Compression-induced failure characteristics of brittle flawed rocks: Mechanical confinement-dependency

The flaw tips in brittle rocks are often the sources of crack initiation and growth due to the stress concentration, which commonly governs the rock strength. However, a unified framework identifying the compression-induced crack types, ultimate failure patterns and the cracking levels of brittle flawed rocks under different mechanical confinements is not yet available. This study conducts the laboratory compression experiments with the AE monitoring to explore the failure characteristics of flawed limestone and its confinement-dependency. Four new crack types including loop crack, secondary transverse crack, near-field transverse crack and far-field transverse crack are found experimentally, and then a modified crack type classification strategy is proposed. Four failure patterns including the sigma(1)-axisymmetric flaw-disturbed spalling for uniaxial compression, the sigma(3)-transverse-symmetric flaw-disturbed spalling for biaxial compression, the sigma(1) -axisymmetric flaw-disturbed shearing for conventional triaxial compression, and the mixed sigma(3)-transverse-symmetric flaw-disturbed shearing and sigma(2)-transverse-symmetric flaw-disturbed spalling for true triaxial compression, are documented for the first time. Moreover, an acousto-mechanics-based classification methodology of rock cracking levels is established, as well as an AF (average frequency)-RA (rising angle)-based Kernel density estimation method for interpreting the rock cracking nature and the strength mechanism. This paper gets insights into the mechanical confinement-dependency of the rock failure characteristics incorporating the pre-existing flaws and help interpret the field observations.