Shape-Dependent Behavior of Intact Rock Under Creep Loading

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Abstract Many underground mining operations require the design of pillars that their long-term structural integrity under creep loading is essential for sustainable operation of mines and safety of personnel. Characterizing the mechanical behavior of rocks at different shapes over the long-term can assist in efficient design of underground pillars. Over the past 4 decades, a large number of studies have examined the shape effect and long-term behavior of rocks separately, while consideration of these two together is essential. Accordingly, a comprehensive investigation is needed to assess the influence of shape on the mechanical behavior of intact rocks under long term or creep loading. Thus, in this work, an extensive laboratory experiments were conducted on a shaly sandstone, known as “Gosford” sandstone, with various length-to-diameter or “slenderness” ratios under both quasi-static and creep compressive loadings. The uniaxial compressive tests were performed on a number of cylindrical samples with constant diameter of 54 mm and varying slenderness ratios of 0.5, 1, 2, and 4. Also, a set of single and multi-step creep experiments were carried out on the samples with different slenderness ratios. Eighteen cylindrical samples were subjected to single-step creep loading at the slenderness ratios of 1, 2, and 4 and their corresponding instantaneous strains, apparent secondary creep strain rates, axial creep strains at the failure and times to failure were analyzed. The results showed that a decrease in the slenderness ratio led to an increase in the uniaxial compressive strength (UCS) of tested samples under quasi-static loading. Also, under single-step creep loading, the samples with the slenderness ratios of 2 and 4 exhibited classical creep behavior including distinct primary, secondary, and tertiary phases, whereas samples with the 1:1 ratio demonstrated localized failure. The multi-step creep tests endorsed these findings in which, the samples with smaller slenderness ratios resulted in larger cumulative strains and higher apparent secondary creep strain rate at the various creep stress ratios (the ratio of applied stress over the mean UCS). Finally, it was concluded that the resulting failure patterns from the tested samples are highly shape dependent under both quasi-static and creep loading conditions.