A New Perspective on the Semi-quantitative Meso-structural Failure Mechanism of Deep Weak Interlayer Zone Under Different Stress Paths

To essentially explore and quantitatively clarify the mesoscopic failure mechanism of deep weak interlayer zone (WIZ) induced by complex stress levels and stress paths (i.e., particle breakage and orientation, pore morphology, etc.), a semi-quantitative mesoscopic structural damage analysis methodology has been proposed, by involving SEM-MATLAB image processing technique with representative meso-structural parameters after sufficient analysis of basic geotechnical properties of WIZ. Results show that the natural WIZ exhibiting a flocculated structure could be characterized as a well-graded geotechnical material forming main clay minerals, in which most pores are intergranular, with the pore size distribution concentrated in 0.007–200 μm. Higher initial confining pressure and axial loading tend to intensify the particle breakage degree and particle size distribution characteristics of WIZ more than that of axial and circumferential unloading, in which the stress path II of axial pressure loading and confining pressure unloading under the initial confining pressure of 25 MPa is the most severe with average particle area reduced by 56% and particle Korcak fractal dimension increased by 36%. The broken particles undergoing a series of irreversible dislocation, tumbling and rotation under the action of shear and tensile stress, tend to orient in the direction of 0°–15°, in which particles in stress path IV aggregate in two directions of 0–15° and 60–90° due to the bidirectional unloading. The unloading stress path IV shows the most distinct directional orientation and orderliness, with particle anisotropy increased by 267% and directional probability entropy reduced by 13%. Particle breakage and orientation in WIZ are accompanied by obvious filling, expansion and propagation of the meso-pores and meso-cracks, in which stress path IV under lower confining pressure most affects the morphological complexity of pore and crack boundaries with the pore morphology fraction dimension increased by 13.5%. The quantitative theoretical correlation of macro-meso parameters has been established by the stepwise regression analysis of two most relevant and representative correlation indexes (i.e., Korcak fractal dimension and pore morphology fractal dimension) with the ultimate bearing strength of WIZ, which has been proved to have high fitting accuracy by comparing the regression results with the test measured values. The meso-structural damage mechanism of WIZ under stress paths II and IV could, respectively, match the failure law of structural stress-induced collapse in the spandrel and the plastic squeezing-out failure of WIZ on the high sidewall of underground excavations. Research could provide feasible ideas for the relationship between macroscopic failure and mesoscopic damage of WIZ, as well as the effective basis for the further discussion of macro-meso constitutive model establishment.