measure of
sample end friction
61mm diameter
samples – 60mm strain
gauges
Sample
Testing
Apparatus
300
Sample Barrelling
Sample Hourglassing
250
UCS (MPa)
200
Samples Tested with Lead Foil
150
100
High Velocity Small Samples
High Velocity Small Samples - 1 End Gauged
Low Velocity Large Samples
High Velocity Large Samples
50
0
0
0.5
1
Strain Ratio
1.5
2
Minimum Samples Strain Ratio vs UCS
300.0
Sample Hourglassing
Sample Barrelling
250.0
UCS (MPa)
200.0
Samples Tested w ith Lead Foil
150.0
100.0
Tensile Strength = 12 Mpa
8 x to = 96 MPa
50.0
0.0
0
0.5
1
Strain Ratio
1.5
2
Conclusions
• Unconfined compressive strength (UCS)
tests are conducted with a significant
degree of confinement at the sample
ends.
• This confinement is a function of the
friction induced between the sample
ends and the loading platens during
testing.
• This confinement significantly increased
measured sample UCS values
Conclusions
• Brazilian tests indicated a consistent tensile
strength of 12 MPa – The minimum UCS
strengths obtained were 8 times the tensile
strength.
• Testing is ongoing
• Triaxial tests with polished ends will be done
to determine a comparable confinement to
standard UCS testing procedures
• Other rock types will be tested
• Comments are welcomed
Strain ratio for
Rough Sample
Ends
(61mm diameter
samples)
Concentric grooved
platens
Strain ratio for
Smooth / Standard Ends
Strain ratio for
Polished Sample
Ends
1.113
1.029
Smooth striated
platens
1.449
1.847
1.778
Polished platens
(35mm diameter
samples)
Polished platens
1.802
1.111
.822
.482 1mm lead foil
.502 1mm lead foil
.489 1mm lead foil
1.065
1.12
1.063
.799
.838
.845
.908*
.837*
1.409*
.637 .03mm lead foil*
.818 .015mm lead foil
.524 .015mm lead foil
1.552 .03mm lead foil
.79 .03mm lead foil
.743
.862
.505