Strain-softening failure mode after the post-peak as a unique mechanism of ruptures in a frozen soil-rock mixture

Abstract A large number of traffic systems in China pass through a large area of frozen soil, most of which contains frozen soil–rock mixtures (FSRMs). For a profound understanding of the mechanical behaviours and damage mechanisms of geotechnical and geoenvironmental projects, it is necessary to understand the stress–strain characteristics and failure modes of FSRMs. This study focuses on the uniaxial compression strength (UCS), Young's modulus, and complete stress–strain curve under loading, considering the post-peak behaviour. More than 250 FSRM specimens were tested for UCS evaluations, while considering the effects of different freezing temperatures (−5, −10, −20, −30, and − 40 °C), initial water contents (15.0, 25.0, and 30.0%), and volumetric block proportions (0, 30, 40, 50, and 100%). The stress–strain curve of the FSRM was characterised by six distinct stress levels and deformation stages, different from those of rock or soil in the normal state. Particularly, the turning point from volume compression to expansion was observed after the post-peak, at which superficial cracks were initiated. Ice crystals provided ductility of the FSRM and maximised its load capacity to prevent premature cracks on the FSRM surface. The block stone hindered the linear propagation of cracks. The failure patterns of the FSRM for the UCS evaluation could be classified into four types. The UCS was influenced by mainly the strength of the mixture composed of ice crystals and soil particles and friction and occlusal strength between blocks. Notably, the gradual thawing of the FSRM might cause the largest deformation at the bottom of the active layer in summer and autumn, according to the experimental results. Thus, it is very important to understand the strength characteristics, deformation stages, and failure patterns associated with the geomechanical behaviour of the FSRM for engineering design and applications in a permafrost region.