Effect of 3D water table profile of horizontal drains on slope stability and idealization of 3D-FEM flow modeling to 2D-FEM flow modeling

Horizontal drains (HDs) are commonly used in the groundwater regime management of landslides. The groundwater table (GWT) profile of slopes with HDs have a complicated formation in three-dimensional (3D) space, requiring 3D analyses to obtain accurate results. However, owing to the complexity of 3D simulations, idealized two-dimensional (2D) cross sections are widely used in numerical simulations of such slopes. Unfortunately, stabilities are overestimated by 2D simulations because the 3D variation of the GWT is neglected. Finite element analysis is performed in this study to evaluate the effect of 3D variation of the GWT on the stability of slopes with HDs and to evaluate the effectiveness of 2D idealizations. The results demonstrate that idealized 2D analyses neglect the high pore water pressures between HDs, thereby overestimating the slope stability, especially with high rainfall intensities and large drain spacings. Alternatively, accurate results can be obtained in 2D analyses by manually estimating an average GWT profile using the Crenshaw and Santi method for steady-state conditions. Each method has its own limitations and, therefore, the selection of an appropriate method should be made based on the specific conditions and requirements of the problem.