Abstract: The permeability coefficient function of unsaturated soil is difficult to be measured and a long time is required. Wetting front advancing method(WFM) can be applied to measure the hydraulic conductivity function(HF) of unsaturated soil in a short time. However, this method relies on an artifical recognition of wet front, which has certain limitations. In addition, the measurement accuracy of WFM is not clear and needs to be verified. In this paper, impacts of parameters include of initial water content, critical water content of wetting front, rainfall intensity, location of sensor etc on the accuracy of WFM is analyzed. The Seep/W software is used to simulate the infiltration process occurs in the homogeneous soil column. The data is analyzed by WFM to calculate the permeability coefficient of the soil. Simulation results are compared with the input permeability coefficient(i.e. the real solution). The calculation accuracy of the wetting front method is evaluated and the source of error is discussed. The results show that WFM can obtain relative high accurate calculation results. Using the characteristic moisture content at the wetting front to calculate wetting front advancing rate, it overcomes the limitations of original wetting front advancing method with artificial observation and greatly expands the applicability of WFM. The sensor spacing has no direct effect on the calculation accuracy of the permeability coefficient function of wetting frontal advance method. The lower the initial water content, the greater the rainfall infiltration rate, and the greater the span of the permeability coefficient function. Based on the analysis in this paper, the following suggestions are proposed for test design of WFM. The soil column with a length of 30-40 cm and 3-4 sensors are suggested. Any initial water content is acceptable and the dry sample is the best choice. To avoid the surface ponding problem, the rainfall intensity should be smaller than the saturated permeability coefficient of soil.

Key words: unsaturated permeability coefficient, rainfall infiltration, wetting front, wetting front advancing method, soil pile test