The Confining Effect of End Roughness on Unconfined

Compressive Strength

Zig SzczepanikDoug MilneChis HawkesDept. of Civil and Geological Engineering

• 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.

Background to Current Research

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)

a b cd e f g

h

j

i

kl

m n o p

Profile length

Roughnessaverage (Ra)

Centreline

n

yyyyRa ncba

K

Roughness measurements

Diameter = 35.1 mmPass length = 12.5 mm

Measured Roughness

Platen Type / Sample End Friction

Average Roughness

Polished platens 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 mid-point 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

0

50

100

150

200

250

300

0 0.5 1 1.5 2

Strain Ratio

UC

S (

MP

a)

High Velocity Small Samples

High Velocity Small Samples - 1 End Gauged

Low Velocity Large Samples

High Velocity Large Samples

Samples Tested with Lead Foil

Sample BarrellingSample Hourglassing

• 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

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

Strain ratio for Smooth / Standard Ends

Strain ratio for Polished Sample Ends

(61mm diameter samples)

Concentric grooved platens

1.113

1.029 Smooth striated platens

1.449

1.847 1.778 Polished platens 1.802 1.111 .822 .482 1mm lead foil .502 1mm lead foil .489 1mm lead foil (35mm diameter samples)

Polished platens 1.065 .799 .743 1.12 .838 .862 1.063 .845 .505 .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