The artificial ground freezing (AGF) method is a frequently-used reinforcement method for underground engineering that has a good effect on supporting and water-sealing. When employing the AGF method, the mesoscopic damage reduces the strength of the frozen sandy gravel and consequently affects the bearing capacity of the frozen curtain. However, a few studies have been conducted on the mesoscopic damage of artificial frozen sandy gravel, which differs from fine-grained soil due to its larger gravel size. Therefore, based on triaxial compression tests and CT scanning tests, this paper investigates both the mesoscopic damage mechanism and variations in artificial frozen sandy gravels. The findings indicate that there are contact pressures between gravel tips within the frozen sandy gravel, with damage primarily concentrated around these gravels during incompatible deformation within a four-phase medium consisting of ice, water, soil, and gravel. Furthermore, numerical simulation validates that failure typically initiates at delicate contact surfaces between gravel and soil particles. For instance, when the axial strain reaches 8%, the plastic strain at the location of gravel contact reaches 4.6, which significantly surpasses most of the surrounding plastic strain zones measuring around 1.3. Additionally, the maximum local stress within the soil sample is as high as 48 MPa. This failure event is distinct from viscoplastic failure observed in frozen fine-grained soil or brittle failure seen in frozen rock. The findings also indicate that the mesoscopic damage is about 0.3 when the axial strain is 10%. The study's findings can serve as a valuable guide for developing finite element models to assess damage caused by freezing in sandy gravel using AGF method.

Artificial frozen sandy gravel exhibits the characteristics of wide distribution of particle size and complex composition, which are quite distinct from frozen fine-grained soils such as clay and silt. It may be more accurate to use both macroscopic and microscopic scales to evaluate the damage of artificial frozen sandy gravel. Therefore, this paper proposes an investigation on the macro-plastic damage and micro-crack damage of artificial frozen sandy gravel through triaxial compression and X-ray CT scanning tests. The two types of damage are obtained from completely different macro-plastic and micro-crack damage theoretical calculation methods. It can be concluded that the evolution law of the two damages is similar, but the value is different. Moreover, the defined cross-scale modified damage which is fitted through the calculated macro-plastic damage and micro-crack damage is proposed. The fitting functions reveal the evolution law of frozen sandy gravel damage more accurate, which is beneficial to the safety of the artificial ground freezing project and provides a valuable reference for subsequent numerical simulations of the frozen sandy gravel constitutive relationship.