Specimen shape and cross-section effects on the mechanical properties of rocks under uniaxial compressive stress

Uniaxial compressive properties of rocks are very important for designing and constructing engineering projects. Based on the available standards for determining these properties, high quality core specimens with proper geometry are needed. In many cases, the standard specimens, especially in clay-bearing, fractured, and weathered rocks, are always not able to be prepared. On the other hand, in some natural conditions, rocks with different size, shape, and cross-section are undergoing uniaxial compressive loading. Therefore, in order to evaluate the uniaxial compressive strength dependency behaviors of rocks on the shape and cross-section of tested specimens, some marble specimens with three different cross-sections, including circular, square, and rectangular, as well as four different shape ratios (height to diameter/width ratio) of 0.5, 1, 2, and 3 were prepared and tested. Axial and lateral strains, acoustic emission (AE), and camera photographs were recorded during the tests. Rock strength behavior was evaluated based on several stress thresholds, including crack closure stress (σ_cc), crack initiation stress (σ_ci), damage stress (σ_cd), and peak stress (σ_ucs). The results indicated that σ_cc was not dependent on the cross-sectional shape of specimens. With increasing shape ratio, σ_cc gradually increased, while σ_cd and σ_ucs greatly decreased, and σ_ci remained at a constant value. The cross-sectional shape effect became operative when r was less than or equal to 1. Moreover, the values of σ_cd and σ_ucs of rectangular prism specimens and square prism specimens are lower than those of cylindrical specimens, indicating that the unstable crack propagation of prism specimens occurs earlier. The difference gap of σ_cd and σ_ucs between specimens with different cross-sectional shapes was dramatically decreased with increasing shape ratio. The AE and camera recorded data indicated that the fracture modes of rectangular and square prism specimens are more likely to change from shearing to slabbing fracture when the shape ratio decreased from 3 to 0.5. The main crack developed surface turned from wide surface to narrow surface with the shape ratio of rectangular prism specimens changing from 3 to 1 and 0.5. The research results are of referential meaning to the design of pillars in underground hard rock mines.