Nuclear magnetic resonance analysis of loess microstructure evolution and internal erosion driven by water seepage

Water-induced loess geo-disasters are closely related to the loess microstructure evolution after water injection. To study the infiltration process and microstructural changes of intact loess, we conducted water injection seepage tests of the Malan loess in the Loess Plateau of China. We used magnetic resonance T₂ spectra and magnetic resonance imaging to monitor the water absorption rate, pore size distribution, and the shape of wetting front of loess samples with different seepage rates. Water preferentially entered large and medium pores; subsequently, influenced by adsorption, water diffused and accumulated into micro- and small pores. Continuous seepage resulted in skeletal collapse of the loess, including an increase in minimum pore size, enhancement of pore connectivity, and closure of large pores. Particle size analysis and scanning electron microscope images of loess samples after seepage indicated that fine particles gradually migrated downwards along the direction of seepage, eventually accumulating between, and blocking, large pores. As the clay fraction increased by 4.29% and 7.85%, the shear strength decreased by 22.3% and 59.6% respectively. In addition， the pore water pressure increased by 66.7% and 300%, respectively. The softening of loess strength caused by accumulation of fine particles and the water-resisting effect caused by clogged pores reduced the stability of loess slopes.