为研究不同含水率下黄土的抗剪强度和细观结构特征，以秦王川重塑黄土为研究对象，利用四联自动直剪仪和核磁共振仪进行了直剪和核磁共振试验。试验结果表明：秦王川黄土重塑土的总体抗剪强度随含水率的增大而降低，黏聚力随含水率的增大呈先增大后减小的变化趋势，内摩擦角呈先减小后增大再减小的变化趋势；含水率变化对土体黏聚力的影响较大，而对内摩擦角的影响较小；土样内部孔隙率随含水率的增大而增大，水分不断从小尺寸孔隙向中大尺寸孔隙扩散，且土体内部的小尺寸孔隙有不断扩展和贯通的趋势；随着含水率的增大，土样的延性和塑性也得到增强，应力—应变曲线特征由应变软化转变为应变硬化；随着含水率的增大，土样内部水分的形态也逐渐变化，由强结合水主导不断向弱结合水主导发展，最后变成自由水主导，导致土颗粒之间的胶结作用力、水膜作用力以及基质吸力减弱，进而导致土样总体抗剪强度降低。本研究成果可为地区黄土工程建设提供数据支持和理论参考。

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 （T₂） 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.