Study on the strength deterioration characteristics and microscopic mechanisms of moraine soil under freeze-thaw cycles

To investigate the strength degradation characteristics and microscopic damage mechanisms of moraine soil under hydro-thermo-mechanical coupling conditions, a series of X-ray Diffraction (XRD), standard triaxial testing, Scanning Electron Microscopy (SEM), and Nuclear Magnetic Resonance (NMR) experiments were conducted. The mechanical property degradation laws and evolution characteristics of the microscopic pore structure of moraine soil under Freeze-Thaw (F-T) conditions were revealed. After F-T cycles, the stress-strain curves of moraine soil showed a strain-softening trend. In the early stage of F-T cycles (0–5 cycles), the shear strength and elastic modulus exhibited damage rate of approximately 10.33% ± 0.8% and 16.60% ± 1.2%, respectively. In the later stage (10–20 cycles), the strength parameters fluctuated slightly and tended to stabilize. The number of F-T cycles was negatively exponentially correlated with cohesion, while showing only slight fluctuation in the internal friction angle, thereby extending the Mohr-Coulomb strength criterion for moraine soil under F-T cycles. The NMR experiments quantitatively characterized the evolution of the internal pore structure of moraine soil under F-T cycles. As the number of F-T cycles increased, fine and micro pores gradually expanded and merged due to the frost-heaving effect during the water-ice phase transition, forming larger pores. The proportion of large and medium pores increased to 59.55% ± 2.1% (N=20), while that of fine and micro pores decreased to 40.45% ± 2.1% (N=20). The evolution of pore structure characteristics was essentially completed in the later stage of F-T cycles (10–20 cycles). This study provides a theoretical foundation and technical support for major engineering construction and disaster prevention in the Qinghai-Xizang Plateau.