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 sample preparations were conducted within the ASTM standards for unconfined compressive strength (UCS) testing. 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.


Research into factors influencing unconfined compressive strength (UCS) tests have been conducted at the University of Saskatchewan for over 10 years. Early testing was conducted to look at crack initiation and propagation in granitic samples (Eberhardt 1998). Subsequent creep testing was conducted to determine if the strength of granitic samples was reduced under long term loads in excess of the theoretical strength, causing unstable crack growth (Sczcepanik et al. 2003).

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