True-triaxial strength and failure characteristics of sandstone within brittle-ductile domain: An experimental and theoretical study

As the depth of underground construction increases, rocks progressively transition from brittle to ductile behavior. Therefore, understanding the true-triaxial strength and failure characteristics of rocks within brittle-ductile domain is crucial for underground engineering applications. A series of true triaxial tests, following a constant Lode angle loading path, were conducted on high-porosity green sandstone. To accurately characterize the variation in true-triaxial strength within the brittle-ductile domain, this study introduces a generalized three-dimensional strength criterion. This criterion incorporates a two-parameter deviatoric function and a segmented meridian function. The strength data of green sandstone and Bentheim sandstone are symmetrically distributed around the three-dimensional failure envelopes, validating the applicability and accuracy of the proposed criterion. The macroscopic and microscopic failure characteristics of green sandstone are analyzed. The failure modes of green sandstone can be broadly classified into three types. Type I is characterized by multiple interacting shear bands, Type II by several discrete shear bands, and Type III by the absence of discernable shear bands. Optical microscopy reveals shear cracks in both Type I and Type II, with those in Type II being significantly narrower than in Type I. A prominent microscopic feature of Type III is pressure-induced grain crushing. The transition among failure modes is closely related to the boundaries of the brittle-ductile domain. With increasing hydrostatic pressure, failure modes transition sequentially from Type Ⅰ to Type Ⅱ and then to Type Ⅲ, with clear pressure boundaries demarcating each mode.