The development of agriculture in the arid and semiarid regions of China mainly depends on agricultural irrigation. Until 2016, water required for agricultural irrigation has accounted for more than 90% of the total water consumption. But traditionally extensive broad irrigation causes frequent loess geological hazards in irrigation area and it threatens security of local life and property. In this paper, we selected the Heifangtai irrigation district in Yongjing County, Gansu Province, where frequent instabilities occur, as the study area. We used laboratory tests and numerical simulation to examine the mechanism of loess landslides owing to the irrigation hydrological cycle. Irrigation changes the local natural hydrogeological conditions because of the loose and macroporous structure of loess. The numerous pores and fissures constitute preferential migration pathways of irrigation water; thus, irrigation can increase the groundwater level and hydraulic gradient. Broad irrigation is the main inducing factor of geological hazards (including landslides, collapses, and soil salinization) in arid and semiarid regions, and the development of fissures and sinkholes increases the risk of landslides. New water‐saving irrigation methods need to replace the traditional irrigation methods and improve the utilization of water resources. A monitoring and warning system in susceptible areas should be established to ensure the sustainability of local agriculture.

With the implementation of China’s western development and “One Belt, One Road” initiative, there are more and more projects in the collapsibility loess area, and the collapsibility loess problem encountered in the construction is becoming more and more prominent. In this paper, the collapsibility loess of the south extension section of Xi’an Metro Line 2 is investigated, and its collapsibility characteristics are studied through a large‐scale site immersion test. The test site was a 15 m diameter circular test pit, which took 35 days for water injection and 60 days for observation after water stopped. The test results showed that the maximum self‐weight collapsibility of the soil layer in the test pit is 32 mm, and the deformation amount is 10.05 mm in Q₃ and 9.55 mm in Q₂. The maximum deformation amount of 32 mm is less than 70 mm in the shallow marker; it may be caused by the paleosol layer as a bridge to provide a support to the overlying soil layer. The shape of the sphere of influence after immersion resembles a trumpet, slightly protruding outwards from the paleosol. The scope of influence between the infiltrated and saturated zones gradually increases with depth, and the saturated zone is generally smaller than the infiltrated zone. The research results of this paper can provide technical support and reference for the construction of Xi’an Metro Line 2 and other related projects in the region.

Chinese Loess Plateau (CLP) has become one of the high‐incidence areas of landslide disasters in China, especially at the edge of the tablelands where irrigation has been practiced at a large scale since the 1960s. Heifangtai tableland, located in the CLP in Yongjing County, Gansu Province, has been selected as a case, where more than 90 slidings occurred in the past three decades. Field monitoring and laboratory tests were conducted to obtain the soil‐water characteristic curve, unsaturated soil shear strength, irrigation water infiltration, and the groundwater level change. Based on these results, a three‐dimensional numerical model of the slope was established, and the change of seepage field and slope stability before and after irrigation was investigated using simulations and compared with the observed data. The results show that flood irrigation raised the water table, and the increase of soil moisture in the flooded area was more significant than that in the nonirrigated area. The rising speed of the groundwater level was about 0.25 m/yr, with an amplitude of 0.5 m. Near the slope, the hydraulic gradient of the concave slope was steeper than the convex slope. The shear strength of the loess decreased with increasing soil moisture, and the tensile strength was about 15% of the cohesion. Numerical results showed that after three years of continuous flood irrigation at the back of the slope, the slope stability coefficient decreased by 0.12. After irrigation, the potential slip zone slightly expanded. The reason why landslides often occurred at the back of the gully or the landslide was that the water table was shallower on concave slopes and the soil moisture of the concave slopes was more susceptible to irrigation.