Abstract: The saturated loess layer is susceptible to liquefaction failure, as evidenced by the liquefaction loess landslide in Zhongchuan triggered by the Jishishan M6.2 earthquake, and the Heifangtai loess landslides serve as typical examples. Nonetheless, the infiltration mechanism in loess remains elusive. A field irrigation test was carried out at the rear boundary of a representative landslide in Heifangtai, with the water infiltration process monitored using the 3D electrical resistivity tomography method. The cumulative rate of resistivity change was employed to depict the overall infiltration process, while the disparity in resistivity change rates between adjacent measurement intervals was utilized to identify the primary seepage pathways. The infiltration mechanism in loess was examined under varying recharge intensities. The main conclusions are as follows: (1) The time-series 3D electrical resistivity tomography method can effectively detect the infiltration process in loess. (2) There co-exists matrix flow and preferential flow in the test, the matrix flow is the main infiltration mode during the test. The preferential flow shows the gradual seepage around the preferential channel extending to a depth of 4 m, and the seepage area is affected by the distribution of preferential channel. (3) The infiltration mode in the fissured loess is affected by the intensity of water recharge. With the flood irrigation or rainfall water ponding, the water in the fissures can directly penetrate into the deep soil forming a preferential flow. While in case of rainfall without water ponding, there is no obvious preferential flow observed, and the water infiltrate only with matrix flow. The results can provide reference for the study of field infiltration test and infiltration mechanism in loess.

Key words: Heifangtai, loess landslide, field infiltration test, preferential flow, electrical resistivity tomography