Failure of debris flow fan is distinct from the common rock and soil slope failure due to the unique geotechnical features of debris flow sediment.Unfortunately,this type of failure was not comprehensively and systematically studied in the past.In our recent research,according to the unique characters of debris flow fan the Spencert slicing method with varying side force inclination is used to search for the noncircular critical slip surface.This is done using Spreadsheet-automated constrained optimization.The results are then compared with the in-situ slip surface.A practical subroutine is developed to interpret the stability of slope with varying physical parameters along the depth by using Visual Basic Application embedded in Microsoft Excel.The in-situ experiment shows that the water content hardly change 50 cm beneath the slope surface and no obvious tension cracks are observed behind the slope.However in the soil layer where the actual sliding occurred,great alteration of water content under concentrated artificial rainfall happened,which is in consistent with the results of numerical simulation.With the increase in height of the soil layer,the calculated safety factor gradually decreases and an optimized noncircular slip surface with an average height of 2.4 m is obtained satisfying all the constrains and assumptions.It can be inferred that the slope failure of the debris flow fan is attributed to the dynamic seepage force caused by hydraulic gradient;and the slicing method is not suitable for the research of the mechanism of failure of debris flow fan.