The Confining Effect of End Roughness on Unconfined Compressive
Z. Szczepanik, D. Milne & C. Hawkes
Department of Civil and Geological Engineering, University of Saskatchewan, Canada
st Proceedings of the 1 Canada-US Rock Mechanics Symposium, Vancouver, Cana-
da, 2007, pp. 191-198.
ABSTRACT: The influence of sample end effects on the unconfined compressive strength of rock core is well recognised. ASTM standards exist to ensure that minimum standards of sample smoothness are maintained to minimise the influence of friction between the samples ends and loading platens. Sample end preparation is also
done to avoid stress concentrations on irregularities on the end surfaces. This paper describes tests that have
been conducted on relatively uniform grey granite from northern Manitoba, Canada to investigate the influence
of sample end effects. End conditions were varied by polishing the sample ends and by using loading platens with varying degrees of roughness. In one series of tests, lead foil was placed between the sample ends and the
loading platens to further decrease frictional effects. In all tests, except the lead foil tests, procedures and sam-
ple preparations were conducted within the ASTM standards for unconfined compressive strength (UCS) test-
The test results presented show that sample “hourglassing”, as measured using circumferential strain gauges
located near sample ends and at sample mid-points, resulted in strengths as low as 50% of the standard UCS values. Rougher sample ends and platens produced sample “barrelling” with strengths the same, or slightly
higher than results from standard tests. These results suggest that standard UCS tests are conducted with a
significant degree of effective sample confinement generated by sample end friction.
1 INTRODUCTION 2 SAMPLES AND PLATENS USED FOR THE
TESTING PROGRAM Research into factors influencing unconfined com-
pressive strength (UCS) tests have been conducted at 2.1 Sample Descriptions
the University of Saskatchewan for over 10 years. Samples of a medium-grained grey granite from Early testing was conducted to look at crack initia-northern Manitoba have been tested. The granitic tion and propagation in granitic samples (Eberhardt, rock was divided into two groups based on the 1998). Subsequent creep testing was conducted to P-wave velocity measured for each sample. The determine if the strength of granitic samples was re-slower velocity samples ranged from 3161 metres per duced under long term loads in excess of the theoret-second (m/s) to 4373 m/s and the faster velocities ical strength, causing unstable crack growth (Sczce-were between 4496 m/s to 5134 m/s. Two groups of panik et al., 2003). Based on the results of these sample sizes were tested as well. The smaller samples tests, research is concentrating on the influence of were 35 mm in diameter and the larger samples had sample end conditions on sample strength (Sczcepa-diameters of 61 mm. All tests had a length to diame-nik et al., 2005). Test results have shown a relation-ter ratio between 2.0 and 2.5, which is within ASTM ship between the ratio of circumferential strain at the (1987) specifications. sample mid point and sample ends versus the sample UCS, and that varying sample to platen contact fric-tion can change the resulting sample UCS by up to 2.2 Sample Instrumentation
about 100%. This paper presents the results of con-All samples were strain gauged. Circumferential tinued tests in this area. Modifications have been strain gauges were mounted 10 mm from each sam-made to test procedures to try and vary sample end ple end and at the sample midpoint. friction as much as possible.
？ For the 35 mm diameter samples, both 14mm and effects between the sample ends and platens and did 90 mm long strain gauges were used. For the not conform to ASTM standards.
samples instrumented with 14mm long gauges, 2 Three platen conditions were also used for testing. gauges were installed at each location and 2 axial In all cases the steel platens had a hardness in excess strain gauges were also used. For the tests con-of 58 (Hardness Rockwell C) HRC, as specified in ducted with 90 mm long gauges, only 1 circumfe-the ASTM standards (1987). No effort was made rential gauge was used at each location. to match the elastic properties of the rock and pla-
tens. Instead, the contact friction between the sam-？ For the 61 mm diameter tests, 90 mm long strain ple and platens was varied. Polished and striated gauges were used with circumferential gauges 10 platens were used, all of which conformed to ASTM mm from each sample end and the sample mid-standards. The polished platen was prepared on a point. Two axial strain gauges were also used. thin section polishing wheel whereas the striated pla-Two gauges were used at each location to provide ten (Figure 2) was prepared on a fine grinding wheel. redundancy. A third platen type was used that consisted of
concentric grooves with a roughness in excess of 2.3 Surface Roughness Measurements ASTM standards. This platen is shown in Figure 3.
The variation in platen types was used to vary Both sample end conditions and platen conditions sample to platen contact friction. The three types of were varied and a method of quantifying the surface sample end finish and three platen types that were conditions of the sample ends and platens was used are summarized in Table 1, along with corres-adopted. A portable surface roughness tester (profi-ponding ranges in average roughness. The sample lometer) was used to measure sample end roughness tested using lead foil are also listed in this table. The to the nearest 0.01 μm. The roughness tester meas-thickness of these foils were 0.015 mm, 0.03 mm, or ured roughness along a 12.5 mm profile length. Av-in three cases, 1 mm. erage surface roughness, Ra, was recorded. Ra is calculated by first determining an average straight
profile to represent the surface trace. The areas
above and below the profile are calculated and added
together. This total area is divided by the straight line
profile length to determine the average profile
roughness parameter Ra. To account for any direc-
tional anisotropy in the roughness of the sample
ends, roughness was measured at 60? increments on
the sample surface to produce a “roughness rosette”.
An average roughness value for each sample end was
obtained by averaging the roughness rosette values.
Figure 1 shows how the Ra value is calculated.
jFigure 2. Striated platen showing scratch marks. Scale bar
shows 1 cm boimpacgnfe hdlkCentrelineRoughnessaverage (Ra)