黑方台地区因灌溉水下渗，导致地下水位上升，深部土体长期处于饱水状态。地下水溶滤作用带走大量盐分，同时改变了孔隙水溶液中的化学成分，影响着黄土的抗剪强度。采用环剪仪、激光粒度分析仪、Zeta电位仪、电感耦合等离子体发射光谱仪、离子色谱仪等装置从强度和物理化学作用等方面对其进行研究。试验结果显示，黄土抗剪强度及其参数（内摩擦角）－浸水时间曲线呈“勺形”。结合相应的物理化学测试结果，分析了黄土强度弱化的浸水时效机制：浸水初期，胶结物（易溶盐）迅速溶解，黄土微结构破坏，内摩擦角显著降低；同时孔隙水离子浓度增大，与黏粒反离子层发生离子交换作用，黏粒结合水膜厚度变小，致使内摩擦角稍有增大；随着浸水时间的增加，中溶盐石膏逐渐溶解于氯化钠溶液中，导致黄土中粗颗粒进一步分散解体，黏粒含量增加，双电层总厚度有所增大，内摩擦角稍有降低。

The infiltration of irrigation water on Heifangtai loess plateau raises the groundwater level and saturates the deep soil all the time. Lixiviation caused by underground water takes away much salt and changes chemical components in pore water at the same time, which affects the strength of loess. In this study, the shear strength and physicochemical characteristics of loess are investigated using ring shear apparatus, laser particle size (LPS) analyzer, Zeta probe (potential measuring apparatus), inductively coupled plasma emission spectrometer (ICP-OES), ion chromatograph. The test results show that the curve of strength (internal friction angle)－soaking time is in a “spoon” shape. Based on the results of physical and chemical tests, mechanism of strength weakening of loess soaked in water is discussed. The cements (soluble salt) among loess particles dissolve in water rapidly, breaking the microstructure and making the internal friction angle decrease. At the same time, ion concentration in the pore water increases, and the ions exchanged with outer layer of clay particles, leading to a decrease in the thickness of adsorbed water of clay particle so as to make the internal friction angle increase slightly. With the increase of soaking time, gypsum dissolves in sodium chloride solution and coarse particles are dispersed into clay particles further, then the total thickness of electrical double layer increases, so the internal friction angle decreases slightly.