Background The Huanghuai mining area represents an important coal base in eastern China. However, the Cenozoic calcareous clay in the area exhibits high plasticity, low permeability, and potential expansion and deformation during freezing, leading to unstable freezing performance. Consequently, artificial ground freezing (AGF) in the area generally faces a risk of cost surge or out of control due to the excessive thickness or limited strength of frozen walls.

Methods  This study investigated calcareous clay remolded based on soil samples from the Huanghuai mining area. Using a low-field nuclear magnetic resonance (LF-NMR) spectrometer and a high-pressure triaxial system, this study systematically explored the variation patterns of unfrozen water content during freezing under temperatures ranging from 20°C to −20°C, confining pressures from 1 MPa to 4 MPa, and initial water contents of 17.5%, 22.5%, and 31.5%. Accordingly, the impacts of these different factors on the triaxial strength of the frozen calcareous clay were determined.

Results and Conclusions  The results indicate that during the freezing of the remolded calcareous clay, unfrozen water content in the clay evolved through three stages—rapid decline, slow decline, and stabilization, sequentially, with a higher initial water content corresponding to a more distinct rapid decline stage. Accordingly, a modified power function of unfrozen water content (w_u) and freezing temperature (t) was developed, with a fitting accuracy (R²) of greater than 0.97. Triaxial test results reveal that the failure stress of the frozen calcareous clay increased significantly with decreasing freezing temperature. In contrast, the stress increased nonlinearly with initial water content, while the increased amplitude decreased gradually. In the case where the confining pressure exceeded 3 MPa, the strength reversal phenomenon occurred due to both pressure-induced ice crystal melting and pore water lubrication at interparticle contacts, with the critical pressure interval determined at 2.5‒3.5 MPa. By innovatively introducing the concept of coupling between unfrozen water content and cementation area into the Mohr-Coulomb failure criterion, this study created a model enabling the strength of frozen calcareous clay to be predicted based on merely three parameters: freezing temperature, initial water content, and confining pressure. The correlation coefficient (R²) and average relative error between the model-calculated and measured strength were determined at > 0.95 and < 5%, respectively. From the perspective of thermo-mechanical coupling, the prediction model established in this study allows for the quantitative characterization of the nonlinear and critical evolutionary patterns of calcareous clay strength with temperature, initial water content, and confining pressure during freezing. This model provides a theoretical basis and data support for analyzing the mechanical responses of calcareous clay in low-temperature and high-pressure environments.