This study investigates the shear strength characteristics of loess under varying water content，with a specific focus on loess from Gansu Province.Through direct shear experiments and nuclear magnetic resonance tests，the research reveals the impact of water content on the shear strength of loess and its underlying microscopic mechanisms.The experimental findings indicate that increasing water content results in elevated internal porosity，an initial increase followed by a decrease in cohesion force，a reduction in the internal friction angle，and enhanced ductility of the soil samples.The mechanism underlying this change involves the alteration in water binding mode，matrix suction，and the degree of cementation between particles due to increased water content.When the initial water content of the soil sample is low，an increase in water content leads to the thickening of the strongly bound water film between particles，thereby enhancing the cementation effect and generating matrix suction，which in turn increases cohesion force.Concurrently，the lubrication of particles by water results in a reduction of the internal friction angle.When the optimal water content is surpassed，the soil sample exhibits a progressive expansion of cracks，an increase in the free water between particles，and a subsequent weakening of the cementation between particles and the matrix suction.This leads to a reduction in cohesion force and an increase in the internal friction angle due to the expansion of soil particles.Finally，when the water content approaches the liquid limit of the soil sample，the soil particles become obstructed by free water，leading to a reduction in the water film force between particles and rendering the cementation effect ineffective.Consequently，the matric suction is reduced to zero，resulting in cohesion approaching zero.Additionally，the occluding force between particles diminishes，causing a further decrease in the internal friction angle.This study also employed nuclear magnetic resonance （NMR） testing to further analyze the microstructural changes and internal water distribution within the loess.The experimental results indicated that an increase in water content led to a significant rise in the main peak of the NMR transverse relaxation time （T ₂） spectral distribution.Additionally，the second wave peak exhibited a pronounced rightward shift，suggesting that water infiltrated the smaller pores.Furthermore，the small-sized pores within the soil appeared to expand and merge into medium-sized pores.The proportion of various pore types in soil samples under different water content conditions was also quantified through calculations.The findings indicate that an increase in water content leads to a reduction in the proportion of micropores within the soil，while the proportion of macropores increases，thereby enhancing the overall porosity of the loess soil sample.This phenomenon also contributes to the further diminution of the shear strength of the loess samples.

http://www.goldsci.ac.cn/article/2024/1005-2518/1005-2518-2024-32-5-860.shtml