The Confining Effect of End Roughness on Unconfined Compressive Strength

1. The Confining Effect of End Roughness on Unconfined Compressive Strength Zig Szczepanik Doug Milne Chis Hawkes Dept. of Civil and Geological Engineering

3. 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.

4. SampleInformation • 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

5. 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

6. 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

7. 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

8. 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

9. Measuring Roughness

10. Profile length j b o i m p a c g n f e h d l k Centreline Roughness average (Ra) + + + K + y y y y = a b c n Ra n Average roughness, Ra (after Hebert, 2004)

11. Roughness measurements Pass length = 12.5 mm Diameter = 35.1 mm

12. Measured Roughness

13. 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

14. 61mm diameter samples – 60mm strain gauges

15. Sample Testing Apparatus

20. 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

21. 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