Coring damage mechanism of the Yan-tang group marble: combined effect of stress redistribution and rock structure

Abstract

Micro-cracks induced by the coring process in rock samples retrieved from high in situ stress conditions may seriously influence the evaluation of rock properties. This paper presents work performed to study the coring damage mechanism of marble samples containing a kind of inclined grey (or brown) ribbon-like stripes, whose mineral components are calcite and dolomite. These samples were obtained from the Jin-ping Second Stage Hydropower Station (JPII) in China, and the coring site is located in the strata of the 5th layer of the Yan-tang group marble in the Middle Triassic system (T 2y 5 ). A special stress-relief coring scheme was carried out to collect rock samples under different stress levels at one position, then the coring damage in these samples was examined by sonic wave testing and acoustic emission monitoring under uniaxial loading conditions, and a numerical simulation was also conducted to investigate the stress path experienced by a rock core during sampling. The results indicate that for the specific Jin-ping Yan-tang group marble (T 2y 5 ) sample, the coring damage can be attributed to two kinds of micro-cracks: horizontal micro-cracks induced by stress redistribution, and vertical or sub-vertical micro-cracks mainly derived along the special rock structure. The horizontal cracks, generated by the tensile stress induced during coring, are mostly trans-granular ones, and can be regarded as the main reason for coring damage. In contrast, the vertical (or sub-vertical) micro-cracks are mainly caused by the combined effect of coring-induced high deviatoric stress and the inclined grey-brown ribbons, and it usually occurs along grain boundaries. At a relatively low stress level, the shear failure may preferentially occur along special rock structures. However, the impact of the micro-structure on cracking reduces significantly with the increase of stress level, and then the induced tensile stress may become the dominant factor leading to horizontal cracks or disking.